Study on Softening Phenomenon of Welded Joint of Low Alloy High Strength Wear Resistant Steel

    
    Low alloy high strength wear resistant steel has become an important and widely used material in various fields in recent years due to its excellent strength, toughness, weldability and wear resistance, which can meet the needs of modern manufacturing. However, when welding the joints of wear-resistant steels, there are often problems such as softening phenomena, which not only reduce the comprehensive performance of the steel but also seriously reduce the service life of the products. Therefore, the study of the softening phenomenon and the optimization of welding process have important practical significance.
    
    Generally speaking, softening phenomenon of welded joints of low alloy high strength wear resistant steels includes four aspects: heat affected zone softening, weld bead softening, fusion boundary softening and coarse grain in weld. Among them, heat affected zone softening and weld bead softening are the main reasons for softening.
    
    Figure 1 Heat affected zone softening
    Heat affected zone softening mainly refers to the phenomenon that the hardness of the heat affected zone is lower than the original hardness of the steel material. Due to the overheating of the steel, the microstructure of the heat affected zone is changed, with the austenite being dissolved, the carbides being significantly reduced and the ferrite volume fraction being increased, leading to the decrease in hardness.
    
    Figure 2 Weld bead softening
    The softening phenomenon of the weld bead is mainly caused by the dissolution of the carbide and the structural stretching in the heat affected zone and austenite zone during the welding process, which causes the hardness of the weld bead to be significantly lower than the original base material. The weld bead softening phenomenon is more obvious when welding with low alloy high strength abrasion resistant steel, which is mainly due to the large number of carbides distributed in the austenite and martensite.
    
    In order to avoid or reduce the softening phenomenon in welding of low alloy high strength abrasion resistant steel, reasonable choices of welding materials and parameters should be made. It is generally believed that preheating and stress relief can help to prevent the softening phenomenon. Preheating can slow down the temperature decreasing rate of the welded joint, reduce the diffusion of carbon, so prevent the change of microstructure caused by over-cooling, thereby inhibiting the softening phenomena caused by precipitation of carbides. In addition, appropriate weld speed can reduce the welding heat input and prevent overheating of welded joints. Besides, reasonable weld parameters should also be adopted.
    
    In addition, when welding low alloy high strength wear resistant steel, the selection of welding materials is also very important. Welding materials with lower carbon Content should be used. For the welding of low alloy high strength wear resistant steel, high strength steel welded joints should be made according to the actual situation, and low hydrogen electrodes should be selected appropriately in order to ensure the comprehensive performance of the welded joint.
    
    In conclusion, the softening phenomenon of the welded joint of low alloy high strength wear resistant steel cannot be avoided in the whole process, but can be minimized by reasonable choice of welding materials and parameters.

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