HEAT TREATMENT

Dr. Muhammad Riaz Bhatti

04/07/23 1

Objective

To know:

• Why Heat Treatment is required?

• What is the relationship b/w structure & properties?

• How Micro-structure can be changed to obtain the

desired properties?

• What are the heat treatment processes used for

steels

04/07/23 2

Contents

• Why Heat Treatment is required?

• How Micro-structure can be altered?

• Spectrum of heat treatments used for steels

• Heat Treatment - definition

• Heat Treatment-processes

• Surface Treatment Processes

• Phase Diagram for Iron and Carbon

• Austenite transformation04/07/23 3

Contents

• Graphical summary of the process of heat

treatments of steels on an equilibrium diagram

• Heat Treatment-processes in detail

• Decarburizing of steel during Heat Treating

• Hardening of Steel

• Mass effect & Hardenability

• Time-Temperature transformation (TTT )

• Jominy End-Quench test

• Quenching Media04/07/23 4

Contents

• Design Concerns, Residual Stresses, Distortion

& Cracking

• Tempering of Martensite

• Symbols used in the Heat Treatment of steel

• Micro-structure of different phases of steel

• Austenite – Pearlite & Ferrite / Cementite

transformation

• Diffusion of Carbon in Pearlite

• Quenching rates &Cooling Rates

• TTT Diagram04/07/23 5

Why Heat Treatment is required?

• Three reasons for heat treatment

– To soften before shaping

– To relieve the effects of strain hardening

(stresses)

– To acquire the desired properties (strength

and toughness) in the finished product.

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Spectrum of heat treatments used on ferrous metals

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Heat TreatmentDefinition

• “A combination of heating & cooling operations,

timed & applied to metals or alloys in a solid

state in a way that will produce desired

properties “.

• All basic heat treating processes for steel

involve the transformation or decomposition of

austenite.

• The nature & properties of any transformation

products determine the physical & mechanical

properties.04/07/23 8

Heat treatment processes(commonly applied to steels)

• Annealing

• Normalizing

• Quenching

• Tempering

• Spheroidizing

04/07/23 9

Surface Treatment Processes (commonly applied to steels)

• Surface Hardening

– Carburizing

– Nitriding

– Carbonitriding

– Cyaniding

• Induction Hardening

• Flame Hardening04/07/23 10

Phase Diagram for Iron & CarbonPure iron can have two crystal structures at equilibrium – FCC (austenite)

above about 910C, BCC (ferrite) below this

• When we add carbon, a third phase, iron carbide (Fe3C), also called

cementite, becomes possible1. All austenite

2. Ferrite begins to form at the grain boundaries of the austenite

3. The remaining austenite transforms into ferrite plus carbide

1

2

3

1

3

2

04/07/23 11

Austenite transformation

AUSTENITE QUENCH MARTNSITE

SLOW COOL FERRITE + CARBIDE

04/07/23 12

Graphical summary of heat treatment processes of steels on an equilibrium diagram

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Heat Treatment of Steel

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Annealing

• Heating the steel above upper critical

temperature and then cooling in furnace (very

slow cooling, cooling rate ~ 10oC / hour).

• Objective of Annealing

– reduces the hardness, stresses and

– improves ductility, machinability, electrical

& magnetic properties.

• Structure after annealing is coarse pearlite

04/07/23 15

Annealing• Types

– Process annealing -- to restore some ductility

to work piece allowing it to be worked further

without breaking

– Full annealing – to create entirely new

homogeneously & uniform structure resulting

more ductile material that has greater stretch

ratio & reduction of area properties but with

low yield & tensile strength04/07/23 16

Stages of AnnealingThere are three stages of recrystallization/

Annealing :• Recovery

– Removal of defects ( dislocations) & internal stresses

• Recrystallization– Nucleation of strain-free grains & their

growth to replace the deformed ones

• Grain growth– Microstructure starts to coarsen

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Annealing by way of doing the operation

• Box annealing – Parts placed in airtight container

to prevent oxidation

• Bright annealing – performed in controlled

atmosphere (Ar, N2) to prevent the formation of

oxide on the surface , used for medium carbon

steel

• Diffusion annealing of semiconductor – Si

wafers are annealed, so that dopant atoms, B, P

can diffuse into substitutional positions in the

crystal lattice, resulting drastic changes in the

electrical properties of the semiconductor material04/07/23 18

Normalizing• Heating the steel 50oC above upper critical

temperature and then cooling in air (cooling rate

~100 oC / hour).

