Evaluation of properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings

Prior work on a failed HY-80 Bridge Access Trunk (BAT) casting indicated the cause of failure to be improper processing techniques. As-received HY-80 casting material showed a non-homogenous microstructure with two distinct microconstituents present: undertempered martensite and a layered martensite–ferrite structure. Heat treatment temperatures from in the intercritical range (721–799 °C) produced microstructures that differ from the desired uniform microstructure of tempered martensite and were similar to those found in the failed casting. In order to further examine the relationship between processing and microstructure, it was decided to vary the time for which the steel was held in the intercritical temperature range. This additional work was warranted by the medium intercritical heat treatment results in the previous study [Holthaus JE, Koul MG, Moran AL. Property and microstructure evaluation as a function of processing parameters: large hy-80 steel casting for a US navy submarine. Eng Fail Anal 2006;13(1):1397–409] and its similarity to the failed casting microstructures. An important finding of this study is that, contrary to normal behavior during tempering, HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. To confirm the hypothesis that the presence of brittle martensite formed by improper heat treatment was the cause of failure, SEM section fractography was employed to directly examine the microstructure underlying the fracture surface and to identify a correlation between microstructure and fracture mode.