Relationship between Grain Size and Sample Thickness on the Creep-Rupture Performance of Thin Metallic Sheets of INCONEL Alloy 740H

A study was conducted on INCONEL (R) alloy 740H (R) to examine the role of grain size and sheet thickness on the alloy's creep-rupture behavior. Three different starting sheet thicknesses were utilized and multiple heat-treatment conditions anticipated for compact heat exchanger (CHX) manufacturing were applied to produce a range of grain sizes. Creep-rupture testing was conducted at 750 degrees C for times up to about 6000 h and the results were compared to wrought databases. The data show that both creep strength and ductility were important factors in the overall creep performance of the sheets. Reductions in performance were observed due either to accelerated creep when grain size was fine or loss of rupture ductility when grain sizes approached the sheet thickness. Some combinations of heat-treatment and thickness were able to produce typical expected wrought creep properties. Historically a 'rule-of-thumb' requirement for creep testing suggests 3-5 grains per sample minimum dimension to ensure homogeneous behavior. This research shows that to ensure representative wrought creep performance (i.e. no effect of sample size), the sample minimum cross-section should be 10 times the average ASTM grain size. Statistical analysis of the microstructures suggests the population of larger grains as a controlling feature in creep failure.

Automated summary

This study investigated the influence of grain size and sheet thickness on the creep-rupture behavior of INCONEL (R) alloy 740H (R). It was found that both creep strength and ductility were crucial for the overall creep performance of the sheets. Reductions in performance were observed when the grain size was fine or when the grain sizes approached the sheet thickness, leading to accelerated creep or loss of rupture ductility.