QETA/018 Manual Metal Arc (MMA)

Welding501/1130/9

EAL Level 3 Diploma in Engineering Technology (QCF)

Session Aims

18.1 Understand the principles of Manual Metal Arc (MMA) Welding18.2 Understand Metallurgy associated with welding18.3 Understand welding health and safety18.4 Understand equipment associated with Manual Metal Arc (MMA) Welding18.5 Understand the consumables used in Manual Metal Arc (MMA) welding18.6 Understand welding procedures and methods of testing applied to Manual Metal Arc Welding

18.1Understand the principles of Manual Metal Arc (MMA) WeldingYou will be able to:• Describe electrical/electric arc theory• Describe fusion arc welding principles• Describe specific principles applicable to MMA Welding• Identify types of welds and joints

The Nature of Electricity (the Flow of Electrons)Electron flow is what we think of as electrical current. We are familiar with two types of electron flow, Direct Current, or DC, and Alternating Current, or AC. Direct Current is the kind of electrical flow we get from batteries and solar cells, when electrons travel in only one direction.

An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second. Electric current is measured using a device called an ammeter.

The Nature of Electricity (the Flow of Electrons)Electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators.The particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom, and can move freely about within the metal. These conduction electrons are the charge carriers in metal conductors.

AC and DC CurrentAC and DCIf the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5V. The diagram shows an oscilloscope screen displaying the signal from a DC supply.

If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply.

Sinusoidal Waveform and PolaritySinusoidal Waveform (or Sine Wave)The sine wave or sinusoid is a mathematical curve that describes a smooth repetitive oscillation. It is named after the function sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields.

The sine wave is important in physics because it retains its wave shape when added to another sine wave of the same frequency and arbitrary phase and magnitude. It is the only periodic waveform that has this property.https://en.wikipedia.org/wiki/File:ComplexSinInATimeAxe.gif

Electrical PolarityElectrical polarity (positive and negative) is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only.

Electrical Polarity

Voltage, Current, Resistance, Power and Energy

VoltageVoltage, also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field. The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time) through a conducting or semiconducting medium for a given resistance to the flow. Voltage is symbolized by an uppercase italic letter V or E. The standard unit is the volt, symbolized by a non-italic uppercase letter V. One volt will drive one coulomb (6.24 x 1018) charge carriers, such as electrons, through a resistance of one ohm in one second.

Voltage, Current, Resistance, Power and Energy

CurrentCurrent is a flow of electrical charge carriers, usually electrons or electron-deficient atoms. The common symbol for current is the uppercase letter I. The standard unit is the ampere, symbolized by A. One ampere of current represents one coulomb of electrical charge (6.24 x 1018 charge carriers) moving past a specific point in one second. Physicists consider current to flow from relatively positive points to relatively negative points; this is called conventional current or Franklin current. Electrons, the most common charge carriers, are negatively charged. They flow from relatively negative points to relatively positive points.Electric current can be either direct or alternating. Direct current (DC) flows in the same direction at all points in time, although the instantaneous magnitude of the current might vary. In an alternating current (AC), the flow of charge carriers reverses direction periodically. The number of complete AC cycles per second is the frequency, which is measured in hertz. An example of pure DC is the current produced by an electrochemical cell. The output of a power-supply rectifier, prior to filtering, is an example of pulsating DC. The output of common utility outlets is AC.

Voltage, Current, Resistance, Power and Energy

ResistanceResistance is the opposition that a substance offers to the flow of electric current. It is represented by the uppercase letter R. The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega.When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm. (For more discussion of the relationship among current, resistance and voltage, see Ohm's law.)

Voltage, Current, Resistance, Power and Energy

PowerElectrical power is the rate at which electrical energy is converted to another form, such as motion, heat, or an electromagnetic field. The common symbol for power is the uppercase letter P. The standard unit is the watt, symbolized by W. In utility circuits, the kilowatt (kW) is often specified instead;1 kW = 1000 W.One watt is the power resulting from an energy dissipation, conversion, or storage process equivalent to one joule per second. When expressed in watts, power is sometimes called wattage. The wattage in a direct current (DC) circuit is equal to the product of the voltage in volts and the current in amperes. This rule also holds for low-frequency alternating current (AC) circuits in which energy is neither stored nor released. At high AC frequencies, in which energy is stored and released (as well as dissipated or converted), the expression for power is more complex.

Voltage, Current, Resistance, Power and Energy

Energy• Energy causes things to happen around us. Look out the window.• During the day, the sun gives out light and heat energy. At night, street lamps use electrical

energy to light our way.• When a car drives by, it is being powered by gasoline, a type of stored energy.• The food we eat contains energy. We use that energy to work and play.The definition of energy in the introduction:"Energy Is the Ability to Do Work."Energy can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy

Energy makes everything happen and can be divided into two types:• Stored energy is called potential energy.• Moving energy is called kinetic energy.

Energy is measured in many ways.One of the basic measuring blocks is called a Btu. This stands for British thermal unit and was invented by, of course, the English.Btu is the amount of heat energy it takes to raise the temperature of one pound of water by one degree Fahrenheit, at sea level.One Btu equals about one blue-tip kitchen match.One thousand Btus roughly equals: One average candy bar or 4/5 of a peanut butter and jelly sandwich.It takes about 2,000 Btus to make a pot of coffee.Energy also can be measured in joules. Joules sounds exactly like the word jewels, as in diamonds and emeralds. A thousand joules is equal to a British thermal unit.1,000 joules = 1 Btu

Voltage, Current, Resistance, Power and Energy

Voltage, Current, Resistance, Power and Energy

A piece of buttered toast contains about 315 kilojoules (315,000 joules) of energy. With that energy you could:• Jog for 6 minutes• Bicycle for 10 minutes• Walk briskly for 15 minutes• Sleep for 1-1/2 hours• Run a car for 7 seconds at 80 kilometres per hour (about 50 miles per hour)• Light a 60-watt light bulb for 1-1/2 hours• Or lift that sack of sugar from the floor to the counter 21,000 times!

Voltage, Current, Resistance, Power and Energy

Nature and characteristics of the Arc (AC & DC)

An electric arc or arc discharge is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current through normally nonconductive media such as air. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".

A drawn arc can be initiated by two electrodes initially in contact and drawn apart; this can initiate an arc without the high-voltage glow discharge. This is the way a welder starts to weld a joint, momentarily touching the welding electrode against the workpiece then withdrawing it till a stable arc is formed. Another example is separation of electrical contacts in switches, relays and circuit breakers; in high-energy circuits arc suppression may be required to prevent damage to contacts.

Industrially, electric arcs are used for welding, plasma cutting, for electrical discharge machining, as an arc lamp in movie projectors and follow spots in stage lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is made in this way as it requires a large amount of energy to promote an endothermic reaction (at temperatures of 2500 °C).Spark plugs are used in internal combustion engines of vehicles to initiate the combustion of the fuel in a timed fashion.Spark gaps are also used in electric stove lighters(both external and built-in).

Nature and characteristics of the Arc (AC & DC)

Arc Power and Energy (Arc Voltage and Heat Input)Arc VoltageIt is the voltage that appears across the contacts of the circuit breaker during the arcing period. As soon as the contacts of the circuit breaker separate, an arc is formed. The voltage that appears across the contacts during arcing period is called the arc voltage. Its value is low except for the period the fault current is at or near zero current point. At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of arc.

Heat InputHeat input (the present best practice term, as it provides a more relevant way of comparing arc welding processes) considers the effect process efficiency has on the energy that actually reaches the workpiece to form the weld. HI is given by the following relationship to Arc Energy or AE. The heat source of an Arc can get up to 6000 degrees Celsius.

Magnetic Arc BlowMagnetic arc blow or "arc wander" is the deflection of welding filler material within an electric arc deposit by a build up of magnetic force surrounding the weld pool. Magnetic arc blow can occur because of:• Workpiece connection• Joint design• Poor fit-up• Improper settings• Atmospheric conditionsArc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative).

