Effect of cooling rate after high-temperature heating on the ductility of 19Cr-0.020C-0.015N-0.4Nb stabilized ferritic stainless cast steel

In as-cast material, cracking can occur during assembly or welding processes depending on the manufacturing conditions. This study proposed heat treatment strategies to improve the ductility of as-cast stabilized ferritic stainless cast steel containing Nb, particularly focusing on cooling rate after solution treatment. The specimen cooled at 1.0 °C/min (“Slow-cooled sample”) exhibited ductile fracture, while the specimen cooled at 2.0 × 10 4 °C/min (“Rapid-cooled sample”) exhibited low ductility and cleavage fracture. The fracture initiation site of the Rapid-cooled sample was a coarse oxide particle near the grain boundary. In the ferrite matrix surrounding the oxide particles in the Rapid-cooled sample, more dislocations were observed compared to the Slow-cooled sample. These dislocations are likely introduced by local thermal stress generated during rapid cooling, resulting in the suppression of plastic accommodation around the oxide particles during plastic deformation. Consequently, stress concentration at the crack tips of the oxide particles and their Nb(C, N) coating increases, causing the cracks to propagate into the ferrite matrix. Accordingly, we concluded that the dislocations introduced around the oxide particles are one of the major factors reducing the ductility of the Rapid-cooled sample. • Ductility improved highly with slower cooling rate after high-temperature holding. • Cleavage fracture initiated at a coarse oxide particle near the grain boundary. • Higher cooling rate resulted in more dislocations near the oxide particles.