Narendra B. Dhokey¹; ¹Government College of Engineering

A new class of High Nitrogen Martensitic steels (HNMS) is used in corrosive environment and at subzero temperatures. High hardness and low level of retained austenite in such steel is required for high stress applications like ball bearings of space crafts. In this report, HNMS steel was hardened (1075oC) followed by low temperature treatment at minus 80oC and then double tempering (500oC). Characterization was done by SEM microscopy, hardness measurement and fracture toughness testing in accordance with ASTM E1820. The combined effect of enhanced precipitation and tough tempered martensitic matrix has been discussed in light of mechanism on fracture toughness improvement. It was concluded that there was a threefold increase in fracture toughness of HNMS after a deep cryogenic treatment.