The Objective of normalizing are:

To eliminate coarse grain structure obtained

during castings, forgings, rolling & stamping etc

To increase strength of medium carbon steel

To improve machinability of low carbon steels.

To improve the structure of welds.

To reduce the internal stresses04/07/23 19

Normalizing• Process is used by rolling mills,

forging/foundry shops to hot worked parts/casting for grain size /alloy uniform distribution

• Structure after normalizing is fine pearlite.• Better surface finish &

better mechanical properties• Hardness & strength more than the

Annealed steel.• Improved machinability of hypo-eutectoid

steels.

• Economical process than annealing04/07/23 20

Figure - Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

Normalizing / Annealing

04/07/23 21

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Figure - The effect of carbon and heat treatment on the properties of plain-carbon steels.

04/07/23 22

Spheroidizing

• Prolong heating between 650-700oC (i.e., just

below A1 line) for 12 hrs,

• No phase change, but due to surface energy

(surface tension) effects, the cementite layers

of pearlite gradually break up & assume a

globular / Spheroidal form.

• Objective:

• Spherodized structure improves machinability

as well as ductility of high-Carbon steels.04/07/23 23

Spheroidite

• If tempered for a long

time, Fe3C forms

“spheres” and grows

inside Ferrite.

• Very soft, easy to

machine

04/07/23 24

Hardening

• A process in which steel is heated to the austenitizing temperature and quenched (rapidly cooled) in water, oil or molten salt baths to form hard martensite.

• Steels with enough Carbon and alloy content will direct harden.

04/07/23 25

Hardening

• Purpose can be to increase :

– Hardness (e.g., gauge block requires dimensional invariance 60-65HRC)

– Wear resistance (as required by tools)

– Strength (Y.S., toughness required for springs)

– Physical properties (eg., coefficient of thermal expansion & magnetic properties)

04/07/23 26

Hardening• The hardening process is based on the

decomposition of eutectoid.

• This reaction is dependent upon the following factors:– Adequate Carbon contents to produce hardening

– Austenite decomposition to produce pearlite, bainite & martensite structures

– Heating rate & time

– Quenching medium

– Quenching rate

– Size of part

– Surface conditions04/07/23 27

Hardening Process (Heating & Quenching)

• Heating the steel above upper critical temperature

and then cooling in water or in oil (very fast cooling).

• After quenching, steel is very hard and brittle and

practically of no use.

• Structure after quenching is fine martensite which is

complex, hard and brittle structure.

• The rate of quenching depends on the fluid media

used and the degree of agitation.

• Water quenches are the most severe, followed by

oil, molten salt, and gas quenching.04/07/23 28

Determination of hardening temperature for 0.6 % Carbon steel

04/07/23 29

Effect of Carbon on the Hardness of Steel04/07/23 30

Figure - Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

04/07/23 31

Decarburizing of steel during Heat Treating

Before heat treatment After heat treatment

Besides oxidizing, steels also preferentially lose some carbon during a

high temperature heat treatment in air (in industry, steel is usually heat

treated under a non-oxidizing atmosphere to prevent decarburization).

Grind this decarburized layer away before taking your hardness

measurement to get a representative value.04/07/23 32

Hardening methods• Quenching in single medium, water

– Most extensively used

– Disadvantage:- cooling rate very high, cracks, distortion & other defects may occur

In order to avoid these defects other hardening methods are:

• Quenching in two methods- – First quench to 300-400 ° C & then transfer to less intensive

quenching medium (air or oil). The purpose is to reduce internal stresses associated with austenite to martensite transformation

– Applied to Carbon steel tools, e.g., taps, dies, milling cutters etc to avoid cracking / warping

04/07/23 33

Hardening methods• Hardening with self tempering

– Article is held in quenching medium until it is completely cooled, but is withdrawn to retain a certain amount of heat in core which accounts for tempering,

– Applied for chisels, hammers, centre punches & other tools that required high surface hardness in conjunction with tough core

• Stepped quenching or martempering– After heating at the required temp, quenching at 150 to

300°C then cooling in air / oil– Less Vol change / warping / cracks

• Isothermal quenching or austempering– Performed in same manner as martempering but with longer

holding time at 250 to 400C (above the martensite point to ensure a sufficiently complete austenite decomposition

04/07/23 34

Tempering process

• Reheat the quenched steel up to intermediate

temperature (below lower critical temperature)

and then cool.

