Welding consumables Part 5.docx

8/10/2019 Welding consumables Part 5.docx

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Welding consumables Part 5 - MIG/MAG andcored carbon steel wiresJob Knowledge

Part 1 Part 2 Part 3 Part 4

To ensure that there is a consistency in composition and properties between wires from avariety of manufacturers, specifications have been produced that enable a wire to be easilyand uniquely identified by assigning the consumable a 'classification', a uniqueidentification that is universally recognised.

The two schemes that are dealt with in this article are the EN/ISO method and the AWSscheme. There are such a large number of specifications covering the whole range offerrous and non-ferrous filler metals, both solid wire and cored, that it will not be possibleto describe all of these here. This article therefore reviews just the carbon steelspecifications.

The identification of the solid wires is relatively simple, as the chemical composition is themajor variable although both the EN/ISO and the AWS specifications detail the strengththat may be expected from an all-weld deposit carried out using parameters given in thespecification. It should be remembered, however, that most welds will contain some parentmetal and that the welding parameters to be used in production may be different fromthose used in the test. The result is that the mechanical properties of a weld can besignificantly different from those quoted by the wire supplier, hence the need to alwaysperform a procedure qualification test when strength is important. In addition, themechanical properties specified in the full designation include the yield strength. (In the

EN/ISO specifications, the classification may indicate either yield or ultimate tensilestrength).

When selecting a wire remember that the yield and ultimate tensile strengths are veryclose together in weld metal but can be widely separated in parent metal. A filler metalthat is selected because its yield strength matches that of the parent metal may not,therefore, match the parent metal on ultimate tensile strength. This may cause the cross

joint tensile specimens to fail during procedure qualification testing or perhaps in service.

The EN/ISO specification for non-alloyed steel solid wires is BS EN ISO 14341. Thisspecification classifies wire electrodes in the as-welded condition and in the post weldheat-treated condition, based on classification system, strength, Charpy-V impactstrength, shielding gas and composition. The classification utilises two systems basedeither on the yield strength (System A) or the tensile strength (System B):
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System A - based on the yield strength and average impact energy of 47J of all-weldmetal.

System B - based on the tensile strength and the average impact energy of 27J of all-weld metal.

In most cases, a given commercial product can be classified to both systems. Then eitheror both classification designations can be used for the product.

The symbolisation for mechanical properties is summarised in Table 1A for classificationsystem A and Table 1B for classification system B. For classification system B, the 'X' canbe either 'A' or 'P', where 'A' indicates testing in the as-welded condition and 'P' indicatestesting in the post weld heat-treated condition. The symbol for chemical composition issummarised in Table 3A and 3B of BS EN ISO 14341 based on each classification system.For classification system A, the standard lists eleven compositions, too many to describecompletely here. Six of the wires are carbon steel with varying amounts of deoxidants, twowires contain approximately 1% or 2.5% nickel and an additional two wires contain around0.5% molybdenum. The designation of these wires is for example G3Si1, 'G' identifying itas a solid wire, '3' as containing some 1.5% manganese and Si1 as containing around0.8% silicon; G3Ni1 is a wire with approximately 1.5% manganese and 1% nickel.

Table 1A Symbols for mechanical properties based on classification system A

SymbolMin YieldStrengthN/mm 2

UTS

N/mm 2

MinElongation

%Symbol Charpy-V Test 47 J atTemp C

35 355 440 to570 22 Z No requirements

38 380 470 to600 20 A +20

42 420 500 to640 20 0 0

46 460 530 to

68020 2 -20

50 500 560 to720 18 3 -30

4 -405 -506 -607 -708 -809 -9010 -100

Table 1B Symbols for mechanical properties based on classification system B

SymbolMin YieldStrengthN/mm 2

UTS

N/mm 2

MinElongation

%Symbol Charpy-V Test 27 J atTemp C

43X 330 430 to600 20 Z No requirements

49X 390 490 to670 18 Y +20

55x 460 550 to740 17 0 0

57x 490 570 to

77017 2 -20

3 -30


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4 -405 -506 -607 -708 -809 -9010 -100

A full designation could therefore be ISO 14341-A-G 46 5 M G3Si1 where the '-A'designates the classification system A, the '-G' designates solid wire electrode/or deposits,and the 'M' designates a mixed gas. An example of a System B designation could be ISO14341-B-G 49A 6 M G3, where 'A' indicates testing in the as-welded condition.

The AWS specification AWS A5.18 covers both solid, composite stranded and cored wirescomprising six carbon steel filler metals for MAG, TIG and plasma welding in both US andmetric units.

The classification commences with the letters 'E' or 'ER'. 'E' designates an electrode. 'ER'indicates that the filler metal may be used either as an electrode or a rod. The next twodigits designates the tensile strength in either 1000s of psi.(ksi) or N/mm 2 eg ER70 (70ksiUTS) or ER48 (480N/mm 2 UTS). However, note that there is only one strength level in thespecification.

The next two characters identify the composition, essentially small variations in carbon,manganese and silicon contents, the wire type (solid wire (S) or metal cored or compositewire (C)) and the Charpy-V impact values.

With one exception, the solid wires are tested using 100% CO 2 , the cored wires withargon/CO 2 or as agreed between customer and supplier, in which case there is a finalletter 'C' designating CO 2 or 'M', a mixed gas.

The permutations in these identifiers are too many and too complicated to be able todescribe them all in sufficient detail but as an illustration, a typical designation would beER70S-3, a 70ksi filler metal, CO 2 gas shielded and with minimum Charpy-V energy of 27Jat -20C. E70C-3M identifies the wire as a solid wire 70ksi UTS metal cored filler metal,27J at -20C and tested with an argon/CO2 shielding gas.

The EN/ISO specification for non-alloy steel flux and metal cored wires is BS EN ISO17632. This covers gas shielded as well as self-shielded wires. The standard identifieselectrode based on two systems in a similar way as BS EN ISO 14341, indicating thetensile properties and the impact properties of the all-weld metal obtained with a givenelectrode. Although the specification claims that the wires are all non-alloy, they cancontain molybdenum up to 0.6% and/or nickel up to 3.85%. The classification commenceswith the letter 'T', identifying the consumable as a cored wire.

