Paulo J. Modenesi

TIG welding with single-component fluxes

Abstract Gas tungsten arc welding is fundamental in those applications where it is important to control the weld bead shape and the metallurgical characteristics. This process is, however, of low productivity, particularly in the welding of large components. The activated flux TIG (ATIG) welding process, developed by the Paton Welding Institute in the 1960s, is now considered as a feasible alternative to increase the process productivity. ATIG welding uses a thin layer of an active flux that results in a great increase in weld penetration. This effect is, generally, connected to the capture of electrons in the outer parts of the arc by elements of high electronegativity, which constrict the arc causing an effect similar to that used in plasma arc welding. Generally, the literature does not present the flux formulations for ATIG welding, the few formulations that were found to have a complex nature. The present work evaluates the use of ATIG welding for the austenitic stainless steels with fluxes of only one major component. The changes in weld geometry were compared to variations in the electrical signals from the arc and the arc shape. The effect of the flux on the weld microstructure was also studied. The results indicate that even the very simple flux that was used can greatly increase the penetration of the weld bead.

Statistical modelling of narrow-gap GTA welding with magnetic arc oscillation

Abstract Statistical experimental design and linear-regression modelling were used to study the effect of the welding parameters on the bead shape in a narrow gap-GTAW process with magnetic are oscillation. It was shown that, for the conditions studied, are oscillation has little influence on the lateral fusion of the joint, although it favourably influences the shape of the bead. The undercutting level tended to increase rapidly when the gap width was reduced. Optimized welding conditions are presented, based on models calculated from the experimental data.

The influence of small variations of wire characteristics on gas metal arc welding process stability

Abstract Gas metal arc welding (GMAW) is highly sensitive to small changes in operational variables. This sensitivity can make it difficult to optimize operational conditions and to diagnose process faults. This paper evaluates the influence of small differences in wire characteristics on GMAW-CO2 operational conditions. A total of 16 samples of AWS type ER70S-6 wire were produced from the same steel batch with small and controled differences in diameter, mechanical strength, cast and helix. For each sample, bead-on-plate welding trials were carried out using the same wire feeding rate (6 m min−1) over a voltage range of 13–27 V. Welding current and voltage were recorded by a microcomputer using an A/D system. Spatter produced during each welding trial was collected, weighed and compared with the weight of the weld bead. Data were evaluated using factorial analysis and graphical techniques to assess the effect of different wire characteristics on the welding process. Results showed that differences in wire diameter produced the most important changes in process characteristics. These changes were related to differences in fusion rate and caused a small drift in optimal operational conditions. The other characteristics analyzed produced only minor changes in process characteristics.

Computer simulation of stress distribution during Vickers hardness testing of WC-6Co

This paper describes a numerical simulation and experimental study of the Vickers indentation testing of WC-6Co specimens. The numerical analysis was implemented by a three-dimensional finite element (FE) model using the commercial solver MARC™. Hardness values predicted by this model agreed well with those obtained experimentally. It was also observed that the load-displacement curves obtained numerically were quite similar to those presented by the literature for the Vickers testing. The maximum principal stress field was used to locate the most expected areas for crack formation and propagation during the Vickers indentation testing of WC-6Co.

DETERMINATION OF CCT DIAGRAMS BY THERMAL ANALYSIS OF AN HSLA BAINITIC STEEL SUBMITTED TO THERMOMECHANICAL TREATMENT

CCT diagrams are broadly used to predict the microstructure and mechanical properties after thermal treatments. Most of these curves are determined by dilatometry with or without deformation prior to cooling. Dilatometry, however, even when straining is present, applies little deformation to the sample as compared to that imposed during an industrial process such as hot rolling. An alternative and attractive technique, however, is torsion testing, and this has ben successfully used to simulate industrial rolling schedules. The use of torsion testing and thermal analysis may yield cooling curves from a deformed austenite, which may be more suitable for the rolling conditions. Therefore, it seems preferable to apply the last technique to obtain information about the behavior of the austenite transformation after the samples had been deformed by torsion according to a rolling schedule. Among the industrial rolling processes available for simulation, controlled rolling of microalloyed steels remains great technological interest because it results in a fine grain microstructure, providing a high strength and good material toughness. HSLA low carbon steels with bainitic or polyphase microstructure have been the subject of countless scientific works in the last decades.

Comparative study of methods for determining the diffusible hydrogen content in welds

Summary This paper presents a comparative study on the glycerine, mercury and gas chromatography methods for determining the diffusible hydrogen content in welded joints. The effects of the variables of time and temperature for hydrogen collection are also considered. AWS type E9018‐M electrodes were wetted, then dried at different temperatures for different times in order to obtain a wide range of diffusible hydrogen contents. The results showed that the collection time may be reduced when the sampling temperature is increased. The gas chromatography and mercury methods give similar results, much higher than those obtained with the glycerine method, without the need to use liquid nitrogen for preparation of test specimens. The gas chromatography method presents less scatter of results and a shorter sample collection time due to the increased sampling temperature. The use of equations found in the literature for comparison of the results obtained by the different methods is also discussed.

