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HeatTreatment of

Steel

Lecture 9

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Heat-Treatment

Heat treatment is a method used to alter thephysical, and sometimes chemical propertiesof a material. The most common applicationis metallurgical

It involves the use of heating or chilling,normally to extreme temperatures, to achievea desired result such as hardening orsoftening of a material

It applies only to processes where the heatingand cooling are done for the specificpurpose of altering properties intentionally

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Types of Heat-Treatment (Steel)

Annealing / Normalizing,

Case hardening,

Precipitation hardening,

Tempering, and Quenching

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Time-Temperature-

Transformation (TTT)Curve TTT diagram is a plot of temperature versus the

logarithm of time for a steel alloy of definite

composition. It is used to determine when transformations

begin and end for an isothermal heat treatmentof a previously austenitized alloy

TTT diagram indicates when a specifictransformation starts and ends and it also showswhat percentage of transformation of austeniteat a particular temperature is achieved.

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Time-Temperature-

Transformation (TTT)Curve

The TTT diagra

m for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at

900C

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Decarburization during Heat

TreatmentDecrease in content of carbon in metals is

called Decarburization

It is based on the oxidation at the surface of

carbon that is dissolved in the metal lattice In heat treatment processes iron and carbon

usually oxidize simultaneously

During the oxidation of carbon, gaseous

products (CO and CO2) develop In the case of a scale layer, substantial

decarburization is possible only when thegaseous products can escape

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Decarburization Effects

The strength of a steel depends on thepresence of carbides in its structure

In such a case the wear resistance is

obviously decreased

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Annealing It is a heat treatment wherein a material is

altered, causing changes in its propertiessuch as strength and hardness

It the process of heating solid metal tohigh temperatures and cooling it slowly sothat its particles arrange into a defined

lattice

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Types of Annealing

1. Stress-Relief Annealing (orStress-relieving)

2. Normalizing

3. Isothermal Annealing

4. Spheroidizing Annealing (orSpheroidizing )

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1. Stress-Relief Annealing It is an annealing process

below the transformationtemperatureAc1, with

subsequent slow cooling,the aim of which is toreduce the internal residualstresses in a workpiecewithout intentionallychanging its structure andmechanical properties

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Causes of Residual Stresses1. Thermal factors (e.g., thermal stressescaused by temperature gradients within the

workpiece during heating or cooling)2. Mechanical factors (e.g., cold-working)

3. Metallurgical factors (e.g., transformationof the microstructure)

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How to Remove Residual Stresses?

R.S. can be reduced only by a plasticdeformation in the microstructure.

This requires that the yield strength of the materialbe lowered below the value of the residual

stresses. The more the yield strength is lowered, the greater

the plastic deformation and correspondingly thegreater the possibility or reducing the residual

stresses The yield strength and the ultimate tensile

strength of the steel both decrease withincreasing temperature

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Stress-Relief Annealing

Process Forplain carbon and low-alloy steels the

temperature to which the specimen is heatedis usually between 450 and 650C, whereas for

hot-working tool steels and high-speed steels itis between 600 and 750C

This treatment will not cause any phasechanges, but recrystallization may take place.

Machining allowance sufficient tocompensate for any warping resulting fromstress relieving should be provided

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Stress-Relief Annealing R.S.

In the heat treatment of metals, quenching orrapid cooling is the cause of the greatest residualstresses

To activate plastic deformations, the local

residual stresses must be above the yield strengthof the material.

Because of this fact, steels that have a high yieldstrength at elevated temperatures can withstand

higher levels of residual stress than those thathave a low yield strength at elevatedtemperatures

Soaking time also has an influence on the effect

of stress-relief annealing

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Relation between heating

temperature and Reduction

in Residual Stresses

Higher temperatures andlonger times of annealingmay reduce residualstresses to lower levels

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Stress Relief Annealing -

Cooling The residual stress level after stress-relief annealing will

be maintained only if the cool down from theannealing temperature is controlled and slow enoughthat no new internal stresses arise.

New stresses that may be induced during coolingdepend on the (1) cooling rate, (2) on the cross-sectional size of the workpiece, and (3)on thecomposition of the steel

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2. Normalizing

A heat treatment process consisting ofaustenitizing at temperatures of 3080Cabove theAC3 transformationtemperature followed by slow cooling(usually in air)

The aim of which is to obtain a fine-grained, uniformly distributed, ferritepearlite structure

Normalizing is applied mainly tounalloyed and low-alloy hypoeutectoidsteels

For hypereutectoid steels theaustenitizing temperature is 3080Cabove theAC1 orACm transformationtemperature

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Normalizing Heating and

Cooling

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Normalizing Austenitizing

Temperature Range

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Effect of Normalizing on Grain Size

Normalizing refines the grain of a steel that hasbecome coarse-grained as a result of heatingto a high temperature, e.g., forforging orwelding

Carbon steel of 0.5% C. (a) As-rolled or forged;(b) normalized. Magnification 500