Development of a small punch testing method to evaluate the creep property of high Cr ferritic steel: Part I-Effect of atmosphere on creep deformation behavior

In a preliminary study carried out for evaluating creep properties with the small punch (SP) creep testing method, creep tests were performed using reduced activation ferritic/martensitic steel (RAFMs) F82H specimens both in an argon atmosphere and in vacuum (less than 10^-3 Pa). The results showed that the rupture time was approximately 2.5 times longer in the argon atmosphere compared to that in the vacuum. Changes in the deflection rate with the deflection and time and changes in the minimum creep deflection rate with the rupture time were almost independent of the test atmosphere, although the minimum creep deflection rate decreased as the rupture time increased. After comparing the SP creep test results, which were sorted by using the Larson-Miller parameter, and the standard creep test results, the ratio (F/σ) between the load (F) in the SP creep test and the stress (σ) in the standard creep test was determined to be 2.1 and 2.3 for the argon atmosphere and vacuum, respectively. Consequently, it was found that the ratio, namely, the load/stress conversion coefficient, depended on the test atmosphere. This result implied that the stress and/or strain distribution on the SP specimen varied with the type of atmosphere. This change with the test atmosphere seemed to be closely associated with changes in the friction between the specimen and the ball.