Variation in microstructures and mechanical properties in the coarse-grained heat-affected zone of low-alloy steel with boron content

The correlation between mechanical properties and boron content in the coarse-grained heat-affected zone (CGHAZ) of low-alloy steel was investigated using 10 ppm and 30 ppm boron added low-alloy steels. A Gleeble system was used to simulate various CGHAZs as a function of heat input. The segregation behavior of boron in the CGHAZ was estimated through secondary ion mass spectrometry (SIMS) and particle tracking autoradiography (PTA). Vickers hardness and Charpy impact tests were performed in order to assess the mechanical properties of the CGHAZs. In the steel containing 30 ppm of boron, boron segregation was relatively high even with low heat input, and increased compared to 10 ppm boron added steel. Furthermore, the boron segregation level maintained a maximum at intermediate heat input conditions from 60 to 500 kJ/cm. This is believed that the boron segregation can be reached a maximum level for relatively low heat input despite the faster cooling rate because of an increase in the boron segregation rate. Also, impact toughness decreased with increased boron content at identical heat input conditions. It is believed that the effect of segregation and precipitation of boron on deterioration of impact toughness is insignificant. However, deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as the martesite-austenite (M-A) constituent, and the hardening of the matrix with increased boron content.