Mechanical Behaviors And Mechanisms Of Nitrogen Effect Of High Nitrogen Austenitic Stainless Steels

High nitrogen austenitic stainless steels (HNSs) possess 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. However, high nitrogen bearing also causes some drawbacks, such as brittle fracture at low temperature for HNSs. In this dissertation, the mechanical behaviors of the HNSs with different nitrogen contents were systematically and deeply investigated, and the mechanism of nitrogen effect in the steels were analyzed and discussed.Based on the phase diagram and the nitrogen solubility calculated using the thermodynamic model, the chemical compositions of the HNSs used in this study were designed to be as Fe-18Cr-16Mn-2Mo steels. The HNS ingots with nitrogen of 0.52% (M52), 0.66% (M66) and 0.81% (M81), respectively, were produced in an induction furnace and and an electro-slag remelting furnace under nitrogen atmosphere. In addition, a Fe-18Cr-13Mn-2Mo-N HNS with 1.2% nitrogen was produced by solid state nitriding at high temperature. The effect of hot deformation on microstructure and mechanical properties of the steels was studied. The results showed that avoiding nitride precipitation, keeping full austenitic structure and refining grain size were the keys to improve the mechanical properties of the HNSs. By controlling the heat treatment temperature, holding time and cooling rate could eliminate the ferrite formed during hot deformation and obtain single austenitic structure.Through studies on the microstructure and mechanical properties of the cold worked HNSs, the effect of nitrogen on the mechanical properties and work-hardening ability of the HNSs were analyzed. Tensile test showed that the yield strength (YS) and work-hardening ability of the HNSs were effectively increased with increase of nitrogen. The YS of the HNSs was rapidly increased with increase of cold deformation, but the increasing rate was decreased gradually with increase of cold deformation.As seen under optical microscopy, both mechanical twinning and dislocation slipping were the main mechanisms for HNSs at the initial deformation stage, and the mechanical twins obstructed the slipping strongly. With increase of cold deformation, the twins became bended. With further deformation, the mechanical twins were split by slipping lines and the twin boundaries became blurred. The TEM observation on microstructure of the cold deformed HNSs showed that the stacking fault energy (SFE) of the HNSs increased with nitrogen and the planar slipping of dislocations became more pronounced with increase of nitrogen. The mechanical twins formed with cold deformation and became smaller in size with increase of cold deformation, and width of the new twins were only tens nanometer with cold deformation up to 60%. After 60% cold deformation, the microstructure of the HNSs was still single austenite, as analyzed by X-ray diffraction (XRD) and TEM, and no strain induced martensite was found.Based on the above results, the mechanism of nitrogen effect on the HNSs was analysed. The short range ordering (SRO) and the lower SFE were considered to be the main mechanisms to promote the dislocation planar slipping. The lower SFE could cause the extension of dislocations, and prevent the cross slipping. The SFE of the HNSs was found to be increased with increase of nitrogen. At the same time, the SRO became the controlling mechanism with increase of nitrogen, which blocked the dislocation slipping and promoted the planar slipping strongly, which then increased the YS and work-hardening ability of the HNSs. According to the above analysis, a model to describe the relationship between the cold deformation and the YS was proposed, and the strengthening mechanism at different cold deformation stages and the effects of nitrogen were also discussed.The impact fracture behaviors of HNSs were studied by Charpy V-notch test. The results showed that typical ductile-to-brittle transition (DBT) phenomenon occurred in HNSs. The fracture surface consisted of flat facets surrounded by dimples, and the flat facets always contained much straight slip lines. M66 HNS and an 316LN stainless steel with 0.14%N were used to study the cryogenic mechanical behaviors and the mechanism of nitrogen effect. 316LN steel showed ductile fractures in the temperature range of 77-293K. XRD confirmed that lots of strain (or stress)-induced martensite existed near the fracture surface of 316LN steel, but no such transformation in M66 HNS.Tensile test was carried out in temperature range of 77-293K for M66 HNS and 316LN steel. The results showed that the temperature dependences of yield strength for the two steels were almost the same. The true fracture stress of 316LN steel increased rapidly with decrease of temperature. However, the true fracture stress of M66 HNS at 77K was even lower than that at 293K. XRD showed that almost all the austenite transformed into martensite for 316LN steel at 77K, but only austenite was found in M66 HNS. There was a three-stage work-hardening behavior in the instantant work-hardening rate vs. true strain curves for 316LN steel at 173K or below, which became more remarkable with decrease of temperature. However, no such behavior occurred in the M66 HNS, and a brittle fracture occurred for it without necking at 77K.The microstructures of the uniformed elongation section of 316LN steel and the M66 HNS were investigated by TEM. For the 316LN steel, the density of the mechanical twins increased with decrease of temperature, and the mechanical twins were cut by slipping bands. For the M66 steel, the planar slipping of dislocations was more pronounced at lower temperature, and the slipping band was distorted at the intersection point. Lots of stacking faults and paralleled planar slipping were formed with temperature decreased to 77K in the M66 steel. This indicated that the SFE should decrease with decrease of temperature, which promoted the twinning and planar slipping.Based on the above research results, the mechanisms of crack nucleation and growth in nitrogen bearing steels were analyzed from the view points of the strain-induced martensite transformation and the planar slipping of dislocations. For the 316LN steel, the strain-induced martensite was considered as the main mechanism to reduce the stress concentration, hinder the crack growth and prevent the brittle fracture. For the M66 HNS, however, the planar slipping accelerated the crack nucleation, then decreased the brittle fracture stress and promoted the brittle fracture to occur. A modified scheme was proposed to explain the fracture behavior of these two austenitic stainless steels.