effect of cooling rate and quench on microstructure and hardness

Isfahan University of Technology

Materials Department

Heat treatment laboratory

Effect of cooling rate and Quench on microstructure

and hardness

Reza Rashidi

WWW.MATERIALS.IUT.AC.IR 1

I. IntroductionII.ExperimentalIII.ConclusionIV.References

Contents :

REZA.RASHIDI2012@GMAIL.COM 2

Introduction: Pure Iron P

ure

Iro

n Ferrite Iron α (BCC) 0-912˚C

Austenite Iron γ(FCC) 912-1392˚C

Delta Iron δ(BCC) 1392-1536˚C

HISTORICAL NOTEThe first three letters of the Greek alphabet are alpha, beta, andgamma (α, β, γ), but there is no structure of iron called beta iron.When the structure of iron was being discovered in the late 19thcentury, the magnetic transition in iron that occurs at 770 °C(1420 °F) caused scientists to theorize a structure of iron theycalled beta iron, which was later shown not to exist.

Fig .1. Temperature dependence of the mean volume per atom in iron crystals(Hume-Rothery,The Structure of Alloys of Iron, Pergamon Press, Oxford, UK, 1966).

REZA.RASHIDI2012@GMAIL.COM 3

Introduction: Iron –Carbon (Steel)

Austenite Up to 2.11%C fcc

Ferrite Up to 0.022%C

bcc

Cementite 6.67%C orthorhombic

Equ

ibri

um

ph

ase

REZA.RASHIDI2012@GMAIL.COM 4

Introduction: Iron –Carbon (Steel)

Critical temperature

REZA.RASHIDI2012@GMAIL.COM 5

Microstructure of Steel :equilibrium cooling rate

hypo eutectoid eutectoid hypereutectoid

Pearlite

Coarse

Fine

TABLE .1 ■ The effect of carbon on the strength of steels

Slow Cooling (Coarse Pearlite) Fast Cooling (Fine Pearlite)

Carbon %

Yield

Strength

(psi)

Tensile

Strength

(psi)

%

Elongation

Yield

Strength

(psi)

Tensile

Strength

(psi)

%

Elongation

0.20 42,750 57,200 36.5 50,250 64,000 36.0

0.40 51,250 75,250 30.0 54,250 85,500 28.0

0.60 54,000 90,750 23.0 61,000 112,500 18.0

0.80 54,500 89,250 25.0 76,000 146,500 11.0

0.95 55,000 95,250 13.0 72,500 147,000 9.5

After Metals Progress Materials and Processing Databook, 1981.

REZA.RASHIDI2012@GMAIL.COM 6

Microstructure of Steel: disequilibrium cooling rate

1. Martensite Austenite(fcc)

0.8%C

Slow cooling rate

Enough time for diffusion

Ferrite (bcc)

0.02%C

Austenite(fcc)

0.8%C

Rapid cooling rate

There is not enough time for diffusion

Martensite (bct)

As %C increase

c/a (tetragonalite)

Increase

Strength increase

REZA.RASHIDI2012@GMAIL.COM 7

Microstructure of Steel : disequilibrium cooling rate

1. Martensite

Lath

Plate

Mixed

Lath Mixed plate

REZA.RASHIDI2012@GMAIL.COM 8

Microstructure of Steel :disequilibrium cooling rate1. martensite

Quench Temperature control the amount of Martensite

Ms and Mf, fall rapidlyas wt %C in austenite increases

Retained austenite as a function of carbon content in Fe-C alloys.

REZA.RASHIDI2012@GMAIL.COM 9

2. Bainite (α + cem)

Upper bainite Lower bainite

at fast cooling rates, there will be a competitionalong the old austenite grain boundaries, withpearlite forming in some places and bainite formingin other places

REZA.RASHIDI2012@GMAIL.COM 10

Microstructure of Steel :disequilibrium cooling rate

Microstructure of Steel

Cooling curve :

REZA.RASHIDI2012@GMAIL.COM 11

Characterization of Quench Bath Cooling Performance

Factors Affecting Cooling Rates:1. The ability of the heat to diffuse from the interior to the surface of the steel specimen 2. The ability of the quenching medium to remove heat from the surface of the part

Transfer of heat

CONDUCTION

convection

Severity of Quench: H=F/KF: heat transfer factor K: thermal conductivity

Cooling stage:A:vapor blanket cooling stageB:vapor transport cooling stage….most heat transfer rateC:liquid cooling stage

Movement of piece Air Oil Water

None 0.02 0.3 1.0

Moderate --- 0.4—0.6 1.5—3.0

Violent ---- 0.6—0.8 3.0—6.0

REZA.RASHIDI2012@GMAIL.COM 12

The Hardness Test:

vickers

Rockwel

Brinell

• diamond indenter shaped in the form of a pyramid

REZA.RASHIDI2012@GMAIL.COM 13

References :

Steel Metallurgy for the Non-Metallurgist-2007Steels Microstructure and Properties-2006Steels Processing Structure and Performance-2005The Science and Engineering of Materials - Askeland 6th EditionPrinciples of the heat treatment of plain carbon and low alloy steels

REZA.RASHIDI2012@GMAIL.COM 14

Thanks For Your Attention

REZA.RASHIDI2012@GMAIL.COM 15