Advanced Procedures for Long-Term Creep Data Prediction for 2.25 Chromium Steels

Abstract

A critical review of recent creep studies concludes that traditional approaches such as steady-state behaviour, power law equations and the view that diffusional creep mechanisms are dominant at low stresses should be seriously reconsidered.Specifically, creep strain rate against time curves show that a decaying primary rate leads into an accelerating tertiary stage, giving a minimum rather than a secondary period. Conventional steady state mechanisms should therefore be abandoned in favour of an understanding of the processes governing strain accumulation and the damage phenomena causing tertiary creep and fracture. Similarly, creep always takes place by dislocation processes, with no change to diffusional creep mechanisms with decreasing stress, negating the concept of deformation mechanism maps. Alternative descriptions are then provided by normalizing the applied stress through the ultimate tensile stress and yield stress at the creep temperature. In this way, the resulting Wilshire equations allow accurate prediction of 100,000h creep data using only property values from tests lasting 5000h for a series of 2.25 chromium steels, namely Grades 22, 23 and 24.

A Modern Philosophy for Creep Lifing in Engineering Alloys

“…With surprising consistency the Q c * value was found to be approximately 280 kJ/mol for tests conducted above the initial yield stress of the material, 230 kJ/mol for the intermediate region, and, 280 kJ/mol for low stress/higher temperature tests [47].…”

“…The first essentially reduces the dataset to obtain the best model fit regardless of creep mechanism and would prove difficult to independently reproduce. The second method uses a known material property limit to split data but does not have the flexibility to apply to different materials such as superalloys, and furthermore to multiple regions as seen in Grade 22, 23, and 24 [47].…”

“…Initial application of the Wilshire Equations proved difficult, since rather than the two distinct regions seen in previous alloys described above, a third region was observed for longer duration/high temperature tests [47]. Evaluation of test pieces revealed that this behavioural change was most likely due to the degradation of the bainitic microstructure in the material, with the high temperature and long exposure leading to an overaged ferritic microstructure with coarse molybdenum carbide particles also present.…”

“…Through wider application, the method of implementing the Wilshire equations has evolved especially as demonstrated when applied to the 2.25Cr series of alloys, Grade 22, 23 and 24 [47]. These steels provide a test to the Wilshire equations as Grade 22 (2.25Cr-1Mo) steel has long been utilised and has a wealth of results thanks to NIMS providing both short and longterm data.…”

A Discussion of Non-Constant Creep Activation Energy

“…Now having large datasets of multiple materials it can be seen that the Gibbs energy approach may provide a better description of creep activation energy. By physically limiting the system through evaluation at normalised stress rather than absolute stress, we have produced more realistic activation energies in a number of cases [22][23][24]. By having realistic activation energies, creep modelling should be able to better characterise the physical system especially in terms of region splitting where a model is partitioned based on a change in creep mechanism [10].…”