Abstract
Thermomechanical processing of low carbon bainitic steels is used to obtain a bainitic microstructure with good strength and toughness by continuous cooling after forging without the need of further heat treating, hence reducing manufacturing costs. However, hot forging parameters can significantly influence the microstructure in the forged material. A series of heat treating and forging experiments was carried out to analyze the effect of austenitizing time and temperature on the grain growth and the effect of forging temperature on the Prior Austenite Grain Size (PAGS) and continuously cooled microstructure. The forged microstructures were characterized by optical microscopy, microhardness tests, and X-ray diffraction. The results indicate that at 1200 °C austenitizing temperature abnormal grain growth takes place. Forging temperature significantly affects the PAGS and the subsequently formed microstructure. At high forging temperature (1200 °C), an almost fully bainitic microstructure was obtained. As the forging temperature was reduced to 1100 and 1000 °C, the PAGS refined, while the polygonal ferrite faction increased and the amount of retained austenite decreased. Further evaluations showed that a decrease in the forging temperature results in a higher carbon concentration in solution in the retained austenite leading to a stabilization effect.
Highlights
In the last decade, the application of advanced bainitic steels in thermomechanical processing (TMP) has gained significant importance [1]
At 1000 ◦ C, a slight growth was observed at 40 min of holding time (≈4 μm) in comparison to the initial grain size
200 μm μm and and the the matrix matrix 45. This heterogeneity in the grain size indicates that abnormal grain growth took place at 1200 ◦ C; This heterogeneity in the grain size indicates that abnormal grain growth took place at 1200 °C; this affect was not observed at lower temperatures
Summary
The application of advanced bainitic steels in thermomechanical processing (TMP) has gained significant importance [1]. Understanding how the process parameters affect recrystallization and the final microstructure is crucial to ensure a well-defined processing window of forged components with suitable mechanical properties. Yang et al [12] investigated the workability of a low carbon bainitic steel through processing map analysis and showed that the amount of strain significantly affected the suitable hot deformations parameters, known as stable regions in a processing map. The TMP of bainitic steels requires further investigation to avoid undesirable microstructures related to grain coarsening, incomplete recrystallization, and heterogeneous microstructure in the forged component. We discuss the effect of austenitizing and forging parameters on the growth and recrystallization of PAGS and continuously cooled microstructure of a low carbon bainitic steel through optical microscopy, microhardness, and X-ray diffraction analysis
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