Abstract Scope |
Introduction
Systems of offshore oil production with floating platforms are becoming predominant worldwide. In service, the structural elements reach a maximum depth of about 20 meters. To these depths, wet welding can be considered as an attractive technique for in situ repairs, since it has the advantages of simplicity, low cost, and accessibility. This work presents results from a research program whose main objectives were to develop consumables capable of producing structural quality underwater welds by the SMAW process. The most important one was the development of low alloy ferritic oxyrutile coated electrodes capable of producing welds with a tensile strength greater than 70ksi, good ductility, low porosity, and low diffusible hydrogen content down to 20m. Relationships between weld metal chemical composition and mechanical properties are presented. Envelopes of weld metal %C, %Ni, or %Mo versus Tensile Strength and Elongation are suggested. In light of these results, the possibility of producing welded joints meeting the requirements of Class A welds is discussed. In general, the test coupons were welded in laboratory. Field welding trials with diver-welders were also performed for comparison.
Experimental Procedures
The welding tests were performed in a hyperbaric tank using a mechanized gravity system at simulated water depths of 20m maximum. An electronic CC source operating in DCEN was used. The welding current ranged from 160A to 190A and the voltage ranged from 23V to 28V. The test coupons were V grooved butt joints 250mm x 250mm x 16mm or 19 mm thick.
The base metals were ASTM A 131 / A 131M - 08 ship steels in the following grades:
A: Tensile Strength = 440Mpa, C% = 0.167, Si = 0.209%, Mn = 0.780, and CE = 0.290;
AH36: Tensile Strength = 5180Mpa, C = 0.13%, Si = 0.296%, Mn = 0.980, and CE = 0.30.
Tests performed were: all-weld-metal tensile test, transversal tensile test, Charpy V, bending, macrography, micrography, and hardness HV10.
The metallographic samples were cut transversally to the weld bead, polished with diamond paste, and etched with Nital 2%. The amounts of the weld metal micro-constituents, ferritic grain size, and porosity were measured by optical microscopy. Chemical composition was determined by optical spectroscopy. The weld metal hardness (HV10) measurements were carried out in the metallographic samples. Tensile tests, both longitudinal and transversal, followed the specifications of the AWS D3.6M: 2017 code. Reduced cylindrical specimens with 25mm base length and 5mm diameter were used in the all-weld-metal tensile tests.
The experimental 3.25mm oxyrutile electrodes were manufactured in an ESAB industrial production line and insulated with vinyl varnish. The quantities (wt%) of the raw materials present in the coatings were hematite: 20 to 47; rutile: 14 to 50; quartz: 19 to 34. Limestone, Fe-Mn, feldspar, nickel powder, and Fe-Mo participated in small amounts.
The diffusible hydrogen was measured by the gas chromatography method in samples welded at 0.5m water depth.
Results and Discussion
With increasing Ni, Mo, and C contents, Tensile Strength and Hardness increase and Toughness decreases.
Yield Strength increases with Ni and Mo contents and is not significantly influenced by C content.
Elongation decreases very little with Mo and C contents and is not influenced by Ni content.
Mo seems to be the most influential element on mechanical properties as it favors the formation of isolated carbides, ferrite-carbide aggregates, the formation of acicular ferrite, and the reduction of the grain size in the weld metal re-heated zone. Elongation and hardness are a little dependent on Pcm. Tensile strength and Yield Strength, on the other hand, increase with increasing Pcm, and the toughness decreases.
Porosity had little influence on elongation. Few results were below the value of 18% and all were above 14%. Tensile strength and Yield Strength presented no significant influence of porosity.
As for toughness, all the Charpy V results at 0ºC exceeded the average value of 27J specified for Class A welds
The standard deviations of the yield strength, tensile strength, elongation, and reduction of area showed a small variability of these properties, despite the intrinsic instabilities of the wet welding process. The influences of pressure on the main mechanical properties of the weld metal of joints welded with the developed electrode at equivalent depths of 0.5m, 10m, and 20m were also evaluated. There was a small influence of depth, up to 20m.
In the field tests at 5m and 10m, the results of weld metal mechanical properties can meet the requirements of Class A in the qualification of welding procedures. Nevertheless, maximum hardness and bending capacity, which are strongly influenced by the properties of the Heat Affected Zone, are those with the highest incidence of failure and most of the time not approved in class A.
Conclusion
An oxyrutile electrode capable of producing weld welds with the ductility, mechanical strength, and toughness required to meet Class A welds according to AWS D3.6 code was developed. Tensile strength is compatible with AWS E70XX electrodes. The diffusible hydrogen contents (in the range of 20ml / 100g to 30ml / 100g) is low, compared to the commercial rutile-based electrodes, which are, in general, typically between 600 ml / 100g and 100 ml / 100g. Therefore, weldments without hydrogen cracks in the weld metal and HAZ can be produced in the wet welding of ship steels up to the depth of 20m. The elongation results obtained (in the order of 20%) are explained by the absence of micro-cracks in the weld metal. The literature shows that, in the case of rutile electrodes, elongation results above 12% are rarely achieved. These results are original and important contributions to the development of welding procedures that meet the corresponding Class A requirements of the AWS D3.6M:2017 code in the welding low and high strength ship steels (ASTM A 131 / A 131M - 08 grades A and AH36.
Depth had little influence on the main mechanical properties of the weld metal as well as the porosity up to the welding depth of 20m.
Keywords
Underwater Wet Welding
Covered Electrodes
Diffusible Hydrogen
Oxyrutile electrodes |