Page 140 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 4.10 Case Hardening l I9

Aging. Because the precipitation process is one of time and tem-
4OO_ 15O°C
perature, it is also called aging, and the property improvement is
known as age hardening. If carried out above room temperature, fe
D.
the process is called artificial aging. However, several aluminum
5 soo
alloys harden and become stronger over a period of time at room C/J
(D
temperature; this process is called natural aging. Such alloys are first 9
quenched, and then, if desired, they are shaped by plastic deforma- ‘a 200-
E
190°
tion at room temperature; finally, they are allowed to gain strength > \ _§
and hardness by aging naturally. Natural aging can be slowed by 345° 230°
refrigerating the quenched alloy (cryogenic treatment). 100-
In the precipitation process, if the reheated alloy is held at the
0.01 0.1 1.0 10 102 103 104
elevated temperature for an extended period of time, the precipi-
Aging time (hr)
tates begin to coalesce and grow. They become larger, but fewer, as
is shown by the larger dots in C in Fig. 4.21b; this process is called
FIGURE 4.22 The effect of aging time and
overaging, and the resulting alloy is softer and weaker. temperature on the yield stress of 2014-T4
There is an optimal time-temperature relationship in the aluminum alloy. Note that, for each temper-
aging process that must be observed in order to obtain desired ature, there is an optimal aging time for maxi-
properties (Fig. 4.22). Obviously, an aged alloy can be used only up mum strength.
to a certain maximum temperature in service; otherwise, it will over-age and so
lose its strength and hardness. Although weaker, an over-aged part has better dimen-
sional stability.

Maraging. This is a precipitation-hardening treatment for a special group of high-
strength iron-base alloys. The word maraging is derived from martensite age hard-
ening, a process in which one or more intermetallic compounds are precipitated in a
matrix of low-carbon martensite. A typical maraging steel may contain 18% Ni in
addition to other elements, and aging is done at 48O°C. Hardening by maraging
does not depend on the cooling rate; consequently, uniform and full hardness can be
obtained throughout large parts with minimal distortion. Typical uses of maraging
steels are in dies and tooling for casting, molding, forging, and extrusion (Parts II
and III).

4.l0 Case Hardenlng

The heat-treatment processes described thus far involve microstructural alterations
and property changes in the hulk of the material or component by means of
through hardening. It is not desirable to through harden parts, because a hard part
lacks the necessary toughness for these applications; a small surface crack could
propagate rapidly through such a part and cause total failure. In many cases, how-
ever, alteration of only the surface properties of a part (hence, the term surface or
case hardening) is desirable. This method is particularly useful for improving resist-
ance to surface indentation, fatigue, and wear. Typical applications for case harden-
ing are gear teeth, cams, shafts, bearings, fasteners, pins, automotive clutch plates,
tools, and dies.
Several case-hardening processes are available (Table 4.1):

a. Carburizing (gas, liquid, and pack carburizing);
b. Carbonitriding;
c. Cyaniding;
d. Nitriding;