Magnetic arc blow is popularly attributed to a change in the direction of current as it flows into and through the workpiece. Magnetic arc blow is known to begin at field densities as low as 10 gauss and becomes severe at densities of, equal to or greater than, 40 gauss; it is directional and can be classified as forward or backward moving along the joint, but can occasionally occur to the sides depending on the orientation of the poles to the workpiece.

Magnetic arc blow is more common in DC welding than in AC welding.

Magnetic Arc Blow

Fusion Arc Welding Principles

Weld FormationTo strike the electric arc, the electrode is brought into contact with the workpiece by a very light touch with the electrode to the base metal then is pulled back slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld pool. As the electrode melts, the flux covering disintegrates, giving off shielding gases that protect the weld area from oxygen and other atmospheric gases. In addition, the flux provides molten slag which covers the filler metal as it travels from the electrode to the weld pool.

Once part of the weld pool, the slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, it must be chipped away to reveal the finished weld. As welding progresses and the electrode melts, the welder must periodically stop welding to remove the remaining electrode stub and insert a new electrode into the electrode holder..

This activity, combined with chipping away the slag, reduces the amount of time that the welder can spend laying the weld, making SMAW one of the least efficient welding processes.

The actual welding technique utilized depends on the electrode, the composition of the workpiece, and the position of the joint being welded. The choice of electrode and welding position also determine the welding speed. Flat welds require the least operator skill, and can be done with electrodes that melt quickly but solidify slowly. This permits higher welding speeds

Fusion Arc Welding Principles

‘Metal Arc’An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. The workpiece and the electrode melts forming the weld pool that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapours that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.

Function of ElectrodesWelding electrodes in arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.

Function of ElectrodesThe choice of electrode for SMAW depends on a number of factors, including the weld material, welding position and the desired weld properties. The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidizers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality. Electrodes can be divided into three groups—those designed to melt quickly are called "fast-fill" electrodes, those designed to solidify quickly are called "fast-freeze" electrodes, and intermediate electrodes go by the name "fill-freeze" or "fast-follow" electrodes. Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized, while fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying.

The composition of the electrode core is generally similar and sometimes identical to that of the base material. But even though a number of feasible options exist, a slight difference in alloy composition can strongly impact the properties of the resulting weld. This is especially true of alloy steels such as HSLA steels.

Likewise, electrodes of compositions similar to those of the base materials are often used for welding nonferrous materials like aluminium and copper. However, sometimes it is desirable to use electrodes with core materials significantly different from the base material. For example, stainless steel electrodes are sometimes used to weld two pieces of carbon steel, and are often utilized to weld stainless steel workpieces with carbon steel workpieces.

Electrode coatings can consist of a number of different compounds, including rutile, calcium fluoride, cellulose, and iron powder. Rutile electrodes, coated with 25%–45% TiO2, are characterized by ease of use and good appearance of the resulting weld. However, they create welds with high hydrogen content, encouraging embrittlement and cracking. Electrodes containing calcium fluoride (CaF2), sometimes known as basic or low-hydrogen electrodes, are hygroscopic and must be stored in dry conditions. They produce strong welds, but with a coarse and convex-shaped joint surface. Electrodes coated with cellulose, especially when combined with rutile, provide deep weld penetration, but because of their high moisture content, special procedures must be used to prevent excessive risk of cracking. Finally, iron powder is a common coating additive that increases the rate at which the electrode fills the weld joint, up to twice as fast.

Function of Electrodes

Principles of MMA Welding

Open Circuit and Arc VoltageThe highest voltage is the open circuit voltage of the power source. Once the arc is struck the voltage rapidly falls as the gases in the arc gap become ionised and electrically conductive, the electrode heats up and the size of the arc column increases. The welding current increases as the voltage falls until a point is reached at which time the voltage/current relationship becomes linear and begins to follow Ohms Law. What is important to note from Fig. 1 is that as the arc length changes both the voltage and welding current also change – a longer arc giving higher voltage but with a corresponding drop in welding current and vice versa.

ISO Weld SymbolsISO Weld Symbols

Please refer to the following links:http://www.draftsperson.net/images/6/63/Weld_Symbols.png

http://triblab.teipir.gr/files/Welding/Lab/CH3_1_Welding_joint_symbols.pdf

Refer to the different types of welded joints such as Butt, Tee, Lap and Corner.

Fillet and Butt Weld CharacteristicsFillet Weld Characteristics:• Leg Length• Throat Thickness• Penetration• Number of Runs• Surface Finish• Weld Toes and Weld Profile

Butt Weld Characteristics:• Types of Preparation• Number of Runs• Excess weld metal• Penetration• Surface Finish

18.2 Understand metallurgy associated with welding1. Describe the effects of heating and cooling metals2. Describe the effects of welding on metals3. Describe how cracking occurs in welds4. Describe residual stress

The Effects of Heating and Cooling Metals

Heating Metals and Colling slowly and rapidlyHeat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.With the exception of stress-relieving, tempering, and aging, most heat treatments begin by heating an alloy beyond the upper transformation temperature. This temperature is referred to as an "arrest" because, at the upper transformation temperature nothing happens. Therefore, the alloy must be heated above the temperature for a transformation to occur. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution.

Because a smaller grain size usually enhances mechanical properties, such as toughness, shear strength and tensile strength, these metals are often heated to a temperature that is just above the upper critical temperature, in order to prevent the grains of solution from growing too large. For instance, when steel is heated above the upper critical temperature, small grains of austenite form. These grow larger as temperature is increased. When cooled very quickly, during a martensite transformation, the austenite grain-size directly affects the martensitic grain-size. Larger grains have large grain-boundaries, which serve as weak spots in the structure. The grain size is usually controlled to reduce the probability of breakage.

The diffusion transformation is very time-dependent. Cooling a metal will usually suppress the precipitation to a much lower temperature. Austenite, for example, usually only exists above the upper critical temperature. However, if the austenite is cooled quickly enough, the transformation may be suppressed for hundreds of degrees below the lower critical temperature. Such austenite is highly unstable and, if given enough time, will precipitate into various microstructures of ferrite and cementite. The cooling rate can be used to control the rate of grain growth or can even be used to produce partially martensitic microstructures. However, the martensite transformation is time-independent. If the alloy is cooled to the martensite transformation (Ms) temperature

before other microstructures can fully form, the transformation will usually occur at just under the speed of sound

The Effects of Heating and Cooling Metals

The Effects of Heating and Cooling Metals

Microstructures of Welded Joints

Unlike in casting, during welding, where the molten pool is moved through the material, the growth rate and temperature gradient vary considerably across the weld pool. Geometrical analyses have been developed that relate welding speed to the actual growth rates of the solid at various locations in the weld pool.

Along the fusion line the growth rate is low while the temperature gradient is steepest. As the weld centre line is approached, the growth rate increases while the temperature gradient decreases. Consequently, the microstructure that develops varies noticeably from the edge to the centre line of the weld. Most of these microstructural features can be interpreted by considering classical theories of nucleation and growth.

In welds, weld pool solidification often occurs without a nucleation barrier. Therefore, no significant undercooling of the liquid is required for nucleation of the solid. Solidification occurs spontaneously by linear growth on the partially melted grains. This is the case during autogenous welding. In certain welds, where filler metals are used, inoculants and other grain-refining techniques are used in much the same way as they are in casting practices. In addition, dynamic methods for promoting nucleation such as weld-pool stirring and arc oscillation have been used to refine the weld metal solidification structure. Although the mechanisms of nucleation in weld metal are reasonably well understood, not much attention is given to modelling this phenomenon.

