Study On Improving The Thermal Fatigue Properties Of Welded HHD

Mold which takes an important role in the modern industrial production is not only an important equipment of non-chip processing technology , but also the key equipment of improving the quality of products, saving raw materials and reducing costs of production. Hot die steel works in very poor and harsh conditions, repeatedly heated by hot metal and coolled by coolling medium in the work which induced tensile/compressive stress and tensile/compressive strain on the cavity surface.All of these could easily make molds failure, such as thermal fatigue. Due to the complexity of mold cavity structure, long working hours and expensive costs of manufacturing process, the research of repairing and healing thermal failure has become particularly important. Therefore, in addition to study methods to improve the thermal fatigue properties of die steel,weld the failure die steel and study the methods to improve the thermal fatigue performance of welded die steel is also of great significance to the national economy. At present, the methods of improving the thermal fatigue properties of die steel mainly include heat treatment and electropulsing treatment , and welding is usually used to repair failure die steel.In this paper,we weld the HHD steel by TIG, analyse properties of microstructure of the welding specimen ,and treat the welding specimen by low-temperature annealing and electropulsing which are under different parameters and roles. Researching the factors of thermal fatigue properties for welded HHD steel ,as well as the impact of law and mechanism of thermal fatigue properties's parameters after low-temperature annealing and electropulsing through the changes of thermal fatigue cracks, hardness and resistance. Established a correlation model, using ANSYS software to simulate temperature field and stress field distribution of thermal fatigue specimen at the monment of transient pulsed current, providing related theoretical basis for the actual test results.The results show that non-uniform distribution of mesh carbides and a large amount of segregation appear on the welded zone of the welded HHD samples, especially the grain boundary of the welded zone. Because of the effect of the heat treatment which caused by multi-layer weld ,the grain size decreases from outside of the welded zone,and the number of grain boundary carbides and inclusions reduces.Low-temperature annealing could eliminate internal stress generated by cold processing, welding or casting. The carbide and composition of segregation in the grain boundary reduce by using low-temperature annealing.Under the same processing conditions, the surface of 520℃annealing sample has minor surface cracks compared to the sample which did not be treated, the main cracks grow slowly, the decreased trend of surface hardness for thermal fatigue specimen reduces and the growth rate of resistance lower.As a supplementary form of energy ,pulse current could provide energy for welded HHD steel to restore or heal fatigue specimen. In the same experimental conditions, specimen which is treated by trigger voltage of 7V and pulse current time for 460ms, initiation of it's thermal fatigue short cracks are later, and growth rate are slower through analysising the pulsed current parameters. the results show that as a result of current forming a partial high-density flow area in the cutting-edge incision of specimen, the most high temperature in the cutting-edge area of the sample heated up near the melting point of 1519℃, and then produced a strong stress field near the incision by using Ansys to simulate temperature and stress fields which are near the notch tip region of specimen. While thermal fatigue cycles increasing to 1,500, the hardness of specimen reached more than HRC 50 .After 500 ultra-high thermal fatigue cycles, cutting-edge thermal fatigue cracks has still not reached a destructive extent , the phenomenon of surface cracks were relatively minor, the thermal fatigue stability and hardness matched best ,the specimen had the best resistance to thermal fatigue.Processing specimen by the same pulse current parameters which are the trigger voltage of 7Vand pulse current time for 460ms after different thermal fatigue cycles to study the role of timing of electropulsing . It was found that dealing specimen with electropulsing at the end of 100 thermal fatigue cycles could weaken effect of fatigue properties ,which caused by the internal defects ,and lag behind the thermal fatigue crack initiation .With the thermal fatigue cycles increasing to 2,000, the effect of electropulsing becomes weak. Electropulsing specimen at the end of 500 thermal fatigue cycles,there will not be obviously expansion after thermal fatigue crack growth to a certain extent,and the increasing rate in hardness is the largest, upward trend of resistance slow,the sepcimen has the best thermal fatigue properties.Reasonable low-temperature annealing can improve microstructure of the welded hot die steel, refine grain,delay growth of thermal fatigue cracks, reduce the effect of microstructure to thermal fatigue properties and improve die life. Optimization of pulsed current parameters and the choice of the role of pulse current can delay the initiation time of thermal fatigue cracks, reduce the thermal fatigue crack growth rate, get preferable thermal stability and mechanical properties of fatigue and significantly increase the thermal fatigue properties of the sample, be propitious to enhance the service life of molds.