METALLIC MATERIALS

MATERIAL SCIENCE AND METALLURGY

Classification of Metallic materials

Metals/Alloys

Ferrous Non-Ferrous

Steel

Cast iron

Low carbon Medium

carbon

High carbon

Gray C.I.Nodular C.I.

White C.I.

Malleable C.I.

Definition• Metallic materials -Metals are element/substances which

readily give up electrons to form metallic bonds and conduct electricity.

• Alloy- When two or more pure metals are melted together to form a new metal whose properties are quite different from those of original metals, it is called an alloy.

• Alloys can be formed by blending two or more metals and atleast one being metal. The properties of an alloy can be totally different from its constituent substances, e.g. 18-8 stainless steel, which contains 18%, chromium and 8% nickle, in low carbon steel, carbon is less than 0.15% and this is extremely tough, exceedingly ductile and highly resistant to corrosion. We must note that these properties are quite different from the behavior of original carbon steel.

Properties of Metallic Materials

(a) metals are usually good electrical and thermal conductors.(b) at ordinary temperature metals are usually solid.(c) to some extent metals are malleable and ductile.(d) the freshly cut surfaces of metals are lustrous, (e) when struck metal produce typical sound.(f) most of the metals form alloys.(g) Have relatively high density.(h) They exhibit crystalline structure.(i) Have ability to be deformed plastically.

Ferrous material

• These are metals and alloys containing a high proportion of the element iron.

• They are the strongest materials available and are used for applications where high strength is required at relatively low cost and where weight is not of primary importance.

• As an example of ferrous metals such as : bridge building, the structure of large buildings, railway lines, locomotives and rolling stock and the bodies and highly stressed engine parts of road vehicles.

Classification of Ferrous Material• One main drawback of ferrous alloys is their environmental

degradation i.e. poor corrosion resistance. Other disadvantages include: relatively high density and comparatively low electrical and thermal conductivities. In ferrous materials the main alloying element is carbon (C). Depending on the amount of carbon present, these alloys will have different properties, especially when the carbon content is either less/higher than 2.14%. This amount of carbon is specific as below this amount of carbon, material undergoes eutectoid transformation, while above that limit ferrous materials undergo eutectic transformation. Thus the ferrous alloys with less than 2.14% C are termed as steels, and the ferrous alloys with higher than 2.14% C are termed as cast irons.

• On the basis of the percentage of carbon and their alloying elements present, these can be classified into following groups:

• (a) Mild Steels/Low carbon steel: The percentage of carbon in these materials range from 0.15% to 0.25%. These are moderately strong and have good weldability. The production cost of these materials is also low.

• (b) Medium Carbon Steels: These contains carbon between 0.3% to 0.6%. The strength of these materials is high but their weldability is comparatively less.

• (c) High Carbon Steels: These contains carbon varying from 0.65% to 1.5%. These materials get hard and tough by heat treatment and their weldability is poor. The steel formed in which carbon content is upto 1.5%, silica upto 0.5%, and manganese upto 1.5% alongwith traces of other elements is called plain carbon steel.

• (d) Cast Irons: The carbon content in these substances vary between 2% to 4%. The cost of production of these substances is quite low and these are used as ferrous casting alloys.

Principle Metal Crystal Structures• In crystalline structure , the atoms are located at regular and

recurring positions in three dimension . The pattern may be replicated millions of times within a given crystal. The structure can be viewed in the form of a unit cell, which is the basic geometric grouping of atoms that is repeated.

• There are some metals that are undergo a change of structure atdifferent temperatures. Iron metal for example is arranged in a body centered-cubic ( BCC ) at room temperature, when the metal is heated and reaches a temperature of 910˚C, the atoms rearrange themselves into Face-Centered-Cubic ( FCC ) crystals. If the metal is heated to the still higher temperature of 1400˚C the atoms again rearrange themselves, this time back into Body-Centered-Cubic form.

There are three principle crystal structures for metals:

(a) Body-centered cubic (BCC)(b) Face-centered cubic (FCC)(c) hexagonal close packed structures (HCP)

Body-centered cubic (BCC)

Face-centered cubic ( FCC )

HEXAGONAL CLOSE PACKED STRUCTURES

Fracture

• Fracture can be defined as the separation of a body under stress into two or more parts.

or • Separation of a body into pieces due to stress, at temperatures

below the melting point.• It is considered as the end result of plastic deformation

processes which results in the creation of two or more new surfaces.

