Metallurgy and Mechanism of Underwater Wet Cutting Using Oxidizing and Exothermic Flux-Cored Wires

This paper considers the metallurgical processes of dissociation, ionization, oxidation, deoxidation, and dissolution of oxides during underwater wet cutting. A multiphase mechanism of underwater wet cutting consisting of working and idle cycles of the electrical process in a pulsating vapor gas bubble is proposed. A model of arc penetration into metal due to metal oxidation and stabilization of the arc by the inner walls of a narrow kerf is proposed. For underwater cutting of 10 KhSND, 304L steel, CuAl5, and AlMg4.5Mn0.7 alloy, we provide a principle of modeling the phase composition of the gas mixture based on high oxygen concentration, improving ionization, enthalpy, heat capacity, and thermal conductivity of plasma through the use of a mixture of KNO3, FeCO3, and aluminum. The method of improving the thermophysical properties and ionization of plasma due to the exothermic effect when introducing Fe3O4, MoO2, WO2 oxides and Al, Mg, Ti deoxidizers is proposed. Although a negative effect of refractory slag was revealed, it could be removed by using the method of reducing surface tension through the ionic dissolution of refractory oxides in Na3AlF6 cryolite. In underwater cutting of 10 KhSND and 304L, the steel welding current was 344–402 A with a voltage of 36–39 V; in cutting of CuAl5 and AlMg4.5Mn0.7 alloy, the welding current was 360–406; 240 A, with a voltage of 35–37; 38 V, respectively, with the optimal composition of flux-cored wire: 50–60% FeCO3 and KNO3, 20–30% aluminum, 20% Na3AlF6. Application of flux-cored wires of the KNO3-FeCO3-Na3AlF6-Al system allowed stable cutting of 10KhSND, AISI 304L steels, and CuAl5 bronze with kerf width up to 2.5–4.7 mm.

Fatigue and Strength Performance of Underwater Fillet Welds and Broco® Underwater Cutting Edges

This paper presents the results of fatigue and strength tests for: 1) cruciform joints with the fillet welds performed underwater, and 2) test coupons with one sided cut by Broco® underwater cut. The laboratory testing was conducted in air. The results show that the under-water fillet welds can have root cracking. For large root cracks this results in fatigue performance that is lower than the equivalent in-air welds without root cracks. In the absence of root cracks the DNV [3] approach for cruciform joints is shown to be applicable. The Broco® underwater cut edge cuts performed above the DNV category F3 [3] fatigue performance for the in-air environment, but below the F1 category. Hardness testing on the cut edge heat affected zoned surface showed values as high as 38 HRC. This is typically a concern for hydrogen stress cracking in offshore environments, and thus the using any category above the F3 curve likely is not justified. For in air testing the strength of the underwater welds and the Broco® underwater cut edges showed no appreciable strength reduction vs. what would be expected from conventional flame cut edges.