Transcript DAY 15: HARDENABILITY   Hardenability CCT Curves HARDENABILITY We have seen the advantage of getting martensite, M.

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 g  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
Rank steels in order of hardenability.
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!
ALLOYING AND HARDENABILITY
CARBON AND HARDENABILITY
HARDNESS AND HARDENABILITY
Predict the center hardness in a water quenched 3” bar of 8640
Jominy Distance =17mm
Water Quenched
Oil Quenched
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 the
0.77% Eutectoid Composition Steel
What would we get if
we 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. Different
Microstructures
at different
cooling rates.
2. Different
microstructures
possible in same
piece
3. 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.

PRECIPITATION HARDENING
• Particles impede dislocations.
• Ex: Al-Cu system
700
T(°C)
• Procedure:
600 
+L
--Pt A: solution heat treat
A
500
(get  solid solution)
--Pt B: quench to room temp.
400 C
--Pt C: reheat to nucleate
small q crystals within
300
0 10
 crystals.
(Al) B
Adapted from Fig.
11.22, Callister 7e.
q+L
q
+q
20
30
40
50
wt% Cu
composition range
needed for precipitation hardening
• Other precipitation
systems:
• Cu-Be
• Cu-Sn
• Mg-Al
CuAl2
L
Adapted from Fig. 11.24, Callister 7e. (Fig. 11.24 adapted from
J.L. Murray, International Metals Review 30, p.5, 1985.)
Temp.
Pt A (sol’n heat treat)
Pt C (precipitate q)
Pt B
Time
14
HEAT TREATING ALUMINUM
Solution Treat
Age
Quench
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PRECIPITATE EFFECT ON TS, %EL
• 2014 Al Alloy:
• %EL reaches minimum
with precipitation time.
400
300
200
100
204°C
149°C
1min
1h 1day 1mo 1yr
precipitation heat treat time
%EL (2 in sample)
tensile strength (MPa)
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
30
20
10
0
204°C
149°C
1min
1h 1day 1mo 1yr
precipitation heat treat time
18
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.)
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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