Steels_ Metallurgy and Applications, Third Edition

78 Steels: Metallurgy and Applications

c (a) Steel B

a~ 0.30 ..........

m

020

~0.10~--0 400~ --

u. A 375 ~ /

9 350 ~

0 / I 1. I I I I. I t

1 10 100

Holding time (min)

Figure 1.86 Change in retained austenite volume fraction with isothermal holding time
for a steel containing 1.2% silicon for different holding temperatures (After Matsumura 139)

.-. 1.8

(I)

"E | 1.6

._= 1.4

.r

~ | 1.21

c-

O

o 1.0 L

(a) Steel B

I

!. .;"

9 % ....

I

" Z~ 425~

~i~, q..,. ,.. ~.=. w~" "# ' 0 400~

"-lP" " k 375~

9 350~

I ...i I I l

1 10 100

Holding time (minutes)

Figure 1.87 Variation of carbon content in retained austenite for different isothermal
holding times for a steel containing 1.2% silicon for different holding temperatures (After
Matsumura 139)

Figure 1.86 illustrates how the volume fraction of austenite retained at room

temperature in a steel containing 1.2% manganese and 1.2% silicon varies with

time and temperature of austempering, and the equivalent variation of the carbon

content of the retained austenite is given in Figure 1.87. An example of the

effect of holding time at 400~ on the nature of a stress-strain curve is given

in Figure 1.88. It is seen that for the low holding time of 1 minute, the curve is

typical of a classical dual-phase ferrite-martensite steel with continuous yielding.

Higher hold times lead to an increasing yield point elongation and a higher

elongation. Figure 1.89 shows that an increasing hold time leads to a progressive

decrease in tensile strength and an increase in yield stress, but that the elongation

values pass through a peak for an intermediate holding time of 6 minutes.