| Cr-Ni-Mo series of super martensitic stainless steels with higher strength, toughness, good cavitation and water erosion resistance and weldability are increasingly applied in construction of nuclear power engineering, large turbine, high pressure feed water pump and oil country tubular goods. With the increase in application of Cr-Ni-Mo series of super martensitic stainless steel, the higher grade of strength and corrosion resistance is increasingly demanded. In recent years, N as cheap alloying element is widely used in austenitic and duplex stainless steel to replace Ni, not only increasing the strength notably, but also improving the corrosion resistance effectively. Regarding to this background, the present work is focused on changeing traditional quenching and tempering heat treatment system to quenching and partitioning (Q&P) heat treatment system on high nitrogen super martensitic stainless steels to improve its comprehensive mechanical properties and corrosion resistance.Based on0Cr16Ni5Mo-0.12N as tested steel, the effects of different Q&P heat treatment system on phase composition and the content, distribution and morphology of retained austenite were investigated by charactering the tested steel samples using laser scanning confocal microscope (LSCM), scanning and transmission electron microscope (SEM and TEM) and X-ray diffraction (XRD). The influence of different Q&P heat treatment system on mechanical properties and corrosion resistance of tested steel was examined by conducting tensile, Chary impact and electron chemical polarization tests. Finally, one heat treatment system which can synthetically improve the mechanical properties and corrosion resistance of super martensitic stainless steel was found. The Conclusion was obtained as follows:1. Tested steel normalized from1000℃have typical lath martensitic structure and a thimbleful of δ ferrite. N as strong austenite stabilizing element, adding0.12%can effectively suppress the occurrence of δ ferrite in0Cr16Ni5Mo steel. The Ms point of tested steel is164℃by measuring. N can falls down the Ms point as well as C. 2. After Q&P process, the microstructure of0Cr16Ni5Mo-0.12N steel consists of lath martensite and retained austenite dual phase structure. Q&P heat treatment is divided into quenching stage and partitioning stage. Through incomeplete quenching, tested steel acquires martensite matrix with high dislocation density and untransformed austenite with poor stability in quenching stage. N atom diffuses from martensite to untransformed austenite and presents interstitial solid solution condition in austenite phase, makes austenite stabilizing. The microstructure of tested steel includes of lath martensite matrix with dislocation substructure, retained austenite and twinned martensite. Film-like and lath-like retained austenite distributes in inter-lath and both of ends of martensite, which fully explains stabilization effect of N atom on austenite.3. Based on the most volume fraction of retained austenite be obtained when the quencheing temperature at80℃in Q&P process of tested steel, both of increasing or decreasing of quenching temperature falls down the volume fraction of retained austenite. The most volume fraction of retained austenite be obtained when the partitioning temperature at410℃, N atom can diffuses from martensite to untransformed austenite and shows uniform solid solution state in the partitioning process, makes the untransformed austenite can be existed steadily at room temperature. By the time, supersaturated untransformed austenite will translate into twinned martensite.4. The mechanical properties of0Cr16Ni5Mo-0.12N steel after Q&P process compared to that of0Cr16Ni5Mo-0.12N steel after normalizing, in addition to impact toughness worsen a certain extent with partitioning temperature increasing and partitioning time prolonging, tensile strength, yield strength and elongation of tested steel are superior to the normalized, accounting for the mechanical properties of martensitic stainless steel be improved after Q&P process. Q&P process parameters have little effect on yield strength, tensile strength and elongation of tested steel, but the impact toughness decrease significantly with the partitioning temperature and partitionging time increasing due to the appearance of twinned martensite.5. After Q&P process heat treatment on tested steel, because of N atom shows solid solution state, resistance to pitting corrosion of tested steel is obviously better than normalizing and tempering heat treatment. Q&P process parameters have conspicuous effect on the polarization curve. After QT is80℃, PT is380℃, Pt is20min heating treatment, the pitting potential value of tested steel is the highest as508mV among all heat treatment processes. |
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