The classification uses the same symbols for mechanical properties as shown in Table1A&B and a somewhat similar method to describe the composition as BS EN ISO 14341.Thus MnMo contains approximately 1.7% manganese and 0.5% molybdenum; 1.5Nicontains 1% manganese and 1.5% nickel. In addition to the symbols for properties andcomposition, there are symbols for electrode core composition. Table 2 summarises thesymbols for electrode core type and welding position in accordance with classificationsystem A. Classification system B uses Usability Indicators as oppose to a one-lettersymbol for electrode core type, which can be found in Table 5B of BS EN ISO 17632.

Table 2 Symbols for electrode core type and position based on classificationsystem A

Flux Core Welding Position

Symbol Flux Core Type ShieldingGas Symbol Welding position


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R Rutile, slow freezing slag Required 1 AllP Rutile, fast freezing slag Required 2 All except V-down

B Basic Required 3 Flat butt, flat and HVfilletM Metal powder Required 4 Flat butt and fillet

V Rutile or basic/fluoride Not required 5 V-down and (3)

W Basic/fluoride, slow freezingslag Not required

Y Basic/fluoride, fast freezingslag Not required

Z Other typesIn addition, there are symbols for gas type. These are 'M' for mixed gases, 'C' for 100%CO 2 and 'N' for self-shielded wires and 'H' for hydrogen controlled wires. A full designationmay therefore be ISO 17632-A -T46 3 1Ni B M 1 H5 in accordance with classificationsystem A. For classification system B, an example may be ISO 17632-B -T55 4 T5-1MA-N2-UH5, where 'T5' is the usability designator, 'A' indicates test in the as-weldedcondition, 'N2' is the chemical composition symbol, and 'U' is an optional designator.

The American Welding Society classification scheme for carbon steel flux cored wires isdetailed in the specification AWS A5.36. This also contains information from A5.18, butdoes not officially supercede it. The full designation is ten characters in length beginning'E' for an electrode then designators for strength, welding position, cored wire, usability,shielding gas, toughness, heat input limits and diffusible hydrogen, the last fourdesignators being optional.

There are two strength levels - E7 (70ksi UTS) and E6 (60ksi UTS) followed by adesignator for welding position,'0' for flat and horizontal and '1' for all positions, includingvertical-up and vertical-down.

The next symbol 'T' identifies the wire as being flux cored and this is followed by either a

number between 1 and 14 or the letter 'G' that identifies the usability. This number refersto the recommended polarity, requirements for external shielding, and whether the wirecan be used to deposit single or multi-pass welds. 'G' means that the operatingcharacteristics are not specified. The sixth letter identifies the shielding gas used for theclassification, 'C' being 100% CO 2 , 'M' for argon/CO 2 , no letter indicating a self-shieldedwire.

The non-compulsory part of the designation may include the letter 'J', confirming that theall-weld metal test can give Charpy-V values of 27J at -40C; the next designator may beeither 'D' or 'Q'. These indicate that the weld metal will achieve supplementary mechanicalproperties at various heat inputs and cooling rates. The final two designators identify thehydrogen potential of the wire.

A full AWS A5.36 designation could therefore be E71T-2M-JQH5. This identifies the wire asa cored, all positional wire to be used with argon/CO 2 shielding gas on electrode positivepolarity. The weld metal should achieve 70ksi tensile strength, 27J at -40C, 58 to 80ksiyield strength at high heat input, a maximum 90ksi at low heat input, and a diffusiblehydrogen content of less than 5ml of H2/100g of deposited weld metal.

This article was written by Gene Mathers , reviewed and modified by Runlin Zhou .

SUMMARY

This lecture commences with a discussion of the need for civil and structuralengineers to have a basic knowledge of the metallurgy of steel. Then thecrystalline nature of irons and steels is described together with the influence


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of grain size and composition on properties. The ability of iron to have morethan one crystalline structure (its allotropy) and the properties of the

principal crystalline forms of alloys of iron and carbon are discussed.

The metallurgy and properties of slowly cooled steels are reviewed,including the influence of grain size, rolling, subsequent heat treatment andinclusion shape and distribution. Rapidly cooled steels are treatedseparately; a brief description of quenching and tempering is followed by adiscussion of the influence of welding on the local thermal history.Hardenability, weldability and control of cracking are briefly discussed.Finally the importance of manganese as an alloying element is introduced.

1. INTRODUCTION

1.1 Why Metallurgy For Civil and Structural Engineers?

The engineering properties of steel, i.e. strength, ductility and resistanceagainst brittle fracture, depend on its crystalline structure, grain size andother metallurgical characteristics.

These microstructural properties are dependent on the chemical compositionand on the temperature-deformation history of the steel. Heat treatments thatoccur during welding may also have a large influence on the engineering

properties.

When selecting steel for welded structures, it is important to have at least a basic knowledge of metallurgy. This knowledge is required especially whenlarge and complicated structures are being designed, such as bridges,offshore structures, and high rise buildings.

Selecting materials, welding processes and welding consumables usuallyrequires consultation of "real" metallurgists and welding specialists. A basicknowledge of metallurgy is essential for communication with thesespecialists.

Finally, a basic knowledge of metallurgy also enables civil and structuralengineers to have a better understanding of the engineering properties ofsteel and the performance of welded structures.

1.2 The Scope of Lectures in Group 2

Lecture 2.1 deals with the characteristics of iron-carbon alloys. Where possible, direct links are indicated to the engineering properties andweldability of steel. These subjects are covered in Lectures 2.2 and 2.6respectively.
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Lecture 2.3 describes steelmaking and the forming of steel into plates andsections. The various processes for controlling the chemical compositionand the different temperature-deformation treatments are discussed. Most ofthe underlying principles described in Lecture 2.1 are applied.

Steels are available in various grades and qualities. The grade designates thestrength properties (yield strength and ultimate strength), while the quality ismainly related to resistance against brittle fracture. Grades and qualities areexplained in Lecture 2.4 . A system for choosing the right quality accordingto Eurocode 3 (Annex C) [1] is presented. Some guidelines for the selectionof steel grade are given.

2. STRUCTURE AND COMPONENTS OF STEEL

2.1 Introduction

To get an impression of the metallurgical structure of steel, a piece of steel bar can be cut to expose a longitudinal section, the exposed surface groundand polished and examined under a microscope.