Often, weld solidification models assume epitaxial growth and for most of the cases the assumption seems to be appropriate. However, to describe the effects of inoculants, arc oscillations, and weld pool stirring, heat and mass transfer models have to be coupled with either probabilistic models such as cellular automata or deterministic models using the fundamental equations of nucleation as described elsewhere

Microstructures of Welded Joints

Microstructures and the Changes produced by welding

During growth of the solid in the weld pool, the shape of the solid-liquid interface controls the development of microstructural features. The nature and the stability of the solid-liquid interface is mostly determined by the thermal and constitutional conditions (constitutional supercooling) that exist in the immediate vicinity of the interface. Depending on these conditions, the interface growth may occur by planar, cellular, or dendritic growth. Dendritic growth of the solid, with its multiple branches, is shown in Figure 3. Another example of changes in solidification morphology directly related to welding conditions is shown in Figure 4. This figure shows a spot weld on a nickel-based superalloy in which the morphology changes from cellular to dendritic as the growth velocity increases toward the centre of the spot weld after the spot weld arc is extinguished. The micrograph also shows the elimination of a poorly aligned dendrite

Microstructures and the Changes produced by welding

Cracking in WeldingDefinition - What does Cold Cracking mean?• Cold cracking is cracking that occurs as the result of hydrogen

dissolving in the weld metal and then diffusing into the heat affected zone (HAZ). Cold cracks mostly develop long after the weld metal solidifies, but sometimes appear sooner. Cold weld cracking occurs at temperatures well below 600°F. It is considered a serious welding defect because it can significantly affect the integrity of infrastructure.• Cold cracking is also known as hydrogen-induced cracking, delayed

cracking or underbead cracking.

What exactly is Cold Cracking??Cold cracking is especially common in thick materials, as they tend to create areas of high restraint and can serve as a heat sink that leads to fast cooling rates. Rapid cooling causes the microstructure in the HAZ to form a new crystalline microstructure called martensite, which very hard, very brittle and lacks ductility. Martensite provides a location for diffusible hydrogen to gather, which causes residual stresses to build in the HAZ. Once these residual stresses reach a critical level, cold cracking occurs.Cold cracking may occur for the following reasons:• There is hydrogen in the weld material or atmosphere• Susceptible microstructure (martensite)• Mechanical stresses (thermal or residual stresses)• The following can be done to prevent cold cracking of a metal:• Pre-heating the base material in order to reduce the speed of cooling, preventing the formation of

martensite on the weld and allowing the hydrogen to be removed from the weld• Reducing tension concentration, avoiding discontinuities on the weld or carefully selecting the

disposition of the welds and the assembly sequence of the structure• Using welding consumables with low hydrogen to minimize the hydrogen diffusion on the weld• Selecting the appropriate welding process

Cracking in Welding

How Hydrogen appears in Cold Cracking

Weld metal hydrogen contentThe principal source of hydrogen is moisture contained in the flux, i.e. the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is influenced by the electrode type. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.It is important to note that there can be other significant sources of hydrogen, e.g. from the material, where processing or service history has left the steel with a significant level of hydrogen or moisture from the atmosphere. Hydrogen may also be derived from the surface of the material or the consumable.• Sources of hydrogen will include:• oil, grease and dirt• rust• paint and coatings• cleaning fluids

How Hydrogen appears in Cold Cracking

Lamellar TearingAs lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance.Causes• It is generally recognised that there are three conditions which must be satisfied for lamellar

tearing to occur:• Transverse strain - the shrinkage strains on welding must act in the short direction of the

plate ie through the plate thickness• Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions• Material susceptibility - the plate must have poor ductility in the through-thickness direction• Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in

the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen

• Factors to be considered to reduce the risk of tearing• The choice of material, joint design, welding process, consumables, preheating and

buttering can all help reduce the risk of tearing.

Lamellar Tearing

Hot Cracking (1)Solidification cracks can appear in several locations, and orientations, but most commonly are longitudinal centreline cracks (coincident with the intersection of grains growing from opposite sides of the weld), or 'flare' cracks, again longitudinal, but at an angle to the through-thickness direction ( Fig.1).

Where there is a central segregate band in the plate, cracking may extend from this position at the fusion boundary ( Fig.2). The cracks in all locations can be buried (Fig.3) or surface-breaking.

Hot Cracking (1)

Hot Cracking (2)Cracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%.Weld metal composition is dominated by the consumable and as the filler is normally cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Plate composition assumes greater importance in high dilution situations such as when welding the root in butt welds, using an autogenous welding technique like TIG, or a high dilution process such as submerged arc welding.

Reheat Cracking Issues

If possible, avoid welding steels known to be susceptible to reheat cracking. For example, A 508 Class 2 is known to be particularly susceptible to reheat cracking, whereas cracking associated with welding and cladding in A508 Class 3 is largely unknown. The two steels have similar mechanical properties, but A508 Class 3 has a lower Chromium content and a higher manganese content.Similarly, in the higher strength, creep-resistant steels, an approximate ranking of their crack susceptibility.

Thus, in selecting a creep-resistant, chromium molybdenum steel, 0.5Cr 0.5Mo 0.25V steel is known to be susceptible to reheat cracking but the 2.25Cr 1Mo which has a similar creep resistance, is significantly less susceptible.Unfortunately, although some knowledge has been gained on the susceptibility of certain steels, the risk of cracking cannot be reliably predicted from the chemical composition. Various indices, including ΔG1, PSR and Rs, have been used to indicate the susceptibility of steel to reheat cracking. Steels which have a value of ΔG1 of less than 2, PSR less than zero or Rs less than 0.03, are less susceptible to reheat cracking

Reheat Cracking Issues

Reheat Cracking

The welding procedure can be used to minimise the risk of reheat cracking by• Producing the maximum refinement of the coarse grain HAZ• Limiting the degree of austenite grain growth• Eliminating stress concentrationsThe procedure should aim to refine the coarse grained HAZ by subsequent passes. In butt welds, maximum refinement can be achieved by using a steep-sided joint preparation with a low angle of attack to minimise penetration into the side-wall, ( Fig 2a). In comparison, a larger angle V preparation produces a wider HAZ, limiting the amount of refinement achieved by subsequent passes, ( Fig 2b). Narrow joint preparations, however, are more difficult to weld, due to the increased risk of lack of side-wall fusion.

Reheat Cracking

Reheat Cracking• The weld toes of the capping pass are particularly vulnerable, as the

coarse grained HAZ may not have been refined by subsequent passes. In susceptible steel, the last pass should never be deposited on the parent material, but always on the weld metal, so that it will refine the HAZ.• Grinding the weld toes with the preheat maintained has been

successfully used to reduce the risk of cracking in 0.5Cr 0.5Mo 0.25V steels.

Thermal Cycle in Welding

• In the present investigation, thermal simulated specimens were used to investigate the effect of welding cooling time and peak temperature on characteristic fracture toughness and microstructure feature of heat-affected zone (HAZ) for an 800 MPa grade high strength low alloy (HSLA) steel. It is found that the fracture toughness is the best for the simulated coarse-grained HAZ.

• In addition, the size of prior austenite grain, and the volume fraction of bainitic ferrite and constituent increase with increasing the cooling time. However, the volume fraction of martensite decreases with increasing the cooling time. Remarkable decrease of toughness is observed with increasing the size of austenite grain and the volume fraction.

Understand Welding Health & Safety You will be able to:• Work safely whilst performing welding and welding related activities• Describe Health and Safety issues associated with Welding related

activities• Describe the safe working practices observed when using the Manual

Metal Arc (MMA) welding process• Describe Electrical hazards associated with welding plant and safe

working procedures adopted

Health and Safety Legislation and Current Regulations applicable to the Welding Process

Refer back to your previous notes on from QETA/001 module:• Health and Safety at Work Act 1974• Personal Protective Equipment Regulations• Control of substances hazardous to health regulations• Management of Health and Safety at Work Regulations• Reporting of Injuries, diseases and dangerous occurrences regulations

(R.I.D.D.O.R)• Provision and use of work equipment regulations• Noise at Work Regulations

Personal Protective Equipment (PPE)

Personal Protective Equipment (PPE) and the reasons for need:• Protection of others from Hazards• Hot Materials• Sparks• Falling Objects• Heat• Burns• Safe Start-up and shutdown procedures

Arc RadiationArc Radiation and the hazards caused by:• Visible Light - Intense visible light or 'blue light', passes through the cornea and

lens and can dazzle and, in extreme cases, damage the network of optical nerves in your eye. • Infra-Red - is perceptible as heat. The main hazard to the eyes is that prolonged

exposure causes a gradual but irreversible opacity of the lens. The infrared radiation emitted by normal welding causes damage only within a short distance from the arc. There is an immediate burning sensation in the skin surrounding the eyes should they be exposed to arc heat. The natural human reaction is to move or cover up to prevent the skin heating, which also reduces eye exposure.• Ultra-Violet – UV is generated by all arc processes. Excess exposure causes skin

inflammation, and possibly even skin cancer or permanent eye damage. However the main risk amongst welders is for inflammation of the eye, commonly known as 'arc eye' or 'flash‘.

