  The High Nitrogen Martensitic Stainless Steel (HNMS) was subjected to three
different austenitizing cycles of 1050$^\circ$C, 1075$^\circ$C and
1100$^\circ$C followed by subzero treatment at -70$^\circ$C. The fraction of
retained austenite has been reduced after sub-zero treatment as revealed by
microstructural evolution. The material was subsequently tempered at different
temperatures ranging from 180$^\circ$C to 650$^\circ$C and the change in
micro-structure, hardness, tensile strength and toughness were investigated
after each heat treatment cycle. Optical microscopy, electron microscopy with
EDS 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 evaluate
mechanical properties after the heat treatment which has showed the better
mechanical properties for the samples solutionised at 1075$^\circ$C. Secondary
hardening has been observed on tempering above 450$^\circ$C which can be
attributed to the precipitation of secondary phase inter-metallic compounds.
Hardness attains a peak value at peculiar temperature range after which it
decreases on further tempering which is most likely because of the loss of
coherency of the precipitates with the metal matrix. This has been further
confirmed by the XRD of the specimens before and after tempering. The study
stablishes the structure-property correlation of HNMS for different heat
treatment cycles. The results indicate that a good combination of hardness and
strength can be achieved after solutionizing at 1075$^\circ$C followed by
double tempering at 525$^\circ$C.
