Grain Growth Behavior of Austenite in Medium Manganese Martensitic NM500 Steel

The medium manganese martensitic NM500 steel is a light-weight steel used in construction that is high in strength and wear resistant. It is produced by controlled cooling in an austenite-martensite field. During its production, the steel undergoes a continuous solid-state transformation that is also referred to as grain growth. Grain growth is defined as the increase in grain size due to the diffusion of interstitial atoms across grain boundaries. This article seeks to investigate the grain growth behavior of the austenite during the production of the medium manganese martensitic NM500 steel.
    
Discussion    
The medium-manganese martensitic NM500 steel is developed by an austenite-martensitic transformation in which the austenite grain size is critical for its properties. During the solid-state transformation, the grain size increases as the microstructure forms. The grain growth behavior is characterized by two different stages. The first stage is that of primary grain growth and second stage is that of secondary grain growth.
    
Primary grain growth is the first stage of grain growth and is characterized by rapid growth in grains size until the grain boundaries begin to interact. This stage is accompanied by a significant reduction in the grain boundary energy. The driving force for this stage is the minimization of the grain boundary energy, i.e., it is driven by the reduction in the grain boundary energy. The rate of growth is determined by the amount of energy available to drive the grain boundary movement. In the medium manganese martensitic NM500 steel, the primary grain growth occurs between the temperatures of 500°C to 700°C.
    
Schematic Representation of Primary Grain Growth and Secondary Grain Growth in the Medium Manganese Martensitic NM500 Steel
    
Secondary grain growth is the second stage of grain growth and is characterized by a much slower rate of growth than the primary grain growth. This slower rate of growth is driven by the diffusion of atoms across the grain boundaries. The driving force for this stage is the minimization of the total energy of the grains. The rate of growth is determined by the rate of diffusion of interstitial atoms across the grain boundaries. In the medium manganese martensitic NM500 steel, the secondary grain growth occurs between the temperatures of 600°C to 800°C.
    
Conclusion
From this study, it can be concluded that the medium manganese martensitic NM500 steel undergoes a two-stage solid-state transformation during which the grain size increases. The first stage of grain growth is driven by the reduction in grain boundary energy and is known as primary grain growth. The second stage of grain growth is driven by the diffusion of interstitial atoms and is known as secondary grain growth.

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