• It is caused by physical and chemical forces.• It takes place in following way-– Crack initiation– Crack Propagation

Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle.• Ductile fracture - most metals (not too cold):– Extensive plastic deformation ahead of crack.– Crack is “stable”: resists further extension unless

applied stress is increased.

Ductile Fracture (Dislocation Mediated)

(a) Necking, (b) Cavity Formation,(c) Cavity coalescence to form a crack, (d) Crack propagation, (e) Fracture

Brittle fracture

• Brittle fracture is characterized by very low plastic deformation and low energy absorption prior to breaking.

• A crack, formed as a result of the brittle fracture, propagates fast and without increase of the stress applied to the material.

• The brittle crack is perpendicular to the stress direction.

Ductile Fracture• It involves larger plastic

deformation.

• It is always preceded by localized deformation called “necking”

• It occurs normally in FCC metals.

• It occurs in transgranular.• It occurs by slow tearing of

the metal with expenditure of considerable energy.

Brittle Fracture• It is associated with

minimum plastic deformation.

• It does not involve “necking”

• It occurs normally in BCC and HCP metals.

• It occurs in intergranular.• It occurs suddenly without

any warning.

Examination of Metals

• Micro-Those which require high magnification to be visible are termed microstructures. Microscopes are required for the examination of the microstructure of the metals. Optical microscopes are used for resolutions down to roughly the wavelength of light (about half a micron) and electron microscope are used for detail below this level, down to atomic resolution.

• Macro-Structures which are coarse enough to be discernible by the naked eye or under low magnifications are termed macrostructures. Useful information can often be gained by examination with the naked eye of the surface of metal objects or polished and etched sections.

Macro- (Macrography, Macrostructure) Examination

• Macro- (Macrography, Macrostructure) Examination is a method of examination of large regions of the specimen surface or fractured section with the naked eye or under low magnification. The following macrostructure details may be studied:

• Macro-segregation of the alloying elements or impurities (sulfur in steel) Large non-metallic inclusions such oxides, sulphides or slag.

• Forging flow lines.• Cast grain structure.• Physical defects such gas pockets, shrinkage cavities, cracks.

• Three different procedures are used for observation various characters of macrostructure:

1. Surface examination2. Macroetching3. Fracture Examination

1. Surface examination

• The sample surface is usually ground (polish is not necessary)

• The finish grinding is Grade 320 paper. • Large cracks, shrinkage cavities, gas pockets

may be investigated.

2. Macro-etching• Macroetching helps to reveal much information of cast, hot-rolled and forged

details. • Sulfur print is a macroetching technique of steel samples, allowing to determine

sulfur distribution and segregation in the sample: • The ground specimen is degreased and washed;• Pieces of photographic paper (bromide) are soaked in 2-3% solution of

sulphuric acid for two minute;• The paper is taken from the solution and excess acid is removed from the

surface;• The paper is placed on the sample surface, providing an intimate contact

between the sensitive side of the paper and the sample surface.• The sulphuric acid reacts with the sulphides in the steel specimen, forming

hydrogen sulphide, which then reacts with the silver bromide of the photographic paper, forming dark traces corresponding the sulfur segregation;

• After 2-5 min. the paper and specimen are separated and the print paper is rinsed in water and fixed in hypo solution (20%) for 5 min.

• Finally the print is washed for 30 min and dried.

Macro-etching reagents

• (quantities of the components are in weight parts) • For steels

– Crystal structure: Iodine10 parts, Potassium Iodide20 parts, Water100 parts

– Flow lines, cracks, porosity: Hydrochloric acid50 parts, Water50 parts, Boiling for 5-15 min.

– Dendritic structure: Copper ammonium chloride 9 parts, Water91 parts, Immersion for 0.5-4 hours.

• For stainless and austenitic steels: Hydrochloric acid 15 parts, Nitric acid5 parts, Water100 parts.

• For aluminum alloys: Hydrochloric acid 10 parts, Nitric acid1 part, Water200 parts.

• For copper alloys (dendritic structure): Ferric chloride 25 parts, Hydrochloric acid 25 parts, Water100 parts.

3. Fracture Examination

• Fracture examination is a method relating to observation of the broken surfaces of the failed part.

• The appearance of the surfaces may be a result of brittle fracture, fatigue with its characteristic fracture, intercrystalline fracture, segregation, slag particles, cracks, starting from physical defects, rough surface, corrosion and other causes.

