Embrittlement of 2^1/4 CrMoV steel bolts after long exposure at 540 °C

Abstract

Bolts made of2 ^1/4 CrMoV steel became gradually embrittled and exhibited an increasing fraction of intercrystalline fracture after long exposure (more than 25,000 h) at 540 °C. This has been found to be caused by the segregation of phosphorus to the prior austenite grain boundaries. This was accompanied by the depletion of molybdenum in the ferrite down to about 0.3 pct owing to the formation of a molybdenum-containing carbide Fe₃Mo₃C(M₆C). Reheating of the embrittled bolts to 680 °C removed the segregation of phosphorus at the grain boundaries and therefore also removed the intercrystalline embrittlement. However, the molybdenum content of the ferrite was lowered further to about 0.2 pct; hence the scavenging of phosphorus by molybdenum was further reduced, and therefore the embrittling tendency of these bolts during reuse was increased. Even after reaustenitizing, quenching and tempering, the service life of these bolts was found to be shorter than in the original condition; this is presumed to be due to an inhomogeneity of molybdenum in the austenite not removed by the reaustenitizing treatment.