Method for assessing remaining lifespan of bolt used at high temperatures

The present invention is intended to provide a method for assessing remaining lifespan of a bolt with higher precision than conventional methods, which can be applied to a bolt comprising a high alloy with fewer tissue changes caused by creep damage. The present invention provides a method for assessing the remaining lifespan of a bolt used at high temperatures, including: carrying out a creep test for a bolt material that constitutes a bolt to be assessed and determining a life-time and a life-elongation rate; determining a time-temperature parameter based on the life-time and the creep test temperature; creating a lifespan-assessment diagram by plotting the time-temperature parameter with respect to the life-elongation rate; measuring a elongation rate between before and after the use of the bolt to be assessed; and assessing the remaining lifespan of the bolt with the use of the elongation rate between before and after the use of the bolt to be assessed and the lifespan-assessment diagram.