Steels_ Metallurgy and Applications, Third Edition

Engineering steels 217

4.0

3.5

3.0

2.5

E"

t,,,,

"~ 2.0

C

2

1.5

1.0

0.5

o 90% martenslte

9 80% martenslte

o 50% martensite

• %

g\s

r{t

I

I

_ I

I

I

I

I

,,I,, I I I I I 1 I I I I I

0 0.1 0.2 o.a" 0.4 o.s 0.6 0.7 0.8 0.9 1.0

Carbon (%)

Figure 3.18 Effect of carbon on boron multiplying factor (After LleweUyn and Cook s)

A consequence of the above effect is that the case hardenability of a carburized
boron-treated steel is lower than that of a boron-free steel of comparable core
hardenability. However, the limiting section sizes that will provide adequate case
depths are probably well in excess of those that present a problem with regard
to core hardenability.

Tempering resistance

Although the objective of quenching a steel is generally to produce a martensitic
structure of high strength, steels are rarely put into service in the as-quenched
condition because this represents a state of high stress, low toughness and poor
ductility. After quenching, components are therefore tempered at an elevated