Metallurgy & Metallurgy & Material ScienceMaterial Science

Dr.S.JoseDept of Mechanical Engg.,TKM College of Engineering, Kollamdrsjose@gmail.com

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Ferrous Materials Non – ferrous alloys Bearing Materials Fusible alloys Composites

Metal matrix composites Smart Materials.

Module III

Ferrous Materials In engineering applications, lion’s

share is served by ferrous materials. Factors account for it

availability of abundant raw materials economical extraction ease of forming versatile mechanical and physical

properties.

Ferrous Materials Drawbacks of ferrous materials:

poor corrosion resistance high density & low specific strength low thermal and electrical conductivities

Classification steels and cast irons – categorized based

on carbon content. Steels: %C is upto 2.14% Cast irons: %C is above 2.14%

Ferrous Alloys Cast irons are called so because they are

usually manufactured through casting technique owing to their brittle nature due to presence of iron carbide.

Steels are serving major part of present engineering applications.

However, cast irons mostly serve as structural components. eg: automobile motor casings, lathe bed,

sliding guides in machinery

Steel An alloy whose major component is iron,

with carbon being the primary alloying material.

Carbon acts as a hardening agent, preventing iron atoms from sliding past one another.

Carbon atoms occupy interstitial sites of Fe. Varying the amount of carbon and its

distribution in the alloy controls the qualities of the resulting steel.

Steel Steel with increased carbon content can

be made harder and stronger than iron, but is also more brittle.

Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired.

A more recent definition is that steels are iron-based alloys that can be plastically formed

Steel - classification Steels are classified based on their C

content/alloying additions Plain-carbon steel

Mild (low carbon) steel: < 0.3 wt % C Medium carbon steel: 0.3 to 0.6 wt% C High carbon steel: 0.6 to 2.14 wt% C

Alloy steels HSLA steel Tool steels Stainless steel

Plain-carbon steel An alloy of iron and carbon, where other

elements are present in quantities too small to affect the properties.

Steel with a low carbon content has the same properties as iron, soft but easily formed. As carbon content rises the metal becomes harder and stronger but less ductile.

Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30% to 1.70% by weight.

Plain-carbon steel A limitation of plain carbon steel is the very

rapid rate of cooling needed to produce hardening.

In large pieces it is not possible to cool the inside rapidly enough and so only the surfaces can be hardened.

This can be improved with the addition of other elements resulting in alloy steels

Trace impurities of various other elements can have a significant effect on the quality of the resulting steel

Low-carbon steel Steel with a low carbon content has the same

properties as iron, soft but easily formed. As carbon content rises the metal becomes

harder and stronger but less ductile. Carbon present is not enough to strengthen

them by heat treatment, hence are strengthened by cold work.

They are easily weldable and machinable. Typical applications: tin cans, automotive body

components, structural shapes, etc.

Mild steel The most common form of steel as it

provides material properties that are acceptable for many applications.

Mild steel has medium carbon content (up to 0.3%) and is therefore neither extremely brittle nor ductile.

It becomes malleable when heated, and so can be forged. It is also often used where large amounts of steel need to be formed, for example as structural steel.

Mild Steel They are less ductile and stronger

than low carbon steels. Heat treatable (austenitizing,

quenching and tempering). Hardenability is increased by adding

Ni, Cr, Mo. Used in various tempered conditions. Typical applications: gears, railway

tracks, machine parts.

High Carbon Steels They are the strongest and hardest of carbon

steels. Heat treatable. Used in tempered or hardened

conditions. Toughness, formability and hardenability are

quite low. Not recommended for welding. Alloying additions – Cu, V, W, Ni, Cr, Mo Typical applications: Knives, razors, hacksaw

blades, etc where high wear resistance is the prime requirement.

Alloy Steels

Limitations of plain carbon steel are overcome by adding alloying elements

The alloying elements improve various properties HSLA steel Tool steels Stainless steel

HSLA steel High strength low alloy steel is a type

of steel alloy that provides many benefits over regular steel alloys.

HSLA alloys are much stronger and tougher than ordinary plain carbon steels.

They are used in cars, trucks, cranes, support columns, pressure vessels, bridges and other structures that are designed to handle a lot of stress.

HSLA steel A typical HSLA steel may contain 0.15% carbon,

1.65% manganese and low levels (under 0.035%) of phosphorous and sulphur.