• Fracture examination should take into account the working condition, the history of the part, possible overloads, applied to the part, misalignments and other working factors.

• Fracture examination should be supported by other metallography examinations.

SPARK TEST

• Spark testing is a method of determining the general classification of ferrous materials.

• It is a reliable and inexpensive method to identify the metals and alloys.

• Spark testing most often is used in tool rooms, machine shops, heat treating shops, and foundries.

METHOD

• It normally entails taking a piece of metal, usually scrap, and applying it to a grinding wheel in order to observe the sparks emitted. These sparks can be compared to a chart or to sparks from a known test sample to determine the classification. Spark testing also can be used to sort ferrous materials, establishing the difference from one another by noting whether the spark is the same or different.

• Different types of steel gives different type of spark.

Advantages and Limitations

• Spark testing is used because it is quick, easy, and inexpensive. Moreover, test samples do not have to be prepared in any way, so, often, a piece of scrap is used. There is no need of dark room for it. The main disadvantage to spark testing is its inability to identify a material positively; if positive identification is required, chemical analysis must be used. The spark comparison method also damages the material being tested, at least slightly.

Types of Spark and Colors

• Most observed sparks are-Fork, Shaft, Bud break arrow, Stream, Dashed and Appendages.

• The major types of colors of spark are- Red, Orange, White, Yellow.

EXAMPLES

SULPHUR PRINTING

• Sulfur print is a macroetching technique of steel samples, allowing to determine sulfur distribution and segregation in the sample.

• PRINCIPLE- Sulphide, when attacked with diluted acid, evolves hydrogen sulphide gas which stains bromide paper and therfore can readily detected in ordinary steel and cast iron.

• FeS + H2SO4 FeSO4 + H2S

PROCESS• The chosen surface is ground flat and smoothened by machine or by means of

emery paper to a grade of 0.• The ground specimen is degreased and washed;• Pieces of photographic paper (bromide) are soaked in 2-3% solution of

sulphuric acid for two minute;• The paper is taken from the solution and excess acid is removed from the

surface;• The paper is placed on the sample surface, providing an intimate contact

between the sensitive side of the paper and the sample surface.• The sulphuric acid reacts with the sulphides in the steel specimen, forming

hydrogen sulphide, which then reacts with the silver bromide of the photographic paper, forming dark traces corresponding the sulfur segregation;

• After 2-5 min. the paper and specimen are separated and the print paper is rinsed in water and fixed in hypo solution (20%) for 5 min.

• Finally the print is washed for 30 min and dried.

PHOSPHORUS PRINTING

• Phosphorus printing is carried out in order to find the amount and distribution of phosphorus in steel.

• The steel surface is prepared as in sulphur printing.

PROCESS• Bromide paper is soaked in a solution of ammonium molybdate — five gms

per 100 ml of water to which 35 ml of concentrated nitric acid is added.• Thus soaked bromide paper is placed in contact with the surface of steel.• After about five minutes, this paper is removed and developed in an

aqueous solution of 35% hydrochloric acid to which a small quantity of alum and 5 ml of saturated stannous chloride solution is added. Developing time is about four minutes.

• During developing, the complex phosphomolybdate areas formed on the bromide paper earlier are partly reduced. This changes the colour of the areas from yellow brown to blue. These blue areas on the paper indicate the phosphorus rich portions on the surface of steel.

• Since with the increase in phosphorus usually the intensity of blue color also increases, the amount of phosphorus can be estimated from the relative intensity of the blue color on the areas of the paper.

MAGNETIC TESTING

• The Magnet test is used to distinguish ferrous from non-ferrous metals; or, rather obviously, magnetic metals from non magnetic metals.

• Ferrous metals are alloys of iron, and they usually stick to a magnet…

• There are actually 3 (THREE) common elements that pull to a magnet, and those are iron, nickel, and cobalt.

• Non-ferrous metals are generally non-magnetic (except for

nickel and cobalt).   

PROCESS

• Find a metal sample• Find a magnet• Place the magnet against the metal sample.• Pull the magnet away from the metal sample.• Obey the magic rule…• If the magnet sticks to a metal alloy, it is a ferrous (or nickel

or cobalt) alloy.

CHEMICAL ANALYSIS-STEEL

• The chemical analysis can provide percentage of carbon for a given steel.

• There are 3 methods for it-– Gravimetric method– Magnetic method– Volumetric method