INFLUENCE OF SMALL DEVIATIONS IN STEEL CHEMICAL COMPOSITION ON HARDENABILITY

摘要

Chemical composition of steels used for heavy-duty parts of car gearboxes is usually defined by technical standards, in which certain deviations from nominal composition are allowed. However practical experience shows that in many cases even such small deviations can significantly influence the steel's behaviour during hardening. In particular, the final hardness and hardness profile are often affected by this phenomenon. Gear pinions used in automobile gear boxes were case hardened using a low pressure carburizing process. Because of significant variation in final hardness values, the scrap rate was more than 20%. Analysis of the entire manufacturing process showed that the hardness variation results from fluctuation of content of several alloying and accompanying elements, especially of the copper content which varied from 0.025 to 0.050 % which does not exceed the tolerances of the applicable technical standard. Using several technical updates, the hardening process quality was stabilized and the scrap rate was reduced to less than 2 %.