Mechanism Of The Effect Of Microstructure And Nitrogen On Mechanical Behaviors Of High Nitrogenaustenitic Stainless Steels

Effects of microstructure and nitrogen content on the mechanical properties of highnitrogen austenitic stainless steels were studied systematically in this paper. On the basisof Mn18Cr18N and0Cr21Ni6Mn9N steels, microstructure controlling was conducted by aseries of heat treatment processes and the corresponding mechanical properties were tested.Optical metallography (OM) and SEM were employed to observe the deformingmicrostructure and fracture morphology after tensile and impact tests. Effect of nitrogencontent on mechanical properties and deforming behaviors was studied by Vickershardness testing, along with dents-nearby slip band configuration analyzing. Mechanismsof the effects of microstructure and nitrogen were investigated.The results of solution/annealing heat treatments conducted on generator guard ringmaterial Mn18Cr18N produced by hydraulic pressure showed that recrystallizationhappened between950℃and1150℃. The temperature of1050℃providing sufficientthermodynamics and kinetics for grain growth produced uniform grade6grain. For thespecimens with grain size through grade7to3, Vickers hardness dropped from280to260，and impact energy increased from3.35J·mm-2to3.92J·mm-2. So the characteristic ofexcellent toughness not affected by grain boundaries was testified in Mn18Cr18N steel.Annealed more than1h at1050℃, Mn18Cr18N could retain appropriate strength lowerthan500MPa and adequate ductility beyond60%for hydraulic pressure process.Observations of such phenomena as slip bands nearby Vickers dents, tensile/impactfractures and deforming microstructures on the longitudinal section of tensile specimensimplied that the strengthening mechanism of grain refining corresponded to the followingtheories: Cottrell’s Dislocation Piling-up and Work Hardening. Mn18Cr18N steelexhibited favorable compatibility of cracks without showing brittle fracture and theductility was not worsen by broken carbonitrides observed in the tensile fracture.Annealing twin manifested property of blocking slip deformation; therefore strengtheningeffect of such phenomenon could be concluded.0Cr21Ni6Mn9N specimens with similar grain size containing0.26%N,0.28%N and0.29%N respectively were tested by Vickers tester. The results showed that Vickers hardness increased as nitrogen content increased, by which effect of solution strengtheningof nitrogen was testified to be remarkable. Slip bands nearby Vickers dents became finerand denser as nitrogen content increased, meanwhile slip deformation transmitted smallerdomain. The reasons of nitrogen strengthening0Cr21Ni6Mn9N could be out of itsprompting short range order and dislocation emitting from grain boundary.