| In recent years,the gradual popularity of private cars has not only greatly promotes the rapid development of the automobile manufacturing industry,but also strongly stimulated the explosive growth of the consumption of stamping die products.Automobile die materials mainly use forging alloy die steel,it has a long manufacturing cycle,low material utilization and poor economic efficiency.And casting alloy steel dies have gradually replaced forging alloy steel dies due to the advantage of near-final forming,simple process,reasonable material utilization,short manufacturing cycle,and low cost.Therefore,They have important engineering and economic value in realizing "casting instead of forging" for automobile and other stamping dies.In the casting process of the manufacturing process,surface defects including blow hole,slag inclusion,and dispersed shrinkage of the casting die steel are inevitable.At the same time,casting die steel is prone to surface damage such as chipping,wear,and deformation during long-term service.These defects and damages will directly cause the scrap of casting alloy die steel castings or dies,resulting in huge economic losses.Therefore,the repair of these defects and damage is a major engineering problem facing the industry.The welding repair method of casting die steel castings and dies,the characterization and performance evaluation of the repair zone microstructure are still the scientific issues that need to be solved urgently.In this paper,the research object is a new type of casting die steel YBD-4,and the microstructure and mechanical properties of the new type of casting die steel's casting defects and service damage repair zones are studied.Including the study of the influence and mechanism of die steel microstructure on the welding repair performance,the optimization of welding process parameters to improve the mechanical properties of the welding repair zone,the comparative study of the effect of heat treatment process on the microstructure and mechanical properties of the repair zone,finite element simulation analysis of the temperature field and stress field of the die steel welding repair zone,research on theoretical model for evaluation of die steel welding repair zone,and surface strengthening to improve the performance of die steel welding repair zone.Based on a newly developed method for repairing casting defects,this article carried out a research on the repairability of a new type of cold working die steel YBD-4,which is defined as the round pit repair method.Simulating defects such as blow holes and dispersed shrinkage on the die surface,the influence of round pit size on the microstructure and hardness was investigated.The research results show that the size of the round pits can significantly affect the microstructure and microhardness distribution of the repair zone.The size of the pits increases,and the microstructure gradually transforms from fine acicular martensite + a small amount of retained austenite to coarse acicular martensite + retained austenite.The hardness fluctuations gradually become smaller,and the maximum hardness decreases significantly.Controlling the size of the pit can adjust and control the performance matching of the repair zone and the base metal.The distribution of microhardness in the repair zone of casting die steel defects shows that the maximum microhardness,the minimum microhardness and the base metal microhardness are the three main characteristic parameters,which are also the key parameters affecting the wear resistance of the repair zone of casting die steel.This paper establishes a performance evaluation method for the welding repair zone of die steel based on three main characteristic parameters: maximum microhardness,minimum microhardness and base metal microhardness,and then a theoretical model of microhardness fluctuation coefficient was proposed.The fluctuation coefficient model was used to evaluate the performance of round pit repair zones with different sizes and different heat treatment processes.The experimental results show that when the diameter of the round pit is controlled around ?20 mm,the best performance matching can be obtained with the minimum fluctuation coefficient.The heat treatment process can reduce the fluctuation coefficient and improve the die performance.During the long-term service,surface scratches and cracks are the main failure mode of the casting dies.In view of the above problems,the microstructure and properties of the service damage repair zones of the new casting die steel YBD-4 were studied.The results indicate that the hardness of the weld zone is much higher than that of the base metal,656 HV,and the maximum value is up to 863 HV,and the hardness of the welding repair zone gradually decreases from the weld to the tempered zone,and finally decreases to 300 HV.When the heat input is 6.6 k J/cm,the hardness fluctuates and the fluctuation coefficient is smaller as compared with other heat inputs,and a better repair performance can be obtained.In the process of welding repair,welding heat input affects the temperature field of the repair zone,and then affects the microstructure evolution as well as the residual stress distribution in the repair zone.In order to reveal the influence of temperature field on the residual stress distribution in the welding repair zone,the commercial finite element software ABAQUS was used to simulate the influence of temperature field on the residual stress distribution in the damage repair of the new casting cold working die steel.The calculation results show that preheating can effectively improve the residual stress in the repair zone,and the residual stress under non-preheating condition is twice higher than that under preheating at 300°C.By simulation calculation,the optimal welding preheating temperature was determined to be 300°C.The surface hardness of welding repair zone of casting die steel determines its wear resistance.Therefore,improving the surface hardness can improve its wear resistance and further improve the service performance of the repair zone.In this study,both shot peening and spinning strengthening were used to strengthen the service damage repair zones.Then the mechanical property measurement of the surface hardening layer were carried out.The results show that the microhardness of the welding softening part in the quenched and tempered repair zone is significantly improved,and the microhardness is increased by 18% after shot peening and 40% after surface spinning strengthening.The wear resistance of the repair zone was tested by pin-disk test and the results show that the wear performance of the softened zone significantly decreases by 41%.Both methods can introduce a certain degree of residual compressive stress into the surface layer and adjust the surface stress distribution.The residual compressive stress of 50-140 MPa was introduced after shot peening,and the compressive stress of 300-560 MPa was introduced by the surface spinning strengthening.The improvement of surface strength and the introduction of residual compressive stress are helpful to improve the service life of casting dies.In summary,welding repair can make up for casting defects and service damage of casting dies,but the size of the repair zone and preheating process significantly affects the repair performance and repair effect.Based on the maximum microhardness,minimum microhardness and base metal average microhardness of the welding repair zone,the fluctuation coefficient model can evaluate the repair effect,which is verified by the wear test results.The research results of this paper provide a scientific basis for the evaluation of defects and damage repair performance of the new casting die steel,and provide an evaluation method and theoretical model for the evaluation of repair performance,which plays a good role in promoting the large-scale industrial application of the new casting die steels in the future. |
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