D AY 15: H ARDENABILITY Hardenability CCT Curves.

DAY 15: HARDENABILITY

Hardenability CCT Curves

HARDENABILITY

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.

FACTORS WHICH IMPROVE HARDENABILITY

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

JOMINY TEST FOR HARDENABILITY

Hardenability not the same as hardness!

THE RESULT IS PRESENTED IN A CURVE

Note:1.Distance from quenched end corresponds to a cooling rate, and a bar diameter2.Notice that some steels drop off more than others at low cooling rates. Less hardenability!

Rank steels in order of hardenability.

ALLOYING AND HARDENABILITY

CARBON AND HARDENABILITY

HARDNESS AND HARDENABILITY

Predict the center hardness in a water quenched 3” bar of 8640

Water Quenched Oil Quenched

Jominy Distance =17mm

ALLOYING AND HARDENABILITY

Hardness at Center of a3 inch bar is about 42 HRC

DEPTH OF HARDENING

CONTINUOUS COOLING TRANSFORMATION

CCT Curves – Here is the one for the 0.77% Eutectoid Composition Steel

What would we get if we cooled at1.150 oC/s2.50 oC/s3.5 oC/s

ANOTHER CURVE

Here’s One for an Alloy Steel

Note:1.Different Microstructures at different cooling rates.2.Different microstructures possible in same piece3.Comparison with previous steel, note the effects of alloying

IN 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.

14

0 10 20 30 40 50wt% Cu

L+L

+L

300

400

500

600

700

(Al)

T(°C)

composition range needed for precipitation hardening

CuAl2

A

Adapted from Fig. 11.24, Callister 7e. (Fig. 11.24 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)

PRECIPITATION HARDENING

• Particles impede dislocations.• Ex: Al-Cu system• Procedure:

Adapted from Fig. 11.22, 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

HEAT TREATING ALUMINUM

Solution Treat

Quench

Age

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18

• 2014 Al Alloy:

• TS peaks with precipitation time.• Increasing T accelerates process.

Adapted from Fig. 11.27 (a) and (b), Callister 7e. (Fig. 11.27 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.)

PRECIPITATE EFFECT ON TS, %EL

precipitation heat treat timetensi

le s

trength

(M

Pa)

200

300

400

1001min 1h 1day 1mo1yr

204°C

non-

equi

l.

solid

sol

utio

nm

any

smal

l

prec

ipitat

es

“age

d”

few

er la

rge

prec

ipitat

es

“ove

rage

d”149°C

• %EL reaches minimum with precipitation time.

%EL

(2 in s

am

ple

)10

20

30

0 1min 1h 1day1mo1yr

204°C 149°C

precipitation heat treat time

AGING AND OVERAGING

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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