High-strength and high-toughness weld of S690QL HSLA steel via a high-speed high frequency electric cooperated arc welding without additional heating measures

Problems in balancing joint strength and toughness, caused by the strict requirements of welding heat, have been a long-troubling problem that attracts close attention in the High Strength Low alloy (HSLA) steel welding field. Additional heating measures have been considered as an only and necessary solution with limited effect to deal with the above issue, which not only complicate the welding process but also have some incidental impacts. In this paper, a novel method named High Frequency Electric Cooperated Arc Welding (HFAW) was employed to join typical HSLA steel S690QL without additional pre- or post-heating measures. Relationships between the welding thermal characteristics, microstructure and mechanical properties of different parts in the joint were revealed and compared. Results showed that sound HFAW joint was achieved with a welding heat input and speed of 3630.6 J/cm and 2.3 cm/s, while that in conventional GMAW was 7077.1 J/cm and 0.5 cm/s, respectively. The width of the softening zone in the optimal HFAW joint was narrow to 0.95 mm. Lath Martensite and Bainite were generated in the coarse grain zone with an average grain size of 14.9 μm, rather than coarse feathery Bainite in the GMAW joint (with doubled grain size). Benefitted from the fine grain size, better phase composition, narrow and slight softening, the HFAW joints exhibited excellent tensile strength, moreover, the impact toughness at hardened zone and softening zone reached 0.87 J/mm2 and 1.15 J/mm2, which were at least 60% higher than that in the GMAW joint (0.58 J/mm2 and 0.61 J/cm2).