F. J. Martin, R. A. Bayles, P. M. Natishan, R. Rayne, Naval Research Laboratory, Washington, DC; G. M. Michal, F. Ernst, H. Kahn, A. H. Heuer, Case Western Reserve University, Cleveland, OH; M. Gaudette Koul, U. S. Naval Academy, Annapolis, MD
Materials properties of Alloy A286 surface hardened by Low-Temperature Carburization (LTC) are discussed. Surface modification of Alloy A286 (UNS S66286) is performed using a LTC strategy, whereby a high concentration of carbon in solid solution (more than 10 at% directly below the surface) is diffused into near-surface portions the metallic structures. The depth of the carburized layer is on the order of 30µm. In spite of the tempering required for LTC, the bulk mechanical properties of the alloy appear to be unchanged. Surface hardness increases significantly and is accompanied by a large compressive biaxial stress. During mechanical testing, the carburized layer remains ductile and intact until the ultimate tensile stress is reached, after which strain localization and case cracking appear prior to specimen failure. Corrosion behavior of the carburized alloy surface is that of a passive material, more resistant to corrosion in seawater than that of the base Alloy A286 alloy. Corrosion fatigue resistance of LTC-altered coupons also increases, presumably due to the combined influence of surface compressive stress and improved corrosion resistance. The carburized case has a negligible influence on hydrogen embrittlement sensitivity, showing neither benefits from residual surface compressive stresses nor any signs of premature surface cracking despite the existence of post-UTS case cracking.
Summary: Report on materials properties of low-temperature carburized high strength stainless steel alloy A286, showing improvements in corrosion resistance, corrosion fatigue, and a higher near-surface hardness.
