Hardness based creep life prediction for 2.25Cr-1Mo superheater tubes in a boiler

In the present work, the applicability and limitation of a hardness based approach to evaluate creep strength of boiler tubes fabricated from 2.25Cr-1 Mo steel have been examined. For superheater tubes in a boiler, a screening technique to judge the damage level would be useful since the number of tubes to be assessed is numerous. It was confirmed that creep strength of tube materials was well correlated with hardness independently of service histories. However, the obtained relationship should not be extended to hardened materials associated with higher dislocation densities. It was found that cold work prior to creep tests remarkably decreased rupture lives despite high hardness values. The extent of change in precipitates, in terms of carbides spherodization and growth of PFZ, was more pronounced in a pre-strained material, presumably due to contribution of pipe diffusion. A similar tendency is also observable in quenched and tempered 2.25Cr-1 Mo plate materials, in which densely populated dislocations were introduced at fabrication. For the critical judgement, a more straightforward method, for example examining the removed tubes in service in an iso-stress manner, should be employed. However, time consuming and expensive tests do not necessarily generate reliable answers. Since a test piece machined from an actual boiler tube inevitably has got a small cross-sectional area, rupture life in air is significantly reduced by oxidation. The metal loss, which. is a function of temperature and testing duration, can be larger in a test at low temperature rather than that in a short-term test at high temperature. Metal loss at failure for an iso-stress tested specimen can be reduced with increase in testing temperature, suggesting that a creep test under accelerating temperature can derive more realistic prediction. To evaluate the genuine creep strength, the rupture lives in NIMS database obtained in air were converted into those in vacuum using the damage mechanics by Kachnov and Rabotnov. The following equation was obtained to predict the rupture life in vacuum on the basis of hardness measurement prior to service. Larson Miller Parameter (LMP) = (log t(rv) + 20)T = 18858-6183 log(sigma/H-vo)- 1 807 log(2)(sigma/H-vo) where t(rv) is the rupture life in vacuum, T is the temperature in Kelvin and H-vo is the Vickers hardness.