Influence of Hot Forging Parameters on a Low Carbon Continuous Cooling Bainitic Steel Microstructure

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.

Constitutive Modelling of High Temperature Flow Behaviour for a Low Carbon High Silicon Bainitic Steel

“…A critical characteristic of the presented material which differs from the others studied bainitic steels, is the low carbon content, below 0.2% C. Also, it is already known in recent reports that the continuous cooling of DIN 18MnCrSiMo6-4 bainitic steel has a suitable amount of silicon (>1.0%) used to avoid cementite precipitation which is replaced by stabilized austenite as the second constituent in bainite 4 . Thus, offering a considerable increase in yield strength and ductility.…”

“…For that reason, this shortcoming can be improved by microstructure optimization, avoiding subsequent heat treatments. Thereby, the microstructure can be influenced by grain refining mechanisms, which brings different bainitic morphologies after continuous cooling 4 .…”

“…Table 1 shows the chemical composition of the low carbon bainitic steel DIN 18MnCrSiMo6-4, which was developed and produced by the Swisstec (Swiss Steel), Emmenbrücke, Switzerland. Figure 1 shows the as-received microstructure composed by Granular Bainite (GB), which is characterized by Bainitic Ferrite (White Regions), Retained Austenite (RA) and Polygonal Ferrite (PF) 4 .…”

Advances in Design by Metallic Materials: Synthesis, Characterization, Simulation and Applications

“…The collection includes papers regarding the most multifaced aspects of metals as synthesis [1][2][3], treatments [2][3][4], experimental characterization [4][5][6][7], material models [7][8][9] and engineering applications [10][11][12] providing a clear cross-section of the wide variety of topics and research arguments under investigation in the scientific community now.…”

“…Regarding metal processing, in [2] the authors investigated the process of hot forging and how its changes can influence the metal microstructures in the case of a carbon continuous cooling bainitic steel. Other unconventional processes and treatments are proposed, too, as, for instance, in [4] where the protective effect of tungsten carbide provided by HVOF coating on martensitic stainless-steel with respect to jet slurry erosion is discussed.…”