The Effect of Boron on the Impact Energy of Low-Carbon Steel Weld Metal at Low Temperatures

Abstract

In this research, the effect of boron (B) on the impact energy at low temperatures of carbon steel weld metal was investigated. B was added into the flux core weld wire in the form of boron oxide (B2O3). The welding operation was carried out using the submerged arc method on steel plates. After that, the scanning electron microscopy, optical microscopy (OM), microhardness, chemical analysis and impact Charpy test were performed to investigate the B effects. The OM results showed that until 20 ppm B, acicular ferrite in the microstructure was increased, whereas Widmanstatten, polygonal and grain boundary ferrites were decreased. In addition, by increasing B from 20 to 110 ppm, the dominant structure of ferrite was made of Widmanstatten and polygonal ferrites. Microhardness test also showed that the samples containing 20 ppm B had the lowest hardness. Charpy impact test revealed that the sample containing 20 ppm B at three temperatures (25, 0 and – 20 °C) had the best results, whereas in the other samples, by increasing B, a reduction was observed in the value of impact energy. It was also found that the portion of the brittle fracture in the samples containing 20 ppm B (with the highest impact energy) was 10% at the temperature of – 20 °C, whereas this portion in the samples containing 110 ppm B (with the lowest impact energy) was 50%.