EFFECT OF OXIDIZING ELECTRODES AND POLARITY ON HYDROGEN MITIGATION IN UNDERWATER WET WELDING

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

05/05/1997

RESUMO

Weld metal hydrogen pickup in underwater wet welding is unavoidable due to the presence and dissociation of water vapor surrounding the welding arc. In conjunction with the fast quenching nature of the water environment, the high hydrogen content in the weldment leads to hydrogen cracking in the heat-affected zone. This undesirable occurrence has been successfully overcome with the use of oxidizing-type electrodes which lowers the diflusible hydrogen content. Despite their unique behavior in terms of both low diffusible and low total hydrogen content, there seems to be very little research work undertaken to identify the influence of the slag and the effect of polarity on hydrogen absorption during underwater wet welding. The aim of this investigation has been placed on the thorough understanding of hydrogen pickup by the slag as well as the effect of welding current and polarity (Direct Current Electrode Negative - DCEN and Direct Current Electrode Positive - DCEP) on weld metal diffusible hydrogen. To accomplish this purpose, twenty experimental oxidizing electrodes with systematic ferric oxide (Fe2O3) additions, ranging from 0 to 70 wt. pct., to the flux system were investigated. The mole fraction ratio of CaO:SiO2 in the flux ranged from 0.05 to 0.35, independently of ferric oxide additions. Underwater gravity welds were deposited on ASTM A36 steel coupons at 0.27 m (city) water depth using a gravity feed system. All welds were made using similar conditions. Weld metal diflusible hydrogen content was determined using the mercury displacement method according to current AWS standard. The measured diffusible hydrogen contents showed that DCEN produced welds with lower diffusible hydrogen contents than DCEP. As an example, with 36 wt. pct. Fe2O3 addition in the flux, the diffusible hydrogen contents were 23.2 ml/100g of deposited metal (DCEN) and 30.5 ml/100g of deposited metal (DCEP). Additionally, higher hydrogen values were always related to lower ferric oxide contents initially present in the flux, for example, 70.8 ml/100g of deposited metal (DCEP - 0 wt. pct. Fe2O3) and 30.5 ml/100g of deposited metal (DCEP - 36 wt. pct. Fe2O3). Amazingly, diffusible hydrogen as low as 13.2 ml/100g was obtained with the B3 electrode (53 wt. pct of Fe2O3 and CaO:SiO2 equal to 0.15). As such, the program results reaffirmed the effectiveness of oxidizing electrodes in controlling hydrogen pickup in underwater wet welds. X-ray diffraction analysis (XRD) conducted on different slags showed that the minimum diffusible hydrogen values were always associated with the presence of fayalite. Complementing XRD analysis, Mossbaüer spectroscopy tests carried out on different slags showed that all ferric oxide initially present in the slag had transformed to iron oxide (FeO), free or combined.The results also showed that weld metal hydrogen pickup was strongly dependent on water solubility in slags. Finally, variations in weld metal hydrogen as well as slag hydrogen content with both polarity mode and iron oxide content in the slag were successfully predicted using an electrochemical model for the slag/metal interface equilibrium. In this investigation, the slag/metal interface has been identified as responsible in controlling the hydrogen pickup. The model assumed that hydrogen is present in the slag as (OFT) ions and that FeO displays ideal solution behavior.

ASSUNTO(S)

engenharia de materiais e metalurgica soldagem wet welding

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