Retained austenite in high strength bainitic steel

    
    Retained austenite is a special form of austenite material found within certain types of steel alloys, such as the high-strength bainitic steel. It is created during the bainite transformation process which produces strong, tough, and resilient steel with excellent mechanical properties. In this article, the formation and structure of retained austenite in high strength bainitic steel will be discussed, as well as its influence on the mechanical properties and microstructure of the alloy.
    
    First, it is important to understand the bainite process. Bainite is formed by a thermodynamic process called isothermal transformation. Ferrite and cementite particles, the primary constituents of stabilized bainitic steel, are formed by diffusive transformation of austenite at temperatures between 460 to 540°C. During this transformation, the austenite partially changes to a mixture of ferrite and cementite along with remaining isolated islands of austenite called “retained austenite”.
    
    Retained austenite is the result of several factors, including chemical composition of the alloy, cooling rate, temperature and time of cooling. Retained austenite has a different crystal structure than the ferrite/cementite mixture present in bainitic steels; it forms as a result of a prolonged transformation process that produces both ferrite/cementite and retained austenite within the microstructure of the alloy.
    
    Retained austenite particles are usually surrounded by ferrite/cementite plates in a columnar arrangement (see Figure 1). The size and shape of the retained austenite particles vary due to the chemical composition of the alloy, cooling rate and temperature. The ferrite/cementite plates have a lamellar structure, with uniform thickness (Figure 2).
    
    Figure 1. Columnar microstructure arrangement of retained austenite within high strength bainitic steel.
    
    The presence of retained austenite has a beneficial effect on the mechanical properties of high-strength bainitic steel. The presence of retained austenite improves the steel’s fatigue strength by providing additional energy absorption and reducing crack propagation. Additionally, it improves the steel’s toughness, as the retained austenite particles act as “stress relievers”, reducing the stresses throughout the microstructure.
    
    Retained austenite in high-strength bainitic steel also increases the strength of the alloy by providing additional plasticity and ductility. This is due to the fact that the retained austenite has a lower yield strength than the ferrite/cementite microstructure; hence, when subjected to stress, it deforms more easily. This allows the alloy to undergo plastic strain before fracture and also lengthens the crack propagation rate.
    
    Figure 2. Lamellar microstructure of ferrite/cementite plates adjacent to retained austenite particles in high-strength bainitic steel.
    
    In conclusion, retained austenite is an important component of high-strength bainitic steel alloys. It is created during the bainite transformation process and is characterized by a different crystalline structure than ferrite/cementite. The presence of retained austenite in high-strength bainitic steel is advantageous because it improves the mechanical properties of the alloy, such as fatigue strength, toughness, strength, plasticity and ductility.

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