• The structure is called tempered martensite.

• After tempering, steel becomes tough and looses

some hardness & become useable now.

• Tempering affects hardness, strength (yield,

UTS), & % elongation04/07/23 35

Why Tempering is done?

• After hardening steel must be

tempered to

– Reduce brittleness

– Relieve the internal stresses

– modify structure to obtain pre-

determined mechanical properties

• Martensite needs to be tempered

to get better ductility. This

happens when Fe3C is allowed to

precipitate from the supercooled

Martensite.04/07/23 36

Transformation diagram illustrating the formation of tempered martensite

04/07/23 37

• If we reheat martensite above the martensite transformation temperature, but below the austenite temperature, it will turn into ferrite plus carbide.

• However, if the carbide is very finely dispersed, this material will have properties similar to martensite – almost as hard and strong, but more ductile.

• Heating to slightly above the martensite temperature accomplishes this fine dispersion.

• This type of heat treatment is called tempering.

TEMPERING

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.

Figure - The effect of tempering temperature on the mechanical properties of a 1050 steel.

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41

Bainite

coarse fine

Martensite

Moderate cooling (AS)

Isothermal treatment (PCS)

Tempered

Martensite

Pearlite

Slow

Cooling

Rapid

Quench

Spheroidite

Re-heat

Re-heat

Austenite

04/07/23 41

Austenite ()

Bainite ( + Fe3C plates/needles)

Pearlite ( + Fe3C layers + a proeutectoid phase)

Martensite (BCT phase diffusionless

transformation)

Tempered Martensite ( + very fine

Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

ength

Duct

ilit

yMartensite

T Martensite bainite

fine pearlite coarse pearlite

spheroidite

General Trends

42

SUMMARY: PROCESSING OPTIONS

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SummaryH/T operations vs properties

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Annealing Normalizing Quenching

Hardness Low Medium High

Toughness High Medium Low

Tensile strength

Low Medium High

Ductility High Medium Low

04/07/23 44

Mass effect of Heat-treatment• Mass effect is the variation in hardness across a

section of the components having higher thickness through heat treatment.

• Hardness of carbon steel depends upon:– Carbon content &– Rate of cooling

• A part having less thickness will cool more quickly than a part having higher thickness , if both are cooled in a same quenching media– Result

• Outer hard layer of martensite & • Inner core of soft structure

04/07/23 45

Mass effect of Heat-treatment

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Mass effect of Heat-treatment• Carbon steel can be hardened throughout its x-

section uniformally, if it is cooled very rapidly from

austenite region to room temperature by ensuring

completion of 100 % martensitic transformation.

• However, for large diameter bars or large forgings

such uniformity in hardening is impossible to attain,

& interior regions are always softer than the outer

portions.

• Thus the effect of mass of component undergoing

hardening has to be taken into account.04/07/23 47

What is “Hardenability”?

• The hardenability of a metal is its capability to be hardened

by heat treatment.

• It should not be confused with hardness, which is a measure

of the material's resistance to indentation or scratching.

• Jominy Bar is used to show how cooling rate affects

hardness

• Alloyed steels (Cr, Mo, Ni, etc.) have higher hardenbility at

same cooling rates than carbon steels

04/07/23 48

Jominy Test

04/07/23 49

Typical Jominy Curves

• 4340: Very hardenable, More expensive

• 1040: Less hardenable, Less expensive

04/07/23 50

Jominy TestGenerally, the faster steel cools, the harder it will be. The Jominy bar measures the hardenbility of a steel.

Softest

Hardest

04/07/23 51

Figure - The set-up for the Jominy test used for determining the hardenability of a steel.

Jominy Test

04/07/23 52

Jominy Test•To test the hardenability of a ferrous alloy, a Jominy test is used.

• A round metal bar of standard size is transformed to 100% austenite through heat treatment.

•It is then quenched on one end with room temperature water. As a result, the cooling rates throughout the material will vary significantly, being highest at the end being quenched.

•The hardenability is then found by measuring the hardness throughout the bar.

•The farther away from the quenched end that the hardness exists, the higher the hardenability.04/07/23 53

Design modification

Fig. (a) Shape containing nonuniform sections & a sharp interior corner that may crack during quenching. This is improved by using a large radius to join the section. (b) Original design containing sharp corner holes.