At modest magnifications, a few particles are seen which are extended in thedirection of rolling of the bar, see Slide 1. These particles are inclusions.They are non-metallic substances which have become entrained within themetal during its manufacture, mostly by accident but sometimes by design.

Their presence does not affect the strength but has an adverse effect onductility and toughness. Particular types of inclusion can greatly enhance themachinability of steels and may therefore be introduced deliberately.
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Slide 1 : Longitudinal stringers of inclusions in hot rolled steel. (x 500)

To reveal the true structure of the metal, the polished surface must bechemically etched. When this is done, a wide diversity of microstructuremay be seen which reflects the composition of the steel and its processing,see Slides 2 - 5.

The microstructure has a significant effect on the engineering properties asdescribed in later sections of this lecture.

2.2 The Components of Steel

Steels and cast irons are alloys of iron (Fe) with carbon (C) and variousother elements, some of them being unavoidable impurities whilst others are

added deliberately.Carbon exerts the most significant effect on the microstructure of thematerial and its properties. Steels usually contain less than 1% carbon byweight. Structural steels contain less than 0,25% carbon: the other principalalloying element is manganese, which is added in amounts up to about1,5%. Further alloying elements are chromium (Cr), nickel (Ni),molybdenum (Mo) etc. Elements such as sulphur (S), phosphorus, (P),nitrogen (N) and hydrogen (H) usually have an adverse effect on theengineering properties and during the steel production, measures are taken

to reduce their contents. Cast irons generally contain about 4% carbon. Thisvery high content of carbon makes their microstructure and mechanical

properties very different from those of steels.

Each of the microstructures shown in Slides 2, 3, 4 and 5 is an assembly ofsmaller constituents. For example, the 0,2% C steel of Slide 2 is

predominantly an aggregate of small, polyhedral grains, in this case


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Slide 2 : Microstructure of hot rolled steel containing 0,2% carbon showingferrite (white) and pearlite colonies (dark). (x 200)

Slide 3 : Microstructure of hot rolled steel containing 0,36% carbon showingincreased proportions of pearlite (dark). (x 500)


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Slide 4 : Microstructure of heat treated hot rolled steel containing 0,36%carbon showing spheroidised pearlite (dark) in a ferrite matrix. (x 750)

Slide 5 : Microstructure of quenched hot rolled steel containing 0,36%carbon showing bainite (x 200)

The steel of Slide 2 is an example of a polycrystalline substance which has been made visible by polishing and etching.

(a) The surface is polished but not etched.


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(b) The surface is polished and etched. Different reflections of the lightindicate different orientation of crystals (polycrystalline structure).

(c) Some etchants affect only the grain boundaries. These etchants are usedwhen it is required to investigate the grain structure, e.g. to estimate thegrain size.

(d) The appearance of etched grain boundaries of Figure 1c.

(e) The appearance of a steel with 0,15% carbon (enlargement 100x). Thedark areas are pearlite. The grain boundaries are clearly indicated. The darkareas indicate the presence of carbon.

By adjusting the history of rolling and heating treatment experienced by the

steel during its production, the grain size can be altered. This technique isuseful because the grain size affects the properties. In particular, the yieldstrength is determined by the grain size, according to the so-called Petchequation:

y = o + kd -1/2

where y is the yield strength

o is effectively the yield strength of a very large isolated crystal: for mild

steel this is 50N/mm 2

d is the grain size in mm

k is a material constant, which for mild steel is about 20N/mm -3/2

Thus, if the grain size is 0.01 mm, y 250N/mm2.

2.3 The Crystal Structure

The internal structure of the crystal grains is composed of iron atomsarranged according to a regular three-dimensional pattern. The pattern isillustrated in Figure 2. This pattern is the body-centred cubic crystalstructure; atoms are found at the corners of the cube and at its centre. Theunit cell is only 0,28nm along its edges. A typical grain is composed ofabout 10 15 repetitions of this unit.


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This crystal structure of iron at ambient temperature is one of the majorfactors determining the metallurgy and properties of steels.

Steels contain carbon. Some of it, a very small amount, is contained withinthe crystals of iron. The carbon atoms are very small and can fit, with somedistortion, into the larger gaps between the iron atoms. This arrangementforms what is known as an interstitial solid solution: the carbon is located inthe interstices of the iron crystal.

In the steels of Slides 2, 3 and 4, most of the remaining carbon has formed achemical compound with the iron, Fe 3C, iron carbide or cementite. Ironcarbide is also crystalline but it is hard and brittle. With 0,1%C, there is onlya small amount of Fe 3C in steel. The properties of such steel are similar tothose of pure iron ) [2]. It is ductile but not particularly strong and is used for

many purposes where ability to be shaped by bending or folding is thedominant requirement.

For a steel of higher carbon content, say 0,4%, as shown in Slide 5, a lowmagnification shows it to be composed of light and dark regions - about50:50 in this case. The light regions are iron crystals containing very littledissolved carbon, as in the low carbon steel. The dark regions need closerexamination. Slide 6 shows one such region at higher magnification. It isseen to be composed of alternate layers of two substances, iron and Fe 3C.The spacing of the laminae is often close to the wavelength of light andconsequently the etched structure can act as a diffraction grating, givingoptical effects which appear as a pearl-like iridescence. Consequently, thismixture or iron and iron carbide has acquired the name 'pearlite'. The originof the pearlite and its effect on the properties of steel are revealed byexamining what happens during heating and cooling of steel.


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Slide 6 : Polycrystalline structure of steel containing 0,4% carbon. (x 400)

3. IRON-CARBON PHASES

3.1 Influence of Temperature on Crystal Structure

The crystal structure of steel changes with increasing temperature. For pureiron this change occurs at 910 C. The body-centred cubic (bcc) crystals ofFigure 2 change to face-centred cubic (fcc) crystals as illustrated in Figure 3.For fcc crystals the atoms of iron are on the cube corners and at the centresof each face of the cube. The body-centred position is empty.


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A given number of atoms occupy slightly less volume when arranged as fcccrystals than when arranged as bcc crystals. Thus the change of the crystalstructure is accompanied by a volume change. This change is illustrated inFigure 4. When a piece of pure iron is heated, expansion occurs in the

normal way until the temperature of 910 C is reached. At this temperaturethere is a step contraction of about % in volume associated with thetransformation from the bcc to fcc crystal structure. Further heating givesfurther thermal expansion until, at about 1400 C the fcc structure reverts tothe bcc form and there is a step expansion which restores the volume lost at910 C. Heating beyond 1400 C gives thermal expansion until meltingoccurs at 1540 C. The curve is reversible on cooling slowly.


