Effect of zirconium addition on the impact toughness of the heat affected zone in a high strength low alloy pipeline steel

The effect of zirconium additions on the impact toughness of the heat affected zone in a high strength low alloy pipeline steel was studied, and the corresponding toughening mechanism examined when the welding was conducted with large heat input. The welding of steels was simulated on a Gleeble 2000. Microstructural observations, energy dispersive X-ray spectroscopy and diffraction analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The impact toughness of the heat affected zone was improved by addition of zirconium into steel, especially when a large welding heat input was used. This improvement is attributable to MnS precipitation on the pre-formed ZrO2 as well as the formation of intragranular ferrite.