Method of Hardening Case-Carburized Steels

A 4-step heat treatment for hardening case-carburized steels has been developed and is here described and compared with conventional hardening. In step 1 of this new method, steel is case carburized and oil quenched; much higher than normal carburizing temperatures can be used. Step 2 eliminates retained austenite by heating in the bainite region, for example, 700 F for 2 h. Step 3 is a rapid austenitizing treatment followed by an oil quench, and step 4 a temper at ∼400 F.

Three representative grades of low-alloy steels were investigated to determine their response to this new hardening treatment and to explore the variables which influence the product. It is shown that the treatment permits faster carburizing without affecting the quality of the case by carburizing at least as high as 1900 F.

The new 4-step hardening method is shown to have the following principal advantages over conventional methods:

1. 1.
   It develops higher hardness in the outer case.
2. 2.
   It refines the grain size of both case and core.
3. 3.
   It makes it feasible to use higher carburizing temperatures and hence shorten the carburizing cycle.
4. 4.
   It develops a case microstructure relatively less affected by chemical composition of the steel or by carburizing variables. It offers promise of improving the performance of case-carburized parts (gears, bearing races, etc.).