Microstructure analysis of friction welded joints of 45 steel and W9MnCrV

August 23 09:03:05, 2024

Friction welding is a hot-press welding method that uses metal welding surface friction heating. It has good welding quality and stable welding and is suitable for welding dissimilar metals. Friction welding can not only weld ordinary dissimilar steel but also weld dissimilar steels and dissimilar metals with high temperature and high temperature mechanism and physical properties, such as welding of copper and stainless steel. Therefore, it is of great practical significance to study the material structure of friction welding to select the process reasonably and exert the maximum use value of the material. In this paper, a large number of observations and studies on W9MnCrV and 45 steel friction welded joints were carried out by SEM.

Materials and Methods

The analysis samples include three types: one is W9MnCrV and 45 steel friction welded joints (880Â°Cï½ž890Â°C, 5ï½ž6h) in the annealed state; the other is the W9MnCrV and 45 steel friction welded joints directly after air cooling after welding; Fracture sample. Observed with a S2530 scanning electron microscope.

Sample preparation: The analytical sample preparation method for SEM observation of W9MnCrV and 45 steel friction welded joints is the same as the preparation method of metallographic samples (except for fracture samples).

Experimental etchant: 5% HNO3 + 95% C2H5OH; corrosion time is 60 s.

Scanning electron microscope observation and analysis

Figure 1 shows that the weld superheated zone of the W9MnCrV and 45 steel friction welded joints in the annealed state has obvious black coarse particles, which may be due to insufficient forging pressure to cause oxidation or other impurities not to be completely squeezed. It can also be seen that the microstructure in the direction of 45 steel in the superheated zone is almost ferrite, and the weld zone in the high-speed steel edge has a distinct decarburization layer. This decarburization phenomenon is related to the burning and oxidation of the material during the welding process. Due to the decarburization in the weld zone, the material hinders the grain growth during quenching, which is also the reason why the austenite grains are particularly coarse after quenching, and even the coarse grains after tempering are still faintly visible. It can also be seen from Figure 1 that the decarburization of 45 steel is more serious than that of high speed steel and may have a certain influence on the welding quality. Because of the decarburization in the near weld zone, the austenite grains are coarsened after quenching, and this phenomenon cannot be completely eliminated during tempering. At the same time, due to the different microstructures of the weld zone, the expansion coefficients of different tissues under different conditions are different, and there may be different structural transformations and volume changes during quenching, resulting in heat-affected deformation.

Fig.1 SEM image of W9MnCrV and 45 steel friction welded joints (Bar=5Î¼m)
Figure 2 shows that after the high speed steel is annealed, there is also a carbonized finely divided area, followed by a decarburized layer. This is mainly due to the combination of thermal deformation of high-speed friction and upsetting pressure, which makes the carbides in this area more and smaller, and uneven distribution occurs, which is distributed along the radial direction of the welding surface. The annealed structure of high-speed steel is Soxhlet. Body and carbide.

Fig. 2 SEM image of carbonized fine-grained zone and decarburized layer after annealing of high-speed steel (Bar=10Î¼m)
Fig. 3 is a SEM photograph of a sample which is air-cooled after welding. The air-cooled material after welding exhibits martensite, carbide and black structure, and the 45 steel side is a ferrite body composed of ferrite and pearlite, while the needle-shaped ferrite is distributed in the pearlite. The reason for its formation may be that during the welding, since the welding temperature is higher than the quenching temperature, a quenched structure is generated in the vicinity of the weld, and austenite grains having uneven size appear. The main reason for the formation of black tissue is that due to the decarburization during the welding process, the decarburization site cannot be converted into pearlite under the conditions of high temperature short-time heating and air-cooling, and thus it is transformed into martensite. organization.

Fig. 3 Martensite, carbide and black structure of air-cooled material after high-speed steel welding, the annealed carbonized fine-grained zone and decarburized layer scanned structure; 45 steel side is composed of ferrite and pearlite Body (Bar=25Î¼m)
Two different results are produced by annealing and air cooling the weldment of the weldment specimen. In order to reduce weld cracks in the product, annealing must be performed after welding. According to the above analysis of the air-cooled sample, it is known that at the time of welding, since the metal in the vicinity of the abutting surface is heated to the quenching temperature, a quenched structure (high-speed steel side) is formed in the vicinity of the weld, and 45 steel is formed by iron due to partial overheating. The weiss body structure composed of the elemental body and the pearlite body, because the specific volume of these kinds of tissues is different, the linear expansion coefficient is different, which produces a very complicated tissue stress, and on the high speed steel side, since the martensite plasticity is small, The internal stress generated in this zone will also be the largest. After post-weld annealing, martensite transformation can be prevented, tissue stress and thermal stress can be eliminated, and a large number of weld cracks are prevented.

Figure 4 is a magnified photograph of the air-cooled martensite structure after high-speed steel welding in the white frame of Figure 3 (Bar = 5 Î¼m)
Figure 5 is a fracture scan of the W9MnCrV and 45 steel friction welded joints. It can be seen from the figure that the 45 steel is significantly less than the high-speed steel after the tear. The fracture fracture is mainly characterized by ductile fracture. the Lord.

Fig. 5 SEM photograph of the fracture of W9MnCrV and 45 steel friction welded joints (Bar=100Î¼m)
in conclusion

Scanning electron microscopy of W9MnCrV and 45 steel friction welded joints showed that there was a significant decarburization zone at the weld of the material.
Â· Post-weld insulation and direct annealing are important measures to prevent the welding material from generating a large number of stress cracks.
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