lecture #1c

Chapter Outline

What is Alloy? Composed of 2 or more chemical elements, at least one of it is metal.

Contains 2 basic forms; solid solution and intermetallic compounds

Solute and solvent

Crystal structure maintained during alloying called solid solution

Substitutional Solid Solution size similar to solvent Interstitial solid solution size much smaller than solvent atom

Intermetallic Compound

Complex structure which consist of two metals

Solute are present in solvent atom

Strong hard, brittle, and hard High melting point

Good oxidation resistance Low density

E.g: Ti3Al, Ni3Al, Fe3Al

Two-Phase System

(a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead-copper alloy. The grains represent lead in solid solution in copper, and the particles are lead as a second phase.

(b) Schematic illustration of a two-phase system consisting of two sets of grains: dark, and light. The dark and the light grains have separate compositions and properties.

Alloy consist of two or more solid phase Two solid phase called two phase system Homogenous portion which each has it own characteristics. e.g: sand and water, or ice in water Such as small amount of lead in solid solution copper, lead dispersed through out of structure.

● Phase diagrams show the mixture of phases present in thermodynamic equilibrium

● Tells you what phase(s) present for a given temperature and composition roadmap

● Temperature vs. Composition● One basis is: Temp. vs Time cooling

curves● It is very valuable to be able to construct

a phase diagram and know how to use it to predict behaviour of materials

Phase Diagram

Binary Isomorphous Alloy System

Binary Isomorphous Alloy System: Nickel-Copper Alloy Phase Diagram

Lever rule

Wt fraction of solid phase = Xs = wo – wl / ws – wl

Wt fraction of liquid phase= Xl= ws-wo / ws-wl

A copper nickel alloy contains 47wt % Cu and 53 wt% Ni and is at 1300oC.

1. What is the weight percent of copper in the liquid and solid phase at this temperature?

2. What weight percent of this alloy is liquid and what weight percent of solid?

● The intersection of the 1300OC tie line with the liquidus gives 55wt% Cu in the liquid phase and the

intersection of the solidus of 1300OC tie line gives 42wt% Cu in the solid phase.

• From figure wo=53%, wl=45% and ws=58%.

(i) Wt fraction of liquid phase, Xl = ws-wo/ws-wl

= 58-53/58-45= 0.38 (Wt of liquid phase is 38%)

(ii) Wt fraction of solid phase, Xs

= wo-wl/ws-wl

= 53-45/58-45= 0.62 (Wt % of solid phase= 62%)

Lines on the Phase Diagram

● liquidus - line representing the temperatures at which various compositions begin to freeze upon cooling (liquid and mush)

● solidus - line representing the temperatures at which various compositions finish freezing upon cooling (mush and solid)

● solvus - line representing the limit of solid solubility; a line separating solid phases (1 phase and 2 phases)

What phases are present?

Point Z lies in the field where two phases, B + L, are in equilibrium, therefore the two phases present have to be solid B and Liquid.

● What are the proportions of the phases present?

● To determine the proportions of B + L at Z, carry out the following steps:

● Draw a line through Z, parallel to the base of the diagram (This line is at a constant temperature and is an isothermal) This line should extend only to the boundaries of the B + L field - Points X and Y.

● Measure the three line segments - ZX, Zy and XY and ratio these lengths using the lever rule.

● % B = ZX/XY * 100 = 38% B● %L = ZY/XY * 100 = 62% L

Binary Eutectic Alloy System: Lead-Tin Phase Diagram

● Eutectic reaction:

L (61.9%Sn) --> α (19.2%Sn) + β (97.5%Sn)

183o

C

(o C)

α

α

α+ β β

+β

oC

232oC

‘A’ has a low temperature α phase – dissolve about 5% B

‘A’ has a high temperature γ phase – dissolve as much as 40% B

‘B’ can dissolve up to 10% A, the phase is labelled β.

Try this:

1 kg of an alloy of 70% Pb and 30% Sn is slowly cooled from 300oC. Refer to the lead-tin phase diagram and calculate the following:

a) The wt% of the liquid and proeutectic alpha at 250oC

b) The wt% of the liquid and proeutectic alpha just above the eutectic temp (183oC) and the weight in kg of these phases

c) The weight in kg of alpha and beta formed by the eutectic reaction.

