Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of two high strength low alloy steels, boron-free and boron-containing, was investigated by means of weld thermal simulation test. The result shows that, for the boron-free steel, a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5 (the cooling time from 800 to 500 °C), while lath bainite for shorter t8/5. For the boron-containing steel, granular bainite is dominant for a wide range of t8/5. Continuous cooling transformation (CCT) study on the CGHAZ indicates that the transformation start temperature decreases by about 50-100 °C under different t8/5, for the boron-containing steel compared with the boron-free steel. The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5 for granular bainite transformation. However, the addition of boron deteriorates the impact toughness of CGHAZ, which may be due to a markedly increased fraction of martensite-austenite (M-A) constituents and decreased fraction of high angle grain boundaries.