Effect of flux density and the presence of additives in ATIG welding of austenitic stainless steel

Active flux TIG (ATIG) welding is a simple variant of the conventional TIG process that allows increased penetration of the weld and enables welding in one pass, with total penetration and without chamfer opening, for joints with thicknesses of 5 mm or more. Different mechanisms have been proposed to explain this effect, with emphasis on contraction of the arc due to the presence of negative ions and alteration in the movement of liquid metal in the weld pool, associated with variations of surface tension as a function of temperature. This study evaluates the effect of the amount of one flux of one component (Cr2O3) placed on the surface of the work piece, and the additions of KClO4 and Al2O3, on the shape of the weld bead. Three sets of bead-on-plate weld tests were performed on 5-mm-thick ABNT 304 stainless steel plates. In the first set, the amount of flux used varied; in the second set, the effect of the additions of KClO4 was studied; and in the third, Al2O3. Electric current and voltage were measured during the welding, and width, penetration and area were measured on cross sections of the weld bead. Results indicated a small variation in the welding voltage (increase less than 1 V) during the transition from TIG to ATIG welding. Surface flux concentration affected the weld penetration, with a rapid increase of penetration and weld bead area occurring, to concentrations between 7.5 and 15 g/m2, followed by a milder variation for greater concentrations (up to 120 g/m2). On the other hand, whereas the addition of KCLO4 clearly reduced the increase of penetration caused by the flux, the addition of Al2O3 had a less significant effect on the process.

Efeito da densidade do fluxo e da presença de aditivos na soldagem ATIG de aço inoxidável austenítico

Active flux TIG (ATIG) welding is a simple variant of conventional TIG welding that allows a major improvement in weld bead penetration. Different mechanisms have been proposed to explain the effect of flux. The most accepted ones consider the arc contraction by negative ions vaporized from the flux and liquid metal flow alterations in the weld pool caused by changes the surface tension values. This paper evaluates the effect of one component (Cr2O3) flux concentration and additions of KClO4 and Al2O3 on ATIG welding bead shape. Three sets of bead-on-plate weld tests were performed on 5 mm thick AISI 304 steel plates. Electric current and voltage were measured during each welding trial and the resulting bead geometry was evaluated in cross sections of the weld. Results indicated only minor variations in voltage during the transition from TIG to ATIG welding. Surface flux concentration affected weld bead penetration, and maximum penetration was obtained with flux densities between 15 and 60 g/m2. On the other hand, the addition of KCLO4, despite this being a strong oxidizer, reduced weld penetration. A similar effect was linked to additions of Al2O3 to the flux.

Bead characterization on FCAW welding of a basic tubular wire

This work aimed to study the effect of some operational conditions on the characteristics of beads produced by a Brazilian-made basic tubular wire (ASME SFA-5.20: E71T-5/E71T-5M), 1.2 mm in diameter, intended for welding common structural carbon steels with low and medium levels of carbon. Welding tests were carried out, in a flat position, on thick plates (12 mm thick) of common low-carbon steel using a source operating in constant voltage mode and with continuous monitoring of the arc current, arc voltage, and feed speed (fusion) values for the wire. The composition of the shielding gas (75%Ar–25%CO2 and 100%CO2), the electrode polarity (positive and negative), and the wire feed speed (7 and 9 m/min) were varied. The other welding parameters were kept fixed, including the energized lengths of the electrode (16 mm) and the arc (3.5 mm). The effects of the operational conditions on the main characteristics of the bead were evaluated including its geometry (penetration, reinforcement, width, welded area, deposited area and dilution), presence of discontinuities, microstructure, and hardness. For the basic wire, the operational conditions with the highest productivity (the highest deposit rate) associated with a bead with suitable characteristics for welding thick plates of structural steels were assessed.

Comparison of operational performance and bead characteristics when welding with different tubular wires

The aim of this work was to make a comparative study of the characteristics of the weld bead produced by nationally manufactured tubular wires; all rutilic (ASME SFA-5.20: E71T-1/E71T-9/E71T-9M), basic (ASME SFA-5.20: E71T-5/E71T-5M) and ‘metal cored’ (ASME SFA-5.18: E70C-3M), 1.2 mm in diameter, intended for the welding of structural steels with low and medium levels of carbon. Welding tests were carried out, in the flat position, on thick plates (with a thickness of 12 mm) of common low-carbon steel using a source operating in ‘constant voltage’ mode, with monitoring of the current and voltage signals of the arc and feed speed (fusion) of the wire. The following were varied in welding with each type of tubular wire: the composition of the shielding gas (75%Ar–25%CO2 and 100%CO2) and the feed speed of the wire (7 and 9 m/min). The other parameters were kept fixed, including the polarity of the electrode (DC+) and the energized lengths of the electrode (16 mm) and of the arcs (3.5 mm). For the different tubular wires, there was a comparative analysis of the principal weld bead characteristics, including its geometry (penetration, reinforcement, width, fused area, deposited area and dilution), presence of weld discontinuities, microstructure and hardness. Operational conditions that yielded weld bead characteristics that favoured the welding of thick plates of structural steels were determined.