Study on Welding Properties of NM400 Steel for Bulldozer Blades with Low Alloy and High Strength
NM400 Elemental Analysis:
The major elements in NM400 are C, Mn, Si, Ni, Cr, Mo and Nb. Carbon (C) mainly affects the hardenability, strength and durability of the alloy steel. If its content is too high, the strength and wear resistance of the steel will increase, but its plasticity and welding performance will decrease. Moreover, the carbide generated by the high content of carbon will reduce the toughness of NM400. The content of manganese (Mn) is 0.50-1.70%. Manganese can play an important role in improving the hardness and strength of the steel, but if the content is too high, it will cause cold brittleness. Silicon (Si) can significantly improve the strength and promote the formation of bar-type pearlite. Besides, it can improve the abrasive resistance of the welded joint. Nickel (Ni) can improve not only the strength and toughness, but also the corrosion resistance, machinability and oxidation resistance of the steel. The content of nickel should not be too low or too high. Chromium (Cr) is an important metal element used to form carbides, improve the strength, wear resistance and corrosion resistance of the alloy steel. But its content should not exceed 2.50%, or the toughness and welding performance will be obviously reduced. Molybdenum (Mo) can form a hard and wear-resistant molybdenum carbide and improve the strength of NM400. Meanwhile, it can increase the low temperature impact toughness and corrosion resistance of the steel.
NM400 steel Microstructure:
NM400 is a kind of low alloy and high strength abrasion resistant steel. It has good mechanical properties, plasticity, toughness and weldability. Its typical microstructure contains ferrite, Bainite and a small amount of martensite. Its microstructure is composed of a fine-grained ferrite matrix hardened by the precipitated carbide particles. The finely dispersed carbide particles make the alloy steel have high wear resistance and crack resistance.
NM400 steel Heat Treatment Process:
The commonly used heat treatment process for NM400 is quenching and tempering. The quenching temperature is 820-870℃ and the tempering temperature is 550-680℃. The quenching process can improve the strength and hardness of the steel, and the tempering process can refine the grain size, reduce the brittleness, improve the impact toughness and plasticity of the steel.
NM400 steel Welding Properties:
NM400 has good welding performance. Generally speaking, preheating is not required for welding. But preheating can reduce the cooling rate during welding, reduce the welding deformation, ensure stable welding performance, reduce the amount of weld metal, and eliminate steam and water vapor in the welding zone. The welding rod should be selected according to the welding power source, welding position, base material and plate thickness. Generally, the most commonly used electrode is E8018-B2 electrode; and if the plate is thicker, E8018-C3 or E10-1 electrode can be used. The welding parameters, such as welding current, welding voltage, welding speed and wire feeding speed, should be adjusted and controlled according to different welding positions and welding environment.
Conclusion:
NM400 alloy steel has excellent mechanical properties, high strength and good weldability. The commonly used heat treatment process for it is quenching and tempering, and the commonly used welding rod is E8018-B2 electrode. The welding parameters should be adjusted and controlled according to different welding positions and welding environment in order to ensure the welding quality of NM400.
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