Types of Hazards from Fumes during Welding

Types of fumes:• Particulate - Particulate fume is made up of, discrete, solid particles, As the

particles are tiny and most of the fume falls into the 'respirable' size range. Respirable particulate fume can be breathed in, reach the lungs, and stay there.• The fume is made up mainly of oxides and silicates from the metals present in

the consumable and, to some extent, the parent material being used.• Whether the fume is likely to cause damage depends largely on the material,

or the concentration of it and the length of exposure to it.

Gaseous - consists of either one or more pollutant gases, mixed around the welding area. In its gaseous state it can easily enter the lungs. Whether the fume causes damage, depends on what the gas is, and the concentration consumed and the length of time you are exposed to it.

Fumes may be formed by welding, or the radiation from it, or the air surrounding the arc, something within the flux or coatings or contaminants on the component. Gaseous fumes are not emitted by the metal or the consumable.

Types of Hazards from Fumes during Welding

• Fume Gases - The fume given off by welding and hot cutting processes is a varying mixture of airborne gases and very fine particles which if inhaled can cause ill health. Gases that may be present in welding and cutting fume are:• nitrous oxide (NOx),• carbon dioxide (CO2),• carbon monoxide (CO)• shielding gas (eg Argon, helium)• ozone (O3)

Types of Hazards from Fumes during Welding

The visible part of the fume cloud is mainly particles of metal, metal oxide and flux (if used). The exact level of risk from the fume will depend on 3 factors:

• How toxic the fume is• How concentrated the fume is• How long you are breathing the fume

Types of Hazards from Fumes during Welding

Health Effects of Welding FumesHow toxic is the fume?For arc welding, the visible fume comes mostly from the filler wire when it’s exposed to the electric arc. The amount of hazardous substances in the filler wire should be included in the product information that is printed on the original packaging. Many of the common metals used in filler wires are harmful and several have Workplace Exposure Limits (WEL).

Cadmium and Beryllium are rarely found, but are particularly toxic. Chromium, Nickel, Vanadium, Manganese and Iron all have WEL’s. Generally the smaller the number for the WEL the more toxic the substance is. The toxic constituents of fume can be affected by the choice of welding process.

A full list can be found at: http://www.hse.gov.uk/pubns/priced/eh40.pdf

Methods of Fume/Gas Control• Extraction• Local Extraction• Air Fed Heat Shield (Yellow) • Respirator (Red)• Breathing Apparatus

Working in Confined SpaceA Confined Space: is a place which is substantially enclosed (though not always entirely), and where serious injury can occur from hazardous substances or conditions within the space or nearby (e.g. lack of oxygen).

• Asphyxiation Hazard• Risk of Explosion• Risks from Oxygen Enrichment - atmospheric gases are non-toxic but

alterations in their concentrations - especially that of oxygen - have an effect upon life and combustion processes.• Hazards to Health from Pollutants

Hazards from fire and safe working procedures adoptedYou will need to understand:• Flammable Materials• Suitable types of extinguishers• Identification of fire exit and evacuation procedues

Refer to the previous material from QETA001

Electrical Hazards

Electrical hazards associated with welding plant and safe working procedures adopted:• Fire• Electric Shock• Emergency procedures in the event of an electric shock• Uses of fuses• Electrical insulation• Use of earthing• Workpiece (welding)• Plant• Use of circuit breakers (including earth leakage circuit breakers – eclb)• Use of no-load low voltage protection devices

Health and Safety for WeldingHealth and safety issues associated with welding related activities:• Grinding and material removal• Safe disposal of waste• Workshop layout such as:1. Obstacles in the workshop2. Noise or heavy noise areas (muffling or distancing?)3. Hot Metal fragments or workpieces & safe places to put them4. Positioning of cables including welding torch leads & electrical and

gas cables/hoses

Moving LoadsHow to move loads:• Manual Methods• Overhead Cranes• Slings and other lifting aids

Please refer to the notes from QETA001 on Manual handling and mechanical aids for moving loads.

18.4 Understand Equipment Associated with Manual Metal Arc (MMA) WeldingYou will be able to:• Prepare and reinstate the work area and equipment for welding

operations• Set-up and shutdown the welding equipment safely and correctly for

the efficient welding of positional joints• Describe the equipment requirements for the MMA welding process• Describe the construction and operation of components• Describe the need for care and maintenance of equipment

Describe the equipment requirements for the MMA process

Types of Power Sources and their application:• Transformers - A transformer-style welding power supply converts the

moderate voltage and moderate current electricity from the utility mains (typically 230 or 115 VAC) into a high current and low voltage supply, typically between 17 to 45 (open-circuit) volts and 55 to 590 amperes. • Transformers/Rectifiers - A rectifier converts the AC into DC on more

expensive machines.

Inverters - They generally first rectify the AC power to DC; then they switch (invert) the DC power into a stepdown transformer to produce the desired welding voltage or current. The switching frequency is typically 10 kHz or higher.

Although the high switching frequency requires sophisticated components and circuits, it drastically reduces the size of the transformer, as the mass of magnetic components that is required for achieving a given power level goes down rapidly as the operating (switching) frequency is increased. The inverter circuitry also provides features such as power control and overload protection. The high frequency inverter-based welding machines are typically more efficient and provide better control of variable functional parameters than non-inverter welding machines

Describe the equipment requirements for the MMA process

Describe the equipment requirements for the MMA process

Types of Power Sources and their application:• Generators - Welding power supplies may also use generators or alternators

to convert mechanical energy into electrical energy. Modern designs are usually driven by an internal combustion engine. The power is converted first into mechanical energy then back into electrical energy to achieve the step-down effect similar to a transformer. Because the output of the generator can be direct current, or even a higher frequency ac current, these older machines can produce DC from AC without any need for rectifiers of any type, or can also be used for implementing formerly-used variations on so-called heliarc (most often now called TIG) welders, where the need for a higher frequency add-on module box is avoided by the alternator simply producing higher frequency ac current directly.

Power Source CharacteristicsSome Power Source Characteristics:• Power Source Duty Cycle - A duty cycle is the percentage of one period in

which a signal is active. A period is the time it takes for a signal to complete an on-and-off cycle.

• Drooping Characteristic (Constant Current) – The name given to power supplies, as the slant of the volt-ampere curve is drooping. If the welder maintains a constant arc length, the welding current will remain the same. Should the welder raise or lower the torch height, a new voltage intersection line is obtained. This movement raises or lowers the welding power supply output current. The curve is not straight but drooping when this occurs. Because of this, the machines may also be called "droopers".

Power Source Characteristics• Welding current – The definition of the current that passes through

the Arc of a weld whilst welding occurs. • Open Circuit Voltage - As the name implies, no current is flowing in

the circuit, because the circuit is open. The voltage is impressed upon the circuit, however, so that when the circuit is completed, the current will flow immediately.• Arc Voltage - The voltage across a welding arc.

Current control methodsPrinciples of different Current control methods:• Stepped Reactor - A means of controlling transformer output by selecting the

amount of windings to use. Step control is not preferred, because it is not a smooth, continuous control of transformer output.• Moving Core - Moving a core inside the reactor. A continuous current variation is

possible. The moving core changes the air gap which changes the reactance. Larger the air gap, smaller the impedance and higher the current.• Moving Coil - By changing the position of primary or secondary coil the magnetic

coupling will change. Lead screw is used to change the position of coils. Current is high when both the coils are near and less if far. Continuous current variations but require regular maintenance.• Moving Shunt - Changing the magnetic coupling between primary and secondary

by putting a movable magnetic shunt. Continuous current variations but require regular maintenance. Magnetic shunt causes the leakage flux to vary and thereby adjusts the output current.