Thank You

Iron-Carbon SystemCompositions of Phases Microstructure in Steels

Phase Diagram

● Tells you what phase(s) are present for a given temperature and composition

● Temperature vs. Composition

Iron-Carbon System Ferrous alloys, cast iron and cast steels used

extensively due to low cost and versatile properties.

Pure iron 0.008% carbon, steels up to 2.11%carbon, cast iron up to 4% carbon.

Steels and cast iron representated by iron-carbon binary system.

Solid phases in Fe-Fe3C phase diagram:i. α ferriteii. Austenite (γ)iii. Cementite (Fe3C)iv. δ ferrite

Iron-Iron Carbide Phase DiagramBecause of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams.

δ

γ

γ

γ

α+γ

α-Ferrite α ferrite or ferrite, maximum solid solubility of 0.025%C at 723oC. Relatively soft and ductile; magnetic at room temperature to 768oC. Amount of Carbon can affect the mechanical properties offerrite.

Between 1394 to 912oC iron undergoes change from BCC to FCC structure that is γ-iron or most commonly called austenite Solid solubility up to 2.11% carbon at 1148oC. Have interstitial carbon in FCC structure Single phase FCC structure ductile at elevated temperature, good formability, and non magnetic steels

Austenite (γ)

Cementite (Fe3C)Intermetallic compound (between Fe and C) Content of 100% of iron carbide (Fe3C), which is carbon content of 6.67%, and 93.3% Fe. Very hard and brittle.

δ-ferrite

Solid solution of Carbon in α-iron Maximum solubility ~0.09% at 1465oC BCC structure

Reactions in Fe-Fe3C phase diagram

Peritectic Reaction: This reaction occurs at 1495°C and it can be written asLiquid (0.53 % C) + δ (0.09 % C) γ (0.17 % C)

δ Ferrite is high-temperature phase and is not encountered in plain-carbon steels at lower temperatures.

Eutectic Reaction: This reaction occurs at 1148°C and is written asLiquid (4.43 % C) γ austenite (2.08 % C) + Fe3C (6.67 % C)

This reaction is not encountered in plain-carbon steels because their carbon content is too low.

Eutectoid Reaction: This reaction occurs at 723°C and can be written asγ austenite (0.8 % C) α ferrite (0.02 % C) + Fe3C (6.67 % C)

The eutectoid reaction takes place completely in the solid phase, and is important for some of the heat treatments of plain-carbon steels.

Iron-Carbon Alloy Above and Below Eutectoid Temperature

Schematic illustration of the microstructures for an iron-carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727 °C (1341 °F).

Various can be changed depend on the carbon content, amount of plastic deformation (working) and heat treatment. At 0.07% carbon content called eutectic point. Where cooled very slowly from high temperature 1100oC in the austenite phase. At 727oC austenite is transformed into α ferrite(BCC)+ cementite which is the the reaction called eutectoid reaction. Single solid phase change into two solid phase (ferrite +cementite). This structure is called pearlite.

Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide. Magnification: 2500X. Source: Courtesy of USX Corporation.

Mechanical properties of pearlite are intermediate between those ferrite (soft and ductile) and cementite (hard and brittle).

Less then 0.77% carbon consist of pearlite phase + ferrite phase. The ferrite phase is called proeutectoid ferrite, forms at temperature higher than the eutectoid temperature of 727oC in the α+γ region.

More than 0.77% carbon, austenite transform into pearlite and cementite. The cementite int the pearlite is called eutectoid cementite, and the cementite phase is called proeuctectoid cementite.

Hypoeutectoid Plain- Carbon steels

Microstructure of a 0.35% C hypoeutectoid plain-carbon steel slowly cooled from austenite region. The white is proeutectoid ferrite; the dark constituent is pearlite

Microstructure of a 1.25% C hypereutectoid plain-carbon steel slowly cooled from austenite region. In this structure proectectoid cementite appears in white thas has formed at the former austenite grain boundaries. The remaining structure consist of coarse lamellar pearlite.

Hypereutectoid Plain- Carbon steels

QuestionA 0.80 % C eutectoid carbon-steel is slowly cooled from 750 Celcius to a temperature just slightly below 723 Celsius. Assuming that the austenite is completely tranformed to alpha ferrite and cementite;● Calculate the weight percent eutectoid ferrite formed● Calculate the weight percent eutectoid cementite formed

Austenite, Ferrite, & Martensite

The unit cells for (a) austenite, (b) ferrite, and (c) martensite.

