Study On The Fatigue Properties Of Welded Joints Of HG785D Steel

As a kind of effective method for connecting component, welding technology can be used for heating or pressurizing the artifacts, bringing the material to a permanent metallurgical combination. Conventional welding methods are generally based on the strong or low strength matching principle to choose a base metal strength of welding material for welding, but it often brings many problems for welding the high strength steel, such as crack can not well match with the strength and toughness, or the performance of the fusion and heat-affected zone is not ideal, and so on.In this paper, the HG785 D high strength low alloy steel was considered as the research object, for the mechanical performance declined problem which often appeared in welding the high strength low alloy steel, a new scheme of the welding method was proposed, the new scheme suggested the use of two kinds of different intensity of welding material superimposed up for welding, forming a composite welding structure with “soft layer + hard layer + soft layer”, adding a high strength of the buffer layer between the two soft layer, in order to improve the mechanical properties of welded joints. A series of experiments and analysis were performed on the HG785 D steel welding joint under the conventional welding method and the new technologies. The results are as follows:(1) The thermal cycling simulation of the welding process of HG785 D steel is carried out. The simulation results agree well with the measured value, which can provide guiding value for the actual welding process and specification.(2) The impact toughness test was carried out for the joint of HG785 D steel in two kinds of welding methods, the results showed that the absorbed energy differs very little in the heat affected zone of the two kinds of welded joint, while the absorbed energy under the new technology is greater than it under the traditional process in the weld zone of welded joint,which confirmed that the addition of a buffer layer in the weld can be to improve the toughness of weld metal.(3) The fatigue crack extended test was made on the two kinds of welded joints of HG785 D steel and the base material, the crack propagation rate of samples from fast to slow is followed by the conventional welding heat affected zone’s, the composite welding heataffected zone’s, parent material’s, the conventional welding weld zone’s and the composite welding weld zone’s, the fatigue life of composite weld zone has been significantly prolonged.(4) By doing SEM for the fracture surface of all samples, it was obtained that the base metal, the conventional welding weld zone and the composite welding weld zone is dominated by ductile fracture, the conventional hybrid welding seam area is mainly ductile fracture, the conventional welding heat affected zone and the composite welding heat affected zone is from ductile fracture to cleavage fracture.(5) The relationship of the welded joint’s fatigue properties with it’s microstructure and hardness was discussed. The general performance is that: the more content of pearlite, the smaller grain size, the more uniform microstructure and the higher hardness will result in the slower fatigue propagation rate, on the contrary, the more content of ferrite, the bigger grain size, the nonuniform distribution and the lower hardness will cause the faster fatigue growth rate.