It may also contain small amounts of Cu, Ni, V, Cr, Mo, Si or Zi.

HSLAs are therefore also referred to as 'microalloyed', as they are indeed alloyed in extremely small amounts in comparison to other main commercial alloy steels.

HSLA steels are also more resistant to rust than most carbon steels.

Stainless steel An iron-carbon alloy with a minimum of

12% chromium along with other alloying elements Highly corrosion resistant owing to presence of

chromium oxide. The name originates from the fact that stainless

steel does not stain, corrode or rust as easily as ordinary steel

It is "stains less", but is not actually stain proof. Stainless steel is 100% recyclable.

Stainless Steels Its resistance to corrosion and staining, low

maintenance, relative inexpense, and familiar lustre make it suitable for a host of commercial applications.

There are over 150 grades of stainless steel, of which fifteen are most common.

Typical applications – cutlery, surgical knives, storage tanks, domestic items, jewellery.

Three kinds Ferritic& hardenable Cr steels Austenitic and precipitation hardenable Martensitic

Stainless Steels - types Ferritic steels are principally Fe-Cr-C alloys with

12-14% Cr with small additions of Mo, V, Nb, Ni. Austenitic steels contain 18% Cr and 8% Ni plus

minor alloying elements. Martensitic steels are heat treatable. Major

alloying elements are: Cr, Mn and Mo. Ferritic and austenitic steels are hardened and

strengthened by cold work because they are not heat treatable.

Austenitic steels are non-magnetic as against ferritic and martensitic steels, which are magnetic.

Tool Steels A variety of alloy steels that are particularly

well-suited to be made into tools. Their suitability comes from their

distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness).

Tool steel is generally used in a heat-treated state.

Carbon content between 0.7% and 1.4%,

Tool Steels - Types

Water-hardening grades Air-hardening grades Cold-working grades Shock resisting grades High speed grades Hot-working grades

Effects of Alloying Elements

Barrier to Dislocation movement Polymorphic transformation temperature Strengthening of ferrite Formation and stability of carbides Displacement of the eutectoid point Retardation of the transformation rates Lowering of critical cooling rate Improvement in corrosion resistance

Main Alloying Elements in Steel

Manganese Chromium Nickel Molybdenum Titanium Phosphorus Silicon Copper

Sulphur Cobalt Aluminium Vanadium Tungsten Lead Colubium Boron

Manganese (Mn) Added to steel to improve hot working

properties and increase strength, toughness and hardenability.

Improves ductility and wear resistance. Eliminates formation of harmful iron sulfides,

increasing strength at high temperatures. Manganese, like nickel, is an austenite forming

element Usually present in quantities from 0.5% to 2%

Chromium (Cr) Chromium is added to the steel to increase

resistance to oxidation. This resistance increases as more chromium is

added. 'Stainless Steel' has approximately 12%

chromium When added to low alloy steels, improves

hardenability and strength. Resists abrasion and wear (with high carbon).

Nickel (Ni) Added in large amounts, over about 8%, to

high chromium stainless steel to form the most important class of corrosion and heat resistant steels, the austenitic stainless steels.

Increases toughness and strength at both high and low temperatures.

Improves resistance to oxidation and corrosion. Increases toughness at low temperatures when

added in smaller amounts to alloy steels. Strengthens unquenched or annealed steels. Quantity addition is from 1 to 4%

Molybdenum (Mo) When added to chromium-nickel austenitic steels,

improves resistance to pitting corrosion especially by chlorides and sulphur chemicals.

When added to low alloy steels, it improves high temperature strengths and hardness.

When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.

Increases hardenability and strength. Enhances corrosion resistance in stainless steel. Forms abrasion resisting particles. used in small quantities from 0.10 to 0.40%.

Titanium (Ti) Improves strength and corrosion resistance,

limits austenite grain size. The main use of titanium as an alloying

element in steel is for carbide stabilisation. It combines with carbon to form titanium

carbides, which are quite stable and hard to dissolve in steel.

Reduces martensitic hardness and hardenability in medium Cr steels.

Prevents formation of austenite in high Cr steels.

