Effect of Cold-Wire Addition in the TSAW Process on Microstructure and Mechanical Properties of the HAZ of X70 Microalloyed Pipeline Steel

Microalloyed steels can achieve a good combination of strength and toughness through appropriate alloy design and thermomechanical controlled processing (TMCP). However, the mechanical properties can deteriorate as a result of the high heat input and thermal cycles that the steel experiences during welding. It is generally accepted that the portion of the heat affected zone (HAZ) adjacent to the fusion line, i.e., the coarse grain heat affected zone (CGHAZ), which is characterized by coarse grains and martensite-austenite (M-A) constituents, is the region with poorer toughness relative to the rest of the steel. In the present research work, modification to the conventional tandem submerged arc welding (TSAW) process is carried out by the addition of a cold wire during welding (CWTSAW), which induces changes to the geometry and properties of the weld joint. Microstructural analysis, mechanical property investigation and geometry analysis indicate overall improvement in the weld and the HAZ properties after cold wire addition. These improvements are explained in terms of an increase in the deposition rate and a decrease in the amount of heat introduced to the weldment. An X70 microalloyed steel was welded using both TSAW and CWTSAW processes. Charpy-V-notch impact testing and microhardness testing showed improvement in the HAZ mechanical properties for CWTSAW samples relative to TSAW samples. Microstructural analysis, using both optical microscopy and scanning electron microscopy (SEM), indicated the formation of finer prior austenite grains (PAG) and less M-A constituent within the CGHAZ of the CWTSAW samples. These improvements are due to lower actual heat introduced to the weldment and a relatively faster cooling rate.