Effects of chromium and nickel additions on the structure and properties of carburized and hardened low carbon steels

Four 0.10% carbon steel samples containing about 0.35 to 0.85% chromium and about 1.20F6 nickel singly or in combination were carburized in a natural Titas gas atmosphere at a temperature of 1223 K and pressure of about 0.1 M Pa for rime periods ranging from 1 to 5 hours and quenched in 10% brine fro m the carburizing temperature of 1223 K after pre-cooling to 1133 K followe d by tempering at a low temperature of 433 K. The structure and properties of the carburized and heat treated specimens were studied systematically by means of optical microscopy, surface hardness and microhardness measuremen ts. X-ray diffractometry and impact test. It was found that both chromium a nd nickel promote the formation of retained austenite in the case of carbur ized and hardened steels, chromium being more effective. It was also found that nickel and chromium both refine the martensite platelets (needles) in the case of carburized and hardened steels, with nickel being more effectiv e. Microhardness measurements have shown that chromium in solution and as c arbide increases and nickel in solution decreases the hardness of the case, while both of them increases the core hardness although chromium is more e ffective than nickel as far as core hardness is concerned. The hardenabilit y was found to increase with the increase of austenite grain size and with the extent of carbon penetration in the case of carburized steels. Impact t est results have shown that nickel and chromium reduce the roughness of the core of low carbon steels in the carburized and hardened condition. Chromi um is more detrimental to toughness than nickel.