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The property that metals may have different crystal structures, depending ontemperature, is called allotropy.

3.2 Solution of Carbon in bcc and fcc Crystals

When the atoms of two materials A and B have about the same size, crystalstructures may be formed where a number of the A atoms are replaced by Batoms. Such a solution is called substitutional because one atom substitutesfor the other. An example is nickel in steel.

When the atoms of two materials have a different size, the smaller atom may be able to fit between the bigger atoms. Such a solution is called interstitial.The most familiar example is the solution of carbon in iron. In this way thehigh temperature fcc crystals can contain up to 2% solid solution carbon at

1130 C, while in the low temperature bcc crystals, the maximum amount ofcarbon which can be held in solution is 0,02% at 723 C and about 0,002% atambient temperature.

Thus a steel containing 0,5% carbon, for example, can dissolve all thecarbon in the higher temperature fcc crystals but on cooling cannot maintainall the carbon in solution in the bcc crystals. The surplus of carbon reactswith iron to form iron carbide (Fe 3C), usually called cementite. Cementite ishard and brittle compared to pure iron.

The amount of cementite and the distribution of cementite particles in themicrostructure is important for the engineering properties of steel.

The distribution of cementite is highly dependent on the cooling rate. Thedistribution may be explained by considering the so-called iron-carbon

phase diagram, see Section 3.4.

3.3 Nomenclature

The following nomenclature is used by the metallurgist:

Ferrite or -Fe The bcc form of iron in which up to 0,02%C by weight may be dissolv

Cementite Iron carbide Fe 3C (which contains about 6,67%C).

Pearlite The laminar mixture of ferrite and cementite described earlier. The ov

Austenite or -Fe The fcc form of iron which exists at high temperatures and which can

Steel Alloys containing less than 2% carbon by weight.

Cast Iron Alloys containing more than 2% carbon by weight.

Steel used in structures such as bridges, buildings and ships, usually

contains between 0,1% and 0,25% carbon by weight.


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3.4 The Iron-Carbon Phase Diagram

The iron-carbon phase diagram is essentially a map. The most important part is shown in Figure 5. More details are given in Figure 6.


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Any point in the field of the diagram represents a steel containing a particular carbon content at a particular temperature.

The diagram is divided into areas showing the structures that are stable at particular compositions and temperatures.

The diagram may be used to consider what happens when a steel of 0,5%Cis cooled from 1000 C (Figure 6).

At 1000 C the structure is austenite, i.e. polycrystalline fcc crystals with allthe carbon dissolved in them. No change occurs on cooling until thetemperature reaches about 800 C. At this temperature, a boundary is crossedfrom the field labelled Austenite ( ) to the field labelled Ferrite + Austenite( + ), i.e. some crystals of bcc iron, containing very little carbon, begin to

form from the fcc iron. Because the ferrite contains so little carbon, thecarbon left must concentrate in the residual austenite. The carbon content ofthe austenite and the relative proportions of ferrite and austenite in themicrostructure adjust themselves to maintain the original overall carboncontent.

These quantities may be worked out by considering the expanded part of theiron-carbon diagram shown in Figure 7. Imagine that the steel has cooled to750 C. The combination of overall carbon content and temperature isrepresented by point X.


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All the constituents of the microstructure are at the same temperature. A lineof constant temperature may be drawn through X. It cuts the boundaries ofthe austenite and ferrite field at F and A. These intercepts give the carboncontents of ferrite and austenite respectively at the particular temperature.

If, now, the line FA is envisaged as a rigid beam which can rotate about afulcrum at X, the 'weight' of austenite hanging at A must balance the'weight' of ferrite hanging at F. This is the so-called Lever Rule:

Weight of ferrite FX = Weight of austenite AX

The ratio of ferrite to austenite in the microstructure is then given by:

Thus, as the steel cools, the proportion of ferrite increases and the carboncontent of the remaining austenite increases, until cooling reaches 723 C. Atthis temperature the carbon content of the austenite is 0,8% and it can takeno more. Cooling to just below this temperature causes the austenite todecompose. It decomposes into the lamellar mixture of ferrite and Fe 3Cidentified earlier as pearlite.

The proportions of ferrite and pearlite in the microstructure, say at 722 C,are virtually the same as the proportions of ferrite and austenite immediately before the decomposition at 723 C. Thus, referring to Figure 7 and using theLever Rule:

Weight of ferrite F X = Weight of pearlite F P

In this case, there should be about twice as much pearlite as ferrite.

For other steels containing less than 0,8%C, the explanation is identical

except for the proportions of pearlite in the microstructure below 723 C.This varies approximately linearly with carbon content between zero at0,02%C and 100% at 0,8%C. A typical mild steel containing 0,2%C wouldcontain about 25% pearlite.

For steels containing a greater percentage of carbon than 0,8%, the structureis fully austenitic on cooling from high temperatures. The first change tooccur is the formation of particles of Fe 3C from the austenite. This changereduces the carbon content of the residual austenite. On further cooling, thecarbon content of the austenite follows the line of the boundary betweenthe field and + Fe 3C field. Once again, on reaching 723 C the carbon


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content of the austenite is 0,8%. On cooling further, it decomposes into pearlite as before. Therefore, the final microstructure consists of a few particles of Fe 3C embedded in a mass of pearlite, see Figure 6.

4. COOLING RATE

4.1 Cooling Rate During Austenite to Ferrite Transformation andGrain Size

During cooling of austenite, the new bcc ferrite crystals start to grow frommany points. The number of starting points determines the number of ferritegrains and consequently the grain size. This grain size is important becausethe engineering properties are dependent on it. Small grains are favourable.By adding elements like aluminium and niobium, the number of starting

points can be increased. Another important factor is the cooling rate. Whencooling is slow, the new ferrite grains develop from only a few mostfavourable sites. At high cooling rates, the number of starting points will bemuch higher and the grain size smaller. Slides 7 - 9 shows steels withvarious grain sizes, produced at different finish rolling temperatures.

