Steels_ Metallurgy and Applications, Third Edition

272 Steels: Metallurgy and Applications

Medium-high-carbon pearlitic steels

Although most of the steels that are used in engineering applications are

heat treated to form a tempered bainitic/martensitic structure, there are notable

examples in which the required strength is generated in air-cooled, medium-high-

carbon steels with a predominantly pearlitic microstructure. These include

micro-alloy forging grades, rail steels and high-carbon wire rod and these

applications will be discussed in the sections that follow. However, as a

precursor to these discussions, it is worthwhile to review very briefly the physical

metallurgy of pearlitic steels.

In Chapters 1 and 2, the HaU-Petch relationship was introduced, namely:

1

try ---- tri -t- kyd-2

where ay = yield strength

tri = friction stress which opposes dislocation movement

(Peieds stress)

ky = dislocation locking term

d = ferrite grain size (mm)

It was stated that this basic relationship was extended later to take account of

the solid solution strengthening effects of alloying elements and expressed as

follows:

1

try = tri + k' (% alloy) + kyd-~

Following this concept, Picketing and Gladman 4t then produced the following
quantitative relationships for yields and tensile strength:

1

YS (N/mm 2) = 53.9 + 32.3% Mn + 83.2% Si + 354 Nf -I- 17.4d-~

1

TS (N/mm 2) 294 + 27.7% Mn + 83.2% Si + 3.85% pearlite + 7.7d-~

These equations were derived by statistical analysis of steels containing up to
0.25% C and one of the interesting points to emerge was the absence of a term
for carbon or pearlite in the equation for yield strength.
In the 1970s, Gladman et al. 42 went on to develop equations for steels
containing up to 0.9% C and their equation for yield strength was presented
in the following form:

cry = fatra n + (1 - fa)trp n

where try = yield strength of the ferrite-pearlite aggregate

tra = yield strength of ferrite

trp = yield strength of pearlite

fa = volume fraction of ferrite

and by implication (1 - fa) = volume fraction of pearlite.
Thus the yield strength of the aggregate is presented as the sum of the separate
contributions from ferrite and pearlite and weighted according to their volume