D AY 15: H ARDENABILITY Hardenability CCT Curves

H ARDENABILITY We have seen the advantage of getting martensite, M. We can temper it, getting TM with the best combination of ductility and strength. But the problem is this: getting M in depth, instead of just on the surface. We want a steel where Pearlite formation is relatively sluggish so we can get it to the cooler regions where M forms. The ability to get M in depth for low cooling rates is called hardenability. Plain carbon steels have poor hardenability.

F ACTORS W HICH I MPROVE H ARDENABILITY 1. Austenitic Grain size. The Pearlite will have an easier time forming if there is a lot of g.b. area. Hence, having a large austenitic grain size improves hardenability. 2. Adding alloys of various kinds. This impedes the   P reaction. TTT diagram of a molybdenum steel 0.4C 0.2Mo After Adding 2.0% Mo

J OMINY T EST FOR H ARDENABILITY Hardenability not the same as hardness!

T HE R ESULT IS P RESENTED IN A C URVE Note: 1.Distance from quenched end corresponds to a cooling rate, and a bar diameter 2.Notice that some steels drop off more than others at low cooling rates. Less hardenability! Rank steels in order of hardenability.

A LLOYING AND H ARDENABILITY

C ARBON AND H ARDENABILITY

H ARDNESS AND H ARDENABILITY Predict the center hardness in a water quenched 3” bar of 8640 Water QuenchedOil Quenched Jominy Distance =17mm

A LLOYING AND H ARDENABILITY Hardness at Center of a 3 inch bar is about 42 HRC

D EPTH OF H ARDENING

C ONTINUOUS C OOLING T RANSFORMATION CCT Curves – Here is the one for the 0.77% Eutectoid Composition Steel What would we get if we cooled at o C/s 2.50 o C/s 3.5 o C/s

A NOTHER C URVE Here’s One for an Alloy Steel Note: 1.Different Microstructures at different cooling rates. 2.Different microstructures possible in same piece 3.Comparison with previous steel, note the effects of alloying

I N THE AREA OF AGE HARDENING ( PRECIPITATION HARDENING ) : State the factors necessary for age hardening to be possible. Name the three steps in the age hardening process, the microstructural changes associated with each step, and the relative mechanical properties which result from those microstructures. compare and contrast age hardening and quench and tempering in terms of procedure, microstructure and properties.

wt% Cu L +L+L    +L+L (Al) T (°C) composition range needed for precipitation hardening CuAl 2 A Adapted from Fig , Callister 7e. (Fig adapted from J.L. Murray, International Metals Review 30, p.5, 1985.) P RECIPITATION H ARDENING Particles impede dislocations. Ex: Al-Cu system Procedure: Adapted from Fig , Callister 7e. --Pt B: quench to room temp. --Pt C: reheat to nucleate small  crystals within  crystals. Other precipitation systems: Cu-Be Cu-Sn Mg-Al Temp. Time --Pt A: solution heat treat (get  solid solution) Pt A (sol’n heat treat) B Pt B C Pt C (precipitate 

H EAT T REATING A LUMINUM Solution Treat Quench Age

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Al Alloy: TS peaks with precipitation time. Increasing T accelerates process. Adapted from Fig (a) and (b), Callister 7e. (Fig adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, p. 41.) P RECIPITATE E FFECT ON TS, % EL precipitation heat treat time tensile strength (MPa) min1h1day1mo1yr 204°C non-equil. solid solution many small precipitates “aged” fewer large precipitates “overaged” 149°C % EL reaches minimum with precipitation time. % EL (2 in sample) min1h1day1mo1yr 204°C 149°C precipitation heat treat time

A GING AND O VERAGING After quenching, there is thermodynamic motivation for precipitate to form. Precipitates initiate and grow due to diffusion, enhanced by higher temperatures. To get significant strengthening the precipitate should be coherent When the precipitates get too large, they lose coherence and strengthening decreases (overaging)

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