This paper presents a summary of recent testing into bolt relaxation, which occurs at temperatures above 230°C (450°F). Bolt relaxation is problematic for pressure boundary bolted joints as it can lead to joint leakage in the longer term. In addition, it should be a major consideration at the design phase, with selection of the appropriate bolt material limited not by the creep or yield limit of the material, but by the point where bolt relaxation becomes significant.
Preliminary test results for different bolt materials at different temperatures are presented in this paper. The test results allow for some very high level observations regarding the acceptable design limits for different bolt and nut material combinations.