Slide 1

DAY 15: HARDENABILITY

Hardenability

CCT Curves

HARDENABILITY

We have seen the advantage of gettingmartensite, M. We can temper it, getting TMwith the best combination of ductility andstrength.

But the problem is this: getting M in depth,instead of just on the surface. We want a steelwhere Pearlite formation is relatively sluggish sowe can get it to the cooler regions where M forms.

The ability to get M in depth for low cooling ratesis called hardenability.

Plain carbon steels have poor hardenability.

FACTORS WHICH IMPROVEHARDENABILITY

1. Austenitic Grain size. The Pearlite will havean easier time forming if there is a lot of g.b.area. Hence, having a large austenitic grain sizeimproves hardenability.

2. Adding alloys of various kinds. This impedesthe   P reaction.

TTT diagramof amolybdenumsteel 0.4C0.2Mo

AfterAdding2.0% Mo

JOMINY TEST FOR HARDENABILITY

Hardenability not the same as hardness!

THE RESULT IS PRESENTED IN A CURVE

Note:

1.Distance fromquenched endcorresponds toa cooling rate,and a bardiameter

2.Notice thatsome steelsdrop off morethan others atlow coolingrates. Lesshardenability!

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 a

3 inch bar is about 42 HRC

DEPTH OF HARDENING

CONTINUOUS COOLING TRANSFORMATION

CCT Curves – Here is the one for the0.77% Eutectoid Composition Steel

What would we get ifwe cooled at

1.150 oC/s

2.50 oC/s

3.5 oC/s

ANOTHER CURVE

Here’s One for an Alloy Steel

Note:

1.DifferentMicrostructuresat differentcooling rates.

2.Differentmicrostructurespossible in samepiece

3.Comparison withprevious steel,note the effectsof alloying

IN THE AREA OF AGE HARDENING(PRECIPITATION HARDENING) :

 State the factors necessary for age hardening tobe possible.

 Name the three steps in the age hardeningprocess, the microstructural changes associatedwith each step, and the relative mechanicalproperties which result from thosemicrostructures.

compare and contrast age hardening and quenchand tempering in terms of procedure,microstructure and properties.

wt% Cu

+L







+L

300

400

500

600

700

(Al)

T(°C)

composition range

needed for precipitation hardening

CuAl2

Adapted from Fig. 11.24, Callister 7e. (Fig. 11.24 adapted fromJ.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)

Pt B

Pt C (precipitate 

HEAT TREATING ALUMINUM

Solution Treat

Quench

Age

f11_22_pg403

f11_23_pg404

• 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