Acceptability assessment of casting defects under transient thermal loading

Abstract

Weld repairs of castings during manufacture is a well-known and acceptable practice if conducted in accordance with approved standards and procedures. However, either original casting induced cracks or cracking of these weld repairs in high temperature and aging plant is not uncommon. Structural integrity assessments of such components must not only consider reduced material toughness, due to temper embrittlement, but also the stress intensities generated during transient thermal events such as start/stops and quenching incidents. In the first of two case studies the acceptability of weld repair defects in a high pressure turbine outer casing is presented. Metallurgical assessments concluded significant temper embrittlement had occurred which was taken into account in a finite element based structural integrity assessment that considered design operating conditions as well as a hypothetical quenching event. Low pressure turbine bypass valves are considered in the second case study. In this case the material temper embrittlement was found not to be as severe but the criticality of pre-warming to reduce transient thermal stress and by extension crack stress intensities during trips or shutdowns was clearly demonstrated. Remaining life assessments concluded the defects to be acceptable for operation to the next planned outage.