Effect Of Electropulsing And Annealing Treatment On Thermal Fatigue Property Of HHD Steel Weld

As the rapid development of the study on the cast die material recently years, the cast replace forge not only reduces the waste of resources and energy, but also reduces the cost of the mould and shortens the production cycle of the mould. However, it is hard to avoid defects such as shrinkage voids, shrinkage porosity, air holes, inclusion and so on, in the casting procedure of the mould. It also can not be avoided all kinds of failure forms during the mould processing and working. Besides, thermal fatigue (TF) is a leading failure of hot working die steels. So, repairing mould is worth and it is of great importance for the development of the economy to improve the TF property of hot working die steels. Welding methods are an important way to promote the working life of the mould, also can improve its TF property, which are the regular methods of repairing mould at present.It is researched that flux-cored wires which fit for repair welding or surfacing of the new type of HHD steels with TIG welding are adoptd in this thesis. The welding microstructure property is analysed, the weld of HHD steels with TIG welding is treated with annealing treatment and electric current pulsing treatment. From the view point of biomimetics, Optical microscope (OM), scanning electron microscopy (SEM), hardness tests and thermal fatigue tests have been used to study the factors and the mechanisms on TF resistance of HHD steel. During the study pulse current effect on microstructurees and properties, found that From the view ofbiomimetics, it is analyzed that the structure of welded of HHD steel with hot treatment and electric current pulsing treatment, as is studied that the influence of the welded zone function, thermal fatigue crack propagation velocity and hardness,under different parameter of hot treatment and electropusling treatment. According to the research the conclusion gose as follows:It is researched that flux-cored wires which fit for repair welding or surfacing of the new type of HHD steels with TIG welding are adoptd in this thesis. HHD steel is welded by biggish welding current, which adopted with direct current method. The HHD steel microstructures is tempering martensite and followed the bainite with a trace of remained austenites, there are a mass of carbides as a phrase separating out and being dispersed on the maxture. The typical weld microstructures are mainly the platelet martensite and followed the bainite with a trace of remained austenites. During the thermal fatigue test, the initiation of the thermal fatigue cracks end after about 500 thermal cycles, as the increase of hot cycle , the hardness of welded zone of HHD steel descends gradually, the thermal cracks get into the stage of steady propagation.It can be adopted to fine microstructure of weld of HHD steel by annealing treatment, and the thermal fatigue property is promoted as well. The optimum temperature of anneal treatent is 540℃. The hardness of as-welded zone of HHD steel is about between HRC43 and HRC45. The thermal fatigue crack of as-welded zone of HHD steel with 540℃annealing heat treatment is shorter than that without annealing heat treatment, the length of sample with optimum annealing treatent is about 67% of which without annealing heat treatment, after 2000 thermal cycles.It is can be promoted the thermal fatigue life of weld of HHD steel apparently, during thermal fatigue test by electric current pusle biomimetics treatment. Andelectopulsing treatment is a better way in progress the thermal fatigue propery of as-welded zone of HHD steel, in addition multi-pusle is better than single pusle current, at the current density of 90MA/m2. The hardness of as- welded zone of HHD steel descends from HRC45 to HRC35. The of thermal fatigue crack of propagating of HHD steel weld with optimum multi-pulse is lower than which without eletropulsing treatment, the length of sample with optimum eletropulsing treatment is about 76% of which without electric current pulsing treatment, after 2000 thermal cycles.The mechanism of action of electropulsing treatment mainly contains: finer microstructure, the decreace of dislocation by pulling dislocation motion .The thermal fatigue properties cannot be evaluated by a single mechnical property. It can achiveve the better thermal fatigue resistence only by the good mach strengthening, hardness and thermal stability. Then its properties are increased, the TF resistance is improved.