DEVELOPMENT of an ADVANCED STAINLESS CARBURIZING AND NITRIDING STEEL

A strong, tough, carburizable stainless steel is needed for high performance gear and bearing applications exposed to unprotected environments. Surface-hardened stainless steels, such as Pyrowear® 675 and CX13VDW, involve cumbersome carburization and heat treatment processes, and exhibit poor corrosion resistance after carburizing. Through-hardening stainless steels, such as Type 440C and BG-42®, suffer from low impact and fracture toughness and similarly poor corrosion resistance after heat treatment. In some instances the corrosion resistance of these conventional steels is no better than that of carburized alloy steels.

A-21® (UNS S41429) is a new stainless carburizing steel designed to create a deep, hard and stainless case and a strong, tough and ductile core. The unique chemistry results in a fine grain size and permits streamlined carburization and heat treatment. A-21® achieves a surface hardness of 64 Rc while maintaining its stainless properties. It also is nitridable and achieves a nitrided surface hardness of 70 Rc. Salt fog testing per ASTM B117 was done to compare the corrosion resistance of carburized A-21® with that of carburized Pyrowear 675, and 440C and BG42. All alloys were tested in the hardened condition having surface hardness values of 59 Rc to 63 Rc. Results indicate that carburized A-21® exhibited superior corrosion resistance in salt fog testing versus the other stainless steels while maintaining good core mechanical properties of: 138 ksi yield strength, 178 ksi UTS, 14% elongation, 47% RA, 48 ft-lb. CVN impact energy, and 87 ksi √in. fracture toughness.

A-21® could replace carburizing alloy steels, such as AISI 9310, that are currently coated for corrosion resistance, thereby eliminating the need to coat (and inspect), and replace carburizing and through-hardening “stainless” steels with a true stainless steel, thereby improving corrosion resistance in atmospheric, humid, and harsh (i.e., salt water) environments.