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Materials 51
Figure 2–13 300
Tensile strength
The effect of thermal-
mechanical history on the 250 600
Tensile and yield strength, kpsi 150 Brinell hardness 400 Brinell 60 Percent elongation and reduction in area
mechanical properties of AISI Yield strength
4340 steel. (Prepared by the 80
International Nickel Company.) 200 500

100 300 Reduction area 40
20
Elongation
50 0
200 400 600 800 1000 1200 1400
Tempering temperature, °F

Tensile Yield Reduction Elongation Brinell
Condition strength, strength, in area, in 2 in, hardness,
kpsi kpsi % % Bhn
Normalized 200 147 20 10 410
As rolled 190 144 18 9 380
Annealed 120 99 43 18 228

Case Hardening
The purpose of case hardening is to produce a hard outer surface on a specimen of low-
carbon steel while at the same time retaining the ductility and toughness in the core.
This is done by increasing the carbon content at the surface. Either solid, liquid, or
gaseous carburizing materials may be used. The process consists of introducing the part
to be carburized into the carburizing material for a stated time and at a stated tempera-
ture, depending upon the depth of case desired and the composition of the part. The part
may then be quenched directly from the carburization temperature and tempered, or in
some cases it must undergo a double heat treatment in order to ensure that both the core
and the case are in proper condition. Some of the more useful case-hardening processes
are pack carburizing, gas carburizing, nitriding, cyaniding, induction hardening, and
flame hardening. In the last two cases carbon is not added to the steel in question, gen-
erally a medium carbon steel, for example SAE/AISI 1144.
Quantitative Estimation of Properties of Heat-Treated Steels
Courses in metallurgy (or material science) for mechanical engineers usually present
the addition method of Crafts and Lamont for the prediction of heat-treated properties
9
from the Jominy test for plain carbon steels. If this has not been in your prerequisite
experience, then refer to the Standard Handbook of Machine Design, where the addi-
tion method is covered with examples. 10 If this book is a textbook for a machine

9 W. Crafts and J. L. Lamont, Hardenability and Steel Selection, Pitman and Sons, London, 1949.
10 Charles R. Mischke, Chap. 33 in Joseph E. Shigley, Charles R. Mischke, and Thomas H. Brown, Jr. (eds.),
Standard Handbook of Machine Design, 3rd ed., McGraw-Hill, New York, 2004, p. 33.9.