Slide 7 : Microstructure of pearlite. (x 1000)

Another important factor is that, when a fine grained steel is heated to atemperature in excess of about 1000 C, some of the austenite grains growwhile neighbouring grains disappear. This grain growth occurs duringwelding in the so-called heat affected zone (HAZ). This is a 3-5 mm wide


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The dramatic effect of carbon content on toughness is shown in Figure 9.Increasing pearlite content decreases the upper shelf toughness and increasesthe ductile-brittle transition temperature.

Figures 8 and 9 illustrate one of the difficulties in the choice of carboncontent. Increasing the carbon content is beneficial in that it improves yield


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strength and ultimate tensile strength, but is undesirable in that it reducesductility and toughness. A high carbon content may also cause problemsduring welding, see Section 4.3.

In European Norm 10025, Table 3, [3] the chemical composition for flat andlong products is given. An extract is presented in Figure 10. The designationS235 JR, for example, indicates that the yield strength is at least 235

N/mm 2. It is emphasised that the compositional values in the table aremaximum values. Many steelmakers achieve much lower levels, resulting in

better ductility, resistance against brittle fracture, and weldability.


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The lowest carbon content that can be achieved easily on a large scale isabout 0,04%. This content is characteristic of sheet or strip steels intended to

be shaped by extensive cold deformation, as in deep drawing.

Carbon contents of more than 0,25% are used in the wider range of generalengineering steels. These steels are usually put into service in the quenchedand tempered state (see below) for a great multiplicity of purposes inmechanical engineering. High strength bolts for some structural applicationswould also be steels of this type.

4.2.2 The need for control of grain size

The mechanical properties of steel are affected by grain size. Slides 8 and 9show microstructures of two samples of the same batch of mild steel which

have been treated, by methods outlined in Section 4.2.3, to give differentgrain sizes. Reduction in grain size improves yield strength but also has a profound effect on the ductile/brittle transition temperature, see Figure 11.Thus, there are several benefits from the same microstructural charge. Thisis an unusual circumstance in metallurgy where adjustments to improve one

property often mean a worsening of another and a compromise is necessary.An example of such compromise relates to carbon content, alreadydiscussed above.


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Slide 8 : Microstructure of typical hot rolled structural steel containing0,15% carbon and showing white ferrite grains and pearlite colonies. (x

200)

Slide 9 : Refined microstructure of controlled rolled structural steelcontaining 0,15% carbon (white ferrite grains and pearlite colonies. (x 200)


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4.2.3 Grain size control by normalising

In Section 4.2.1 the transformations that can occur when steels are cooledslowly are described. To form ferrite and pearlite from austenite, the carbon

atoms in the steel must change their positions. The diffusion processeswhich transport the atoms within the solid occur at rates which dependexponentially on temperature. The rate of cooling also affects thesetransformations.

If the cooling rate is increased the transformations occur faster. In addition,the diffusional processes cannot keep up with the falling temperature. Thus,a steel cooled very slowly in a furnace keeps close to the requirements of the

phase diagram. But the same steel, removed from the furnace and allowed tocool in air, may undercool before completing its sequence of

transformations. This more rapid cooling has two effects. First it tends toincrease slightly the proportion of pearlite in the microstructure. Secondly it

produces ferrite with a finer grain size and pearlite with finer lamellae. Bothof these microstructural changes give higher yield strength and betterductility and toughness.

Furnace cooled steels are known as fully annealed steels. Air cooled steelsare known as normalised steels.

Grain size can also be affected by the temperature to which the steel isheated in the austenite range. The grains of austenite coarsen with time, therate of coarsening increasing exponentially with temperature. Thecoarsening is important because the transformation to ferrite and pearlite oncooling starts at the grain boundaries in the austenite. If the new structuresstart growing from points which are further apart in a coarse grainedaustenite, the grain size of the resulting ferrite is itself coarser. Thus, steelsshould not be overheated when austenitising before normalising.

The temperature to which the steel is heated before cooling in air is usuallyreferred to as the normalising temperature. The requirements of the last

paragraph mean that this temperature should be as low as possible, as longas the structure is single phase austenite. A glance at the phase diagram ofFigure 5 shows that the normalising temperature decreases as the carboncontent increases from zero to 0,8%. It should lie in the hatched band shownin Figure 12.


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4.2.4 Microstructural changes accompanying hot rolling of steels

Structural steel sections are produced by hot rolling ingots or continuouslycast strand into the required forms. The rolling processes have important

effects on the development of the microstructure in the materials.The early stages of rolling are carried out at temperatures well within theaustenite range, where the steel is soft and easily deformed. The deformationsuffered by the material breaks up the coarse as-cast grain structure but, atthese high temperatures, the atoms within the material can diffuse rapidlywhich allows the deformed grains to recrystallise and reform the equiaxial

polycrystalline structure of the austenite.

Heavy deformation at low temperatures in the austenite range gives finer

recrystallised grains. If the rolling is finished at a temperature just above theferrite + austenite region of the equilibrium diagram and the section isallowed to cool in air, an ordinary normalised microstructure havingmoderately fine-grained ferrite results. Modern controlled rolling techniquesaim to do this, or even to roll at still lower temperatures to give still finergrains.

If the temperature falls so that the rolling is finished in the ferrite + austeniterange, the mixture of ferrite and austenite grains is elongated along therolling direction and a layer-like structure is developed. If now, the sectionis air-cooled, the residual austenite decomposes into fine-grained ferrite and

pearlite, with the later being present as long, cigar shaped, bands in thematerial, as in Slide 10. Structural steels are not harmed by microstructuresof this sort.


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Slide 10 : Microsection through a fillet weld on structural steel showingthree distinct regions: the coarse grained cast structure of the weld deposit,

the heat affected zone, and the unaffected microctructure of the parent steel.(x 200)

If the finish rolling temperature drops further, to below 723 C, theequilibrium diagram shows that the structure should be a mixture of ferriteand pearlite. Rolling in this range is usually restricted to low carbon steelscontaining less than 0,15%C because the presence of pearlite makes rollingdifficult.

If the temperature is above about 650 C, the ferrite grains recrystallise asthey are deformed, as was the case with austenite. The carbide laths (Fe 3C)in the pearlite become broken and give rise to strings of small carbide

particles extending in the direction of rolling, see Slide 11. The ferrite fromthe pearlite becomes indistinguishable from the rest of the ferrite.


