Microstructure And Mechanical Properties Of High Nitrogen Nickel-free Austenitic Stainless Steel

Austenitic stainless steel has been used widely as one of the most important structuralmaterials nowadays. Compared with the conventional austenitic stainless steels, high nitro-gen austenitic stainless steels possess lots of attractive mechanical properties such as highstrength and ductility, high work hardening ability, excellent corrosion resistance and lowmagnetic susceptibility. The excellent performance above let high nitrogen austenitic stain-less steels become an increasingly important new class of structural materials and exploringgood quality of high nitrogen austenitic stainless steel has been intensive focus in the re-searchers domestically and overseas.So far, the mechanical properties and microstructure ofhigh nitrogen austenitic stainless steel have been intensively studied, however, few attentionhas been paid to the influence of solid solution, cold rolling, warm rolling and hot rolling onthe mechanical properties and microstructure systematically. To study these systematically isconducive to develop a deeper understanding of the plastic deformation and related mecha-nism, which can provide theoretical basis for the reasonable application of high nitrogenaustenitic stainless steel.In this paper, solid solution-treated high nitrogen nickel-free austenitic stainless steel(HNS) samples were prepared by solution treatment. Cold-rolled HNS samples with differ-ent strains were produced by cold rolling at room temperature. By aging treatment, HNSsamples with different microstructures were produced. By rolling at400C and600C, thewarm-rolled HNS samples with different strains were prepared. By rolling at1140C, thehot-rolled HNS samples with different strains were prepared. The tensile testing system andnanoindentation were used to carry out a series of tests for mechanical properties. The X-raydiffraction, scanning electron microscopy and transmission electron microscopy were usedto characterize the specimens before and after deformation.The main research results of current study are listed as follows:1) Solid solution-treated HNS samples with single-phase and abundant intracrystalline twin were produced by solution treatment at1150C for8h and water quenching. The aver-age grain size is40m. Uniaxial tensile tests reveal that, with the increase of strain rate, theyield strength and ultimate tensile strength increase (0.2=528-712MPa, UTS=957-1004MPa) while the uniform plastic strain and tensile fracture strain decrease(u=44.6-72.2%, f=60.4-87.6%). It is noted that flow stress performs a relatively high strain rate sensitivity.The strain rate sensitivity and activation volume are0.03and17b3, respectively. This re-veals that the deformation mode is mainly the interaction between dislocation and twin. Sur-face and fracture of the deformed samples demonstrate that the relatively high strength, ul-tra-high elongation and the relation of strength and elongation with strain rate relate toabundant intracrystalline twin and the interaction between dislocation and twin caused bylow stacking fault energy.2) Cold rolled HNS samples with0%-70%rolling strain were produced respectively.The main structure of cold rolled HNS is twin. With the increase of rolling strain, the twinrefined and amount increased. Tensile tests reveal that, with the increase of rolling strain, theyield strength and ultimate tensile strength increase (0.2=589-1885MPa, UTS=1001-2236MPa), while the uniform plastic strain and tensile fracture ductility decrease (u=5.9-64.1%,f=12.3-86.6%). It is noted that the mechanical properties (UTS=2236MPa, f=12.3%)of70%cold rolled HNS have been the most outstanding results compared with the HNS re-ported before. The ultra-high strength and excellent elongation are ascribed to the refinedtwin. For50%cold rolled HNS, which exhibits a relatively high strain rate sensitivity (0.04)and low activation volume (6b3). This shows that the deformation of cold-rolled HNS ismainly controlled by the interaction between dislocation and twin.3) For50%cold-rolled HNS samples after aging treatment from200C to600C for3h, twin gradually disappears and grains become coarse. Aging at800C for3h, the obviousrecrystallization with the formation of equiaxed grains happen, meanwhile, the ferrite andCr2N precipitate. Tensile tests reveal that the yield strength and ultimate tensile strength of50%cold rolled HNS after aging from200C to600C can be improved significantly (0.2=1494-1545MPa, UTS=1834-1974MPa), while the uniform plastic strain has a slightly decrease and then increase (u=6.31-7.85%). The fracture ductility shows a decreasing ten-dency (f=9.04-9.60%). Under800C, the incensement of the strength is due to dislocationpinning caused by small Cr2N and segregation of nitrogen. Apparently decrease of elonga-tion is caused by coared-grain and the elimination of refined twin. At800C, the reduce ofstrength and elongation is caused by the grain boundary precipitation of Cr2N and segrega-tion of nitrogen, which result in embrittlement.4) Hot rolled HNS samples were prepared by30%,50%and70%rolling at1140C.Warm rolled HNS samples were prepared by50%and70%rolling at400C and600C, re-spectively. Tensile tests reveal that the strength of hot rolled HNS increase with the increaseof strain rate and rolling strain, while the elongation shows a decrease tendency. For warmrolled HNS, with the increase of strain rate, the strength still has an enhancement but theelongation decrese. Elevating the warm rolling temperature is harmful for strength and elon-gation. For hot rolled HNS, refining twin and abundant low-angle boundaries in large rollingstrain are contributed to the enhancement of the strength and the decrease of high-angleboundaries is responsible for the decrease of elongation. For warm rolled HNS, the refinedstructure in large rolling strain is beneficial for the increase of strength and the precipitationof banded ferrite is harmful for the elongation.5) Nanoindentation tests under different strain rates show that, compared with the solidsolution-treated HNS,70%cold rolled HNS exhibits significant creep deformation. Thecreep strain is related to the microstructure and loading strain rate. The creep strain and ratebecome much higher when the structure is refined and loading strain rate become higher.Creep deformation of70%cold rolled HNS is mainly dominated from the rapidly relaxationof the dislocations structures which generated in the loading regime. The high stability of thedislocation structures generated during loading should be responsible for the creep defor-mation in the solid solution-treated HNS.