300-Series Stainless Steels
Austenitic stainless steels find many applications in power generation, petrochemical and aerospace industries owing to their combination of creep strength and superior corrosion resistance. But the life and integrity of welded structural systems operating at high temperature is dependent on the accumulation of in-service creep damage under complex loading conditions.
We are currently developing methods for mapping creep deformation in laboratory cross-weld specimens and quantifying the early stages of creep damage accumulation in these materials by comparing laboratory creep specimens with real ex-service components. We are also studying time-dependent creep recovery (anelasticity) that can occur during periods of load reduction, and which can have a significant effect on material performance.
High Chromium Martensitic Steels
High Cr martensitic steels, such as P91 and P92, are widely used in fossil-fired supercritical power stations because of their excellent creep strength at elevated temperatures. Their resistance to irradiation degradation also makes them leading candidate materials for use in the fusion demonstration plant, ITER, being built in France and in GEN IV nuclear reactor systems. However, welded joints made from these materials suffer from premature failure at high temperature owing to development of Type IV cracking in the heat affected zone (HAZ) adjacent to the weld.
Our research is focussed on the role of residual stress, post-weld heat treatment, microstructure and localised properties on the initiation and growth of life-limiting creep damage in welded engineering components made from P91. In this work we are undertaking a programme of creep tests on simulated HAZ microstructures and cross-weld samples (extracted from welded pipes) supported by microscopy, tomography, digital image correlation and residual stress measurements.
Advanced metal alloys are often mechanically processed at elevated temperatures (e.g. by extruding, rolling or forging) because they can be easily shaped at low stresses, and because deformation can be used to induce favourable changes in metallurgical structure. Understanding and modelling these microstructural changes is of significant academic and commercial interest, and allows the manufacturer to produce reliable components of known strength and material properties.
Our research uses small-scale laboratory forging experiments to characterise deformation and microstructure evolution in aero-engine turbine-disc alloys, including the role of recrystallisation and delta-phase precipitation in nickel superalloy IN718, and the influence of phase morphology and strain-partitioning in two-phase titanium alloys such as Ti-6Al-4V.
Electronic solders are exposed to in-service temperatures close to their melting point, and failure of component interconnects often occurs due to thermo-mechanical fatigue. At this time, serious question marks remain about the long-term structural integrity of recently introduced lead-free solders – not least because their mechanical behaviour is a complex function of temperature, stress and microstructure, as well as prior deformation and thermal history.
Our research focuses on quantifying and understanding the mechanical behaviour of lead-free solder alloys, and of the joints containing them, under the complex conditions likely to be encountered in service. Such an approach is important in supporting reliable design and life-prediction methods. We have been using our mechanical data to develop constitutive equations, based on an internal stress approach, describing the creep behaviour of dispersion-strengthened tin-based solder alloys.