γλώσσες
Σελίδες
Νομικός
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
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).
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
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.
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
Microstructure of Steel : disequilibrium cooling rate
1. Martensite
Lath
Plate
Mixed
Lath Mixed plate
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.
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
Microstructure of Steel :disequilibrium cooling rate
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
The Hardness Test:
vickers
Rockwel
Brinell
• diamond indenter shaped in the form of a pyramid
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
Top Related