B Chen

A review of the changes of internal state related to high temperature creep of polycrystalline metals and alloys

Abstract When polycrystalline metals and their alloys are used at high temperature, creep deformation leads to changes in their internal state. The change in internal state manifests itself in many ways, but the two ways that concern us in this review are (i) the creation of internal stress arising from the strain incompatibility between grains and/or the formation of cell/sub-grain structures and (ii) a change in the material resistance. This review aims to provide a clear separation of these two concepts by exploring the origin of each term and how it is associated with the creep deformation mechanism. Experimental techniques used to measure the internal stress and internal resistance over different length-scales are critically reviewed. It is demonstrated that the interpretation of the measured values requires knowledge of the dominant creep deformation mechanism. Finally, the concluding comments provide a summary of the key messages delivered in this review and highlight the challenges that remain to be addressed.

An improved method to identify grain boundary creep cavitation in 316H austenitic stainless steel

Inter-granular creep cavitation damage has been observed in an ex-service 316H austenitic stainless steel thick section weldment. Focused ion beam cross-section milling combined with ion channelling contrast imaging is used to identify the cavitation damage, which is usually associated with the grain boundary carbide precipitates in this material. The results demonstrate that this technique can identify, in particular, the early stage of grain boundary creep cavitation unambiguously in materials with complex phase constituents.

ENGIN-X – instrument for materials science and engineering research

Engin-X is a leading neutron diffractometer for materials science and engineering, with high resolution and versatile capabilities at the ISIS spallation source, UK. Over the past 10 years Engin-X has continually redefined the frontier of stress characterisation capability through investment in state-of-the-art equipment, attracting academic and industrial users from 24 countries. Measurements are typically carried out in collaborative experiments between universities, industry and ISIS to address a wide range of engineering problems: manufacturing challenges surrounding magnesium alloys for the automotive industry, creep deformation of nickel-base superalloys for aero engines, structural integrity of welds for nuclear power plants, residual stresses in a range of samples from complex aerospace components to ancient steel making manufacturing techniques.

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO2 environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

Abstract The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.

Effect of thermo-mechanical history on reheat cracking in 316H austenitic stainless steel weldments

Reheat cracking has been observed in the heat affected zone of the 316H austenitic stainless steel thick section weldments during service at a temperature of ∼500°C. This has been attributed to the creep dominated relaxation of the highly triaxial residual stresses. Here the role of thermo-mechanical variables that contribute to the susceptibility of thick section 316H austenitic stainless steel weldments is briefly reviewed. The influence of the plastic strain, carbide precipitation and impurity element segregation on the subsequent creep deformation behaviour and the susceptibility to creep cavitation damage is discussed. A systematically designed experiment which includes these parameters has been undertaken for a 316H austenitic stainless steel. In addition, residual stress profiles have been introduced into cylindrical pre-treated specimens and the relaxation of these profiles with heat treatment has been measured by neutron diffraction. The experimental results are considered with respect to the effect of the microstructure on subsequent creep deformation and stress relaxation. The susceptibility to intergranular brittle fracture is discussed and an attempt is made to correlate the microstructure and stress state factors encountered in the HAZ with the accumulation of the creep cavitation associated with reheat cracking.Copyright

A Microstructural Sensitivity Study of 316H Austenitic Stainless Steel to Inter-Granular Creep Fracture

A preliminary sensitivity examination of the ductility exhaustion based creep damage prediction model, currently used in the R5 high temperature assessment procedure, showed that material property inputs had significant effects on damage prediction. In the present work, the link between the microstructural factors and the susceptibility to inter-granular high temperature creep failure is considered. The latter was judged to be associated with the low creep ductility. Here, the longitudinal section of a creep specimen and the fracture surface were examined. Auger electron spectroscopy was used to investigate the grain boundary composition in this specimen, which failed after a creep test of 1038h at 550°C under a triaxial stress state. The present results demonstrate that there is a possibility to correlate the susceptibility to high temperature inter-granular fracture from the low temperature fracture investigations. Finally, the susceptibility of the pre-treated 316H stainless steel to inter-granular high temperature failure and the contribution to the creep damage model are briefly discussed.