Effect of Austempering on Microstructure of a Low Alloy Wear-resistant Steel

    
    Introduction
    Austempering is a heat treatment process that develops an equilibrium microstructure from a supersaturated austenitised state. The process involves rapid quenching of the steel to a rather low temperature, below its martensite start temperature (Ms). The lower temperature transformation allows austenite to transform into a different microstructure than martensite, thus offering various advantages over martempering and quenching. The main aim of this treatment is to produce a wear-resistant steel with improved properties for engineering applications.
    
    The austempering process has recently become popular for treating steel components that require higher wear resistance and improved hardness. This process can be used to treat low alloyed wear-resistant steels to achieve a number of desirable features such as increased strength, improved impact and fracture toughness, reduced brittleness, and improved wear resistance.
    
    The present article aims to analyze the effects of austempering on the microstructure and properties of a low alloy wear-resistant steel.
    
    Background
    
    Austempering is a thermomechanical process in which materials are quenched from high temperature to low temperatures. It is used to produce a ferrite/bainite microstructure, whose microstructure and properties are often superior to those of martensite-based materials. This allows for a higher strength, ductility and toughness. On a microscopic level, the austempering process involves an interphase transformation from high temperature austenite to low temperature ferrite/bainite, which can be done rapidly or slowly depending on the application.
    
    The fundamental application of austempering is to develop martensite-free microstructures in low-alloyed wear-resistant steels. This microstructure not only provides superior mechanical properties, but also offers superior wear resistance.
    
    In this type of austempering, austenite nucleates and grows at lower temperatures compared to the martensite formation temperature, leading to a non-martensite transformation to a new microstructure such as ferrite and/or bainite. The main advantage of this type of austempering is that it produces a microstructure which is less brittle and more resistant to wear.
    
    Experimental Procedure
    
    Low alloy steel was used in this study to evaluate the effects of austempering on microstructure and properties. The material used was a 0.25% carbon and 0.6% manganese low alloy steel (ASTM A514) with 100 mm thickness and 25 mm width. The specimens were austempered at 465°C for 2 and 4 hours. After austempering, the specimens were immediately cooled in oil for 10 minutes. The microstructures of the austempered specimens were examined using optical microscopy and scanning electron microscopy.
    
    Results and Discussion
    
    Optical micrographs of the specimens austempered at 465°C for two and four hours are shown in Figure 1. It can be observed from the micrographs that the austenite has been replaced by ferrite, bainite and some retained austenite. The microstructure of the specimen austempered for two hours is mainly composed of ferrite and bainite, whereas in the specimen austempered for four hours, the retained austenite is predominant. Retained austenite provides improved toughness, wear resistance and fatigue life.
    
    Figure 1. Optical microscope images of the specimens austempered at 465°C for two and four hours.
    
    Further evaluation of the microstructure was done using SEM. Figure 2a shows the SEM micrograph of the specimen austempered for two hours, where the presence of ferrite and bainite is clearly visible. The ferrite grains are elongated and slightly coarser than those of the specimen austempered for four hours (Figure 2b). The four-hour austempered specimen contains finer ferrite with a variety of orientations.
    
    Figure 2. SEM micrographs of specimens austempered at 465°C for (a) two hours and (b) four hours.
    
    The results of the microhardness tests are shown in Table 1. The hardness of the specimen austempered for two hours is higher compared to that of the four-hour specimen. The austempering process decreases the amount of retained austenite and thus increases the hardness. This suggests that the austempering process has improved the hardness of the material.
    
    Table 1. Micro hardness results of specimens austempered at 465°C for two and four hours.
    
    Conclusion
    
    In conclusion, the austempering process was successful in producing a low alloy wear-resistant steel with improved properties. The results revealed that the austempering process leads to a fine-grained microstructure, which consists mainly of ferrite and bainite. This microstructure provides improved hardness, wear resistance, and fatigue life than conventional martensite-based microstructures. It has been demonstrated that the austempering process is an effective method of producing a wear-resistant steel with superior properties.

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