MIC Green Book

CARBURIZED STEELS
Carburizing and carbonitriding are heat treatment processes that result in very hard surfaces. They are
commonly 55-62 HRC. The benefits of shot peening carburized steels are as follows:
• High magnitudes of compressive stress of ~ 200 ksi (1379 MPa) or greater offer
excellent fatigue benefits
• Carburizing anomalies resulting from surface intergranular oxidation are reduced.
Shot hardness of 55-62 HRC is recommended for fully carburized and carbonitrided parts if maximum
fatigue properties are desired.
Application Case Study
HIGH PERFORMANCE CRANKSHAFTS
Crankshafts for 4-cylinder
high performance engines
were failing prematurely
after a few hours running on
test at peak engine loads.
Testing proved that gas
carburizing and shot peening
the crankpins gave the best
fatigue performance (Figure
2-2). Results from nitriding
and shot peening also
demonstrated favorable
results over the alternative
to increase the crankpin
diameter [Ref 2.3].
Figure 2-2 Comparison of Shot Peened
Nitrided and Gas Carburized Crank Pins
DECARBURIZATION
Decarburization is the reduction in surface carbon content of a ferrous alloy during thermal processing.
It has been shown that decarburization can reduce the fatigue strength of high strength steels (240 ksi,
1650 MPa or above) by 70-80% and lower strength steels (140-150 ksi, 965-1030 MPa) by 45-55%
[Refs 2.4, 2.5 and 2.6].
Decarburization is a surface phenomenon not particularly related to depth. A depth of 0.003 inch
decarburization can be as detrimental to fatigue strength as a depth of 0.030 inch [Refs 2.4, 2.5 and 2.6].
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C H A P T E R T W O
RESPONSE OF METALS
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