Investigation Of Strength-toughness Of B-HARD360 High Strength Wear-resistant Steel Undermatched Joints By GMAW

High strength low alloy wear resistant steel have been widely used in bridge construction, engineering machiney, vehicles and vessels industries etc., due to their advantages of high wear resistance, high strength and toughness. In recent years, the research and production of high strength low alloy wear resistant steel in our country has made great progress, gradually narrowing the gap on product quality and performance compared with the developed countries. Welding is a indispensable way of processing and manufacting in engineering field, so the research in welding this kind of steel is important. The welding of high strength steel is complicated, major problems may be encountered in joint during welding, including mismatching of toughness and strength, embrittlement and softening problems in the heat affected zone (HAZ), and so on, which become limitations in extending the application of high strength low alloy steel (HSLA) and improving production efficiency. In this paper, HSLA steel B-HARD360 was welded by gas metal arc welding (GMAW) processes without pre-heating. The microstructure and mechanical properties of welded joints were investigated to reveal the relation of welding parameters, welding material, microstructure and performance of joints. Welding joint design adopts the form of "V-shape" butt, choose "low matching" welding wire and the protection gas is CO2.Tensile test results show that the tensile samples all fractured within the weld. Along with the welding heat input increased from 9.7 kJ/cm to 15.5 kJ/cm, the average tensile strength of joint decreased from 694MPa to 658 MPa. Impact test results show that the influence of the welding heat input on impact toughness of weld area is small at room temperature, the average impact absorbing energy is more than 130J, which shows a good impact toughness. When the test temperature is 0℃ and-20℃, the impact toughness value of weld metal decreases gradually with the increase of the heat input. When the test temperature fell to-40℃, joint at the heat input of 12.9kJ/cm owned the highest impact absorbing energy of 98J; yet the joint with heat input of 15.5kJ/cm had the minimum impact absorbing energy of 68J.From weld metal to heat affected zone, the microhardness increased first and then decreased to a minimum, finally reach the base material’level. The average hardness of weld metal is low (220HV-240HV); the peak hardness appears in the heat affected zone,which is a certain distance away from the fusion line; each exists a range of softening zone, the softening degree of heat affected zone under different heat input are similar, which is about 220HV; the hardening region of the first welding bead disappeared, and the softening degree of weld metal and softening zone increased due to the thermal interaction of the after welding bead.The microstructure, impact fracture surface morphology and composition in weld metal were investigated by means of optical microscope and scanning electron microscope (SEM). The microstructure of weld metal are all composed of proeutectoid ferrite, side of lath ferrite and acicular ferrite. With the increase of the welding heat input, the content of acicular ferrite in weld metal reduce gradually, as heat input increased from 9.7 kJ/cm to 15.5 kJ/cm, the content of acicular ferrite decreased from 79.7% to 69.3%. The coarse grain zone composed of low carbon martensite, with the increase of the welding heat input, the grain coarsening. Up bainite and M/A group exist in heat affected zone, which is bad to the mechanical properties of weld joint. There are carbide precipition in the tempered zone, which become the softening area of the joint; with the increase of welding heat input, grains in the HAZ softening area coarsening, and carbide gathered further more.The analysis of the impact fracture morphology under different heat input shows that the fibre area of impact fracture is toughening fossa fracture characteristics, radiation area is the mixed fracture characteristics of toughening fossa fracture and cleavage fracture. With the decrease of test temperature, fracture characteristics changed from toughening fossa fracture+quasi cleavage fracture mode to quasi cleavage fracture+cleavage fracture. Under larger the welding heat input, the cleavage facet size of the weld metal at low temperature is bigger, cleavage crack expands easier, so impact absorbing energy decrease significantly.This paper researches the microstructure characteristics and mechanical properties of welded joint without preheating of HSLA steel B-HARD360, the results provides experimental and theoretical basis for improving the application of high strength low alloy steel and promoting the efficiency of production.