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Slide 11 : Macrosection through a butt weld on hot rolled steel plate, typicalof line pipe weld.

If rolling is done at ambient temperature, the pearlite is broken up in thesame way, but the ferrite can not recrystallise. It work-hardens, i.e. the yield

and ultimate tensile strength of the steel increase, and the ductilitydecreases, see Figure 13. As cold rolling continues, the force required tocontinue deformation increases because of the increasing yield strength.Furthermore, the steel becomes less ductile and may begin to split. Theamount of cold rolling that can be done is therefore very much smaller thanthat which can be achieved when the steel is hot.


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Of course, cold working need not be applied by rolling. Any way ofdeforming the material causes work hardening. For example, high strengthsteel wire is made by cold-drawing, imparting large deformations. Inanother example, one type of reinforcing bar is made by twisting square

section bar into a helical form. The cold-deformation produced in this way isnot large but causes significant work hardening.

To restore the ductility and at the same time reduce the work hardened stateof the material, it is necessary to reform the isotropic, polycrystallinestructure of the ferrite. Re-heating to temperatures between about 650 C and723 C allows the ferrite to recrystallise. The carbide particles are unaffected

by this treatment.

Thus, there is another technique for controlling the grain size of steel. The

greater the amount of deformation before the recrystallisation treatment andthe lower the temperature of the treatment, the finer is the final grain size.Because this type of treatment does not involve the formation anddecomposition of austenite, it is known as sub-critical annealing. Theresulting microstructure has good ductility and deep drawing characteristics.Sheet steels of low carbon content (< 0,1%C) are usually supplied in thiscondition. Objects such as motor car body panels are formed from suchsteels by cold pressing.

If the material is heated into the austenite range, subsequent cooling reformsthe normalised microstructure.

4.3 Rapidly Cooled Steels

4.3.1 Formation of martensite and bainite

Normalising causes steels to undercool below the requirements of the phasediagram before the austenite transforms into fine ferrite and pearlite. Stillfurther increases in cooling rate give further undercooling and still finermicrostructures.

Very rapid cooling by quenching into cold water, causes the formation offerrite and pearlite to be suspended. The internal diffusion-controlledrearrangement of atoms needed to form those products cannot occursufficiently rapidly. Instead, new products are formed by microstructuralshear transformations at lower temperatures. Very fast cooling givesmartensite: its microstructure is shown in Slide 12. When martensite forms,there is no time for the formation of cementite and the austenite transformsto a highly distorted form of ferrite which is super saturated with dissolved

carbon. The combination of the lattice distortion and the severe workhardening resulting from the shear deformation processes necessary to


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achieve the transformation cause martensite to be extremely strong but very brittle.

Slide 12 : Longitudinal section of hot rolled structural steel showing dark bands of pearlite in a ferrite matrix. (x 200)

Less rapid cooling can give a product called bainite,. This is similar totempered martensite where much of the carbon has come out of solution andformed fine needles of cementite which reinforce the ferrite.

4.3.2 Martensite in welded structures

Civil engineering structures are not heat-treated by heating to, say, 900 Cand quenching into water. However, there is one important circumstancewhich can produce martensite in localised parts of the structure, and that is

welding. The weld zone is raised to the melting temperature of the steel andthe immediately adjacent solid metal is heated to temperatures well withinthe austenite range. When the heat source is removed, the whole regioncools at rates determined mainly be thermal conduction into the surroundingmass of cold metal. These rates of cooling can be very large, exceeding1000 C per second in some cases and can produce transformation structuressuch as martensite and bainite. The properties of rapidly cooled steels andthe influence of carbon content on the nature of the transformation product -ferrite and pearlite, or bainite, or martensite - are discussed below.


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Figure 14 shows the hardness of martensite as a function of its carboncontent. Reheating martensite to temperatures up to about 600 C causescementite to precipitate which causes the steel to soften and become muchtougher. This reheating is known as tempering. The extent of these changes

increases as the reheating temperature increases, as shown in Figure 15.Tempering at 600 C produces an extremely tough material. What is more,its ductile-brittle transition temperature is lower than for the same steel inthe normalised condition. Bainite has properties similar to those of temperedmartensite.


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4.3.3 Quenching and tempering

The process of quenching and tempering, when allied to changes of steelcomposition, can produce a bewilderingly wide range of properties. Steels

heat-treated in this way are used for a multiplicity of general engineering purposes which demand hardness, wear resistance, strength and toughness.Once again, compromises must be struck between these desirable properties

but generally quenched and tempered steels exhibit optimum combinationsof strength and toughness. For structural purposes quenched and tempered

plate is used in large storage tanks, hoppers, earthmoving equipment, etc.

Martensite produced in a weld heat-affected zone as a result of single passwelding would be in its hard and brittle untempered condition. Furthermore,the formation of martensite from austenite is accompanied by a volume

expansion of approximately 0,4%. This expansion, together with the uneventhermal contractions taking place as a result of uneven cooling, can producelocal stresses of sufficient magnitude to crack the martensite. Because thistype of cracking occurs after the HAZ has cooled, it is referred to as coldcracking. The cracking problem can be further aggravated if the weld has

picked up hydrogen. Sources of hydrogen during welding might includemoisture from the atmosphere or damp welding electrodes. Hydrogendissolved in the weld metal diffuses to the hard HAZ where it initiatescracks at sites of stress concentration. This diffusion can lead to crackingwhich occurs some time, even days, after the welding is completed. HardHAZs of low ductility are less able to cope with this problem than are softerand more ductile materials. This type of cracking is called delayed crackingor hydrogen cracking.

Avoidance of cold cracking and hydrogen cracking requires that the materialshould not be overhardened. As a rule of thumb, as-welded hardnesses ofless than about HV = 350 are considered to be acceptable. In modern finegrain low carbon steels the "allowable" hardness may be increased to HV =400 or even HV = 450.

The danger of hydrogen cracking may also be present in high strengthquenched and tempered steels, e.g. 10.9 bolts (R e 900 N/mm

2 andR m 1000 N/mm

2). When such bolts are electroplated with zinc orcadmium, hydrogen may be picked up from the plating bath. Usuallycracking does not occur until sometime after tightening bolts when thehydrogen has diffused to the sites of stress concentration at the thread roots.

