d ay 15: h ardenability hardenability cct curves
TRANSCRIPT
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
f11_22_pg403
f11_23_pg404
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)
f11_27_pg406
f11_25_pg405
f11_26_pg405
f11_28_pg413