Current control methods• Variable Resistance - A minimum variation in welding current is of paramount

importance in the production of satisfactory welds, inasmuch as it is one of the three basic factors that enable resistance welds to be made, i.e., current, pressure, and time.• Satiable Reactor - By putting a saturable reactor in the secondary circuit.

Eliminates moving parts but more expensive. Secondary reactor impedance is controlled by regulating the saturation level of the core electrically. DC control coil is used. If DC current is flowing in the coil the impedance is less, more output current and reverse in the case of lesser DC current.• Instrumentation e.g. ammeter, voltmeter – when using a welding transformer, it

is possible to attach and ammeter or voltmeter to the circuit so that you can measure the amperage or voltage of the welding.

Calibration of equipment aka Validation - is the procedure of proving that the welder is to the operating specification. Before it can be carried out it is necessary to establish the specification. This depends on the design, the specification to which its manufactured or a specification chose by the user. Generally, the manufacturer doesn’t state values, but if the equipment is manufactured to a recognised National, European or International standard, a level of accuracy is defined for current and arc voltage.

However, these are the minimum levels of accuracy and are usually insufficient for mechanised welding. Therefore, for more advanced equipment, the manufacturer may produce to a higher specification.

Current control methods

• Output Monitoring - The power source is monitoring the arc and making millisecond changes in order to maintain a stable arc condition. The term “constant” is relative. A CC power source will maintain current at a relatively constant level, regardless of fairly large changes in voltage, while a CV power source will maintain voltage at a relatively constant level, regardless of fairly large changes in current. • CC = Constant Current• CV = Constant Voltage

Current control methods

Construction and Operation of Components

• Welding Lead - is the electrical conductor for the welding current. It consists of a series of fine copper strands wrapped inside a non-conductive, durable jacket (typically some type of synthetic or natural rubber of various colours).• Electrode Holders – is a torch that grips the electrode in an ergonomic grip for

the user to handle safely• Return Lead - the return path for an electrical circuit made by connections to

earth at each end . • Workpiece Earth - Grounding literally refers to connecting any electrical

generator or conductor to the earth (the ground), thereby allowing electrical current to be discharged into the ground, and not transmitted into the air, people or other electrical conducting material.

• Return/Earth Clamps –

• Transformer - A transformer is a device that transfers electrical energy between two or more circuits through electromagnetic induction. This produces an electromotive force across a conductor which is exposed to time varying magnetic fields. Commonly, transformers are used to increase or decrease the voltages of alternating current in electric power applications.• Rectifier - A rectifier is an electrical device that converts AC, which periodically

reverses direction, to DC. The is known as rectification• Equipment Faults – These can be caused by many things such as poor

maintenance and or damage.

Construction and Operation of Components

Describing the need for Care and Maintenance

• Cleanliness - this is an important function within the welding environment, mainly because the dust and dirt that is created by the welding process requires constant attention to prevent it from getting out of hand. It is essential at the end of your shift or use of equipment to ensure that; your bay or work area is cleaned up/swept up, your machine is turned off and back the way you found it, all tools are returned and there are no hazards remaining. This is also out of respect to your colleagues and your workplace.

• Identifying equipment faults - it is important to identify faults to provide maintenance to the machines to ensure they are in good working order. Also, out of respect to your fellow colleagues as well as following the Law, you must report any faults you find with machinery so that they can either be repaired or taken out of action. Not reporting faults you have identified can cause injury and can cost lives.

Describing the need for Care and Maintenance

Describing the need for Care and Maintenance

• Insulation – Insulation is a vital element to the welding process. As the welding process uses high voltage/high amperage electric it can cause death or serious injury if exposed. It is important to check before the usage of any equipment to ensure that all insulation is intact on the equipment you are using to prevent harm to yourself or others. It is imperative to report any faults regardless of how minor they are to the appropriate person in charge of maintenance. Equipment with missing insulation shouldn’t be used.

• Contact Faces and Connections – It is important to keep a clean face on either the material or workbench in order to weld. This is because contaminants can cause the connection to break or have a poor connection and prevent the machine from arcing up. Poor connection can cause poor welding.

• Related Accessories; Cleaning Equipment, Chipping Equipment, Wire Brush and Grinders/Sanders/ Polishers.

18.5 Understand the consumables used in Manual Metal Arc (MMA) WeldingYou will be able to:• Select the correct welding consumables for joints to be welded• Store and handle welding consumables correctly• Understand the classification of electrodes for carbon steels• Describe the functions of electrode coverings• Describe the functions of electrode coverings• Electrode storage, handling and defects

Understand the Classification of Electrodes for Carbon Steel

• BS EN 499 (1995)

Understand the Classification of Electrodes for Carbon Steel

• American Classification – AWS A5. 1-91

BS EN 499 Non-alloyed and fine grained steel electrodes

BS EN 757 High strength steels

BS EN 1599 Creep resisting steels

BS EN 1600 Stainless and heat resisting steels

AWS A5.1/A5.1M Carbon Steel Electrodes for SMAW

AWS A5.4 Stainless Steel Electrodes for SMAW

AWS A5.5 Low Alloy Steel Electrodes for SMAW

Listed below are those EN and AWS specifications that prescribe the requirements for ferrous electrodes.

Understand the Classification of Electrodes for Carbon Steel

Describe Electrode types, size and selection

• Rutile Electrodes – Rutile coatings, as the name suggests, contain a large amount of rutile, titanium dioxide, typically around 50%, in addition to cellulose, limestone, silica mica ferro-manganese and some moisture, around 1 to 2%. Binders are either sodium or potassium silicate. The cellulose and the limestone decompose in the arc to form a gas shield containing hydrogen carbon monoxide and carbon dioxide. The electrodes have medium penetration characteristics, a soft, quiet but stable arc and very little spatter, making them a 'welder friendly' electrode. Striking and re-striking is easy and the electrodes will run on very low open circuit voltages. The electrodes produce a dense covering of slag that is easily removed and gives a smooth evenly rippled weld profile.• The presence of cellulose and moisture means that the electrodes produce

relatively high levels of hydrogen. This restricts their use to mild steels less than 25mm thickness and thin section low alloy steels. They are probably the most widely used general purpose electrode. Rutile coated austenitic stainless steel electrodes can be obtained and can be used in all thicknesses as cold cracking is not a problem with these alloys.

Rutile Electrodes – Rutile electrodes, require some moisture in the coating and they should not be baked. If they become damp, re-drying at around 120°C should be sufficient. Those electrodes with a sodium silicate binder can be used on DC electrode negative or AC. Electrodes with the potassium silicate binder can be used on both polarities and on AC.

The potassium silicate binder electrodes generally have better arc striking and stability characteristics than the sodium silicate binder types and a more readily detachable slag.

Describe Electrode types, size and selection

• Cellulostic Electrodes - contain a large proportion of cellulose, over 30% and generally in the form of wood flour. This is mixed with rutile, manganese oxide and ferro-manganese and is bonded onto the core wire with sodium or potassium silicate. Moisture content of these electrodes is quite high, typically 4 to 5%. The cellulose burns in the arc to form a gas shield of carbon monoxide, carbon dioxide and, in conjunction with the moisture in the coating, produces a lots of hydrogen.• The hydrogen raises arc voltage and gives the electrodes their characteristics of

deep penetration and high deposition rates. The high voltage requires a high open circuit voltage of around 70 volts to allow easy arc striking and to maintain arc stability. The forceful arc also results in appreciable amounts of weld spatter and this limits the maximum current that can be used on the larger diameter electrodes. A thin, friable and easily removed slag is produced, giving a rather coarsely rippled weld profile. The slag is also fast freezing so that, unlike most other electrodes, they can be used in the vertical down position - 'stove piping'.

