Study On Microstructures And Mechanical Properties Of The MAG Welded Joints Of B-grade Steel And Q450NQR1 Steel

Weld joints of B-grade steel and Q450NQR1 steel which are the key material component chassis truck body are investigated in this paper. Treating surface with mechanically polishing and blasting shot which after boom was overall welded were added to the current under-frame crosstie welding process. The fatigue performance of Weld joints of B-grade steel and Q450NQR1 steel was improved by the new added process which verified by the three-point bending fatigue test. The result of the fatigue fracture was different from the actual work condition in the simulation of the factual tooling position under the condition that Q450NQR1 steel was used as base plate, B-grade steel(top center plate) used as web plate. To solve the problem, a three-point bending test in which the fatigue cracks were lead to spread B-grade steel laterally was put forward.Microstructure and mechanical properties for the weld joints of B-grade steel and Q450NQR1 steel under the current welding parameters were researched by tensile, flexural, impact, hardness tests, metallographic and SEM . The fatigue properties of the Lap welded joints of the B-grade steel/Q450NQR1 were investigated by the three-point bending fatigue test. The test result shows: joint tensile strength of the weld joints is better than B-grade steel materials, and the bending properties is good; the impact value near to B grade steel HAZ is lower;Organization is fine dendritic ferrite + pearlite in lap joint welded seam, and some martensite hardening phase was formed near the weld bond of B grade steel;Organization is proeutectoid ferrite +acicular ferrite in butt Weld, and near the HAZ of B grade steel side is ferrite + meshed pearlite with coarser grains. The fatigue cracks of the improved sample mostly occurred on the side of the B-grade steel that was corresponding to the situation of the actual work condition of the freight train. The fatigue strength of the original butt joints, the blasting shot joint, sanding joint were 201.89MPa, 225.36MPa,232.69MPa respectively under the condition that cycle index was 2×106 , fraction surviving was 95%, confidence was 90%. Compared with the welded, weld toe mechanical polished to make joint fatigue strength increased 12%, shot a whole increased by 15%; Under the cyclic loading, the cracks mostly occurred from the toe of the lap weld joints without welding grooves .