Thermal Cracking Of Resistance Spot Welded Dissimilar Steel 301L/Q235B Joints And The Effect On Fracture Behavior

The welding method of the austenitic stainless steel car mainly adopts resistance spot welding.However,there are some carbon steel/stainless steel welded structures in the car body.It is of great engineering and application value to study the welding performance of dissimilar resistance spot welding.But differences in resistivity,thermal conductivity,and thermal expansion coefficient of 301L and Q235B can cause thermal cracks in the spot welded joint.In this thesis,the 301L/Q235B dissimilar steel welding structure in stainless steel car body were studied.The generation of heat cracks of 2 mm+2 mm resistance spot welding joints and 1.5 mm(301L)+4 mm(Q235B)resistance spot welding joints and their effects on failure behavior were discussed.The nugget microstructure of the 301L/Q235B spot welding joint was martensite,austenite and a large number of Cr23C6 compounds.The heat affected zone of carbon steel plate was composed of martensite and ferrite of different sizes.The heat affected zone of 301L plate was austenite and δ ferrite.The hardness of the spot welding nugget was uneven,and the hardness near the edge of the stainless steel nugget was the highest,which was 465.79 HV.The chemical composition inside the nugget was not uniform.The content of Cr and Ni at the edge of the nugget of 301L stainless steel plate was higher,which leads to more Cr23C6 and higher hardness in this region.The thermodynamic calculation results showed that Cr23C6 had the highest growth trend during the solidification of spot welding nugget.The spot welding joint had a softening zone.The softening zone of the 2 mm+2 mm spot welding joint was located in the outer base metal zone of the 301L nugget.The softened zone of the 1.5 mm+4 mmspot welding joint was located at the nugget dendrite joint surface.The degree of softening was approximately 100 HV.The welding crack of the dissimilar steel resistance spot welding joint was thermal crack,the crack was distributed on the edge of the nugget of the stainless steel plate,whose direction was parallel to the crystal direction of the nugget and structure was slender;The formation of thermal crack was due to the aggregation of nugget compounds and stress concentration.The resistance spot welding process parameters had a great influence on the thermal crack size,and were ranked according to the significant degree of influence:electrode pressure,welding current and power-on time.In the case of the same heat input,the electrode pressure had a significant effect on the crack size.Therefore,when the welding current was increased,a reasonable increase in the electrode pressure could effectively suppress crack growth.The static tensile fracture mode of 2 mm+2 mm spot welding joints showed interface fracture and nugget pullout fracture.The maximum tensile strength was 22.73 kN.The interface fracture was a ductile-brittle hybrid fracture.The pullout fracture location was at the edge of the nucleus of the 301L plate nugget,and the fracture mode was brittle fracture.The crack initiation position of the fracture was coincident with the location of the nugget thermal crack.The stress concentration and thermal crack had a great influence on the tensile fracture location.The 1.5 mm+4 mmspot welding joints were partially fractured and fractured in the softened zone along the fitting surface and the fracture characteristics were brittle fracture.The maximum tensile strength was 23.85 kN.Shrinkage size and solidification line softening zone were the main factors affecting the strength of welding joints.The fatigue fracture modes of 2 mm+2 mm spot welding joints were two-plate eyebrow fracture with a fatigue limit of 2.6 kN.The fatigue fracture position of Q235B was always in the overheated zone;The 301L plate had two fatigue fracture locations,the fatigue fracture occurs near the edge of the nugget and was close to the thermal crack occurrence zone when the load was higher than 3.5 kN,the fatigue fracture occured near the heat affected zone of the 301L plate when the load was lower than 3.5 kN.The high-cycle fatigue fracture of the 1.5 mm+4 mm spot welding joint was located in the nugget near the edge of the 301L plate.The fatigue limit of the spot weld sample was 2.786 kN,and the Q235B plate was partially fractured.The compositional unevenness,crack and stress concentration on the 301L side were the main factors affecting the fatigue performance of the 301L side weld zone at high load.The fatigue limit values of the two combinations were higher than 10%of the tensile strength value,and the service performance under the lower fatigue load was better,so it has certain engineering use value.