STEEL - Materials for Civil and Construction Engineers - page 96

the surface of the material being hardened, the surface of the material is
harder and more brittle than the interior of the element, creating nonho-
mogenous characteristics. Due to the rapid cooling, hardening puts the steel
in a state of strain. This strain sometimes causes steel pieces with sharp an-
gles or grooves to crack immediately after hardening. Thus, hardening must
be followed by tempering.

■ 3.3.4 Tempering

The predominance of martensite in quench-hardened steel results in an un-
desirable brittleness. Tempering is performed to improve ductility and
toughness. Martensite is a somewhat instable structure. Heating causes car-
bon atoms to diffuse from martensite to produce a carbide precipitate and
formation of ferrite and cementite. After quenching, the steel is cooled to
about 40°C then reheated by immersion in either oil or nitrate salts. The
steel is maintained at the elevated temperature for about two hours and then
cooled in still air.

■

3.3.5 Example of Heat Treatment

In the quest to economically produce high-strength low-alloy steels, the in-
dustry has developed specifications for several new steel products, such as
A913. This steel is available with yield stresses ranging from 50,000 to
75,000 psi. The superior properties of A913 steel are obtained by a quench-
self-tempering process. Following the last hot rolling pass for shaping, for
which the temperature is typically 850°C (1600°F), an intense water-cooling
spray is applied to the surface of the beam to quench (rapidly cool) the skin.
Cooling is interrupted before the core on the material is affected. The outer
layers are then tempered as the internal heat of the beam flows to the sur-
face. After the short cooling phase, the self-tempering temperature is 600°C
(1100°F) (Bouchard and Axmann, 2000).

3.4 Steel Alloys

Alloy metals can be used to alter the characteristics of steel. By some counts,
there are as many as 250,000 different alloys of steel produced. Of these, as
many as 200 may be used for civil engineering applications. Rather than go
into the specific characteristics of selected alloys, the general effect of dif-
ferent alloying agents will be presented. Alloy agents are added to improve
one or more of the following properties:
1. hardenability
2. corrosion resistance
3. machineability
4. ductility
5. strength