(d) The effect of percentage of carbon (by weight) on the lattice dimensions for martensite.

Note the interstitial position of the carbon atoms. Note, also, the increase in dimension c with increasing carbon

content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism.

Quiz 2

1. Define the following phases that exist in the Fe-Fe3C

phase diagram:

(a) austenite, (b) α ferrite, (c) cementite, (d) δ ferrite.

2. A 0.55 %C hypoeutectoid plain-carbon steel is slowly cooled from 950ºC to a temperature just slightly below 723ºC.

(a) Calculate the weight percent proeutectoid ferrite in the steel.

(b) Calculate the weight percent eutectoid ferrite and the eutectoid cementite in the steel.

Cast Iron Refers to a ferrous alloy composed of iron, carbon (ranging from 2.11% to 4.5%), and silicon (up to 3.5%).

Phase diagram for the iron-carbon system with graphite (instead of cementite) as the stable phase.

Thank You

Metal Alloys:Heat Treatment of Ferrous Alloys

Heat treatment Hardenability Annealing

Austenite, Ferrite, & Martensite

The unit cells for (a) austenite, (b) ferrite, and (c) martensite.

(d) The effect of percentage of carbon (by weight) on the lattice dimensions for martensite.

Note the interstitial position of the carbon atoms. Note, also, the increase in dimension c with increasing carbon

content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism.

Heat Treatment of Ferrous Alloys

Microstructure can be modified by heat treatment techniques, by controlling the heating and cooling of the alloys at various state. It will force phase transformation which effect the mechanical properties. Effects of thermal treatment depend on the alloy, on it composition and microstructure, on the degree of prior cold work, and on the rates of heating and cooling during heat treatment.

Heat Treating: Concept I

To force a metal to do something the normal laws of solubility will not let it do.

Usual case is to cause one phase to dissolve in another when it normally wouldn’t.

Heat Treating: Concept II

So, we must have a two-phase region (at room temperature) with a single phase region at a higher temperature.

When we heat up to higher temperature, the metal becomes one phase.

Then we cool fast enough that we retain the higher temperature phase at the lower temperature.

Where it may exist as that phase or change (transform) into something else that’s useful.

Iron-Carbon Alloy Above and Below Eutectoid Temperature

Schematic illustration of the microstructures for an iron-carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727 °C (1341 °F).

Austenite To

Pearlite Transformation

(a)Austenite-to-pearlite transformation of iron-carbon alloy as a function of time and temperature.

(b)Slow cooling

Strength of Eutectic Alloys

● Interlamellar Spacing● Eutectic Grain Size● Shape of the Eutectic● Amount of Eutectic

Martensite Plain-carbon steel austenite condition rapidly cooled to room temperature by quenching in water.

A supersaturated interstitial solid solution of carbon in body centered tetragonal iron Structure change from austenite to martensite

The temperature,upon cooling, at which the austenite to martensite transformation starts is called the martensite start, Ms.

The temperature at which transformation finishes is called the martensite finish, Mf ,temperature.

The Ms, temperature for Fe-C alloys decrease as the weight percent carbon increases in these alloys.

Quench

A rapid cooling after solution heat treatment.

Is generally done in water or oil; tool steels often cooled in air.

If Austenite quenched rapidly enough and to a low enough temperature, below Ms, martensite is produced.

It is a metastable phase (not an equilibrium phase) hard, brittle, interstitial supersaturated solid solution of carbon in BCT iron.

Tem

pera

ture

(oC

)

Tem

pera

ture

(oF)

o

o

Austenite rapidly cooled to room temperature by quench in water it will changed to martensite. The temperature, upon cooling at which autenite-to-

martensite transformation called martensite start Ms, finish called martensite finish Mf. The Ms decrease as the weight percent of carbon increase

Martensite● Austenite is cooled at a high temperature FCC is transformed to body centered tetragonal (BCT) which this structure is called martensite● Long lamellae and others slightly elongated of its priciples● Hard and brittle and lack of toughness

(b)

(a) Hardness of martensite, as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000X.

Structure of lath martensite

Plate type

The transformation of austenite to martensite in Fe-C alloys is considered difussionless since the transformation taken too

rapidly. Carbon contents in Fe-C martensite of less than about 0.2% C the austenite transform to a BCC ferrite crystal structure.

