Effect of composition variation and service environment on the long term ageing of 316H austenitic stainless steel

Abstract

Type AISI 316H austenitic stainless steel AGR boiler components have shown different creep performances during extended service at ~505 oC. In this thesis, the chemical composition of three ex˗service superheater bifurcations from different steel manufacturers have been analysed, and then compared using a statistical procedure. The results demonstrate that these samples have different specific compositions and yield strengths affecting their creep performance. The microstructures after long term ageing were characterised, and it was demonstrated that certain chemical elements, notably Si and P, have a role in the formation of precipitates, and the partitioning or segregation of elements to these phases at grain boundaries. Different routes of microstructure precipitation have been proposed, and the role of these microstructures on creep time to fracture was addressed regarding the promotion of creep cavitation. The effect of CO2˗rich AGR coolant on the performance of these components was investigated using autoclave samples that had been exposed to temperatures 560 and 600 oC for differing durations. The composition (especially Cr and Si), temperature and ageing times influenced oxidation and carburisation rates, with both mechanisms acting synergistically. Several methods were adopted to quantify oxidation and carburisation, including HS˗AFM to map carbide distributions. The degradation of mechanical properties due to carburisation was evaluated using microcantilever tests to monitor changes in Young’s modulus and creep embrittlement. Three different levels of carburisation were quantified and the results were used to model creep rate in a simulation. The simulation indicated that the creep rate decreased as the precipitation number density increased, with an associated potential reduction in the creep time to fracture. Accordingly, low temperature carburisation was identified as having a potentially significant influence on limiting the long term service performance of 316H boiler components by the promotion of surface creep cracks and an associated reduction of the component section.

Date of Award	20 Jun 2017
Original language	English
Awarding Institution	University of Bristol
Supervisor	Tom Scott (Supervisor) & Peter E J Flewitt (Supervisor)