Effect of Quenching Temperature on Microstructure and Properties of NM550 Steel

With the continuous upgrade and improvement of modern steels, the performance of steel materials has been greatly improved, and NM550 steel has become one of the most advanced steels in the industry. In order to obtain better performance of NM550 steel, comprehensive optimization and improvement of different processing parameters is critical. Quenching temperature is one of the significant parameters to influence the microstructure and properties of NM550 steel, in this article, the effect of quenching temperature on microstructure and properties of NM550 steel will be discussed in detail.
    
Microstructure of NM550 steel is complex and it contains martensite, bainite, and retained austenite. When NM550 steel is quenched at high temperature, martensite and bainite appear more, while retained austenite is less. Increasing the quenching temperature can reduce the microhardness of NM550 steel, and makes the microstructure coarser and more stable. While quenching at lower temperatures, the volume fraction of retained austenite will be increased, leading to the decrease of strength and hardness of NM550 steel, but with better toughness and plasticity.
    
Based on the case of quenching temperature, it is found that the as-quenched microstructure of NM550 steel can be divided into five temperature ranges: cryogenic temperature, very low temperature, low temperature, medium temperature and high temperature, as shown in Table1.
    
Table1: Quenching temperature classification
Quenching Temperature (℃)MicrostructureCryogenic temperature (lower than -195)martensiteVery low temperature (-195~-120)martensite and small amount of retained austeniteLow temperature (-120~-60)martensite, bainite and large amount of retained austeniteMedium temperature (-60~370) martensite, Bainite and very small amount of retained austeniteHigh temperature (370~500)martensite and bainite
    
The mechanical properties of NM550 steel are closely related to its microstructure. With the increase of quenching temperature, the hardness of NM550 steel will decrease and the strength, plasticity and toughness will be improved. Taking the medium temperature range (-60~370 ℃) as an example, tensile strength is higher than 930Mpa, yield strength is 830Mpa and Elongation is 17%. While quenching in the high temperature range (370~500℃), tensile strength is 950-1100Mpa and yield strength is 850Mpa.

In conclusion, the quenching temperature has a significant impact on the microstructure and properties of NM550 steel. Increasing the quenching temperature will cause the decrease of microhardness and the improvement of strength, plasticity and toughness. Therefore, it is very important to control the quenching temperature in order to obtain better performance of NM550 steel.
    

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