Tempered Martensite

Hardness of tempered martensite, as a function of tempering time, for 1080 steel quenched to 65 HRC. Hardness decreases because the carbide particles coalesce and grow in size, thereby increasing the interparticle distance of the softer ferrite.

● Martensite is tempered to improve mechanical properties. ● Tempering by heating: hardness is reduced and toughness is improved.● Heating at 150oC -650oC where it decompose to 2 phase consist of BCC ferrite and small particles of cementite● Increasing tempering time and temperature, hardness decrease due to particles cementite grow bigger

oCoC

oC

oC

oC oF

)oF

)oF

)oF

)oF

)

SpheroiditeMartensite is heated to just below eutectoid temperature and the held for a period of time (anneling) such for a day

Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700 °C (1292 °F). Magnification: 1000X.

Isothermal Decomposition of Austenite Previous section the reaction products from the decomposition of austenite of eutectoid plain-carbon steels for very slow or rapid conditions have been describe Now, what reaction products form when austenite of eutectoid steels is rapidly cooled to temperature below the eutectoid temperature and the isothermally transformed.

Isothermal tranformation experiments to investigate the microstructural changes for the decomposition of eutectoid austenite.

Specimens are 1st austenitized in furnace temperature above the eutectoid temperature. Then rapidly cooled(quenched) in a liquid salt bath at the desired temperature below eutectoid temperature at various time intervals. Then samples quenched into water at room temperature. The microstructure after each transformation time can then be examined at room temperature

After being austenitized, sample are hot quench in to salt bath at 705oC. After 6 min, coarse pearlite formed to a small extent. After 67min, the austenite is completely transformed to coarse pearlite.

S curved next to temperature axis indicate the time necessary for the isothermal transformation of austenite to begin, and the second curve indicate the time required for the transformation to be completedCoarse Pearlite, Fine Pearlite, Bainite, Martnesite

Bainite

● Fine microstructure of ferrite and cementite● Exist when cooling rates are higher ● Stronger and more ductile than pearlite steel at the same hardness level

Upper bainite Lower bainite

Isothermal transformation of eutectoid steels at temperature 723oC and about 550oC produce pearlite microstructure (hot quench). Transformation temperature decrease in this range, the pearlite change from a coarse to fine structure. Rapid quenching from 723oC of eutectoid steel where it is in austenite condition, transform austenite into martensite. If euctectoid steels in the austenite condition are hot quenched to temperature in the 550-250oC range and are isothermally transformed, structure intermediate between pearlite and martensite, called bainite. Upper bainite formed by isothermal transformation temperature between 550-350oC, large, rodlike cementite region. Lower bainite formed from 350-250oC, fine cementite particles.

Continous Cooling Transformation Diagram for a Eutectoid Plain-Carbon Steel

Industrial heat treating operations, in most cases a steel is not isothermally transformed at a temperature above the martensite start temperature but continously cooled from the austenitic temperature to room temperature. The transformation from austenite to pearlite occurs over the range of temperature rather than at a single isothermal temperature. The continuous cooling transformation diagram start and finish lines are shifted to longer times and slightly lower temperature below about 450oC for the austenite to bainite transformation.

Figure show different cooling rates of eutectoid plain carbon steels cooled continuously from austenite region to room temperature. A curve very slow cooling, such as by shutting off power of an electric furnace and allowing the steel cool as the furnace cools. Microstructure would be coarse pearlite. B more rapid cooling such as by removing austenitized steel from a furnace and allowing the steel to cool in the air. Fine pearlite microstructure C starts with the formation of pearlite, but there is insufficient time to complete the austenite -to-pearlite transformation. The remaining austenite do not transform to pearlite will transform to martensite at lower temp. 250oC. Mix of pearlite and martensite. Cooling at a rate faster that curve E (critical cooling rate), will produce fully hardened martensite structure.

Variation on the microstructure of eutectoid carbon steel by continously

cooling at different rates

Hardness and Toughness of Annealed Steels

(a) and (b) Hardness and (c) toughness for annealed plain-carbon steels, as a function of carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has spherelike carbide particles. Note that the percentage of pearlite begins to decrease after 0.77% carbon.

Mechanical Properties of Annealed Steels

Mechanical properties of annealed steels, as a function of composition and microstructure. Note (in (a)) the increase in hardness and strength and (in (b)) the decrease in ductility and toughness, with increasing amounts of pearlite and iron carbide.

