ASTM - Intensive Quenching Systems - Engineering and Design 2010 - N I Kobasko, M A Aronov, J A Powell, G E Totten

CHAPTER 1 n THERMAL AND METALLURGICAL BASICS OF DESIGN OF HIGH-STRENGTH STEELS 15
TABLE 5—Effect of the deformation degree in the case of
high-temperature thermomechanical treatment upon mechanical
properties of 55S2 steel (AISI 9255) (with tempering at
480°F/250°C, austenitizing temperature 1,760°F/960°C) [15,48]
Steel grade k df (%) R m (MPa) R p (MPa) A (%) Z (%)
55S2
(AISI 9255)
Usual
Fragile fracture
25 2,256 2,010 5 10
50 2,276 2,020 7 9
75 2,158 1,982 6 10
55S2Kh Usual Fragile fracture
25 2,433 2,207 9 18
50 2,492 2,266 9 18
75 2,453 2,246 10 19
55S2M Usual Fragile fracture
25 2,394 2,178 8 25
50 2,472 2,256 7 20
75 2,423 2,207 8 27
55S2V 0 2,158 1,942 4 10
25 2,453 2,246 13 26
50 2,550 2,286 10 24
75 2,472 2,256 12 25
55S2FM Usual 2,256 2,040 5 12
25 2,472 2,256 9 18
50 2,531 2,286 8 18
75 2,492 2,266 9 20
deformation at 1,740°F (950°C) for one pass, the fracture
strength increases up to 2,350–2,450 MPa and fatigue limit
increases more than 300 MPa. For the same high-temperature
thermomechanical treatment with the same deformation, but
for two passes, it is 2,650–2,700 MPa. Fatigue stress test data
for steel having 0.62 % carbon and 2.16 % silicon are given in
Fig. 26. The limit strength for the steel used in Fig. 26 after
conventional heat treatment is very low.
1.8 COMBINING THERMOMECHANICAL
TREATMENT WITH THE INTENSIVE QUENCHING
PROCESS COULD BE VERY BENEFICIAL
There are not enough data providing the impact of intensive
quenching combined with the thermomechanical treatment
(TMT) process on mechanical properties of a material. This
is because the TMT process is used mostly for alloy steels to
make low-temperature thermomechanical treatment reliable.
As a rule, alloy steels are quenched in oil.
Let’s compare the mechanical properties of AISI 5140
steel with those of AISI 1040 steel, both subjected to TMT.
The AISI 5140, after TMT, was quenched in oil; the AISI
1040 was quenched in water. The martensite start temperature
for both steels was about 350°C. The heat transfer
coefficient of oil within a range of temperature of 100–
350°C is 300 W/m 2 K, and the heat transfer coefficient within
the same interval for water is about 4,000 W/m 2 K due to the
boiling process, which occurs above 100°C. This means that
the cooling rate within the martensite range differs significantly
between the two.
The equation for cooling rate evaluation, depending on
heat transfer coefficients, is well known [1,49]:
v ¼ abKn
D 2 ðT T m Þ; ð15Þ
where v is the cooling rate in °C/s; a is the thermal diffusivity
of a material in m 2 /s; b is the coefficient depending on the
configuration of steel parts; Kn is the Kondratjev number (a
dimensionless value); D is size (diameter or thickness) in m; T
is temperature in °C; and T m is bath temperature in °C.
For a cylindrical 10-mm specimen, when quenching in
oil (300 W/m 2 K) and water (4,000 W/m 2 K), the Kondratjev
numbers are 0.05 and 0.4 [1]. According to Eq 15, the cooling
rate within the martensite range in cold agitated water is
eight times faster than with the still oil.
Let’s see how the mechanical properties of alloy steels,
subjected to TMT and quenched in oil, differ from the