Steels_ Metallurgy and Applications, Third Edition

Engineering steels 219

Table 3.1

Element Retardation in tempering per 1%

addition

C -40

Co 8

Cr 0

Mn 8

Mo 17

Ni 8

Si 20

V 30

W 10

operating conditions dictate the use of high tempering temperatures, resort is
made to molybdenum and vanadium additions, but at relatively modest levels.
At levels greater than about 1%, molybdenum induces a secondary hardening
reaction.
In addition to its effect on tempering resistance, molybdenum is also added
to steels to suppress temper embrittlement. This involves the co-segregation of
alloying elements such as manganese and silicon and impurity elements such
as antimony, arsenic, tin and phosphorus to the prior austenite grain bound-
aries in steels with bainitic or martensitic microstructures. Temper embrittlement
occurs during exposure in the temperature range 325-575"C, either from oper-
ating within this range or by slow cooling through the range from a higher
tempering temperature. This topic will be discussed further when dealing with
Steels for steam power turbines, later in this chapter.
Whereas nickel is a well known alloying element in the context of engineering
steels, it is expensive and not very effective in enhancing either hardenability or
tempering resistance. However, it is perceived to be a ferfite strengthener and is
used in engineering steels in order to promote toughness.

Surface-hardening treatments

Case carburizing

Case carburizing involves the diffusion of carbon into the surface layers of a low-

carbon steel by heating it in contact with a carbonaceous material. The objective

is to produce a hard, wear-resistant case with a high resistance to both bending

and contact fatigue, whilst still maintaining the toughness and ductility of the low-

carbon core. Carburizing is carried out at temperatures in the range 825-925"C

in solid, liquid or gaseous media but, in each treatment, the transport of carbon

from the carburizing medium takes place via the gaseous state, usually CO.

Carburizing in a solid, granular medium, such as charcoal, is termed pack

carburizing and has been practised since ancient times. However, whereas the

early treatments relied solely on the reaction between charcoal and atmospheric

oxygen for the generation of CO2 and CO, energizers such as sodium or barium

carbonate are now added to the carburizing compound. During the heating-up