Investigation of CGHAZ Simulations of Low-Carbon Bainitic Steels

Abstract

The effect of ten different combinations with various amounts of niobium (0-0.6 wt.%) and chromium (1-4 wt.%) on weldability and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed bainitic steel were investigated. Two compositions were alloyed with boron to increase the hardenability, and two with titanium to improve the toughness properties in heat affected zone. The target of the study was to produce steel with 700 MPa yield strength combined with good impact toughness. Coarse grained heat affected zone (CGHAZ) simulations were performed using the Gleeble 3800 thermomechanical simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t8/5) of 5 s and 15 s to simulate different heat inputs in actual welding procedure. Microstructures were characterized using light optical microscopy, and hardness profiles of simulated heat affected zones were determined as well as Charpy-V impact toughness at-40 °C and-60 °C. Shorter t8/5 time (5 s) produced generally better impact toughness properties compared to longer t8/5 -time (15 s). Steels with 4 % Cr had the highest impact energies. Generally, more softening occurred with longer t8/5-time (15 s). However, Cr and Nb alloying decreased the amount of softening in the CGHAZ region, especially with longer t8/5 -time. These results indicate that even with higher t8/5 -time, it is possible to achieve strength properties equivalent to the base material in the CGHAZ region by Cr and Nb alloying.