Influence of nominal composition variation on phase evolution and creep life of Type 316H austenitic stainless steel components

AbstractThe present work aims to understand the influence of variation in chemical composition in the long term evolution of secondary phases. Three samples with nominal composition of Type 316H but different specific composition have been exposed to 505°C during 150, 145 and 300 kh. The percentage of ferrite and M23C6 carbide have been measured using EBSD and compared with Thermo-Calc predictions. In addition, thin foils were prepared and characterized to identify secondary phases in the samples. The discussion is focused on the influence of the secondary phases on creep deformation and failure

Influence of thermal ageing on the creep behaviour of a P92 martensitic steel

It is well established that thermal ageing reduces the life of engineering components operating at elevated temperature. However, there is still not an accurate approach to estimate the change in the service life as a result of thermal ageing of the material. In this study, the effect of accelerated thermal ageing, for a thousand hours prior to creep testing at a temperature of 650 ℃ for a martensitic P92 steel has been compared to un-aged steels. The effect of thermal ageing on a primary-secondary stress dependent creep model has been explored, by conducting a set of uniaxial creep experiments on both un-aged and aged P92 steel. It was observed that thermal ageing enhanced the creep deformation for a narrow range of stresses, and systematically reduced the creep rupture life. These applied for conditions by up to a quarter of the original life. Results from creep crack growth tests for similarly aged material reduced incubation time and accelerated the crack growth rate. However, creep stress relaxation tests revealed no evidence of the influence of thermal ageing on rate of relaxation

Comparison of measured and modelled residual stresses in a welded P91 steel pipe undergoing post weld heat treatment

The process of fusion arc welding of steel pipes in power generation plants induces residual stresses which may be detrimental to the integrity and endurance of plant pipelines. P91 is high-grade steel used in the construction of pipelines carrying hot steam at high pressure, conditions which cause creep during service. Welded P91 pipes are usually subjected to post-weld heat treatment (PWHT) to mitigate the magnitude of residual stresses and temper the material, hence improving its resistance to creep. In this paper, the finite element (FE) method of modelling residual stresses due to PWHT in a circumferentially butt-welded P91 pipe is presented. The PWHT hold temperature is 760 °C. The paper describes the X-Ray Diffraction (XRD) and Deep-Hole Drilling (DHD) experimental techniques and how they are applied to measure residual stresses in the welded P91 pipe after PWHT. The material property data, necessary for the FE simulation of PWHT, has been obtained from stress-relaxation tests on P91 uniaxial tensile specimens at 760 °C. Good agreements have been achieved between the results of the FE method and the two sets of experimentally-measured residual stresses

On the evaluation of the Bauschinger effect in an austenitic stainless steel—The role of multiscale residual stresses

In this work, a physically based self-consistent model is developed and employed to examine the microscopic lattice response of pre-strained Type 316H polycrystalline austenitic stainless steel subjected to uniaxial tensile and compressive loading. The model is also used to explain the Bauschinger effect observed at the macroscopic length-scale. Formulated in a crystal based plasticity framework, the model incorporates detailed strengthening effects associated with different microstructural elements such as forest dislocation junctions, solute atoms and precipitates on individual crystallographic slip planes of each individual grain within the polycrystal. The elastoplastic response of the bulk polycrystal is obtained by homogenizing the response of all the constituent grains using a self-consistent approach. Micro-plasticity mechanisms and how these influence the Bauschinger effect are illustrated in terms of the role of residual stresses at different length-scales. Overall, predictions are in good agreement with experimental data of the Bauschinger effect and the corresponding meso-scale lattice response of the material, with the latter measured by neutron diffraction. The results demonstrate that transient softening of the material is related to residual stresses at different length scales. In addition, the (Type III) residual stress at the micro-scale slip system level extends the strain range over which the tensile and compressive reloading curves of the pre-strained material merge

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

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