Evaluation of dynamic fracture toughness based reference temperature () of modified 9Cr–1Mo steel in phosphorus embrittled and cold-worked condition

In the present study, the DBTT of a high phosphorus bearing modified 9Cr–1Mo steel in normalized and tempered (N&T) condition and also after 5% and 10% cold work, has been evaluated by extending the reference temperature (T0) based Master Curve approach to dynamic loading conditions. The reference temperature in dynamic loading condition ( T 0 dy ) thus determined is found to be significantly high for the N&T Mod.9Cr–1Mo steel as compared to plain 9Cr–1Mo steel. Cold-work effect was not found to be significant in T 0 dy evaluation of Mod.9Cr–1Mo steel. The high T 0 dy of the present Mod.9Cr–1Mo steel is attributed to a fracture initiation mechanism predominated by decohesion of prior austenitic grain boundaries at the fatigue pre-crack front. This is attributed to segregation of phosphorus along the prior austenitic grain boundaries causing embrittlement, supported by SEM and Secondary Ion Mass Spectrometry (SIMS) observations.