Effect of specimen size on ductile to brittle transition behavior of martensitic 9Cr steels after various heat treatments

The effect of specimen size on the ductile to brittle transition behavior has been investigated using the full-size and 1/3-size Charpy V-notch specimens of martensitic 9Cr-WVTa steels after quenching, tempering and subsequent prolonged aging treatments. The upper shelf energy (USE) and ductile to brittle transition temperature (DBTT) were widely changed by the heat treatments; the USE varied from 90 to 292 J, and the DBTT from 188 to 325 K. The nominal volume (Bb)(3/2), where B is the specimen thickness and b the ligament size, was effective as a normalization factor for the USE only in the large USE region above 210 and 8.5 J for the full-size and 1/3-size specimens, respectively. A new normalization factor depending on the USE level was proposed. Although the relationship between the DBTT for the full-size and 1/3-size specimens was different from steel to steel, the shift in DBTT caused by the heat treatments was described by Delta DBTT1/3-size = (1.2 +/- 0.1)Delta DBTTfull-size for all the steels examined.