Hardenability of ferrous alloy

HARDENABILITY

The capability of an alloy to be hardened by heat treatment

A measure of the depth of hardness that can be obtained by heating and subsequent quenching

Most common test is Jominy hardenability test.

The hardenability of steels depends primarily on:i. The composition of steelii. The austenitic grain sizeiii. The structure of the steel before quenching and iv. The cooling rate

Jominy test

Jominy Test

Annealing

● Temperature higher than recrystallization temperature such as copper ranges 200-300C, Annealing to recover the original properties from 260-650C.

● Full annealing term for annealing of ferrous alloys, generally low carbon and medium-carbon steels. Steel heated at A1or A3, and the cooling slowly in a furnace. Obtained coarse pearlite which soft and ductile and has small uniform grains.

Heat-treating temperature ranges for plain-carbon steels, as indicated on the iron-iron carbide phase diagram.

Normalizing ● The normalizing heat treatment for steel consists

of heating within the austenitic region and then cooling in still air.

● Some of the purposes for normalizing include:

1. to refine the grain structure;

2. to increase the strength of the steel, as compared to annealed steel;

3. to reduce compositional segregation in castings or forgings and thus provide a more uniform structure.

4. to avoid excessive softness from annealing of steels cooling in still air

Normalizing

● Fine pearlite with small uniform grains. Higher strength and hardness lower ductility than full annealing.

● To obtain uniform structure, to decrease residual stresses and to improve machinability

Hardness of steels in the quenched and normalized conditions, as a function of carbon content.

Tempering

● Thermal treatment of steels (generally done below 1330oF).

● The purpose is to reduce brittleness caused by previous rapid cooling that created the martensite.

● Strength/hardness properties are reduced, toughness and ductility improved.

Austempering

● Heated steel is quenched from austenizing temperature rapidly enough to avoid formation of ferrite and pearlite.● Certain temperature until isothermal transformation from austenite to bainite is complete, then cooled to room temperature● Medium of quench is molten salt cooled in still air

MartemperingThe martempering or marquenching process for a plain-carbon

steel produces a martensitic microstructure and consists of: 1. austenitizing the steel;2. quenching the steel in oil or molten salt at a temperature just

slightly above the Ms temperature;3. holding the steel in the quenching medium temperature for a

time period sufficient to provide thermal equilibrium of the steel with the medium, without causing the initiation of the austenite-to-bainite transformation;

4. cooling the steel at a moderate rate to room temperature.

Martempering

The advantages of martempering are i. The minimization of distortion and cracking of the workpiece. ii. By subsequent tempering, the martempered steel develops a tempered martensite microstructure which provides for higher impact energy.

Ausforming

In the austempering process, the steel is austenitized, then quenched in a molten salt bath at a temperature just above the steel Ms temperature, held isothermally until the austenite-to- bainite transformation is complete, then cooled to room temperature in air.

Thank You

• Describe the purpose and effects of heat treatment on plain carbon steel.

• Describe the phase transformation process during heat treatment stage of iron-carbon alloy in relation to composition, microstructure and properties.

Heat Treating: Concept I

To force a metal to do something the normal laws of solubility will not let it do.

Usual case is to cause one phase to dissolve in another when it normally wouldn’t.

Varying the heating and cooling of plain carbon steels to obtain different combinations of mechanical properties.

Hardenability of ferrous alloy

HARDENABILITY

The capability of an alloy to be hardened by heat treatment

A measure of the depth of hardness that can be obtained by heating and subsequent quenching

Most common test is Jominy hardenability test.

The hardenability of steels depends primarily on:i. The composition of steelii. The austenitic grain sizeiii. The structure of the steel before quenching and iv. The cooling rate

Martensite

• A plain-carbon steel in austenitic condition is rapidly cooled to room temperature by quenching in water.

• Structure change from austenite to martensite.

• A supersaturated interstitial solid solution of carbon in body centered tetragonal (BCT) iron.

Martensite

• The temperature, upon cooling, at which the austenite to martensite transformation starts is called the martensite start, Ms.

• The temperature at which transformation finishes is called the martensite finish, Mf, temperature.

• The Ms temperature for Fe-C alloys decrease as the weight percent carbon increases in these alloys.

Effect of carbon content on the martensite-transformation start temperature Ms for iron carbon

alloys.

