Manual Gas Tungsten Arc Welding Research Articles

Duplex Stainless Steel Welds and their Susceptibility to Intergranular Corrosion

Duplex stainless steels (DSS) as alternatives to conventional austenitic stainless steels for the construction of pipelines is becoming more wide-spread, particularly for sour service applications where corrosion resistance / stress corrosion cracking resistance is required in aggressive chloride /sulphide environments. While these steels show many superior characteristics, limitations are associated with the welding of these steels, particularly controlling the weld structure and properties and understanding how the weld metallurgy may influence the susceptibility to intergranular corrosion (IGC). The focus of this paper is to report on the findings from a detailed study of the various weld sections within a DSS pipeline, as a function of heat input and type of weld, in terms of the metallurgical structure, composition and mechanical properties and to assess the susceptibility to IGC. Welding was performed using the manual Gas Tungsten Arc Welding (GTAW) technique at both high and low heat input conditions. Two different join configurations (double bevel single V bevel and double bevel single U joint configuration) were adopted. Structural analysis consisted of (i) optical microscopy of welded specimens; (ii) ferrite content determination; (iii) Vickers hardness measurements; (iv) Charpy impact studies and (v) transverse tensile testing. Two test methods namely a modified ASTM A262 and a modified Double Loop Electrochemical Potentiokinetic Reactivation (DL-ERP) test was used to determine susceptibility to IGC. The electrolyte solution used was 0.5M H2SO4 + 0.001 M TA (thioacetamide). The test was conducted at 60 °C. The potential was scanned from -500 mV (SCE) to +200 mV (SCE) and back to 500 mV (SCE) at a rate of 1.67 mV/s. The ratio of the reactivation charge to the passivation charge was calculated. From the results obtained it can be shown that the fill region was most susceptible to IGC for all weld conditions.

A sensing torch for on-line monitoring of the gas tungsten arc welding process of steel pipes

Non-intrusive and real-time monitoring techniques are increasingly required by manufacturing industry in order to detect flaws in arc welding processes. In this work the development of an optical inspection system, for monitoring the manual gas tungsten arc welding (GTAW) process of steel pipes, is described. The arc plasma visible emission produced during the process was acquired and spectroscopically analysed. Measuring the intensities of selected argon emission lines allowed real time calculation and recording of the axial electron temperature of the plasma. Experimental results showed that the temperature signal varies greatly in the case of instabilities of the weld pool that cause weld defects. A suitable algorithm, based on a statistical analysis of the signal, was developed in order to real time flag defective joints. It is shown that several weld defects such as porosity, dropout, lack of fusion, solid inclusions and craters were successfully detected in a production environment. The performances of the optical sensor were compared with the results of state-of-the-art post-weld controls such as x-rays and penetrating dyes, showing good agreement and thus demonstrating the validity of this quality monitoring system.

Effects of filler metal composition on joining properties of alloy 690 weldments

This work investigates the influence of filler metal composition on the corrosion resistance and mechanical properties of alloy 690 weldments. Alloy 690 (61 wt.% Ni, 30 wt.% Cr) was used as the base metal. Inconel I-52 (61 wt.% Ni, 29 wt.% Cr) and I-82 (73 wt.% Ni, 20 wt.% Cr) rods were used as filler metals. Manual gas tungsten arc welding was performed using four weld passes in three layers for a single-V groove butt weld. The subgrain structure near the centerline of the fusion zone middle layer was cellular and columnar dendritic in the I-52 weld, but was dominantly equiaxed dendritic in the I-82 weld. Both had white particles dispersed in the fusion zone. The I-82 weld had more white particles and a denser subgrain structure. Compositional analysis showed the I-52 weld's interdendritic region had higher Al, Si, Ti, N content than the dendritic core. The I-82 weld's interdendritic region had higher Al, Si, and Nb content than the dendritic core. The Ni and Cr content of interdendritic white particles of the I-52 and I-82 welds decreased to 38, 23 wt.% and 11, 9 wt.%, respectively, much lower than the base and filler metals. This situation causes high corrosion at the white particle sites during Modified Huey testing. Thus, the I-52 weld had better corrosive resistance than the I-82 weld. However, I-82's superior weld joint strength is attributed to its finer fusion zones subgrain structure and giving it higher tensile strength and elongation.