A Microstructural and Damage Investigation into the Low Temperature Impact Behavior of an HSLA Steel
Stelco, a Canadian steel making company produced a hot rolled coil of a G40.21 50WT High Strength Low Alloy (HSLA) steel. The material needed to meet a demanding Drop Weight Tear Test (DWTT) specification at -35℃. The coils produced using a coiling temperature of 540℃ did not meet the DWTT specification with mixed pass/fail results. A second coil produced using a coiling temperature of 500℃ met the DWTT specification, but still with mixed results. To investigate this, a two-part study was undertaken. The first part investigated the effect of coiling temperature on the microstructure and CVN impact behavior. It was found that 540℃ coiling temperature resulted in a coarser grain size but less initial void/inclusion content. Furthermore, CVN testing showed that the material coiled at 540℃ had better impact toughness. The second part investigated the mixed pass/fail results for the DWTT and CVN tests conducted at -35℃ for the material coiled at 500℃. The results showed that there were no obvious differences between the pass and failed specimen in terms of microstructure, initial porosity, damage evolution and impact testing behavior, which suggests that the test temperature of -35℃ was within the ductile-brittle transition of this material.