Effect of Bainite Austempering on Microstructure and Properties of Medium Chromium Wear-resistant Steel

    
    Bainite is a type of microstructure found in steels that can improve various properties of wear-resistant steels. When heat treated correctly, bainite can significantly improve the wear resistance, hardness and strength of materials such as medium chromium wear-resistant steel. It has been found that by austempering medium chromium wear-resistant steel, a bainitic microstructure is formed which yields greatly improved properties. This article will discuss the microstructural and property changes of medium chromium wear-resistant steel due to austempering, specifically the bainite formation.
    
    Bainite is generally formed when a steel is rapidly cooled either during quenching or other heat treatments, such as austempering. This process allows the carbon in the steel to be trapped within the ferrite, thus creating a microstructure with high strength and hardness levels. In austempering, the steel is heated above the austenitizing temperature and then immediately cooled to the bainite transformation range of 150-250 degrees Celsius. At this temperature range, the diffusion of carbon between the ferritic and austenitic grains is slow. As a consequence, the bainite microstructure forms.
    
    When using medium chromium wear-resistant steel and following the standard austempering process, the bainite microstructure consists of two morphological forms – acicular and plate. Acicular bainite (AB) is characterized by needle-like microstructures while plate bainite (PB) consists of thin plates with blocky microstructures. PB tends to form when the cooling rate is slower and the temperature is at the lower end of the bainite formation range.
    
    When austempering medium chromium wear-resistant steel, the bainite formed can significantly increase its wear resistance, hardness and strength. This is due to the high hardness, high strength and low ductility present in the bainitic microstructure. On average, a 0.4 – 0.5% increase in hardness can be observed after austempering with bainitic microstructure formation. Additionally, these steels also display superior wear resistance properties and high tensile strength.
    
    The microstructure of medium chromium wear-resistant steel can also be greatly improved due to the formation of bainite during austempering. AB and PB may be produced depending on the austempering condition and cooling rate. Generally, AB has been found to possess higher hardness levels than PB, and may also provide better wear resistance. Additionally, AB can also improve the mechanical properties such as ductility and strength when present in the microstructure of the steel.
    
    In conclusion, austempering with bainitic microstructure formation is an effective method for improving the wear resistance, hardness and strength of medium chromium wear-resistant steel. The bainitic microstructures, namely AB and PB, also provide beneficial properties such as improved hardness, wear resistance and improved mechanical properties. Thus, medium chromium wear-resistant steel can be effectively treated with this method for the purpose of increased wear resistance.

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