A novel method to characterize low-temperature brittle failure at various pre-strains

Abstract

Abstract. A new characterization method for the determination of the pre-strain dependent ductile-to-brittle-transition-temperature (DBTT) of steel is presented in this paper. From four different types of mild steel circular cups were deep drawn. Out of these cups, rings were cut, which were used as Charpy impact test specimens. The Charpy-notch was placed at two different positions along the circumference of the ring: due to the steel anisotropy (rolling direction) the side walls of the cup have variations in strain, hence, notches are placed at areas with high, or low level of forming pre-strains. Using the ring samples, Charpy impact tests have been carried out at various temperature levels and the absorbed energy was measured as a function of temperature. The test results have been processed into material-specific DBTT values, including the dependency of pre-strain. As a result: high pre-strains generally raise the DBTT and therefore increase the risk of brittle failure in a low-temperature impact load. The pre-strain dependency of the DBTT varies for different types of mild steels – this is likely due to differences in the chemical composition and microstructure. Interstitial free steel with a near zero amount carbon and no added boron is most sensitive to forming pre-strain. The addition of boron in general shifts the DBTT to lower temperatures and reduces the pre-strain sensitivity.