“WELDING CURRENT EFFECT ON DIFFUSIBLE HYDROGEN CONTENT IN FLUX CORED ARC WELD METAL”

CONTENTS

l ABSTRACT

l INTRODUCTION

l EXPERIMENTAL PROCEDURE

l RESULTS AND DISCUSSIONS

l CONCLUSIONS

ABSTRACT

l The application of flux cored arc welding (FCAW) has increased in manufacturing and fabrication. Even though FCAW is well known for its good capability in producing quality welds, few reports have been published on the cause of the relatively high diffusible hydrogen content in the weld metal and its relation with the ingredients used in the wire production and with the welding parameters (mainly welding current).

 

l This paper describes experiments where data obtained from weld metal diffusible hydrogen analysis, metal droplet collection, and high-speed recording of metal droplet transfer were used to evaluate the effect of welding current on diffusible hydrogen content in the weld metal.

 

l The results from gas chromatography analysis showed that weld metal hydrogen content indeed increased with welding current. A polynomial regressional analysis concluded that hydrogen increase with current was better described by a linear function with proportional constant of approximately 0.7 or 70%.

INTRODUCTION

l  Flux cored arc welding (FCAW) exhibits many important characteristics such as: high productivity, good quality weld and low cost. In the process, the wire is fed continuously into the weld pool, and the shielding is achieved by ingredients inside the tubular wire and from an external gas supply (AWS, 1991). This semi-mechanized process is an excellent candidate for automation.

 

l  Despite its many industrial applications, reports (Meyer, 1993; White et al, 1993 and Harwing et al, 1994) have shown that the diffusible hydrogen found in the weld deposits is usually higher than the ones found in the weld metals made using SMAW and GMAW. In practice, the lower values of weld metal hydrogen in GMA and SMA welds are obtained with cleaner wires and drier fluxes and electrode covering. Typical values obtained from different welding process and consumables types are shown in Fig. 1. In this figure, it is possible to observe that even with lower potential hydrogen level, the FCAW process reaches the same or higher weld metal hydrogen level than SMAW. One hypothesis is that this difference could be caused by the flux ingredients inside the wire in the FCAW, which stay unaffected until they reach the arc. In the SMAW process, on the other hand, the baking after electrode manufacturing and the electrode heating during welding reduce significantly the amount of moisture and crystallized water, present in several flux ingredients.

 

EXPERIMENTAL PROCEDURE

l A tubular wire, type E71T-1 (AWS A5.20-95) with 1.2 mm diameter, was used in all the experiments. To avoid air contamination when not in use, the wire coil was kept inside a chamber heated by a lamp to temperatures around 130º C. The experimental procedure was divided in two major parts: in the first, experiments were performed to verify the relationship between the welding current and the hydrogen content in the weld metal; In the second, a high speed cinematography recording and droplets collection were made to further evaluate the relationship.

l For the first part, standard bead-on-plate welds for hydrogen chromatography analysis according to the International Institute of Welding Recommendation (IIW (a), 1983) were deposited with different welding current. Thirty two experiments, divided in eight groups of four samples were performed. Each group refers to a different welding current. The welds were deposited on ASTM A36 steel coupons with standard dimensions of 30x15x10 ± 0.25 mm and run-on and run-off tabs of 10 x 15 x 45 ± 0.25 mm. Before welding, all coupons were heated in a furnace to temperatures up to 600ºC for 1 hour to remove all residual hydrogen. The coupons were then weighed (Wi – initial weight) and to avoid atmospheric contamination, coupons that were not going to be used immediately were kept in a dessicator.

 

RESULTS AND DISCUSSIONS

l For the correlation with diffusible hydrogen and to assure the repeatability in the welding parameter, arc voltage and welding current were monitored during the experiments. The monitoring was performed during 5 intervals of 4 seconds between intervals of 3 seconds, allowing the collection of up to 10,000 points of each parameter in the form of oscilograms. In general it was possible to observe that, even though it was tried to keep the voltage constant, small fluctuations were noticed. For each group of experiment the average values of voltage and current for the 10,000 values monitored for the four samples of each group were adopted. Figure 8 shows the relation between the average arc voltage and the average welding current for the eight groups of experiments.

