Steels- Tempering Martensite and Phase Transformations in Steel Flashcards
What is tempering?
The process of heating a martensitic steel to some temperature below the A1 to make it softer and more ductile
What solid state reactions occur during tempering?
Segregation of C atoms.
Precipitation of carbides (Fe3C, ε).
Decomposition of retained γ to bainite.
Recovery and recrystallisation of the matrix.
Can also have precipitation of alloy carbides (e.g Mo, Cr, Ti, Nb, V) is alloy steels which give rise to secondary hardening (local rise in hardness).
Hardness vs temperature for tempering martensite
Starts high (as-quenched) then decreases slowly during carbon segregation and ε-carbide formation. Then decreases quicker for rod shaped carbide precipitates, then recovery, then spheroidal Fe3C precipitates overlapping. Starts to feel of during recrystallisation and coarsening of Fe3C. Higher curves with greater changes in steepness for martensite with higher C content
Stages of tempering on dilation vs T graph
Starts low and small curve up to a peak. Curves back down in stage I and reaches minimum (above start) in stage II. Then big increase over wide T range during stage III up to a new peak. Comes down a bit to another minimum during stage IV. Then continues to curve up (shallow)
Stage I of tempering
135-250C. The decrease in dilation from first low peak. Large contraction and decrease in resistivity (increased conductivity). Pre-precipitation stage where clusters form
Stage II of tempering
About 250C. Around the first minimum. ε-carbide formation.
Stage III of tempering
240-320C. Large increase in dilation up to peak. Expansion due to decomposition of retained austenite to lower bainite and ε-carbides
Stage IV of tempering
> 320C. The small decrease to a new minimum. Large contraction due to Fe3C precipitation in α’.
What happens in tempering if the steel contains strong carbide forming elements? Which are these?
V, Nb, Ti, Cr, Mo. Alloy carbides can be precipitated during tempering in the range 500-700C. Considered as part of stage IV. Secondary hardening.
What is autotempering?
Segregation of C to dislocations during the quench. This means immediately upon quenching you get formation of tempered martensite. Important for steels with a high Ms (low C)
The two general types of phase transformations in steel
- Reactions near equilibrium conditions. Nucleation and growth processes (requires diffusion). E.g γ-> pearlite, αpro or (Fe3C)pro.
- Reactions removed from equilibrium conditions. E.g γ->α’ (shear transformation, diffusionless, athermal) or bainite (diffusion and shear). Equilibrium phase diagram not adequate to describe these reactions. Requires use of TTT diagrams.
What are TTT diagrams and what do they represent?
Graphically represent the precipitation kinetics for the transformation products associated with the decomposition of γ. Used for looking at transformations over time under isothermal conditions. Temperature vs time (log) with different regions in it
Describe the TTT diagram for a eutectoid steel with no alloy additions
Horizontal line at eutectoid temperature, above which is stable austenite, below is unstable austenite if not in any other region. Have 2 big C curves which enclose the partial transformations region. To the right of both has pearlite at top, then fine pearlite and upper bainite, then lower bainite. Curves stop at the Ms temperature (horizontal line). Other lower horizontal lines for increasing percentage of martensite (f(T) not f(t)).
How does interlamellar spacing in pearlite vary with transformation temperature?
Exponential increase in spacing with T from 500 to over 700C. So coarser pearlite as transformation T increases
Difference in strength and elongation between coarse and fine pearlite
Slow cooling gives coarse pearlite. This gives lower strength and higher % elongation than fast cooling (fine pearlite) for the same wt% C.
Converting from psi to MPa
Divide by 145
How to follow cooling paths on a TTT diagram
When quenching assume straight line down at that time and any stable phase that has already formed remains. When holding at a temperature, follow this horizontally for the duration it is being held and see what region you are in. If you are in the martensitic region there is no variation with time so could just end there and composition not change. If not in martensite regions then need a final quench to avoid further transformation. Any unstable austenite goes to α’ when quenched
What does holding for longer in the same region of a TTT diagram do, even if in a fully transformed region?
Holding for longer allows growth. E.g if was in 100% fine pearlite region, holding for longer gives coarser pearlite due to growth
How does TTT diagram change for hypoeutectoid steel?
Above the eutectoid temperature (A1) is ferrite and austenite. Then another higher horizontal line for A3 temperature, above which is only austenite. On left between A1 and A3 is shallow concave curve on which proeutectoid ferrite starts to form. Ms about 400
How does TTT diagram change for hypereutectoid steel?
Above the A1 is austenite and cementite. Above this is another horizontal for the Acm temperature. There is another concave curve starting just below the A1 and finishing on A3 on which proeutectoid cementite starts to form. Ms about 190
How do alloying elements change the transformation kinetics?
Most will retard the transformation kinetics and so push the C curves to longer times. Increase hardenability (ability to harden the steel)
How does austenite grain size influence the transformation kinetics?
As Dγ increases, the transformation kinetics become slower pushing the C-curve to longer times. This is due to less GB area/unit volume, fewer nucleation sites. Increases hardenability (but not how people do this due to disadvantages of coarse grain structure).
How does Ms depend on wt% C?
Ms decreases as wt% C increases
What does continuous cooling do to transformations?
It suppresses them so the C-curve goes to longer times
How to follow continuous cooling on a TTT diagram
The C-curves will need moving right. Have curve following the temperature at a given time. Faster cools have steeper negative gradient
What does a continuous cooling transformation (CCT) diagram look like?
Horrible. Still have the top two horizontal lines for A3 and A1 (lower). The regions are bound by C-curves (for each extent of transformation) and their Ms lines only go leftwards from the bottom of them. Have different curves down from the A3 temperature for whatever cooling route is being done. The further left it is the faster the cooling. Numbers in circles at ends of these are hardness values.