Describe Electrode types, size and selection

• Basic Electrodes - The description 'basic' originates from the chemical composition of the flux coating which contains up to perhaps 50% of limestone, calcium carbonate. This decomposes in the arc to form a gas shield of carbon monoxide/dioxide.• In addition to the limestone there may be up to 30% of calcium fluoride added

to lower the melting point of the limestone and to reduce its oxidising effect. Also deoxidants such as ferro-manganese, ferro-silicon and ferro-titanium are added to provide de-oxidation of the weld pool.• Other alloying elements such as ferro-chromium, ferro-molybdenum or ferro-

nickel may be added to provide an alloy steel deposit. Binders may be sodium silicate, only for use on DC current, or potassium silicate which enables the electrodes to operate on both DC or AC.

Describe Electrode types, size and selection

• Basic Electrodes - The gas shield from basic electrodes is not as efficient as others and it is necessary to maintain a constant short arc if porosity from atmospheric contamination is not a problem. The electrodes are particularly sensitive to start porosity because of the length of time taken to establish an efficient protective shield. An essential part of welder training is familiarisation with the technique of starting a weld ahead of the required start position and moving back before proceeding in the direction of welding.• The penetration characteristics are similar to those of rutile electrodes

although the surface finish is not as good. The slag cover is heavier than rutile electrodes but is easily controlled, enabling the electrodes to be used in all positions. High limestone coatings have been developed that enable a limited range of electrodes to be used in the vertical-down (PG) position. The weld pool blends smoothly into the parent metal and undercutting should not occur.

Describe Electrode types, size and selection

• Basic Electrodes - The slag is not as easily removed as with rutile or cellulosic electrodes but the low melting point means that slag entrapment is less likely. The chemical action of the basic slag also provides very clean, high quality weld metal with mechanical properties, particularly notch toughness, better than that provided by the other electrode types. A further feature of these electrodes is that the welds are more resistant to solidification cracking, tolerating higher levels of sulphur than a rutile or cellulosic electrode. This makes them valuable if it becomes necessary to weld free cutting steels.• The basic electrode is also known as a low hydrogen rod. The coating contains

no cellulose and little or no moisture provided the electrodes are correctly handled. When exposed to air, moisture pick-up can be rapid. However, baking the electrodes at the recommended baking temperature, about 400°C, will drive off any moisture and should provide lower hydrogen levels. After baking the electrodes need to be carefully stored in a quiver at a temperature of some 120°C to prevent moisture pick-up.

Describe Electrode types, size and selection

• Iron Powder electrodes may be classified as 'high recovery'.• By adding substantial amounts of iron powder, up to 50% of the weight of the flux

coating, to basic and/or rutile electrode coatings its possible to deposit a greater weight of weld metal than before. These are described as having an efficiency above 100% and this 3 digit figure is often included in the classification.• The electrodes have thicker coatings which can make them difficult to use in

restricted access conditions. They are, however, welder friendly with good running characteristics and a smooth stable arc. The iron powder not only melts in the heat of the arc to increase deposition rate but also enables the electrode to carry a higher welding current than other electrodes.• The iron powder is electrically conducting, so allowing some of the welding current

to pass through the coating. High welding currents can therefore be used without the risk of the core wire overheating, this increases both the burn-off and the deposition rates. The Iron Powder electrodes are ideally suited for fillet welding, giving a smooth, finely rippled surface with a smooth blend at the weld toes.

Describe Electrode types, size and selection

• Iron Powder - They are generally more tolerant to variations in fit-up and their stability on low open circuit voltages means that they are very good at bridging wide gaps. • However, the large weld pool means that they are not suited to positional

welding and are generally confined to welding in the flat (PA) and horizontal-vertical (PC) positions.• The last type of electrode covering is described as 'acid'. These electrodes

have large amounts of iron oxides in the flux which would result in a high oxygen content in the weld metal and poor mechanical properties. It is therefore necessary to incorporate large amounts of de-oxidants such as ferro-manganese and ferro-silicon in the flux. Although they produce smooth flat weld beads of good appearance and can be used on rusty and scaled steel items the mechanical properties tend to be worse than rutile and basic coated electrodes. They are also more sensitive to solidification cracking and are therefore little used.

Describe Electrode types, size and selection

Principles of Electrode selection and the various electrode sizes in both SWG and metric measurements.

Please see the following link below for the full guide:http://www.lincolnelectric.com/assets/global/Products/Consumable_StickElectrodes-MildandLowAlloySteels-Excalibur-Excalibur7018-1MR/c2410.pdf#search=c2410%252Epdf

Describe Electrode types, size and selection

18.6 Understand Welding Procedures and testing applied to Manual Metal Arc Welding (1)

You will be able to:• Produce fillet and butt welded joints in 5-12 mm thick low carbon steel

and/or austenitic Stainless Steel and/or aluminium alloy using the manual metal arc process in accordance with the welding procedure• Produce single-vee butt welded joints in the vertical (PF/3G) welding position

in accordance with welding procedure• Produce tee fillet welds in the overhead (PD/4F) welding position in

accordance with the welding procedure• Test and report on welded joints using bend, macro and fracture tests• Evaluate welds produced for compliance with welding standard EN 25817

18.6 Understand Welding Procedures and testing applied to Manual Metal Arc Welding (2)

You will be able to:• Describe the procedural requirements for the MMA welding process• Understand the welding procedure specification (WPS) – EN 288-2 (ISO

15609-1)• Interpretation of welding symbols –EN 22553• Identify the features of a welded joint• Describe the cause and effects of welding defects (BS EN 25817:1992)• Describe methods of weld inspection and testing• Describe types of distortion/shrinkage and correction methods

Describe the procedural requirements for the MMA Welding Process (1)

Objectives of the MMA Process – to fuse a parent metal and another metal using the Manual Metal Arc Process.

Manufacturers & Examining Body – manufacturers have to submit details to awarding/examining bodies to ensure that trainees are trained to the correct standards using current consumables/machines and procedures in order to properly use the manufacturers equipment.

Standards EN 287 (ISO 6947) and ASME IX Quality Assurance - BS EN 287, BS ISO EN 9606 and ASME Section IX would be appropriate for welders on high quality work such as pressure vessels, pressure vessel piping and offshore structures and other products where the consequences of failure, stress levels and complexity mean that a high level of welded joint integrity is essential. In less demanding situations, such as small to medium building frames and general light structural and non- structural work, an approved welding procedure may not be necessary. However, to ensure an adequate level of skill, it is recommended that the welder be approved to a less stringent standard e.g. BS 4872

Describe the procedural requirements for the MMA Welding Process (2)

Role of a welding inspector - A welding inspector is an individual who is responsible for overseeing all the activities and duties of the welding staff and ensuring that everything is in proper working manner. A welding inspector’s job is to make sure that the welders work within the set quality and deadlinespecifications. He is also required to have advanced knowledge of thewelding procedures and processes so that he can guide his subordinateswhen they are faced with confusion.

Role of a welder - The role of the welder is mainly to join metals together by heating their surfaces using a arc welding process and joining them together and it used in everything from car manufacturing to the erection of buildings. Metal joining is a skill that has been in demand for thousands of years

Describe the procedural requirements for the MMA Welding Process (3)

ISO 3834 – Quality Requirements for welding and ISO 14731 Welding Co-ordination:Entitled 'Quality Requirements for Fusion Welding of Metallic Materials', the standard provides details of how to control the various welding and welding-related operations to achieve the desired quality consistently. A key feature of the standard is the requirement to ensure that people with welding responsibilities are competent to discharge those responsibilities.

This is achieved by incorporation of another standard: EN 719/ISO 14731 'Welding Co-ordination -Tasks and Responsibilities'.

