Dilatometric and Microstructural Investigations on Austenite Decomposition under Continuous Cooling Conditions in a Cu-Bearing High-Strength Low-Alloy Steel

High-strength low-alloy (HSLA) steels are an important class of steels that meet combinations of varied properties demanded by critical engineering applications. Microstructures of this class of steels are engineered to meet the targeted mechanical properties. Thorough understanding of phases that evolve during decomposition of austenite in these steels after hot working or after austenitizing treatments is essential to optimize their key processing parameters. This is achieved through establishment of continuous cooling transformation diagrams. These diagrams shed light on details of cooling transformations like the phases formed during transformation, sequence of transformation when multiple phase transformations are involved and the temperatures at which the transformations start and end, all as a function of cooling rate. These diagrams are constructed from dilatometry experiments together with corroborating microstructural investigations and mechanical property assessments. However, analyses of dilatograms, identification and quantification of microstructural constituents and correlating the same with mechanical properties (usually hardness) are not trivial, especially, in low-carbon HSLA steels where the microstructure is often composed of martensite and bainite that have rather similar morphological features and practically difficult to distinguish from each other. Advanced data processing of electron backscattered diffraction (EBSD) data has been used in recent times with considerable success to differentiate bainite and martensite. This work highlights the salient aspects involved in decomposition of austenite in an indigenously developed Cu-bearing HSLA steel. Details of methodology employing grain average misorientation obtained through EBSD experiments to identify and quantify martensite and bainite are discussed in detail.

Automated summary

High-strength low-alloy (HSLA) steels are important for critical engineering applications, and understanding the phases that occur during their decomposition is vital for optimizing processing parameters. Continuous cooling transformation diagrams provide valuable information about cooling transformations and can be constructed from dilatometry experiments. However, distinguishing between martensite and bainite, which often make up the microstructure of low-carbon HSLA steels, can be challenging. Advanced data processing techniques using electron backscattered diffraction (EBSD) have shown success in differentiating between these two phases.