Joint Penetration Monitoring in Low-Frequency Pulsed GTA Root Pass Welding of Medium-Thick Steel Plates

Welder-dependent manufacturing is no longer suitable for the modern production of a high-performance nuclear pressure container. The high-quality root pass welding of medium-thick steel plates is the main challenge to obtain a sturdy reactor vessel, especially to generate one-sided welding with back-formation bead without a backing. Herein, low frequency and large duty-cycle pulsed gas tungsten arc welding (GTAW) was employed to weld the medium-thick steel plates with a 5-mm root face and 2-mm root opening. The arc characteristic and weld pool dynamic behavior in the proposed GTA root pass welding was investigated by a high-speed camera, and a deflection phenomenon of arc tail flame was first found. The correlations of the characteristic parameters of the arc tail flame, including the deflected angle and length, with the weld joint penetration and welding speed were also analyzed in detail. The results showed a negative correlation to the welding speed and a positive correlation with the weld joint penetration. A sound weld bead was formed at a range from 15 deg and 20 mm to 19 deg and 27mm. Based on the above relationship, a new method using these two characteristic parameters was proposed to identify the weld joint penetration in the root pass welding, and its fundamentals were completely demonstrated by the dynamic change of the keyhole. Its feasibility was also demonstrated by the experiment combined with the weld pool dynamic-dependent theoretical analysis.