Page 142 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 4 11 Anneahng

e. Boronizing;
f. Flame hardening;
g. Induction hardening; and
h. Laser-beam hardening.
Basically, these are operations in which the component is heated in an atmosphere
containing elements (such as carbon, nitrogen, or boron) that alter the composition,
microstructure, and properties of surfaces. For steels with sufficiently high carbon
content, surface hardening takes place without using any of these additional ele-
ments. Only the heat-treatment processes described in Section 4.7 are needed to
alter the microstructures, usually by either flame hardening or induction hardening,
as outlined in Table 4.1.
Laser beams and electron beams (Sections 27.6 and 27.7) are used effectively
to harden both small and large surfaces, such as gears, valves, punches, and locomo-
tive cylinders. These methods are also used for through hardening of relatively small
parts. The main advantages of laser surface hardening are close control of power
input, low distortion, and the ability to reach areas that would be inaccessible by
other means. Capital costs can be high, however, and the depth of the case-hardened
layer is usually less than 2.5 mm.
Because case hardening is a localized heat treatment, case-hardened parts have
a hardness gradient. Typically, the hardness is a maximum at the surface and de-
creases below the surface, with a rate of decrease that depends on the composition
of the metal and the process variables. Surface-hardening techniques can also be
used for tempering (Section 4.11), to modify the properties of surfaces that have
been subjected to heat treatment. Various other processes and techniques for surface
hardening, such as shot peening and surface rolling, improve wear resistance and
other characteristics (Section 342).
Decarburization is the phenomenon in which alloys containing carbon lose
carbon from their surfaces as a result of heat treatment or of hot working in a
medium, usually oxygen that reacts with the carbon. Decarburization is undesirable
because it affects the hardenability of the surfaces of the part (by lowering its carbon
content). It also adversely affects the hardness, strength, and fatigue life of steels, by
significantly lowering their endurance limit. Decarburization is best avoided by pro-
cessing the parts in an inert atmosphere or a vacuum, or by using neutral salt baths
during heat treatment.

Annealing
4.l I
Annealing is a general term used to describe the restoration of a cold-worked or
heat-treated alloy to its original properties-for instance, to increase ductility (and
hence formability) and reduce hardness and strength, or to modify the microstruc-
ture of the alloy. The annealing process is also used to relieve residual stresses in a
manufactured part, as well as to improve machinability and dimensional stability.
The term “annealing” also applies to the thermal treatment of glasses and similar
products, castings, and weldments.
The annealing process consists of the following steps:
I. Heating the workpiece to a specific range of temperature in a furnace;
2. Holding it at that temperature for a period of time (soaking), and
3. Cooling the workpiece, in air or in a furnace.