Research Of Microstructure And Properties On High Nitrogen Austenitic Stainless Steels

High nitrogen austenitic stainless steels(HNSs) have some special properties, such as high strength and toughness, excellent corrosion resistance and nonmagnetic property. Furthermore, cost of the raw materials can be greatly reduced by taking nitrogen to replace nickel for stabilization of the austenitic structure, as a kind of structural material, it works with different load in the engineering application. However, the stability of high nitrogen austenitic stainless steels is poorer under the condition of high temperature. So its mechanical properties and structure stability has important research significance.Based on the as-cast 0Cr21Mn17Mo2 NbN high nitrogen steel as the research object, tensile strain hardening behavior were analyzed on the different heat treatment conditions of high nitrogen steels, and the fitting curve of the strain hardening and then its tensile fracture was analyzed; Under different tensile and compression deformation characteristics of the high nitrogen steel is analyzed, its deformation mechanism is discussed. Law of aging precipitation are analyzed, after heat treatment adjustments of aged treatment on high nitrogen layer structure, analyzes the characteristics of precipitates after heat treatment adjustment.The results are showed as follow:(1) Tensile strength of solid solution state from tensile specimen is the highest, the as-cast specimen take second place, the biggest total elongation reached 48.7% and 46.6%, respectively, showed a strong plastic deformation capacity. The mechanical propertie of aged specimen is the worst, the biggest total elongation was 9%. Strain hardening index of solid solution state is up to 0.83, strain hardening index of aged is the smallest. High nitrogen steels after solid solution state fracture surface is composed of different sizes of dimple fracture, the fracture mode is typically microporous gathered fracture, show the obvious characteristic of ductile fracture; Frature of aged high nitrogen steel is intergranular fracture, fracture show obvious characteristics of brittle fracture; Fracture of as-cast high nitrogen steels is mixed with fracture of transgranular fracture and intergranular fracture, it is a kind of quasi cleavage fracture.(2) At the beginning of the compression deformation, solid solution state of high nitrogen austenitic stainless steel mainly slip deformation, slip are mainly composed of a single slip. When defomatiom increased to 30%, deformation twin began to appear. When the deformation increased to 40%, parts of twin boundary was cut slip line. When further defomation increasement, the deformation dislocation make the slip line cut into squares.(3) The preheat treatments upon high-nitrogen austenitic stainless steels 0Cr21Mn17Mo2 NbN, including solution treatment at 1140℃ for 10 hours and then ageing treatment at 750℃, 850℃ and 950℃ for different holding times respectively were carried through. Then further heat treatments at 1020 ℃ for 2 hours and 4 hours respectively following water quenching. The precipitation of nitrides is more distinct at 850℃ under same a age temperature. With the increase of aged time, cellular precipitation developed from grain boundaries into grains and presented strip, short rod and particle shapes; After 850℃ age treatments for 8 hours and 16 hours, the heat treatment at 1020℃ for 4 hours, partly changed nitride flakes among the lamellar structures into particles. With the increase of holding time at 1020℃, the hardness of specimens decreased.