Grain Boundary Damage Evolution and Rupture Life of Service-exposed 1.25Cr-0.5Mo Steel Welds

抄録

For components operated under creep regime, Type IV cracking would be the most likely failure mode if they were fabricated from ferritic steels. In the present work, creep behavior of 1.25Cr-0.5Mo steel weldment operated at 500°C for 28 years has been examined. The morphology of grain boundary damage was observed to clear the cause of Type IV cracking. Intergranular failure at the Intercritical HAZ took place in spite of short testing duration, which was less than 1 000 h. Change in feature of damage was observed with time to rupture. The number of cavities and the area of cavitated at failure increased as time to rupture became longer. Specimens broken with short duration showed few cavities resolvable by optical microscopy adjacent to the final intergranular crack path. On the other hand, more profuse cavitation in terms of number and area was observed in the specimens tested for 1 000 h and longer. However, commonly observable feature in both short and long term specimens was preferential damage occurrence at the triple points, proving that Type IV damage was caused by grain boundary sliding. The change in failure location in laboratory tests, where parent material failure often occurs due to too much acceleration, can be explained by relatively low stress exponent and activation energy associated with grain boundary sliding. Therefore, life assessment by conventionally employed Time-Temperature Parameter such as Larson-Miller Parameter could result in non-conservative estimation since equivalent LMP at failure would be smaller with increase in time to rupture.