Hot forming and welding simulation of 9-12% Cr P92 creep resistant steel

Abstract

In this thesis, the behaviour of three P92 steels with different chromium, molybdenum and tungsten contents, all within the ASME specification were studied. Axisymmetric compression testing and welding simulations were done using Gleeble 3500 thermo-mechanical equipment. Single-hit compression testing for warm deformation (575-650 °C and 0.001-0.5 s-1) and hot deformation (850-1000 °C and 0.1-10 s-1) were done. The welding simulations of the heat-affected zone (HAZ) regions focussed on the intercritical HAZ and fine-grained HAZ, the narrow regions near the parent metal (PM).Warm and hot deformation uniaxial compression tests for the three steels were done. The overall results showed that the flow stress decreased with an increase in the deformation temperature and vice versa. The flow behaviour for the three steels was mainly controlled by dynamic recovery. The flow stress increased with an increase in chromium content, as chromium enhanced precipitation hardening. Two constitutive models were successfully applied to analyse the flow stress, the Arrhenius-type and a simple physically-based model. These models were used for predicting the flow stress behaviour during deformation. Statistical results showed that predicted and experimental flow stress data had a good correlation. A dynamic material model was used for predicting deformation optimisation. The results showed that the optimum processing conditions for the three steels occurred at higher temperatures and lower strain rates. The heat-affected zone (HAZ) regions of the three steels were studied. The peak temperatures for simulation were: 900 °C of the intercritical (ICHAZ) and 950 °C for fine-grained (FGHAZ). The ICHAZ and FGHAZ. The samples were subjected to two sets of tempering after weld simulation: conventional post-weld heat treatment (PWHT) 760 °C/2 h followed by air cooling, or re-austenitisation at 1050 °C/40 minutes then air cooling followed by PWHT (denoted as r-PWHT). The three steels’ results showed that the microstructure of as-simulated ICHAZ and FGHAZ contained undissolved precipitates, while after PWHT and r-PWHT had coarse and fine precipitates along the grain and lath boundaries. The r-PWHT treatment resulted in homogenisation of the simulated weld joint microstructure. The mechanical properties of the three steel were: lower hardness values, and the Charpy toughness values were higher than the recommended value of 47 J for weld joints for the heat-affected zones (ICHAZ and FGHAZ) after post-weld heat treatments