Abstract
The degradation mechanism “relaxation cracking” is acting in austenitic components operating between 550°C (1020°F) and 750°C (1380°F). The brittle failures are always located in cold formed areas or in welded joints and are mostly addressed within 1 year service. More than 10 different names can be found in the literature for this phenomenon. A brittle failure of a heavy wall Alloy 800H reactor vessel after 6 months service was the starting point of a Joint Industrial Programme (JIP) with 30 partners. The research was focused to assess the key factors concerning relaxation cracking and how to control it. It included the effect of chemical composition of the base materials (both Fe and Ni base alloys), heat to heat variation, grain size, cold deformation, welding, operating temperature and heat treatments. More than 40 base materials and 60 welded joints were included. Many austenitic materials showed to be susceptible for relaxation cracking, especially in the welded condition and after cold deformation. Heat treatments are very effective to avoid this degradation mechanism. The JIP ended up with an Equipment Degradation Document and a Recommended Practice. To date relaxation cracking failures can be avoided by a correct selection of base materials, welding consumables and heat treatments. In-field experiences with equipment manufactured based on knowledge gained within the JIP are supporting the outcome of the results, no failures have been encountered anymore. However, failures are still identified at companies who are still not aware of the degradation mechanism relaxation cracking.
