Effect Of Laser Surface Treatment On Microstructure And Properties Of 9Cr Martensitic Heat Resistant Steel

Ultra-supercritical power generation technology can effectively improve coal-fired utilization rate and thermal efficiency,and has become the preferred introduction technology for power generation units in China.9Cr martensitic heat-resistant steel is the most important heat-resistant material for power station units because of its good thermal strength and low expansion coefficient.At present,in the process of localization,a lot of research on R&D work has focused on the optimization of heat-resistant materials in the power station and the organizational evolution of the service process,while the research on the inevitable defects and the later strengthening methods in the casting structure of large castings less.In view of this,this paper explores the effect of laser surface treatment on the microstructure and properties of martensitic heat-resistant steel,and evaluates the feasibility by repairing the effect of repair welding and strengthening.In this experiment,a new type of 9Cr martensitic heat-resistant steel was designed to simulate the cylinder material for ultra-supercritical generator sets.In the first round of ingot structure,it was found that there was a large amount of?-ferrite.After different homogenization annealing temperature,normalizing temperature and tempering temperature,the content of?-ferrite in the structure was significantly reduced.However,the total content of?-ferrite in the full field of view still exceeds the usable range of 3%.The heat-resistant steel of this component cannot support and follow the laser surface treatment experiment;after that,the composition of the original 9Cr martensitic heat-resistant steel is designed.Optimization and improvement,the existence of?-ferrite phase was not found in the as-cast microstructure,indicating that the improvement scheme is reasonable and feasible.Then the whole process heat treatment of homogenization+normalizing+tempering was carried out,and the effects of different tempering temperatures on the microstructure and properties of martensitic heat resistant steel under the same holding time were discussed.Experiments show that in homogenization annealing?heating temperature 1070?,holding time 10h,air cooling?+normalizing?heating temperature 1100°C,holding time 5h,air cooling?+tempering?heating temperature 700?,holding time 2h,air cooling?Under the heat treatment process,the microstructure with uniform microstructure,tempered martensite strips and the highest hardness,and the best heat treatment system for curing the matrix can be obtained.In order to simulate the repair welding behavior of 9Cr martensitic heat-resistant steel,the Fe-based alloy powder was laser-clad after the substrate was subjected to the whole heat treatment.The microstructure and hardness of the cladding layer and the matrix were evaluated by means of optical microscopy,scanning electron microscopy,X-ray diffraction and performance testing.The electrochemical behavior of the cladding layer and the substrate was compared by an electrochemical workstation.The results show that the cladding layer and the matrix have achieved good metallurgical bonding.The dispersed Cr₇C₃ and Cr₃C₂ are found in the cladding layer,which enhances the hardness of the cladding layer and its hardness is higher than the hardness of the matrix.After laser cladding,after stress relief annealing,the hardness of the cladding layer and the transition layer decreased significantly.At the same time,it was found that there was a slight formation of ferrite in the interface layer,which was caused by heat transfer.Electrochemical tests show that the corrosion resistance of the cladding layer is slightly better than that of the substrate.No obvious interaction of the interface is observed in the 3.5NaCl%solution soaking experiment.The cladding layer and9Cr martensitic heat resistant steel can be used.Good match.In order to explore the laser surface strengthening behavior of 9Cr martensitic heat-resistant steel,the surface of the substrate was laser-quenched after heat treatment.Through microstructure observation and hardness test,the microstructure and hardness distribution of each region after quenching were investigated.The results show that a fine total martensite layer is obtained after surface quenching and a higher hardness protection is obtained.