| Low carbon cast martensite stainless steel is a kind of material which is widely used as the large-medium hydraulic turbine runner at home and abroad because of its excellent hardenability, excellent mechanical properties at room and low temperatures, high corrosion fatigue strength and fracture toughness at static and dynamic loading and excellent service performance such as cavitation erosion resistance and abraded erosion resistance. The corrosion fatigue failure caused by the non-uniformity of microstructure and properties in welded zone becomes more serious with the maximization and high efficiency of the hydraulic turbine. The study on the evolution of microstructure and properties has an important significance to prolonging the service life of the hydraulic turbine and the power generation efficiency improvement of hydropower station.In the present study, the continuous cooling transformation diagram of welding heat affected zone (SH-CCT) was measured by thermal simulation for ZG00Cr13Ni5Mo cast low carbon martensite stainless steel. The microstructure of the steel in welding heat affected zone was simulated through a proper heat treatment. Microstructural characteristic was analyzed by optical and transmission electron microscopy. Mechanical and fatigue properties were tested. The influence of chemical composition and microstructure on properties was studied. The obtained main results were as followed.1. The microstructure of ZG00Cr13Ni5Mo stainless steel in simulated weld heat affected zone was composed of lath martensite and 8 ferrite. High density dislocations exist in laths of martensite and form cellular structures due to dislocation entanglement. The amount of 8 ferrite increased with increasing the cooling rate after simulating weld heat affected zone.8 ferrite formed in the simulated weld heat affected zone can not be eliminated through the following heat treatment. Therefore, the cooling rate after welding should be controlled in order to decrease the amount of 8 ferrite. There was a few film-like austenite in the ZG00Cr13Ni5Mo martensite stainless steel and its amount was about 0.6%, which was distributed between the laths of martensite.2. According to SH-CCT, the cooling rate after simulating weld had little effect on the martensite transformation start temperature (Ms) of ZG00Cr13Ni5Mo cast martensite stainless steel, and Ms was about 255℃, which was in the Ms range of 13-4 steel.3. The strength, toughness and ductility of heat affected zone were lower than those before weld. The more quick the cool rate after simulating weld, the more remarked the decrease in the strength, impact toughness and ductility. Therefore, the cooling rate after welding should be controlled. The combined mechanical properties of heat affected zone were remarkably improved by 480℃×2h tempering after welded. The larger cooling rate after welding was beneficial to improve the combined mechanical properties of heat affected zone.4. The fatigue strength (σ-1) of ZG00Cr13Ni5Mo stainless steel subjected to tempering for 2h at 480℃after simulating weld in the simulated Yangtze River water was decreased compared with that of original material. The decrease inσ-1 was larger in material cooled at a larger cooling rate after simulating weld. Therefore, it is necessary to reasonably decreasing the cool rate after welding process for improving the ability of anti-fatigue crack initiation in the weld heat affected zone.5.δferrite in ZG00Cr13Ni5Mo stainless steel could be avoided or its amount decreased through decreasing the ferrite former of Si and increasing the ratio of w(Nieq)/w(Creq), which is beneficial to improvement of strength. The decrease in impurity contents of N, P and S in ZG00Cr13Ni5Mo stainless steel through AOD refining technology and 480℃×2h tempering after welding can obviously improve the toughness and ductility of the heat affected zone.
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