Steels_ Metallurgy and Applications, Third Edition

226 Steels: Metallurgy and Applications

influence of other important factors such as carbudzing treatment and alloy
content/hardenability. This was illustrated in samples of commercial steels which
were air cooled, slow oil quenched, fast oil quenched and water quenched,
following a carburizing treatment at 925~ and furnace cooling to 840~ The
compositions of these steels are given in Table 3.3 and the hardenability curves
are shown in Figure 3.21. The mean dimensional changes that took place after
heat treatment are illustrated in Figure 3.22. In this figure, the various quenching
treatments are shown in terms of the H value, an expression of the severity of
the quenching rate. After air cooling, relatively little movement occurs in any of
the steels, but as the quenching rate is increased to an H value of 0.8, most of
the steels exhibit a significant decrease in OD and B D and a complementary
increase in thickness. However, when the specimens are water quenched, a
marked increase takes place in OD and B D and, despite the marked variation in
hardenability, each of the steels tends to exhibit similar levels of movement.
From the foregoing remarks, it can be appreciated that a number of measures
can be taken to reduce or control the degree of distortion that occurs in carburized
components:

1. In some cases, quenching presses can be used to restrain the amount of move-
   ment that occurs.


2. Given the marked effect of hardenability, automotive manufacturers in partic-
   ular will generally order steels to restricted hardenability bands, i.e. to only a
   half or even a third of the band width of the standard specification. This will
   minimize the variation in distortion such that the changes can be predicted
   and accommodated in the pre-heat treatment geometry.


3. The major effect of cooling rate is widely recognized in the automotive
   industry. Special quenching oils or warm oils are sometimes used in order to
   reduce temperature gradients during quenching, thereby reducing the degree
   of irregular dimensional change.

50 r- 822H17 (En355)

45

835H15 (En39B)

40

fo

30

25 ~-- CM60 f "~,,ji 637H17 (En352) ~ ~=~,~_

20 I I I I I I I t I

0 5 10 15 20 25 30 35 40 45 50

Distance from quenched end (mm)

Figure 3.21 Jominy hardenability of commercial steels (After Llewellyn and Cook ns)