Effect of Heat Treatment Process on Microstructure and Properties of NM500 Wear-resistant Steel

Wear-resistant steels such as NM500 are heat-treated steels used in industries where corrosion and wear resistance are critical. This alloy is composed of iron, carbon, and other alloying elements, and is known for its excellent wear-resistance and toughness. Heat treatment is a critical process for improving the performance of wear-resistant steels by changing their microstructure and properties. In this article, the effects of various heat treatment processes on the microstructure and properties of NM500 wear-resistant steel are discussed.
    
The microstructure of NM500 wear-resistant steel is composed of a ferrite-pearlite matrix with carbides for wear resistance. The carbide distribution is mainly affected by the cooling rate. The cooling rate can be controlled by a heat treatment process, resulting in different microstructures.
    
Normalizing is a type of heat treatment process in which the steel is heated to 950-1050℃, allowed to cool in the air, and then machined into the required shape and size. This heat treatment process is used to reduce internal stress, refine and homogenize the microstructure, adjust the hardness, and improve the mechanical properties of wear-resistant steels. During normalizing, the microstructure of wear-resistant steel changes from a ferrite-pearlite microstructure to a ferrite-bainite matrix. The hardness of the steel is improved, while the impact toughness is reduced slightly.
    
Annealing is another heat treatment process applied to wear-resistant steels. It involves heating the steel to the polymorphic transformation temperature (900°C-960°C) for a prescribed time and then cooling it in furnace or air. During annealing, carbides such as M2C and M7C3 dissolve in the austenite matrix, increasing the ductility and toughness of the steel. The microstructure is changed from a ferrite-pearlite matrix to a fully pearlitic structure, as shown in Fig.2.
    
Quenching is a heat treatment process that involves quickly cooling the steel from an elevated temperature in an oil bath or air to room temperature. This process increases the hardness and wear resistance of NM500 by forming a martensitic microstructure, but reduces its impact toughness.
    
Tempering is usually conducted after quenching to reduce the hardness of the steel and improve its impact toughness. The steel is heated to the tempering temperature (300-400℃) and held at the same temperature until the desired properties are achieved. This process reduces the hardness of the steel while increasing the ductility and toughness.
    
In summary, different heat treatment processes can be used to improve the performance of wear-resistant steel NM500. Normalizing is used to refine and homogenize the microstructure. Annealing increases the ductility and toughness. Quenching is used to increase the hardness and wear resistance, while tempering reduces the hardness and improves the impact toughness.

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