Interpretation of welding symbols, please see the earlier part of this presentation for the full list of welding symbols that are used by the EN, ISO and AWS standards. ISO Weld Symbols

Please refer to the following links:http://www.draftsperson.net/images/6/63/Weld_Symbols.png

http://triblab.teipir.gr/files/Welding/Lab/CH3_1_Welding_joint_symbols.pdf

Refer to the different types of welded joints such as Butt, Tee, Lap and Corner.

Describe the procedural requirements for the MMA Welding Process (3)

Features of a Welded Joint

Some features of a welded joint using various diagrams and drawings.

Various Welding Positions (1)

Various Welding Positions (2)

Describe the welding procedures and testing applied to Manual Metal Arc Welding (1)

Describe the welding procedure specification (WPS)-EN 288-2 (ISO 15609)• Quality Levels – Quality is defined by the specification as to what standard is

required.• Process – This is whether the Weld is to be carried out on MIG/TIG/MMA.• Material/Thickness – on the WPS it will define what thickness and material it

can be carried out on to do the specific weld.• Welding Positions – a position will be defined as per the specification on the

previous two slides, defines how the weld should be positioned for welding.• Range of approval – what criteria is set before it can be approved as ok.• Testing requirements – what testing needs to be carried out on the weld

before it can be satisfactory.

Describe the welding procedures and testing applied to Manual Metal Arc Welding (1)

Describe the welding procedure specification (WPS)-EN 288-2 (ISO 15609)• Mechanical testing requirements – are there any mechanical testing

requirements to the weld at all and what is the specification for this?• Location of the test – is there any location where the weld needs to be carried

out specifically.• Witnessing – does anyone need to witness the weld taking place and is there

any requirements for this witness to have (if any)• Certification – is there any certification that is required prior to undertaking

this WPS?

• Prolongation – The welding operator's approval remains valid for two years providing the employer/welding coordinator confirms that there has been a reasonable continuity of welding work (period of interruption no longer than six months) and there is no reason to question the welding operator's knowledge.• The validity of approval may be prolonged for further periods of two years by

the examiner / examining body providing there is proof of production welds of the required quality, and appropriate test records maintained with the operator's certificate.• Non-destructive testing requirements – what NDT requirement is there for the

weld that needs to be carried out e.g. Macro Etching, Magnetic Particle Testing, Dye Penetrant

Describe the welding procedures and testing applied to Manual Metal Arc Welding (1)

Welding ParametersA WPS may also specify certain welding parameters that need to be abided by, you can see an example of these on your test piece specifications for this module such as:• Current• Voltage• Electrode Type• Electrode Diameter• Welding Speed• Procedure Approval

Pre and Post Welding Operations in terms of…• Cutting and Preparation process; Manual – cutting and prepping

material by hand ready for the welding process to be conducted and Machined – cutting and prepping material via a means of machining that is either programmed or operated• Surface finish/flaws – What is the finish that is required of the welded

joint e.g. any burrs or sharp edges?• Joint Cleanliness – does it need buffing/linishing afterwards?• Pre-welding alignment and accuracy checks – checking to ensure

distortion doesn’t occur in the welding and it stays within variation.

• Pre-setting - The parts are pre-set and left free to move during welding. In practice, the parts are pre-set by a pre-determined amount so that distortion occurring during welding is used to achieve overall alignment and dimensional control.• The main advantages compared with the use of restraint are that there is no

expensive equipment needed and there will be lower residual stress in the structure.• Unfortunately, as it is difficult to predict the amount of pre-setting needed to

accommodate shrinkage, a number of trial welds will be required. For example, when MMA or MIG welding butt joints, the joint gap will normally close ahead of welding; when submerged arc welding; the joint may open up during welding. When carrying out trial welds, it is also essential that the test structure is reasonably representative of the full size structure in order to generate the level of distortion likely to occur in practice. For these reasons, pre-setting is a technique more suitable for simple components or assemblies.

Pre and Post Welding Operations in terms of…

• Use of restraint - Because of the difficulty in applying pre-setting and pre-bending, restraint is the more widely practised technique. The basic principle is that the parts are placed in position and held under restraint to minimise any movement during welding. When removing the component from the restraining equipment, a relatively small amount of movement will occur due to locked-in stresses. This can be cured by either applying a small amount of pre-set or stress relieving before removing the restraint.• When welding assemblies, all the component parts should be held in the correct

position until completion of welding and a suitably balanced fabrication sequence used to minimise distortion.• Welding with restraint will generate additional residual stresses in the weld which

may cause cracking. When welding susceptible materials, a suitable welding sequence and the use of preheating will reduce this risk.• Restraint is relatively simple to apply using clamps, jigs and fixtures to hold the

parts during welding.

Pre and Post Welding Operations in terms of…

• Welding jigs and fixtures - Jigs and fixtures are used to locate the parts and to ensure that dimensional accuracy is maintained whilst welding. They can be of a relatively simple construction, as shown in, but the welding engineer will need to ensure that the finished fabrication can be removed easily after welding.• Flexible clamps - A flexible clamp can be effective not only in applying

restraint but also in setting up and maintaining the joint gap (it can also be used to close a gap that is too wide).• A disadvantage is that, as the restraining forces in the clamps are transferred

into the joint when the clamps are removed, the level of residual stress across the joint can be quite high.

Pre and Post Welding Operations in terms of…

Preheating is the process applied to raise the temperature of the parent steel before welding. It is used for:•To slow the cooling of the weld and the base material, resulting in softer weld metal and HAZ microstructures with a greater resistance to fabrication hydrogen cracking.•The slower cooling rate encourages hydrogen diffusion from the weld area by extending the time period over which it is at elevated temperature (particularly the time at temperatures above approximately 100°C) at which temperatures hydrogen diffusion rates are significantly higher than at ambient temperature. The reduction in hydrogen reduces the risk of cracking.

Pre and Post Welding Operations in terms of…

• Preheat can be applied through various means. The choice of method of applying preheat will depend on the material thickness, weldment size and the heating equipment available at the time of welding. The methods can include furnace heating for small production assemblies or, for large structural components, arrays of torches, electrical strip heaters, induction heaters or radiation heaters.

Pre and Post Welding Operations in terms of…

• Arc and Gas Gouging - Air carbon arc gouging works as follows. An electric arc is generated between the tip of a carbon electrode and the workpiece. The metal becomes molten and a high speed air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to use, has a high metal removal rate, and gouge profile can be closely controlled.• Disadvantages are that the air jet causes the molten metal to be ejected over

quite a large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), it can be extremely noisy.• Air carbon arc gouging does not rely on oxidation, it can be applied to a wide

range of metals. DC (electrode positive) is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Applications include back gouging, removal of surface and internal defects, removal of excess weld metal and preparation of bevel edges for welding.

Pre and Post Welding Operations in terms of…

• Post-weld cleaning – grinding and linishing of the weld to remove burrs and sharp edges as well as spatter (MMA = slag), wire brush, chipping hammer, hammer and chisel (if required), emory cloth and varying grades of sandpaper.

• This can significantly improve the quality and appearance of your weld and help in the process of applying Non-Destructive Testing Methods/NDT Testing.

Pre and Post Welding Operations in terms of…

Types of Weld Defects (1)• Cracks/Cracking/Solidification Cracking - A crack may be defined by a fracture which comes

from the stresses created on cooling or acting on the structure. It’s the most serious type of imperfection. They reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature.• The main cause of cracking is that the weld bead in the final stage of solidification has

insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:• insufficient weld bead size or shape• welding under high restraint• high impurity content or a relatively large amount of shrinkage on solidification.Joint design can influence the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, carries increased risk to solidification cracking. The centre of the weld which is the last part to solidify, is a narrow zone with negligible cracking resistance.

Weld Cavities - are a naturally occurring phenomenon found in arc welding. While they can occur with all arc welding processes, they are more prevalent with processes that are capable of higher deposition rates and producing larger weld puddles (i.e.. welds with a greater volume of liquid metal). In addition, they are more pronounced when welding with a vertical up progression, as the effect of gravity tends to enhance the size of the pipes. Finally, the piping effect will vary, depending on the type of weld metal.