Phosphorus (P) Phosphorus is usually added with sulphur to

improve machinability in low alloy steels When added in small amounts, aids strength

and corrosion resistance. Phosphorus additions are known to increase

the tendency to cracking during welding. Strengthens low-carbon steel. Increases resistance to corrosion.

Silicon (Si) Improves strength, elasticity, acid resistance

and promotes large grain sizes, which cause increasing magnetic permeability.

Used as a deoxidising (killing) agent in the melting of steel.

Silicon contributes to hardening of the ferritic phase in steels.

Alloying element for electrical and magnetic sheet.

Increase hardenability of steels. Strengthens low-alloy steels. Used in the range of 1.5% to 2.5%

Copper (Cu) Copper is normally present in stainless

steels as a residual element. It is added to a few alloys to produce

precipitation hardening properties. Improves corrosion resistance. Usually 0.15 to 0.25% added

Other Alloying Elements

Sulphur (S) When added in small amounts improves

machinability used in the range 0.06 to 0.30%.

Cobalt (Co) Improves strength at high temperatures and

magnetic permeability.

Aluminum (Al) Dexodises and limits austenite grain growth Alloying element in nitriding steel.

Alloying element Range of percentage

Important functions

Sulphur < 0.33 Improves machinability, reduces weldability and ductility

Phosphorus <0.12 Improves machinability, reduces impact strength at low temperature.

Silicon 1.5 to 2.5 Removes oxygen from molten metal, improves strength and toughness, increases hardenability, magnetic permeability

Manganese 0.5 to 2.0 Increases hardenability, reduces adverse effects of sulphur.

Nickel 1.0 to 5.0 Increases toughness, increases impact strength at low temperatures

Chromium 0.5 to 4.0 Improves resistance to oxidation and corrosion. Increases high temperature strength

Molybdenum 0.1 to 0.4 Improves hardenability, enhances the effect of other alloying elements, eliminates temper brittleness, improves red hardness and wear resistance.

Tungsten 2.0 to 3.0 Improves hardenability, enhances the effect of other alloying elements, eliminates temper brittleness, improves red hardness and wear resistance.

Vanadium 0.1 to 0.3 Improves hardenability, increases wear and fatigue resistance, elastic limit.

Titanium < 1.0 Improves strength and corrosion resistance.

Copper 0.15 to 0.25 Improves corrosion resistance, increases strength and hardness

Aluminium 0.01 to 0.06 Removes oxygen from molten metal

Boron 0.001 to 0.05 Increases hardenability

Lead < 0.35 Improves machinability

Cobalt 5 to 10 Improves red hardness, improves wear and corrosion resistance.

Wrought Iron

Wrought iron is commercially pure iron Wrought iron literally means worked

iron. It is so named because it is worked from

a "bloom" of porous iron mixed with slag and other impurities.

Carbon content not more than 0.15%

Wrought Iron

It is a fibrous material with many strands of slag mixed into the metal.

These slag inclusions give it a "grain" resembling wood, with distinct appearance when etched or bent to the point of failure.

It is tough, malleable & ductile and is easily welded.

It is too soft for blades.

Wrought Iron The fibers in wrought iron give it properties not

found in other forms of ferrous metal. Hammering a piece of wrought iron cold causes

the fibers to become packed tighter, which makes the iron both brittle and hard.

Wrought iron lacks the carbon content necessary for hardening through heat treatment,

wrought iron cannot be bent as sharply as steel, for the fibers can spread and weaken the finished work.

Cast Irons They have low melting temperatures, in the

range 1150-1300OC with good fluidity for taking good casting impressions.

It is a low cost material, as it can be produced relatively easily using low cost raw materials and technology.

Though brittle and have lower strength compared to steels, cast irons have higher compressive strength, ability to absorb vibrations (damping capacity), better wear and abrasion resistance, rigidity and machinability.

Cast Irons

With suitable composition and heat treatments, a variety of microstructures can be developed with varying properties.

Alloy cast irons possess high corrosion and heat resistance.

However, they are not ductile to be rolled, drawn or mechanically worked

Hence the only manufacturing process applicable is casting, and so the name.

Classification of Cast Irons

Cast irons are generally classified based on the metallurgical structure and appearance. The following factors control the structure and appearance.

Carbon content Presence of other elements Cooling rate during and after

solidification Heat treatments

White & Gray Cast Irons

Malleable Cast Iron

Ductile cast iron

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