04/07/23 54

Symbols used in the Heat Treatment of steel

• A – Furnace Annealed – Slow cooled

• N - Normalized - Air cooled.

• O - Oil Quenched

• WQ – Water quenched.

• WT(370)– Water quenched, tempered at 370°C

for 1 hour.

• WT(705)– Water quenched, tempered at 705°C

for 1 hour.04/07/23 55

Quenching Media

• The fluid used for quenching the heated

alloy affects the hardenability.

– Each fluid has its own thermal properties

• Thermal conductivity

• Specific heat

• Heat of vaporization

– These cause rate of cooling differences04/07/23 56

Quenching Media

• Cooling capacities of typical quench media are

– Agitated brine 5.– Still water 1.– Still oil 0.3– Cold gas 0.1– Still air 0.02

04/07/23 57

What’s a CCT Diagram?

• Phase Transformations and Production of

Microconstituents takes TIME.

• Higher Temperature = Less Time.

• If you don’t hold at one temperature and allow

time to change, you are “Continuously Cooling”.

• Therefore, a CCT diagram’s transition lines will

be different than a TTT diagram.04/07/23 58

Slow Cooling

Time in region indicates amount of microconstituent

04/07/23 59

Medium Cooling

Cooling Rate, R, is Change in Temp / Time °C/s

04/07/23 60

Fast Cooling

This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

04/07/23 61

Spring Aging• A wound spring can lose its spring tension due to

anelastic behavior, which causes the spring to unwind

or change its shape over time.

• To avoid this springs are placed in an oven at 315 -

375 ºC for 2 hours for spring aging.

• Once the springs are treated to spring aging, they do not

usually change shape.

• Good for dimensional accuracy in formed shape

04/07/23 62

Steam treating • Steam treating is the controlled oxidation of metals to

produce a thin layer of oxide on the surface of a component.

• This process can be used to provide a component with increased corrosion resistance, better wear resistance, increased surface hardness, an attractive surface finish (producing a blue-gray to a blue-black appearance) and, in the case of porous materials such as powder metal, seal the part porosity and increase the density.

04/07/23 63

Cryogenic Treatment • Deep Cryogenic Treatment is an

extended process that very gradually "freezes" or removes heat from the items being treated.

• Typically, the parts are brought down to 300 degrees below zero (F) in a very slow ramp and then held at that temperature for an extended dwell (24 hours), before being returned to ambient temperature.

• The last step is a post temper to +300/ +350 degrees F. The entire process takes 48 to 72 hours.

04/07/23 64

Cont….. Cryogenic Treatment • Time and temperature promote additional metallurgical

transformations to relieve residual stresses, normalize and stabilize metal, increase resistance to wear, and create a modified and uniform grain (or crystal) structure.

• The benefits of this process are increased part life, less wear, and improved performance.

• The most popular applications in motor sports include:

– Brake Rotors & Brake Pads, Engine & Drive Train Components

– Spark Plugs, Rear Ends, Transmissions, Bearings

• Other Automotive Applications: Blocks, Heads, Rotating Components, Valve Train (incl. Valves), Gears (incl. Transmission), Differential Components, Brake Drums, Discs, Calipers, Pads, Axles, etc.

04/07/23 65

Thanks

04/07/23 66

Hardenability A steel’s hardenability is its ability to transform from austenite to

martensite to a certain depth below the surface.

• A steel having a high hardenability will form martensite deep

under the surface even though its cooling is relatively slow,

such as an air cool or a furnace cool.

– High hardenability: high hardness extending well below the

surface of the part, even at slow cooling rates

By contrast, a low-hardenability steel may only partially

transform to martensite near the surface even if given a

severe quench such as a water quench.

– Low hardenability: low or moderate hardness even at fast

quench rates; hardened material may only form a thin “skin”

on the part

04/07/23 67

TTT Diagrams

04/07/23 68

Pearlite Formation

• Austenite

precipitates Fe3C at

Eutectoid

Transformation

Temperature

(727°C).

• When slow cooled,

this is Pearlite04/07/23 69

Diffusion of Carbon in Pearlite

04/07/23 70

Morphology of Pearlite

(a) coarse pearlite (b) fine pearlite 3000X

(a) (b)

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