Steels_ Metallurgy and Applications, Third Edition

318 Steels: Metallurgy and Applications

raised due to reduced levels of chromium and carbon in solid solution. Thus the

use of low solution treatment temperatures provides a means of accommodating

casts of steel in which the alloy content was slightly higher than the optimum

but, even so, this approach still necessitated very tight control over the solution

treatment temperature.

In addition to the use of refrigeration treatments, controlled transformation

steels could also be hardened by ageing or primary tempering at a temperature

of 700"C. This resulted in the precipitation of M23C6 carbide and, as illustrated

in Figure 4.12(b), this causes a substantial increase in the Ms-Mf range due to

the reduced level of alloying elements in solid solution. Thus on cooling from the

tempering treatment, the steel transforms to martensite. In a completely austenitic

steel, the precipitation of carbide is essentially restricted to the grain bound-

aries and this limits the degree to which the transformation range can be raised.

140

120

100

80

60

o 40

20

o

Q.

E -20

-40

-60

• 80
  -100
  -120
  (a) -140

r o

v

r 20~

E

p-

-70oC

(b)

M8 m

M, M~o

F

M. M~ Mlo~ M~

M10 MS0 1 Mg0

M, M~o Mf

Mlo Mso ~ Moo 900~

Mso M9 ~ Mf 925 ~

M9 ~ Mf 950 ~

Mf 1000~

1050~

S ""

M1 o -

M~-

__ __M_~_.-

950~ AC

Ms

M~o

Mso

Mgo

M,

-- 950~ AC

+

-- 700~ 2 h

Figure 4.12 Effect of heat treatment on martensite transformation range: (a) effect of
solution treatment; (b) effect of primary tempering (After lrvine et aL l)