Steels_ Metallurgy and Applications, Third Edition

Engineering steels 209

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Figure 3.9 Schematic SAC hardenability curve

effects that might have been introduced during cutting. Rockwell (HRC) hardness
measurements are then made at four positions on the original cylinder face and
the average hardness provides the surface (S) value. Rockwell testing is then
carded out along the cross-section of the specimen from surface to centre and
provides the type of hardness profile illustrated in Figure 3.9. The total area
under the curve provides the area (A) value in units of Rockwell-inch (using the
original imperial unit) and the hardness at the centre gives the C value. The SAC
value of a steel might be reported as 65-51-39, which would indicate a surface
hardness of 65 HRC, an area value of 51 Rockwell-inch and a centre hardness
of 39 HRC. One interesting feature of the SAC test is that it can reveal central
segregation in a bar as indicated by a hardness peak at the centre position.

Factors affecting hardenability

Grain size

In a homogeneous austenitic structure, the nucleation of pearlite occurs almost

exclusively at the grain boundaries and therefore the larger the grain boundary

surface area, the greater are the nucleation sites for pearlite formation. Thus the

hardenability of a given composition will increase with increasing austenitizing

temperature and austenite grain size. The major effect of austenitizing temperature

on the hardenability of a 0.55% C 1% Cr 0.2% Mo steel is shown in Figure 3.10,

which is based on the work of Grange and presented by Grossmann. 5

Although grain coarsening could be employed as a cheap method of achieving

high hardenability, this approach is rarely adopted because the toughness and

ductility are impaired. Instead, most commercial engineering steels are made to

an aluminium-treated, fine-grain practice in order to produce microstructures that

will provide a good combination of strength and toughness/ductility.