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.

This study aims to develop a high-temperature low cycle fatigue test method using a nonstandard miniature thin-plate specimen in order to characterize the cyclic viscoplasticity behavior of a component material. For demonstration, fully reversed strain-range controlled low cycle fatigue and creep-fatigue tests at 600 C have been performed for a martensitic steel using standard-sized fullscale specimens and miniaturized thin-plate specimens, respectively. Because the displacement is not directly measured from the uniform gauge section of the miniaturized specimen, a geometry-dependent scaling factor is obtained and used to convert the uniaxial strain. The results obtained are shown that the miniaturized test method developed in this work has exhibited a clear possibility to produce comparable low cycle fatigue data with those that are normally obtained by conventional standard specimen tests.

This study aims to develop a high temperature LCF test method using a non-standard miniature thin-plate (MTP) specimen in order to characterize cyclic visco-plasticity behavior of component materials. For demonstration, fully reversed strain-range controlled LCF and creep-fatigue (CF) tests at 600 °C have been performed for a martensitic steel using both standard-sized full-scale (SSFS) and MTP specimens. A scaling factor is determined using cyclic visco-plastic finite element (FE) for geometry constraint evaluation and data conversion based on the reference strain approach. The equivalent energy principal is proposed to assess the geometry constraint effect that non-standard MTP specimen has. The high temperature LCF results from the MTP specimen based on the proposed testing methodology have shown a good agreement with SSFS specimen data under equivalent conditions. The methodology can therefore be used to conduct accurate transferability to achieve equivalent LCF behavior between the conventional standard specimen and the MTP specimen.