| With development of construction machinery manufacturing industry toward "high-end, high-tech, high-value-added" in China, construction machinery was proposed to reducing weight, energy conservation, improving strength and abrasion resistance. As 1200MPa high-strength wear-resistant steel is becoming popular in excavator-bucket manufacturing, welding technology ought to be promoted to meet this challenge. It is prone to appearing cold cracks and brittle fracture in the welding of 1200MPa high-strength wear-resistant steel, for its high hardenability and crack sensibility. In this thesis, B-HARD360CFA high-strength wear-resistant steels used in excavator bucket were welded by gas shielding arc welding (GMAW) without pre-heating and heat treatment. The cold cracks, microstructure, microhardness and matching for strength of welding joints were studied to reveal the impact of welding materials and welding parameters on the microstructure and cold crack sensibility.The y-slit type crack test was conducted to analyze the effect of matching for strength of welding wires and heat input on crack sensibility of B-HARD360CFA high-strength wear-resistant steel joints by GMAW. Under the conditions of nominal strength matching coefficient 0.42 and a large range of heat input (9.7-45.2kJ/cm), results indicated that the rate of surface cracks was 0, while the rate of section cracks was less than 20%, which proved that under matching decreased cold crack sensibility of welding joint. Furthermore, the rate of section cracks could be reduced below 10%, when the heat input varied from 11.7-16.5kJ/cm. Crack initiation and extension were observed by means of optional microscope and scanning election microscopy. It has been found that all cracks originated from fusion zone self-terminating after extending paralleling the fusion line, as well as some micro-cracks extending to the weld metal.Microhardness test and microstructure experiment were conducted to analyze the microhardness and coarse grain zone of heat affected zone. Conclusion could be drawn to that when the heat input rose above 12.3kJ/cm, the maximum microhardness was below 280HV, the value recommended by IIW. It benefited to decrease cold crack sensibility of B-HARD360CFA joint, when the heat input was below 16.5kJ/cm, as the width of soft zone controlled within 3mm, which was less than 25% of the thickness. Weakening the toughness of the joints and increasing the cold crack sensibility, the net-like microstructure was easily introduced in the coarse grain heat affected zone (CGHAZ), with above 19.7kJ/cm heat input.With heat input and inclusions on microstructure taken into consideration, microstructure in weld metal and fusion zone were observed and the relationship between microstructure and cold cracks was studied. Weld metal consisted of acicular ferrite (AF) and proeutectoid ferrite (PF). The grain size and proportion of AF and PF were mainly influenced by heat input. The percentage of AF in weld metal surpassed 50%, when the heat input was above 19.7kJ/cm, increasing the rate of cold crack and once heat input rose to 19.7kJ/cm, lath microstructure would appear in fusion zone. Connection could be made between lath microstructure and net-like microstructure, which was composed by proeutectoid ferrite in weld metal and HAZ, during the heat input climbing to 24.9kJ/cm. |
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