Influence of Heat Treatment Process on Microstructure and Properties of NM400 Low Alloy Steel Plate

Advances in metallurgical technology have resulted in the development of materials with exceptional properties that yield desirable design characteristics in a range of applications. The heat treatment process is one of the key factors which can be utilized to alter the microstructure and properties of metals to optimize their potential usage.
    
NM400 grade low-alloy steels are comprised of alloying elements like chromium, molybdenum, manganese, and nickel for improved wear resistance, toughness, and strength. In this study, the influence of the heat treatment process on the microstructure and properties of NM400 low-alloy steel plate is investigated.
    
In order to study the influence of heat treatment process on the microstructure and properties of the NM400 steel plate, a sample comprising of 8mm thick NM400 steel sheet was firstly subjected to furnace heating to a temperature of 860°C and was held at this temperature in an inert atmosphere for one hour. After that, the sample was then quenched in water and tempered in a furnace at a temperature of 260°C for another hour before being cooled down at room temperature.
    
After the heat treatment process, the microstructural analysis reveals the presence of martensite, bainite, and lath martensite. Figure 1A shows the microstructural evolution of the sample which exhibits a combination of ferrite, pearlite, and martensite. The martensite observed in the sample is composed of two different phases, i.e. a structure of lath martensite and a more coarse structure of bainite. The lath martensite develops from a combination of ferrite and pearlite and is attributed to the rapid cooling of the sample in an aqueous bath following the furnace heat treatment. The bainitic structure, on the other hand, is formed due to the slower cooling that occurs during tempering.
    
A) Microstructural evolution of the NM400 steel plate and B) optical micrograph of the plate after heat treatment process.
    
The heat treatment process also resulted in definite variations in the mechanical properties of the NM400 steel plate. The results show that the hardness of the material increases significantly compared to the hardness measured before heat treatment. This increase in hardness is due to the precipitation of hard, wear-resistant carbides within the microstructure of the steel. The yield strength and tensile strength of the material also increased with the heat treatment process. Finally, a decrease in the ductility of the steel was observed.
    
In conclusion, the effects of the heat treatment process on the microstructure and properties of the NM400 low-alloy steel plate have been studied. The furnace heat treatment of the steel sample resulted in a microstructure comprising of ferrite, pearlite, martensite, bainite, and lath martensite. It was also observed that the mechanical properties of the material were greatly affected by the heat treatment process. The hardness, yield strength, and tensile strength of the steel increased whereas its ductility decreased. Therefore, it can be concluded that the heat treatment process is a highly beneficial technique in altering the microstructure and properties of materials to meet certain performance criteria.

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