Friction welding of 45 steel and w9mncrv by SEM

Microstructure analysis of friction welded joint between 45 steel and w9mncrv by SEM

friction welding is a hot pressing welding method using friction heating of metal welding surface. The utility model has the advantages of good welding quality and stable welding, and is suitable for welding dissimilar metals. Friction welding can not only weld common dissimilar steels, but also successfully develop kilowatt aluminum air battery power generation system based on graphene air cathode (see Figure 2), but also weld dissimilar steels and dissimilar metals with different physical properties at room temperature and high temperature, such as the welding of copper and stainless steel. Therefore, the research on the material structure of friction welding has important practical significance for the reasonable selection of process and the maximum use value of materials. In this paper, the friction welded joint between w9mncrv and 45 steel was observed and studied by SEM

materials and methods

analysis samples include three types: first, samples of friction welded joints between w9mncrv and 45 steel in annealed state (880 ℃ ~ 890 ℃, 5 ~ 6h); The second is the friction welding joint between w9mncrv and 45 steel, which is directly air cooled after welding; Third, fracture samples. Observe with s2530 scanning electron microscope

sample preparation: the preparation method of analytical samples for SEM observation of friction welded joint between w9mncrv and 45 steel is the same as that of metallographic samples (except fracture samples)

corrosive agent for experiment: 5%hno3+95%c2h5oh; The corrosion time is 60s

SEM observation and analysis

Fig. 1 shows that there are obvious black coarse particles in the overheated area of the friction welded joint between w9mncrv and 45 steel in the annealed state, which may be caused by insufficient upsetting pressure so that it is oxidized or other impurities are not completely squeezed. It can also be seen that the microstructure in the overheated area in the direction of 45 steel is almost ferrite, while there is an obvious decarburization layer in the weld area near the high-speed steel. This decarburization phenomenon is related to the burning loss and oxidation of materials during welding. Due to the decarburization in the weld zone, the material hinders the grain growth during quenching, which is also the reason for the particularly coarse austenite grains after quenching, and even the coarse grains after tempering are still faintly visible. It can also be seen from Figure 1. 1. Regularly check whether there is oil leakage at the main engine and oil source. The decarburization of 45 steel is more serious than that of high-speed steel, which may have a certain impact on the welding quality. Because of the decarburization near the weld area, the austenite grains are coarse after quenching, and this phenomenon cannot be completely eliminated during tempering. At the same time, due to the different structures in the weld zone and the different expansion coefficients of each structure under different conditions, there may be different structural transformation and volume change during quenching, resulting in heat affected deformation

Fig. 1 SEM image of friction welded joint between w9mncrv and 45 steel (bar=5 μ m)

Figure 2 shows that there is also a carbonization fine crushing zone after annealing of high-speed steel, followed by a decarburization layer. This is mainly due to the combined action of hot deformation due to high-speed friction and upsetting pressure, which makes the carbides in this area more and smaller, and uneven segregation occurs. They are distributed radially along the welding surface. The annealing structure of high-speed steel is sorbite and carbides

Fig. 2 SEM image of carbonization fine crushing area and decarburization layer of high speed steel after annealing (bar=10 μ m)

Fig. 3 is the SEM photo of the air cooled sample after welding. The microstructure of air cooled material after welding is martensite, carbide and black structure. The 45 steel side is widmanstatten composed of ferrite and pearlite, while acicular ferrite is distributed in pearlite. The reason may be that the welding temperature is higher than the quenching temperature during welding, so the quenching structure and uneven austenite grains appear near the weld. The main reason for the formation of black structure is that the decarburized part cannot be transformed into pearlite and martensite under high temperature and short time heating and air cooling conditions due to the decarburization during the welding process. Therefore, 45 steel presents widmanstatten structure due to overheating

Fig. 3 martensite, carbide and black structure of air cooled materials of high-speed steel after welding, and scanning structure of carbonization fine crushing area and decarburization layer after annealing; One side of 45 steel is widmanstatten (bar=25) composed of ferrite and pearlite μ m)

two different kinds of knots will be produced by annealing and air cooling the weldments of the weldment samples. The US Department of Agriculture announced that the fruit of Anoplophora glabripennis was found in the wood packaging materials of Chinese imported goods. In order to reduce welding cracks in the products, annealing must be carried out after welding. Through the above analysis of air-cooled samples, it can be seen that during welding, the metal near the butt joint surface is heated to the quenching temperature, so the quenched structure (high-speed steel side) is formed near the weld, and the 45 steel is partially overheated to form the widmanstatten structure composed of ferrite and pearlite. Due to the different specific volume and linear expansion coefficient of these structures, very complex structural stress is generated. On the high-speed steel side, Due to the small plasticity of martensite, the internal stress in this region will also be the largest. After post weld annealing, martensite transformation can be prevented, structural stress and thermal stress can be eliminated, and a large number of welding cracks can be prevented

Fig. 4 is an enlarged photo of martensite structure of high-speed steel after air cooling in the white frame of Fig. 3 (bar=5 μ m)

Figure 5 is the fracture scanning photo of the friction welded joint between w9mncrv and 45 steel. It can be seen from the figure that 45 steel has significantly fewer tearing edges than high-speed steel after tearing, and the fracture is mainly ductile fracture

Fig. 5 SEM photo of friction welded joint between w9mncrv and 45 steel (bar=100 μ m)

conclusion

· the SEM observation of friction welded joint between w9mncrv and 45 steel shows that there is obvious decarburization zone in the weld of the material

· post welding heat preservation followed by direct annealing is an important measure to prevent a large number of stress cracks in welding materials

liqiaoling, Yeyun (end), Department of materials engineering, North China Institute of Technology