| The contradiction between continuously increasing requirement of stainless steels and limited Ni resources has become more and more serious in recent years. In addition, human organs exhibit irritability to austenitic stainless steel with Ni as bioengineering materials. As a result, great attention is now focused on resource-saving stainless steels, that is high-nitrogen stainless steel having not only low cost but also excellent mechanical properties, corrosion resistance, oxidization resistance and wear resistance etc.. However, the precipitation of carbides, nitrides and intermetallics may occur in austenite of this advanced steel material during thermal processes, welding and service at the elevated temperatures, which reduce austenitic stability. The precipitation of second phases will heavily damage various working and service properties of materials. Therefore, investigation on precipitation behavior and its effects on properties are of academic significance and practical importance.In the present study, a series of high-nitrogen austenitic stainless steels completely replacing Ni by Mn and N were smelted. Through optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermomechanical simulation, micro-hardness test and thermodynamic calculation, we systemically investigated the thermodynamic and kinetic behaviors of precipitation during isothermal aging, the relationship among alloy compositions, treatment conditions and second phase precipitation, the effect of austenitic stability on hot ductility and the effects of cold deformation and alloying elements of Mn and Mo on precipitation behavior. The major conclusions are as follows:1. The information provided by calculation of thermodynamic equilibrium diagram on Fe-18Cr-Mn-C-N system and Fe-18Cr-Mn-Mo-C-N system is consistent with the experimental results, indicating that this kind of thermodynamic calculation can provide thermodynamic instructions for alloy composition design, establishing of austenization and isothermal aging heat treatment conditions in Fe-18Cr-Mn-C-N system and Fe-18Cr-Mn-Mo-C-N system.2. M2N phase is the main precipitate in low carbon high-nitrogen Cr-Mn-N austenitic stainless steel during aging after solution treatment. The precipitation of M2N phase transits from initial granular precipitation at grain boundary to cellular precipitation growing towards to grain inside with increasing aging time. When small amount of carbon remains in the steels, thimbleful of granular M23C6 phase precipitates at grain boundary. The volume fraction of M23C6 precipitates does not increase with increasing aging time.3. The temperature region of precipitation is between 625℃and 925℃in Fe-18Cr-12Mn-0.48N steel. According to the isothermal precipitation kinetics curve of M2N mensurated from experimental data, the nose temperature of M2N precipitation is determined to be 800℃, the corresponding incubation period is 30min and the corresponding activation energy of precipitation is calculated as 296 kJ/mol.4. The hot ductility curve of Fe-18Cr-12Mn-0.55N high-nitrogen austenitic stainless steel can be divided into three regions:(1)the high-temperature brittlement region higher than 1150℃; (2)the high-temperature ductility region from 850℃to 1150℃; (3)the middle-temperature half brittlement region lower than 850℃. The high-temperature brittlement and middle-temperature half brittlement are caused by the appearances ofδferrite and the precipitation of M2N phase at austenitic grain boundaries, respectively. The excellent hot ductility region between the two brittlement temperature regions results from the stable single phase austenitic microstructure.5. Cold deformation accelerates the precipitation of M2N phase. Beside the precipitation firstly occurs at grain boundaries, the precipitation also occurs at twin grain boundaries in cold-deformated materials. Cold deformation also induces the precipitation ofσphase.6. After 30% cold deformation, the test steel begins to recrystallize at about 750℃. During aging at 750℃the precipitation occurs prior to recrystallization. Large numbers of the second phases preferentially nucleate at defect sites such as dislocations, grain boundaries and subgrain boundaries. The precipitation of these second-phase particles hinders the formation of recrystallization nucleus. During aging at 900℃, the recrystallization and precipitation simultaneously occur, the precipitation of second phases occurs both at subgrain and recrystallized grain boundaries and inside grains. M2N and M23C6 precipitation occurs at grain boundaries andσphase precipitation occurs inside grains.7. In Fe-18Cr-(18/12)Mn-0.4N low carbon high-nitrogen austenitic stainless steels, increasing of Mn content has no effect on the precipitation type and M2N phase is still major precipitate. Increasing of Mn content accelerates precipitation of M2N phase and increases upper limit temperature of M2N precipitation.8. The addition of Mo to Fe-18Cr-18Mn-0.6N high-nitrogen austenitic stainless steel causes the precipitation of another precipitate, intermetallicsχphase. The precipitation ofχphase occurs not only at grain boundaries but also inside grains. The nitrides also precipitate by the discontinuous cellular way in high-nitrogen austenitic stainless steel with Mo.
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