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The High Nitrogen Martensitic Stainless Steel (HNMS) was subjected to threedifferent austenitizing cycles of 1050$^\circ$C, 1075$^\circ$C and1100$^\circ$C followed by subzero treatment at -70$^\circ$C. The fraction ofretained austenite has been reduced after sub-zero treatment as revealed bymicrostructural evolution. The material was subsequently tempered at differenttemperatures ranging from 180$^\circ$C to 650$^\circ$C and the change inmicro-structure, hardness, tensile strength and toughness were investigatedafter each heat treatment cycle. Optical microscopy, electron microscopy withEDS and X-Ray diffraction techniques were used to characterize the material.This has showed the constituents of microstructure were lath martensite,precipitated metal carbides of type $M_{23}C_6$, $M_7C_6$ and carbo-nitrides.Hardness, tensile testing and Charpy impact testing were carried to evaluatemechanical properties after the heat treatment which has showed the bettermechanical properties for the samples solutionised at 1075$^\circ$C. Secondaryhardening has been observed on tempering above 450$^\circ$C which can beattributed to the precipitation of secondary phase inter-metallic compounds.Hardness attains a peak value at peculiar temperature range after which itdecreases on further tempering which is most likely because of the loss ofcoherency of the precipitates with the metal matrix. This has been furtherconfirmed by the XRD of the specimens before and after tempering. The studystablishes the structure-property correlation of HNMS for different heattreatment cycles. The results indicate that a good combination of hardness andstrength can be achieved after solutionizing at 1075$^\circ$C followed bydouble tempering at 525$^\circ$C.

Microstructural Characterization and Mechanical Property Evaluation of High Nitrogen Martensitic Stainless Steel Subjected to Heat Treatment