Structure of lath martensite

Plate type

The transformation of austenite to martensite in Fe-C alloys is considered difussionless since the transformation taken too

rapidly. Carbon contents in Fe-C martensite of less than about 0.2% C the austenite transform to a BCC ferrite crystal structure.

Discussion

• Characterize the lath and plate martensite microstructure.

• Explain the hardness and strength of martensites.

Quench A rapid cooling after solution heat treatment.

Is generally done in water or oil; tool steels often cooled in air.

If Austenite quenched rapidly enough--and to a low enough temperature, below Ms, martensite is produced.

It is a metastable phase (not an equilibrium phase) hard, brittle, interstitial supersaturated solid solution of carbon in BCT iron.

Jominy test

Isothermal Decomposition of Austenite

Previous section the reaction products from the decomposition of austenite of eutectoid plain-carbon steels for very slow and rapid conditions have been describe Now, what reaction products form when austenite of eutectoid steels is rapidly cooled to temperature below the eutectoid temperature and the isothermally transformed.

Isothermal tranformation experiements to investigate the microstructural changes for the decomposition of

eutectoid asutenite.

Specimens are 1st austenitized in furnace temperature above the eutectoid temperature.

Then rapidly cooled(quenched) in a liquid salt bath at the desired temperature below eutectoid temperature at various time intervals.

Then samples quenched into water at room temperature. The microstructure after each transformation time can then be examined at room temperature

After being austenitized, sample are hot quench in to salt bath at 705oC. After 6 min, coarse pearlite formed to a small extent.

After 67min, the austenite is completely transformed to coarse pearlite.

S curved next to temperature axis indicate the time necessary for the isothermal transformation of austenite to begin, and the second curve indicate the time required for the transformation to be completed

Isothermal transformation of eutectoid steels at temperature 723oC and about 550oC produce pearlite microstructure (hot quench). Transformation temperature decrease in this range, the pearlite change from a coarse to fine structure. Rapid quenching from 723oC of eutectoid steel where it is in austenite condition, transform austenite into martensite. If euctectoid steels in the austenite condition are hot quenched to temperature in the 550-250oC range and are isothermally transformed, structure intermediate between pearlite and martensite, called bainite. Upper bainite formed by isothermal transformation temperature between 550-350oC, large, rodlike cementite region. Lower bainite formed from 350-250oC, fine cementite particles.

Upper bainite Lower bainite

Continous Cooling Transformation Diagram for a Eutectoid Plain-Carbon

Steel Industrial heat treating operations, in most cases a steel is not isothermally transformed at a temperature above the martensite start temperature but continously cooled from the austenitic temperature to room temperature. The transformation from austenite to pearlite occurs over the range of temperature rather than at a single isothermal temperature. The continuous cooling transformation diagram start and finish lines are shifted to longer times and slightly lower temperature below about 450oC for the austenite to bainite transformation.

Figure show different cooling rates of eutectoid plain carbon steels cooled continuously from austenite region to room temperature. A curve very slow cooling, such as by shutting off power of an electric furnace and allowing the steel cool as the furnace cools. Microstructure would be coarse pearlite. B more rapid cooling such as by removing austenitized steel from a furnace and allowing the steel to cool in the air. Fine pearlite microstructure C starts with the formation of pearlite, but there is insufficient time to complete the austenite -to-pearlite transformation. The remaining austenite do not transform to pearlite will transform to martensite at lower temp. 250oC. Mix of pearlite and martensite. Cooling at a rate faster that curve E (critical cooling rate), will produce fully hardened martensite structure.

Variation on the microstructure of eutectoid carbon steel by continously

cooling at different rates

Heat treatment of Nonferrous Alloys and

Stainless Steel● Nonferrous alloy cannot be heat treated by technique used on ferrous alloys. Do not undergo phase transformation like those in steels● Hardening and strengthening of these alloys are fundamentally different.● Aluminium alloys, copper alloys, some stainless steels are hardened by a process called precipitation hardening● This heat treatment is technique in which small particles (of a different phase, and called participates) are uniformly dispersed in the matrix of the original phase.● Solid solubility of one element is exceed in other element.

• Involves 3 stages:

i. Solution heat treatment

ii. Quenching

iii. Aging

Percipitation hardening

Solution heat treatment• Sample is heated to a temperature between

solvus and solidus temperatures.• Soaked the sample until a uniform solid-solution

structure is produced.

