Hardness Distribution in Weld Heat Affected Zone of Boron Bearing Low Carbon Steel

Abstract

Hardness distribution in heat affected zone (HAZ) of boron bearing 780 MPa grade high tensile strength steel was investigated. Welded joint was produced by submerge arc welding with 4.5 kJ/mm heat input. The Vickers hardness was 370 on HAZ 2mm distant from fusion line. The Vickers hardness decreased ti 300 on fusion line. In simulated HAZ test, raising the temperature from 1273 K to 1623 K decreased the Vickers hardness from 340 to 280. These results show that simulated HAZ test can simulate hardness distribution in HAZ in the welded joint. In simulated HAZ test at 1273 K, increasing the holding time from 5 seconds to 3600 seconds decreased the Vickers hardness from 370 to 300, although increasing the holding time at 1623 K did not change the hardness. The cause of the hardness change in simulated HAZ test can be explained in the following. McLean’s theory about equilibrium segregation shows that the raising the temperature decreases boron content at grain boundaries. In addition, rapid grain growth during heating causes a concentration of boron at grain boundaries (called dragging effect). In the area heated up to 1273 K, boron concentrates at grain boundaries over equilibrium amount by the dragging effect. After grain growth, boron content at grain boundaries decreases to equilibrium amount at 1273 K by diffusion. On the other hand, the area heated to 1623 K has a lower boron content than 1273 K at grain boundaries. The reason for this is that the high diffusive rate of boron cancels the dragging effect, and boron content decreases to the equilibrium amount at 1623 K lower than 1273 K. It is concluded that the Vickers hardness of 300 on fusion line increased to 370 at HAZ 2 mm distant from the fusion line, and this hardness distribution can be explained by the change of boron content at grain boundaries.