Microstructure and Properties of Welded Coarse Grain Zone of NM450 Wear-resistant Steel

Wear-resistant steel NM450 has a wide range of applications in the various industries due to its excellent wear resistance, heat resistance and corrosion resistance properties. In order to improve its wear resistance, quenching and tempering process is usually used for treatment of NM450 steel. Welding is often used to join NM450 steel during machining, which may potentially cause a risk of reducing its wear-resistant properties. As such, understanding the microstructure and properties of the welded coarse grain zone of NM450 steel is essential for improving its impact and wear resistant performance.
    
Shows a typical actual welding joint of the NM450 wear-resistant steel, which consists of three different zones, namely, the base metal, the heat-affected zone (HAZ) and the weld metal zone (WMZ). The microstructures of the three zones are also shown in Figure 1, where it can be seen that the HAZ is relatively coarse-grained while the WMZ is dense with a mostly fine-grained structure.
    
Typical welding structure of NM450 wear-resistant steel
2 shows the microstructure of the welded coarse grain zone of NM450 steel. It can be seen that the microstructure consists of a ferrite-bainite mixed microstructure with a higher ferrite content. The ferrite has a Widmanstätten structure, while the bainite also consists of lath-like grain structures. It is also observed that there are a large number of carbides, which are distributed along the grain boundaries and act as points of stress concentration. This increases the impact and wear resistance of the material, thus improving its overall performance.
    
Microstructure of welded coarse grain zone of NM450 wear-resistant steel

The tests were conducted to evaluate the mechanical properties of the welded coarse grain zone of NM450 wear-resistant steel. The results showed that the yield strength and ultimate tensile strength of the material are 414MPa and 647MPa, respectively. The results also revealed that the elongation of the material is 9.5%, indicating a good combination of strength and ductility. The wear resistance of the material was evaluated using the ASTM G65 Standard Test Method and results showed that the wear resistance of NM450 is superior to most other wear-resistant steels. This is attributed to the presence of carbides in the microstructure, which increases resistance to abrasive wear.
    
In conclusion, the welded coarse grain zone of NM450 wear-resistant steel has excellent wear and impact resistance properties due to its ferrite-bainite mixed microstructure. The presence of carbides along the grain boundaries increases its wear resistance, while the fine-grained structure enhances its impact resistance. The ultimate tensile strength and yield strength of the material are also higher than most other wear-resistant steels. Overall, the combined properties make NM450 an ideal material for applications which require high wear and impact resistance.

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