An investigation of the effect of pre-straining on the creep behaviour of a P91 steel at 600 °C using impression creep testing

Abstract

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in a grade 91 steel using impression creep testing. The grade 91 material investigated has been tested in two different microstructural conditions, in the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant. The two microstructural conditions were confirmed by hardness mapping and Electron Back Scattered Diffraction (EBSD). Previous investigations have used pre-strained uniaxial creep specimens to investigate this effect, but the present work has utilised the specialised small-scale impression creep testing technique to test material obtained at a number of positions along the axes of failed hot tensile specimens. This allowed impression creep samples to be extracted at various prestrains for investigation and for a wide range of hot tensile pre-strain to be investigated. The two microstructural conditions have shown a divergence in behaviour, with the normal martensitic material showing little change in creep strength with increasing pre-strain and the aberrant material increasing markedly in creep strength with increasing pre-strain.

Significant reduction in creep life of P91 steam pipe elbow caused by an aberrant microstructure after short-term service

“…In the normal tempering process of P91 steel, martensite only undergoes recovery transformation. However, the elbow piece was deformed twice during the bending process, which increases a large amount of internal energy to the martensite structure, providing a driving force for the recrystallization of the aberrant ferrite phase 7 . The formation of aberrant ferrite weakens the effect of dislocation strengthening 7 , the Ostwald ripening of M 23 C 6 weakens the effect of the precipitation strengthening 19 , and the Laves phase reduces the solid solution strengthening 11 .…”

“…However, the elbow piece was deformed twice during the bending process, which increases a large amount of internal energy to the martensite structure, providing a driving force for the recrystallization of the aberrant ferrite phase 7 . The formation of aberrant ferrite weakens the effect of dislocation strengthening 7 , the Ostwald ripening of M 23 C 6 weakens the effect of the precipitation strengthening 19 , and the Laves phase reduces the solid solution strengthening 11 . The three superposition effects significantly influence the strength, plastic toughness, and high-temperature creep strength of P91 steel in this study.…”

“…Plant operators normally expect a service life of at least 100,000 h, and normal tempered martensitic steels such as P91 with complex microstructures meet this qualification 6 . However, the bending process of the main steam pipe involving a large amount of deformation can produce an aberrant microstructure of the heat-resistant steel, which may lead to significant changes in creep characteristics 7 .…”

“…Usually many precipitates in P91 with the pinning effect along prior austenitic grain boundaries (PAGBs) and lath boundaries can inhibit free dislocation movement and subgrain boundary migration, and enhance the creep strength of the normal P91 9 . In the case of the aberrant pipe, the absence of martensitic lath boundaries weakens the effect of dislocation strengthening 7 . At the same time, the absence of martensitic lath boundaries may also result in abnormal growth of the Laves phase.…”

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

“…In regards to aging effects, the long‐term aging will not only coarsen the precipitates in the sample but will also lead to a reduction in the dislocation density 39,40 . This may explain the increased displacement for the aged samples, namely, more plastic strain during the initial loading period, 41,42 compared to the unaged samples for the same test condition. Future work will seek to incorporate this effect of prestraining and dislocation density evolution.…”