lecture12b.pdf

7/28/2019 Lecture12b.pdf

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Fe-C Phase Transformations and

Hardening of Steel, Continued

Prof. Mitra Taheri



From Last Class

• TTT curves for iron-carbon system.

• Retained Austenite: in most steels, especially

those with .4%C or more, austenite is retained

after quenching.

• What formed?



Iron-Carbon Phase Diagram

• Iron Carbon Phase diagramuseful in the study of steels

• Less than 6.7% carbon

considered “commercially

significant”

• Diagram characterized by 3

invariant points: peritectic,

eutectoid, and eutectic

Peritectic: (0.17%C, 1495oC)

Eutectoid: (0.77%C, 727oC)

Eutectic: (4.32%C, 1154oC)

Knowledge of the phase diagram is useful in the analysis of mechanicalproperties of the alloy. Heat treatment => Phase => Properties



Eutectoid Transformations of

Austenite• Proeutectoid => Steel

• Microstructure obtained when austenite slowlycooled depends on original carbon content of steel

• If C < 0.77% then microstructure primarily containsproeutectoid ferrite and pearlite

• If C = 0.77% microstructure contains only pearlite

• If C > 0.77% the microstructure will contain

proeutectoid cementite and pearlite



What is Pearlite?

• Consists of plates of cementite (Fe3C) in a

matrix of ferrite (lighter part)



Image of Hypereutectoid

SEM image shows the cementitedelineating prior austenite grainboundaries with a thin layer.

The amount of proeutectoid phaseis very low, with the majority of thearea being taken by the pearliteeutectoid.

Each pearlite cell has a different

orientation with the ferrite phasebeing selectively etched.



Image of Hypoeutectoid

SEM image shows that theferrite phase in the pearlitehas been selectivelyetched compared to thecementite.

The cementite phaseappears to protrude fromthe surface.

Within the pearlite region

there are several coloniesin different orientations,indicating that the pearlitenucleated on grains of theprimary ferrite.



•Carbon and alloy steels are Martensitic hardened by heating to the Austenitizingtemperature followed by cooling at the appropriate rate.•Ms is when the Martensite transformation starts. Mf is transformation finishes.•The maximum hardness of carbon and alloy steels, after rapid quenching to avoidthe nose of the isothermal transformation curve, is a dependent on the alloy content,predominantly the carbon content. The maximum thickness for complete hardening

or the depth to which an alloy will harden is measure of a steels hardenability.



What Determines the Hardenability of

Steel?

• Chemical composition

• Size of austenite grain size at the instant of quenching

• High hardenability = austenite to martensite

(complete), with no pearlite.• High cooling rates are required in steels with low

hardenability

• Limiting factor = formation of pearlite at high

temps…..so anything that will slow the nucleation of pearlite is a good thing (meaning move the pearliteformation line to the right on a continuous coolingcurve diagram).



Austenitic Grain Size

• Pearlite Nucleates at Austenite Grainboundaries

• This is heterogeneous nucleation!

• Growth rate of pearlite is independent of austenite grain size, but the total number of nuclei per second varies directly with the

surface available for their formation• Those surfaces are the austenite grain

boundaries!



Carbon Content

• Carbon content strongly influences thehardenability of steel

• Formation of pearlite and proeutectoid phases

is more difficult in the higher carbon contentsteels.

• Variation of D1 with carbon content (D1=

critical diameter…related to critical diameterof a specimen to quench fully, etc.

• Figure 19.9 reed hill



Alloying Elements

• Though all alloying elements will have an

effect on hardenability , some increase it and

some decrease it.

• Table 19.5….Grossman Multiplying Factors

• Cobalt decreases, but those that are soluble in

iron increase it.



Tempering

• Steels that undergo a simple hardening quench

are generally a mix of austenite/martensite

• Both structures are unstable, and will slowly

decompose if left at room temperature….theaustenite will change into martensite, and the

martensite will then transform.

• If the sample is mainly martensite, it’s also toobrittle, and because of cracking, is really of no use

industrially



Tempering, continued.

• So this is why we temper!

• Tempering is when the temperature of thesteel is raised to a value below the eutectoid

temperature and held there for some lengthof time….then cooled to room temperature.

• The point of this is to allow for diffusion to

occur to produce a more stable and less brittlestructure, which would be more industriallyrelevant.



Stages of tempering, in a nutshell

1. Precipitation of the transition carbide

2. Decomposition of the Austenite into a mixture

of ferrite and cementite

3. Formation of cementite by conversion of the

transition carbide and segregated carbon into

small rod-shaped cementite particles

*note: stage 3 is suppressed in low-carbon steelsbecause amount of retained austenite is less…so

transition from austenite to other phases is also less!



Effect of Tempering on Properties

• Figure 19.34, reed-hill

Time/Temperature in Tempering

•Figure 19.37, reed-hill



Quenching

• Very important industrial process is thehardening of steel by quenching (what you aretesting in your lab).

• If the quench is rapid enough from theaustenitic field, then there isn’t enough timefor the eutectoidal diffusion-controlledprocess (decomposition) to occur, and thesteel transforms to martensite (or sometimesmainly martensite and some austenite).



Variation in Microstructure as a

Function of Cooling (as in lab 3)• Take a look at Figure 19.2: Critical Cooling

Curve

• Fast = martensite, slow= some amount of

pearlite• Cooling rates are different at different points

in the specimen….i.e., there is a difference in

temperature at any instant in time betweenthe surface and center of the sample (see fig.19.3)



Hardness Variation with

Microstructure

• Change in microstructure is associated withcorresponding change in hardness (this iswhat you should have found in your lab).

• Figure 19.4: Martensite = rockwell C-65,Pearlite = rockwell C-40….the 50% point of martensite/pearlite = C-54.

• This 50% point can be used as a criterion forthe depth of the hardening given theparticular quench



Quenching Fluid and Style

• Dependence of quench on oil, water, agitation

• As we discussed in class last time, water vs. oilyields different resulting microstructures (onset

of martensite)• Decrease in bubbles on the surface increases

cooling rate (surface area covered by coolingliquid)…moving the sample within the cooling

liquid will accomplish this, and is called agitation.• See section on Jominy End Quench (p.610-612,

Reed Hill).



Bain Distortion: essentially the movement of FCC toBCC with minimal atomic movements. Need to spendmore time on this, and will go over during deformationtwinning on Thursday.

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