Types of Weld Defects (2)

If you want to fill in the cavity, then welding techniques can be used. You can back step or come back down into the crater about 12 mm, hold for a second before you stop welding. Another involves stepping over to the side of the weld bead and finishing on the side. For slag producing processes, the second method does not require having to weld back into the slag.

• Solid/Slag Inclusions - Slag is normally seen as lines along the length of the weld. This is readily identified in a radiograph. Slag inclusions are usually associated with the flux processes, i.e. MMA, Flux Cored Arc and submerged arc, but they can also occur in MIG welding.• Causes - As slag is the residue of the flux coating in MMA welding, it is

essentially a de-oxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs.• As they both have an effect on the ease of slag removal, the risk of slag

imperfections is influenced by; Type of flux coating & Welder technique• The type and configuration of the joint, welding position and access restrictions

all have an influence on the risk of slag imperfections.

Types of WeldDefects (3)

• Lack of Fusion and Penetration - is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides root region of the joint are unfused. Typical imperfections can arise in the following situations:• an excessively thick root face in a butt weld • too small a root gap • misplaced welds • failure to remove sufficient metal in cutting back to sound metal in a double sided weld • incomplete root fusion when using too low an arc energy (heat) input• too small a bevel angle• too large a diameter electrode in MMA welding

Types of Weld Defects (4)

• Lack of Inter-run Fusion - imperfections can occur when the weld metal fails• to fuse completely with the sidewall of the joint • to penetrate adequately the previous weld bead• The principal causes are too narrow a joint preparation, incorrect welding

parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when access to the joint is restricted.

Types of Weld Defects (5)

• Imperfect Shape – There are several weld defects based on appearance which can be associated with the term ‘Imperfect Shape’:• Excessive Weld Penetration• Root Concavity• Excessive Convexity• Oversize Fillet Weld• Undersize Fillet Weld• Asymmetric Fillet Weld • Poor Fit Up(Pictures begin top leftthrough to bottom right)

Types of Weld Defects (6)

• Porosity - the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms: • distributed• surface breaking pores• wormhole• crater pipes• Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the

molten weld pool which is then released on solidification to become trapped in the weld metal.• Nitrogen and oxygen absorption in the weld pool usually originates from poor gas

shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.

Types of Weld Defects (7)

• Porosity - Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.• Surface coatings like primer paints and surface treatments such as zinc

coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides. Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Types of Weld Defects (8)

The gas source should be identified and removed as follows: Air entrainment; seal any air leak, avoid weld pool turbulence, use filler with adequate level of de-oxidants, reduce excessively high gas flow & avoid draughts.

Hydrogen; dry the electrode and flux & clean and degrease the workpiece surface.

Surface coatings; clean the joint edges immediately before welding & check that the weldable primer is below the recommended maximum thickness.

Types of Weld Defects (9)

• Distortion - occurs in six main forms; Longitudinal shrinkage, Transverse shrinkage, Angular distortion, Bowing and dishing, Buckling, Twisting• The principal features of the more common forms of distortion for butt and fillet

welds are shown below:• If a metal is uniformly heated and cooled there would be almost no distortion.

However, because the material is locally heated and restrained by the surrounding cold metal, stresses are generated higher than the material yield stress causing permanent distortion. The principal factors affecting the type and degree of distortion, are:• Parent material properties• Amount of restraint• Joint design• Part fit-up• Welding procedure

Types of Weld Defects (10)

Visual Inspection• Prior to welding – check that the materials you are using are suitable for the

WPS, check that you have all the correct consumables to perform the weld process specified in the WPS, check their no contaminants on the work piece or consumables• During Welding – Be observant of what is happening and if it appears that the

weld isn’t conforming to the WPS, stop the process and re-do if necessary if the component can’t be rectified• After welding – check the weld using measuring equipment to ensure it meets

the requirements set out in the WPS and ensure that it is finished aesthetically to the required standard• Measuring Equipment – Fillet Gauge, micrometer, steel ruler, protractor• Optical Aids – Magnifying glass, Magnified Safety glasses, Magnifying Scope

etc.

Destructive Testing sometimes has to be used to test the tensile strength and other features of a weld. These types of destructive testing can include:• Face (of the joint), Side (of the joint), Root (of the joint)• Macroscopic - involves cutting a sample from the joint. Cold cutting methods are best for this, such

as a bandsaw. The surface needs to be polished, file away any burrs and rough marks and use progressively finer grades of emery cloth until you get a smooth even finish. Then, acid solution is applied with a soft clean cloth. The acid used is nitric acid, dissolved in distilled water. The solution is 10% Nitric acid, and 90% water. This is used because it rapidly oxidises. After a time, the parent metal and weld areas will begin to discolour. Afterwards, the sample is rinsed off and carefully dried. The results show a distinctive colour difference between the weld metal and the parent metal. The weld will show up lighter, and the darker material next to it is the rearranged grain structure, due to the heating and cooling cycle. In multiple run welds, the one that is done first shows up slightly darker, due to the root run being reheated during the second pass.• Nick-Break - you take a sample piece, partially cut through it, then break the remainder off so you

can see the ‘see inside the weld’.• Tensile - by gripping the ends of a suitably prepared standardised test piece in a tensile test

machine and then applying a continually increasing uni-axial load until such time as failure occurs.

Destructive Testing

Dye Penetrant - is a widely applied and low-cost inspection method used to locate surface-breaking defects in metals, plastics, or ceramics. This may be applied to all non-ferrous materials and ferrous materials. Dye Penetrant is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.The steps for using Dye Penetrant are as follows: 1. Pre-Cleaning2. Application of Penetrant3. Excess Penetrant Removal4. Application of Developer5. Inspection of Weld6. Post-Cleaning

Non-Destructive Testing (1)

Magnetic Particle Testing - for detecting surface and slightly subsurface discontinuities in ferromagnetic materials and some alloys. You apply a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either AC or some form of DC (rectified AC).The presence of a surface or subsurface defect in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. The particles may be either dry or wet. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.

Non-Destructive Testing (2)

Magnetic Particle Testing -

Non-Destructive Testing (3)

• Ultrasonic Testing - In most common UT applications, very short ultrasonic pulse-waves with centre frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

Non-Destructive Testing (4)

• Radiography/X-Ray Testing - method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.• There are two different radioactive sources available for industrial use; X-ray

and Gamma-ray. These radiation sources use higher energy level, i.e. shorter wavelength, versions of the electromagnetic waves.• The test-part is placed between the radiation source and film/detector. The

material density and thickness differences of the test-part will reduce the penetrating radiation through interaction processes involving scattering and/or absorption. The differences in absorption are then recorded on film(s) or through an electronic means. In industrial radiography

there are several imaging methods available, i.e. Film Radiography, Real Time Radiography (RTR), Computed Tomography (CT), Digital Radiography (DR), and Computed Radiography (CR).

Non-Destructive Testing (5)

Distortion Correction Methodology

• Mechanical techniques - are hammering and pressing. Hammering may cause surface damage and work hardening.• Thermal techniques - is to create sufficiently high local heat stresses so that, on

cooling, the component is pulled back into shape. • Spot heating - is used to remove buckling, for example when a relatively thin

sheet has been welded to a stiff frame. This is corrected by spot heating on the convex side. If the buckling is regular, the spots can be arranged symmetrically, starting at the centre of the buckle and working outwards. • Line Heating - Heating in straight lines is often used to correct angular distortion.

The component is heated along the line of the welded joint but on the opposite side to the weld so the induced stresses will pull the flange flat.• Wedge Shaped Heating - To correct distortion in larger complex fabrications it

may be necessary to heat whole areas in addition to employing line heating. The pattern aims at shrinking one part of the fabrication to pull the material back into shape.

There are several methods for avoiding distortion in your work:• Pre-setting• Pre-bending• Weld Sequencing• Skip Welding• Back Stepping• Tack Welding• Pre and Post Weld Heat

Treatment

• Joint Design• Balanced Welding• Intermittent Welding• Chills• Restraint• Clamping Jigs• Back-to-back assembly

How to avoid Distortion

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