Dominant Factor Governing Toughness in a Cr-Mo Steel for Pressure Vessel with Intermediate Stage Transformation Microstructures

The steels with intermediate stage transformation microstructures (Zw) show a good balance of strength and toughness. Especially, fracture appearance transition temperature (FATT) in Charpy impact test is one of the most important properties of steel to assure material reliability. As microstructural factors to govern FATT, the effective grain size (d(EFF)) on cleavage fracture surfaces is well known in ferrite-pearlitic and martensitic steels. However, the relationship between FATT and d(EFF) have not yet been clarified for the steels with Zw. Low-alloy heat-resistant steels for pressure vessels such as Cr-Mo steels are the type with Zw and demands for better low temperature toughness are becoming severer in recent years. The present study aims to determine the relationship between FATT and d(EFF), and to clarify the microstructural factor to govern toughness in quenched and tempered Cr-Mo steel with a Zw microstructure by varying the prior austenite grain size. Not the prior austenite grain size but the bainite block section width corresponded to d(EFF) in the size distribution. Accordingly, FATT decreased with decreasing block section width.