Steels_ Metallurgy and Applications, Third Edition

288 Steels: Metallurgy and Applications

2600

2200

r

E

03

1800 -

_____

140040 50 dO '70 80 90

% Reduction

Figure 3.45 Effect of coM drawing on the strength of high-carbon pearlitic rod

However, micro-alloying increases the tendency for martensite formation which
has a particularly damaging effect on the wire-drawing characteristics. The
martensite-forming potential of various elements was therefore determined in
controlled cooling experiments and the critical cooling rate at which martensite
first appeared in the pearlitic matrix was determined. With a prior austenite
grain size of ASTM 7, the following critical cooling rate (CCR) relationship
was derived:

CCR (Ks -1) = 97 - (%Si) - 70(%Mn) - 50(%Cr) - 224(%P)

This equation indicates that manganese is potentially more damaging than
chromium but it was also shown that chromium was more effective in producing
a finer interlamellar spacing. Whereas silicon had relatively little effect in
promoting the formation of martensite, it was also shown to be less effective
than chromium in refining the interlamellar spacing. Jaiswal and Mclvor therefore
propose that silicon is used in combination with chromium, or possibly vanadium,
rather than as a single addition.
Given the importance of avoiding the formation of martensite, Jaiswal and
Mclvor recommend that chromium is better used in large-diameter rods where
the cooling rate is relatively slow and therefore less likely to produce martensite.
For small-diameter rods, vanadium is recommended.
In a base steel containing 0.85% C and 0.7% Mn, the above authors state
that the maximum levels of strength shown in Table 3.29 can be obtained by
microalloying.

Table 3.29 Microalloyed High Carbon Rod

Additive Rod diameter TS

(N/mm 2)

0.8% Si, 0.25% Cr Large 1330

0.07% V Small 1300