Effect of specimen size on small punch creep behavior of high nitrogen ferritic heat-resistant steels

The small punch (SP) creep tests were carried out on different high nitrogen ferritic heat-resistant steels in the temperature range of 650-800 °C, in the load range of 55-400 N, using disc-type specimens measuring ∅8 × 0.5 mm (∅8 mm) and ∅3 × 0.25 mm (∅3 mm). The influence of specimen size on SP creep behavior and on the conversion of SP creep rupture results to conventional uniaxial creep rupture results were investigated. The creep deflection and rupture life of ∅3 mm specimens were much smaller/shorter than those of the ∅8 mm specimens. The experimentally determined SP load-uniaxial stress conversion coefficients to correlate SP and uniaxial creep rupture life were different for the two specimens; 2.05 and 0.49 for ∅8 mm and ∅3 mm specimens, respectively. These observed effects were considered as virtual specimen size effects as these could occur not only due to specimen size but also due to geometrical parameters of the test rig. The conversion coefficients were found to be independent of the type of steels investigated. The conversion coefficients determined from finite element analysis, 2.03 and 0.47 for ∅8 mm and ∅3 mm specimens, respectively, were in close agreement with those obtained from experimental results. Based on the close agreement between the experimental creep rupture life and that predicted using finite element analysis results, it was found that there was no significant actual specimen size effect on the creep rupture life.