Quenching • The sample is rapidly cooled to a lower

temperature (room temp) using water (cooling medium).

• The structure: supersaturated solid solution (α phase) Aging

• To form a finely dispersed precipitate in the alloy• The fine precipitate impedes dislocation

movement during deformation which strengthen the alloy.

Aging ●Alloy is reheated to an intermediate temperature and then held there for a period of time.●At room temperature: natural aging●Above room temperature, the process is called artificial aging● Several alluminium alloys harden and become stronger over period of time at room temperature called natural aging● In the percipitation process, reheated at elavated temperature for an extended of time, the percitipate begin to coalesce and grow. Become larger but fewer, this process is called Over aging. Metal softer and and weaker.

Maraging or martensite age hardening●For special group of high-strength iron base alloys. ●One or more intermetallic compounds are precipated in a matrix of low-carbon martensite. ●Aging is done at 480oC. Maraging steels such as in dies and tooling parts.

The effect of aging time and temperature on the yield stress of 2014-T4 aluminum alloy. Note

that, for each temperature, there is an optimal aging time for maximum strength.

Case Hardening

● Alteration only the surface properties (surface indention, fatigue, and wear)● Application and parts such as gear teeth, cams, shafts, bearing, fasteners, pins and etc.● Hardening is not desirable due to hard part lacks of necessary toughness● Small surface crack could propagate rapidly through cause total failure● Various surface-hardening process available; carburizing, carbonitriding, cyaniding, boronizing, flame hardening, induction hardening and laser hardening.

Surface Hardening

Carburizing Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C)

C Heat steel at 870–950 °C (1600–1750 °F) in an atmosphere of carbonaceous gases (gas carburizing) or carbon-containing solids(pack carburizing). Then quench.

A hard, high-carbon surface is produced. Hardness 55 to 65 HRC. Case depth < 0.5–1.5 mm ( < 0.020 to 0.060 in.). Some distortion of part during heat treatment.

Gears, cams, shafts, bearings, piston pins, sprockets, clutch plates

Carbonitriding Low-carbon steel

C and N

Heat steel at 700–800 °C (1300–1600 °F) in an atmosphere of carbonaceous gas and ammonia. Then quench in oil.

Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm (0.003 to 0.020 in.). Less distortion than incarburizing.

Bolts, nuts, gears

Cyaniding Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C)

C and N

Heat steel at 760–845 °C (1400–1550 °F) in a molten bath of solutions of cyanide (e.g., 30% sodium cyanide) and other salts.

Surface hardness up to 65 HRC. Case depth 0.025 to 0.25 mm (0.001 to 0.010 in.). Some distortion.

Bolts, nuts, screws, small gears

Process Metals hardened

Element added

to surface

Procedure General Characteristics Typical applications

Nitriding Steels (1% Al, 1.5% Cr, 0.3% Mo), alloy steels (Cr, Mo), stainless steels, high-speed tool steels

N Heat steel at 500–600 °C (925–1100 °F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No further treatment.

Surface hardness up to 1100 HV. Case depth 0.1 to 0.6 mm (0.005 to 0.030 in.) and 0.02 to 0.07 mm (0.001to 0.003 in.) for high speed steel.

Gears, shafts, sprockets, valves, cutters, boring bars, fuel-injection pump parts

Boronizing Steels B Part is heated using boron-containing gas or solid in contact with part.

Extremely hard and wear resistant surface. Case depth 0.025– 0.075 mm (0.001–0.003 in.).

Tool and die steels

Flame hardening

Medium-carbon steels, cast irons

- Surface is heated with an oxyacetylene torch, then quenched with water spray or other quenching methods.

Surface hardness 50 to 60 HRC. Case depth 0.7 to 6 mm (0.030 to 0.25 in.). Little distortion.

Gear and sprocket teeth, axles, crankshafts, piston rods, lathe beds and centers

Induction hardening

Same as above

- Metal part is placed in copper induction coils and is heated by high frequency current, then quenched.

Same as above Same as above

Austempering

● Heated steel is quenched from austenizing temperature rapidly enough to avoid formation of ferrite and pearlite.● Certain temperature until isothermal transformation from austenite to bainite is complete, then cooled to room temperature● Medium of quench is molten salt cooled in still air

Martempering

The martempering or marquenching process for a plain-carbon steel produces a martensitic microstructure and consists of: 1. austenitizing the steel; 2. quenching the steel in oil or molten salt at a temperature just slightly above the Ms temperature; 3. holding the steel in the quenching medium temperature for a time period sufficient to provide thermal equilibrium of the steel with the medium, without causing the initiation of the austenite-to-bainite transformation; 4. cooling the steel at a moderate rate to room temperature.