 

 

l Table 2 shows the results of diffusible hydrogen in the weld metal obtained for the eight settings of current. The results corresponded to the average values for each parameter or between the four samples welded with each condition. The values are graphically represented. Linear, quadratic and cubic regressions were performed to verify the trend. The four curves represented regressions of minimum quadratic type, for the correspondent polynomial degree, minimizing the function:

 

l Table 3 shows the equations for the constant, linear, quadratic and cubic approximations, with the respective deviations, calculated according to equation 2. From this table it is possible to observe that the deviations for the linear, quadratic and cubic approximations are very close to each other. In other words, the quadratic and the cubic approximations add very little to the linear approximation. The results seem to indicate that the phenomena of diffusible hydrogen increase with welding current is a true linear relationship with proportionality constant of approximately 0.7 or 70%. This finding is important for one to predict the increase of diffusible hydrogen in weld metal made with FCAW if the variation of welding current is known. It can also be used in projects to determine the safety factor for welded structures.

 

l  Then, from the discussion above, for FCAW it should be expected a significant increase in droplet size when current increases and this increase should be of at least 70%, to explain hydrogen increase with current. To evaluate this hypothesis, droplet collection was performed. Figure 10 shows samples of droplets collected at the currents of 105, 120, 135 and 150 A, immediately after they were removed from the collector tank. Figure 11 shows the droplets classification in six groups of specific diameter.

 

l To further evaluate the effect, the high speed movies were made. Figure shows sequences of frames for currents of 105 A, 150 A and 200 A. After analysis, it was clear the cause of hydrogen increase with current: at high currents, the cored flux touches the weld bead. At 150 A the contact is sporadic but at 200 A, it stays touching during the whole time of droplet formation and transfer. Since the flux has ingredients that contain hydrogen, hydrogen passes through the arc without interacting with the arc, going into weld bead intact and increasing the hydrogen content in the weld bead.

 

 

 

l Another important observation from the recorded movies is regarding the droplet size. At low current the metal sheath melts and a portion of the flux, such as a cone, projects to the weld bead direction while the droplets form laterally to the wire and fall to the weld pool as a certain size is reached.

l As current increases, the wire speed increases and the flux pass from cone to a cylinder and further project to the weld pool, as described above. In this condition, a larger portion of the flux is now in contact with the arc. It is believed that buoyancy forces from gases decomposing in the flux can sustain the droplet, retarding its transfer and allowing it to further grow. It is possible to observe that even with higher melting rate, the size of the droplets are bigger at higher current than at lower current, confirming the results obtained at the droplet collection. Figure 16 shows a schematic model of this phenomena observed.

CONCLUSIONS

l   The phenomena of diffusible hydrogen increasing with welding current, already reported before in the literature, is linear and the increase is at a slope of approximately 70%.

l   The increase in droplet size is relatively insignificant regarding current increases in FCAW. While current increases approximately 50% (from 105 to 150 A), droplet size increases only 20%. In other words, the increase in the droplet size cannot explain the 70% of hydrogen increase with current. Therefore, another factor may be the responsible for the hydrogen increase.

l   In general it could be observed that the ratio dc/dw of the droplets is always bigger than 1, indicating that the transfer mode for the range of current used in the experiments is of globular type.

l   High speed recording showed that at high current, the cored flux stay touching the weld pool during welding. Hydrogen passes through the arc without interacting with the arc and goes to the weld bead intact, increasing its content in it.

l   With the high speed recording was also possible to see that as current increases, the droplets sizes also increase, indicating that probably as the gases from the flux dissociating, they generate buoyancy forces capable enough to sustain the droplets allowing them to grow.