Gold Phase Graph Analysis of Q195 Zinc-plated Steel Plates (Welded by Resistance Spot Welding of Two Homogeneous Steel Plates)
Q195 zinc-plated steel plates refer to steel materials with the chemical composition of C≤0.12%, Mn 0.25-0.60%, P≤0.045% and S≤0.045%. Due to their excellent corrosion resistance and effects of zinc-plated layer, Q195 zinc-plated steel plates are widely used as construction, vehicle parts and more. The performance of this type of steel plate is greatly affected by the quality of its welded spot, which should be examined and detected. This paper will present a gold phase graph analysis, a method of non-destructive testing that is used to detect the welded spot of the Q195 zinc-plated steel plates welded by resistance spot welding of two homogeneous steel plates.
In order to conduct the gold phase graph detection for the welded spot of the Q195 zinc-plated steel plates, the microstructure of the welded spot should be prepared. Generally, grinding and polishing are used to preparation the specimens before the gold phase graph analysis. After grinding, the specimens should be polished with #800 and #6000 wet diamonds, then washed to remove the grinding residue, and then gold vacuum plated. Generally, the thcikness of the gold layer should range from 0.1 to 0.2um. A scanning electron microscope (SEM) is used to observe the microstructure of the welded spot, and a gold phase graph detector is used to detect the microstructure. The detection process is: first focusing the gold phase graph detector on the welded spot, and then slowly moving the detector until the best image is achieved, and then take a photograph of the graph.
To analyze the gold phase graph, the Welding-zone (WZ) and the Heat-affected zone (HAZ) should be defined. WZ refers to the area of the steel plate directly affected by the welding process, where the microstructure is changed and consists of solidified molten material and residual deformation/fusion zone. HAZ refers to the zone outside the welding zone, including the initiation zone and propagation zone. In the initiation zone, it is the area of the steel plate with a width of about 3mm on each side of the welding joint, and the grain size is coarse and there are obvious grain deformation and carbides precipitation. In the propagation zone, the grain size gradually decreases and there is a certain degree of grain deformation.
The gold phase graph can provide information on the microstructure pf the welded spot of the Q195 zinc-plated steel plates, which can be used to evaluate the welding quality. Generally, the higher the degree of liberation of graphite edges and clusters, the better the welding quality. On the other hand, a large number of particles and graphite clusters in the graph indicate that the welding quality is poor and the microstructure of the steel may be inhomogeneous.
In conclusion, a gold phase graph analysis is conducted for Q195 zinc-plated steel plates welded by resistance spot welding of two homogeneous steel plates. The microstructure of the welded spot is observed and the WZ and HAZ is defined. The gold phase graph can provide information on the microstructure of the welded spot, which can be used to evaluate the welding quality. A large number of graphite edges and clusters indicates that the welding quality is good, while a large number of particles and graphite clusters in the graph indicates that the welding quality is poor and the microstructure of the steel is inhomogeneous.