4.3.4 Control of martensite formation

Martensite forms because ferrite and pearlite did not! If follows thatmetallurgical factors which promote the formation of ferrite and pearlite


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inhibit the production of martensite. The ability of a steel to form martensiterather than ferrite and cementite is called hardenability. Note that this termdoes not refer to the absolute value of hardness obtained, but to the ease offormation of martensite.

The most convenient method of assessing hardenability is the so calledJominy end quench. A rod-shaped sample is austenitised and then quenched

by spraying water onto one end face such that different cooling rates are produced along the length of the bar. Thereafter, a flat is ground along itslength and the hardness measured as a function of distance from thequenched end.

Some typical results are shown in Figure 16 for three different steels. For acarbon steel containing 0,08%C and 0,3%Mn, cooling rates at 700 C of

greater than about 50 C s-1

are necessary to form martensite. On the otherhand in the 0,29%C, 1,7%Mn steel, martensite forms at much slowercooling rates. It is mainly the increased carbon content that causes thisdifference. In the alloy steel illustrated, martensite is formed even at veryslow cooling rates.


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The significance of these curves depends very much on what is being produced. If it is a thick-section gear wheel, the alloy steel would be ideal. Itcould be cooled gently and still produce martensite, the gentle cooling beingan advantage because it would reduce stresses arising from differentialcontraction rates, and hence reduce the possibility of quench cracking.


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Thereafter, it could be tempered to achieve the desired combination ofstrength and toughness. On the other hand, for a welded joint, the plaincarbon steel would be preferable in which it is difficult to form martensiteand the hardness of any martensite produced would be relatively low.

Welding presents particular problems for the metallurgist. Slide 13 shows amicro section through a typical structural weld. The micro structures rangefrom the coarse grained cast structure of the weld deposit, to the heataffected zone (HAZ) and to the unaffected microstructure of the parentmetal. Both the deposited weld metal and the HAZ must have adequatestrength and toughness after welding.

Slide 13 : Microstructure of martensite (x 500)

For welding, a steel of low hardenability is therefore required. Hardenability

is affected by steel composition, including not only carbon content but otheralloying elements as well. To take all of these factors into account, theconcept of the carbon equivalent value is used. There are a number of waysof calculating carbon equivalents for use in different circumstances. In thecontext of welding:

C.E. =

If the CE is lower than about 0,4%, the steel can be welded with little or notrouble from martensite and HAZ hydrogen cracking. As indicated before,


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the cooling rate is also an important factor, which means that duringwelding, thick plates are more susceptible to hydrogen cracking than thin

plates. To reduce susceptibility to martensite formation, the cooling rate(between 800 C and 500 C) can be reduced by preheating the plates before

welding.5. INCLUSIONS

5.1 Sulphur, Phosphorus and Other Impurities

One tonne of steel, a cube with sides of about 0,5m, contains between1012 and 10 15 inclusions which can occupy up to about 1% of the volume.The total content is largely determined by the origins of the ores, coke andother materials used to extract the metal in the first place, and by the details

of steelmaking practice.

The principal impurities which worry steelmakers are phosphorus andsulphur. If not at very low concentrations, these impurities form particles of

phosphide and sulphide which are harmful to the toughness of the steel.Typically, less than 0,05% of each of these elements is demanded. Low

phosphorus contents are relatively easily attained during the refining of the pig iron into steel, but sulphur is more difficult to remove. It is controlled bycareful choice of raw materials and, in modern steelmaking, by extra

processing steps to remove it.

Manganese is always added to steels. It has several functions but theimportant one in this context is that it combines with the sulphur to formmanganese sulphide (MnS). If the manganese were not present, ironsulphide would form which is much more harmful than MnS.

Some of the inclusions are too small to be seen with optical microscopes andmust be detected by more elaborate methods. Among this group, which aremainly equiaxial in shape, are nitrides of aluminium and titanium which aredeliberately introduced in order to inhibit the processes which lead tocoarsening of grain size.

Other inclusions, large enough to be seen readily with the opticalmicroscope, include entrained particles of slag, deoxidation products andmanganese sulphide. At hot rolling temperatures, these inclusions are plasticand are elongated in the rolling direction. The result is shown in Figure 1.The properties of steels containing such inclusions reflect both the volumeof the inclusions and the anisotropy of their shapes, see Figures 17 and 18.


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In recent years, a number of practices have been introduced which aim toreduce the inclusion content in the molten steel before it is cast into ingots.Sulphur contents of 0,01% or less are now regularly produced. These

processes produce what have become known as 'clean steels'. The

expression is relative. Clean steels still contain many inclusions, but aresignificantly tougher than ordinary steels. Inclusion shape control is also practised in better quality steels. Additions of calcium or cerium and otherrare earth elements to the refined molten steel combine with the sulphur in

preference to the manganese. Sulphides of these elements appear in the finalmicrostructure as equiaxial particles and are not so deleterious to thethrough-thickness ductility of the material as elongated MnS inclusions.Steels treated in these ways are used in applications where toughness is of

paramount importance and where the extra cost can be justified. Examplesinclude high integrity pressure vessels, oil and gas pipelines and the main

legs of offshore platforms. The introduction of continuous casting has alsoimproved the quality of conventional structural steels.

5.2 Manganese in Structural Steels

It has been noted earlier that the residual sulphur impurity in steel is lessharmful when formed into particles of MnS rather than iron sulphide. The

presence of small amounts of manganese in the steel confers several other benefits. In normalised steels, it tends to increase the amount ofundercooling before the start of the formation of ferrite and pearlite. Thisgives finer grained ferrite and more finely divided pearlite. Both of thesechanges improve strength and reduce the ductile/brittle transitiontemperature. The dissolution of the manganese atoms in the ferrite crystalsalso improves the strength of the ferrite. These effects on properties aresummarised in Figures 19 - 21.


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If the manganese content is increased too much, its effect ceases to be beneficial and can become harmful because it increases hardenability, i.e. promotes martensite formation. It is for this reason that a maximummanganese content is specified: For S355 in Table 3 of EN 10025 this

maximum is 1,7% by weight, see Figure 16. A convention has also grownthat distinguishes between plain carbon steels, i.e. steels containing 1%Mn.