The advantages of martempering are the minimization of distortion and cracking of the workpiece. By subsequent tempering, the martempered steel develops a tempered martensite microstructure which provides for higher impact energy.

Ausforming

In the austempering process, the steel is austenitized, then quenched in a molten salt bath at a temperature just above the steel Ms temperature, held isothermally until the austenite-to- bainite transformation is complete, then cooled to room temperature in air.

Tutorial

● Define aging, and what is the different between natural and artificial aging.● Explain the difference between hardness and hardenability● Explain about percipitation hardening and how the microstructure changes?● Describe about case hardening.● Describe the characteristic and structure of pearlite, austentine, martensite, and cementite● Explain about normalizing heat treatment for steel and what are some of it purposes?

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Metal Alloys:Case Hardening of Ferrous Alloys

Case Hardening

● Alteration only the surface properties (surface indention, fatigue, and wear)● Application and parts such as gear teeth, cams, shafts, bearing, fasteners, pins and etc.● Hardening is not desirable due to hard part lacks of necessary toughness● Small surface crack could propagate rapidly through cause total failure● Various surface-hardening process available; carburizing, carbonitriding, cyaniding, boronizing, flame hardening, induction hardening and laser hardening.

Outline of Heat Treatment Processes for Surface Hardening

TABLE 4.1 Process Metals

hardened Element added to surface

Procedure General Characteristics

Typical applications

Carburizing Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C)

C Heat steel at 870–950 °C (1600–1750 °F) in an atmosphere of carbonaceous gases (gas carburizing) or carbon-containing solids (pack carburizing). Then quench.

A hard, high-carbon surface is produced. Hardness 55 to 65 HRC. Case depth < 0.5–1.5 mm ( < 0.020 to 0.060 in.). Some distortion of part during heat treatment.

Gears, cams, shafts, bearings, piston pins, sprockets, clutch plates

Carbonitriding Low-carbon steel

C and N Heat steel at 700–800 °C (1300–1600 °F) in an atmosphere of carbonaceous gas and ammonia. Then quench in oil.

Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm (0.003 to 0.020 in.). Less distortion than in carburizing.

Bolts, nuts, gears

Outline of Heat Treatment Processes for Surface Hardening

TABLE 4.1 Process Metals

hardened Element added to surface

Procedure General Characteristics

Typical applications

Cyaniding Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C)

C and N Heat steel at 760–845 °C (1400–1550 °F) in a molten bath of solutions of cyanide (e.g., 30% sodium cyanide) and other salts.

Surface hardness up to 65 HRC. Case depth 0.025 to 0.25 mm (0.001 to 0.010 in.). Some distortion.

Bolts, nuts, screws, small gears

Nitriding Steels (1% Al, 1.5% Cr, 0.3% Mo), alloy steels (Cr, Mo), stainless steels, high-speed tool steels

N Heat steel at 500–600 °C (925–1100 °F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No further treatment.

Surface hardness up to 1100 HV. Case depth 0.1 to 0.6 mm (0.005 to 0.030 in.) and 0.02 to 0.07 mm (0.001 to 0.003 in.) for high speed steel.

Gears, shafts, sprockets, valves, cutters, boring bars, fuel-injection pump parts

Outline of Heat Treatment Processes for Surface Hardening

TABLE 4.1 Process Metals

hardened Element added to surface

Procedure General Characteristics

Typical applications

Boronizing Steels B Part is heated using boron-containing gas or solid in contact with part.

Extremely hard and wear resistant surface. Case depth 0.025– 0.075 mm (0.001– 0.003 in.).

Tool and die steels

Flame hardening

Medium-carbon steels, cast irons

None Surface is heated with an oxyacetylene torch, then quenched with water spray or other quenching methods.

Surface hardness 50 to 60 HRC. Case depth 0.7 to 6 mm (0.030 to 0.25 in.). Little distortion.

Gear and sprocket teeth, axles, crankshafts, piston rods, lathe beds and centers

Induction hardening

Same as above

None Metal part is placed in copper induction coils and is heated by high frequency current, then quenched.

Same as above Same as above

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