6. CONCLUDING SUMMARY

Steels used for structural purposes generally contain up to about0,25%C, up to 1,5%Mn and with carbon equivalents of up to0,4%. They are mostly used in the hot-rolled, normalised orcontrolled-rolled conditions, although low carbon steels might be

used in the cold-rolled and annealed condition. Productionprocesses aim to produce low inclusion contents and small grainsize to improve strength, ductility, toughness and reduce theductile/brittle transition.

The elastic modulus of steel is virtually independent ofcomposition and treatment.

The upper limits on the proportions of carbon and other alloyingelements are determined by the effect of carbon equivalent onweldability, and by the effect of carbon on the ductile/brittle

transition temperature. All steels contain manganese, partly todeal with impurities, such as sulphur, and partly because itspresence has a beneficial effect on the ductile/brittle transitionand strength.

In recent years the development of so-called micro-alloyed steelsor HSLA (high strength low alloy) steels has taken place. Thesesteels are normalised or controlled rolled carbon-manganesesteels which have been 'adjusted' by micro-alloying to give higherstrength and toughness, combined with ease of welding. Smalladditions of aluminium, vanadium, niobium or other elements areused to help control grain size. Sometimes, about 0,5%molybdenum is added to refine the lamellar spacing in pearliteand to distribute the pearlite more evenly as smaller colonies.These steels are used where the improved properties justify theext

he Metallurgy Of Carbon Steel

The best way to understand the metallurgy of carbon steel is to study the Iron Carbon


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Diagram. The diagram shown below is based on the transformation that occurs as a resultof slow heating. Slow cooling will reduce the transformation temperatures; for example:the A1 point would be reduced from 723C to 690 C. However the fast heating andcooling rates encountered in welding will have a significant influence on thesetemperatures, making the accurate prediction of weld metallurgy using this diagramdifficult.

Austenite This phase is only possible in carbon steel at high temperature. It hasa Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbonin solution.

Ferrite This phase has a Body Centre Cubic structure (B.C.C) which can holdvery little carbon; typically 0.0001% at room temperature. It can exist as either:alpha or delta ferrite.

Carbon A very small interstitial atom that tends to fit into clusters of ironatoms. It strengthens steel and gives it the ability to harden by heat treatment. Italso causes major problems for welding , particularly if it exceeds 0.25% as it

creates a hard microstructure that is susceptible to hydrogen cracking. Carbonforms compounds with other elements called carbides. Iron Carbide, Chrome


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Carbide etc.

Cementite Unlike ferrite and austenite, cementite is a very hard intermetalliccompound consisting of 6.7% carbon and the remainder iron, its chemical symbolis Fe 3C. Cementite is very hard, but when mixed with soft ferrite layers itsaverage hardness is reduced considerably. Slow cooling gives course perlite; softeasy to machine but poor toughness. Faster cooling gives very fine layers offerrite and cementite; harder and tougher

Pearlite A mixture ofalternate strips of ferrite andcementite in a singlegrain. The distance betweenthe plates and their thickness isdependant on the cooling rateof the material; fast coolingcreates thin plates that areclose together and slow

cooling creates a much coarserstructure possessing lesstoughness. The name for thisstructure is derived from itsmother of pearl appearanceunder a microscope. A fully

pearlitic structure occurs at0.8% Carbon. Furtherincreases in carbon will createcementite at the grain

boundaries, which will start toweaken the steel.

Cooling of a steel below 0.8% carbon When a steel solidifies it formsaustenite. When the temperature falls below the A3 point, grains of ferrite start toform. As more grains of ferrite start to form the remaining austenite becomesricher in carbon. At about 723C the remaining austenite, which now contains0.8% carbon, changes to pearlite. The resulting structure is a mixture consisting ofwhite grains of ferrite mixed with darker grains of pearlite. Heating is basicallythe same thing in reverse.


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Martensite If steel is cooled rapidly from austenite, the F.C.C structure rapidly changes toB.C.C leaving insufficient time for the carbon to form pearlite. This results in a distortedstructure that has the appearance of fine needles. There is no partial transformationassociated with martensite, it either forms or it doesnt. However, only the parts of a sectionthat cool fast enough will form martensite; in a thick section it will only form to a certaindepth, and if the shape is complex it may only form in small pockets. The hardness ofmartensite is solely dependant on carbon content, it is normally very high, unless the carboncontent is exceptionally low.


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Tempering The carbon trapped in the martensite transformation can be released by heatingthe steel below the A1 transformation temperature. This release of carbon from nucleatedareas allows the structure to deform plastically and relive some of its internal stresses. Thisreduces hardness and increases toughness, but it also tends to reduce tensile strength. Thedegree of tempering is dependant on temperature and time; temperature having the greatestinfluence.

Annealing This term is often used to define a heat treatment process that produces somesoftening of the structure. True annealing involves heating the steel to austenite and holdingfor some time to create a stable structure. The steel is then cooled very slowly to roomtemperature. This produces a very soft structure, but also creates very large grains, which areseldom desirable because of poor toughness.

Normalising Returns the structure back to normal. The steel is heated until it just starts toform austenite; it is then cooled in air. This moderately rapid transformation createsrelatively fine grains with uniform pearlite.

Welding If the temperature profile for a typical weld is plotted against the carbonequilibrium diagram, a wide variety of transformation and heat treatments will be observed .


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Note , the carbon equilibrium diagram shown above is only for illustration, in reality it will be heavilydistorted because of the rapid heating and cooling rates involved in the welding process.

a)

b)

c)

d)

Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructurenot significantly affected.

Pearlite transformed to Austenite, but not sufficient temperature available to exceedthe A3 line, therefore not all ferrite grains transform to Austenite. On cooling, onlythe transformed grains will be normalised.

Temperature just exceeds A3 line, full Austenite transformation. On cooling allgrains will be normalised

Temperature significantly exceeds A3 line permitting grains to grow. On cooling,ferrite will form at the grain boundaries, and a course pearlite will form inside the


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grains. A course grain structure is more readily hardened than a finer one, therefore ifthe cooling rate between 800C to 500C is rapid, a hard microstructure will beformed. This is why a brittle fracture is most likely to propagate in this region.

Welds The metallurgy of a weld is very differentfrom the parent material. Welding filler metalsare designed to create strong and tough welds,they contain fine oxide particles that permit thenucleation of fine grains. When a weld solidifies,its grains grow from the course HAZ grainstructure, further refinement takes place withinthese course grains creating the typical acicularferrite formation shown opposite.

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