Superior for Stainless: High-Speed Pulsed GTAW Boosts Productivity, Quality while Reducing Distortion
New test data also provides guidelines for setting and understanding pulsed GTAW variables.
Some TIG welders swear that stainless steel warps if you just look at it wrong. More precisely, the metal's low rate of thermal conductivity causes the heat input to remain localized, and localized heating and cooling promotes distortion. Excess heat also reduces the material's ability to resist corrosion.
When welding any of the common grades of stainless in almost all applications, the challenge remains the same: minimizing heat input while creating good fusion and optimizing the bead profile. A skilled operator can control weld bead characteristics to an amazing degree by adjusting arc length, filler metal addition and amperage via the remote foot or hand control.
However, test results indicate that high speed pulsed GTAW-pulsing between a high peak current and a low background current at frequencies of 100 to 500 pulses per second (PPS)-provides operators with the ability to accomplish one or more of the following:
- Narrow bead width by up to 52 %
- Increase penetration by up to 34 %
- Reduce heat input by up to 60 %
- Increase welding travel speeds by up to 35 %
- Promoting better weld quality
The 300-Series stainless steels are the most popular for both pipe and tubing and cover about 70 % of all stainless applications. This article provides suggested parameters for high-speed pulsing on 300 Series stainless steels in hand-held torch applications, shows how easy it is to set parameters and provides the test data to substantiate the results noted above.
To effectively improve productivity and quality, test data shows that the arc must pulse at frequencies of 100 to 500 PPS. Note that achieving these frequencies requires using an inverter-based GTAW welder (conventional TIG technology lacks high-speed power switching capabilities and cannot pulse any faster 10 or 20 PPS).
While the actual mechanics of setting pulsing variables can differ depending on the brand of equipment used (see Fig. 1), the variables tend to be the same or similar: peak amperage, pulsing frequency, peak time and background amperage. Here are suggested parameters for welding any of the 300 Series stainless steels. Note that these guidelines apply to any wall thickness, as well as to pipe, tube or plate.
- Peak Amperage. Set this as you would a conventional GTAW machine, bearing in mind that the average amperage while pulsing should roughly equal the non-pulsing amperage. Give yourself a little flexibility when you use a remote foot control; add about 20 more amps than required.
- Pulse Frequency. Pulse frequency is the number of times per second the power source pulses between peak and background amperage. Start at 100 PPS and adjust up to 500 PPS without changing the other variables to find which frequency provides the best results in a particular application.
- Peak Time. This is the percentage of each pulsing cycle spent at peak amperage. Starting at 40% peak time is a good rule.
- Background Amperage. Background is set as percentage of peak amperage, and 25% is the recommended starting point. Thus, if you set peak at 100 amps and the background at 25 percent, the background current will be 25 amps.
Fig. 1: Today's TIG inverters simplify setting pulsed MIG parameters using intuitive controls.
Clockwise from upper left: peak amperage, pulsing frequency, peak time and
background amperage; the green light indicates the parameter being adjusted.
Variables and their Effects
The two variables that have the biggest effect on outcomes are peak amperage (which is well understood) and pulse frequency. Increasing frequency narrows and concentrates the arc cone, which in turn increases penetration and narrows bead width without increasing total heat input.
Fig. 2 below shows the results of test A, designed to compare the effects of pulse frequency to penetration at a constant travel speed of 18.9 inches per minute (IPM). Fig.3 shows macro images of the actual weld bead profiles from test A.
As shown by test A, high-speed pulsed GTAW, compared to non-pulsed welding, can narrow bead width by up to 52 percent, increase penetration by up to 25 percent and reduce heat input by up to 59 percent. While these tests were performed on plate (to simplify testing and sampling), the results can also be extrapolated to pipe of the same thickness.
Notice that pulsing frequencies of 100 to 250 PPS produced the most significant changes. Pulsing at higher or lower frequencies does not provide any benefit while holding peak time and background amperage constant.
As a note of some consequence, consider that pulsing also reduces the width of the HAZ (see Fig. 4) and provides more directional control over the weld puddle, both of which promote better weld quality. Further, because operators can place welds more precisely, pulsed GTAW helps beginners learn to weld in less time, while experienced welders can increase their travel speed (as the next test indicates).
1. Pulsing variables were set at factory defaults: 40 percent peak time, 25 percent background amperage.
2. Key variables: Travel speed: 18.9 IPM; Gas flow: 15 cubic feet per hour (CFH); Arc length: 1/16 in.; Tungsten: ceriated, 3/32-in., 30-degree angle, 1/64-in. flat.
Fig. 3: Weld beads made with (L-R) 0, 10, 100, 250 and 500 PPS. Notice the deeper, narrower weld bead at 100 and 250 PPS.
Increasing Travel Speed
Test B (Fig. 5) kept average amperage constant, adjusted travel speed to ensure equal visual penetration and tested pulsing frequencies from 0 to 500 PPS using the factory default settings for peak time and background amperage. Test B showed that adjusting pulse frequency provided up to a 23 % increase in travel speed while keeping penetration and average amperage constant. Frequency increases from 250 to 450 PPS also increased in travel speed. At 500 PPS, a decrease in travel speed was necessary to maintain equal visual penetration.
Once test B was completed, further testing was performed to determine if an advantage could be obtained by using 60% peak time and 40% background amperage. Using the same average amperage as test B, test B1 (Fig. 6) results showed that the higher peak time and background amperage gave similar travel speed results to test B up to 450 PPS. However in test B1, when the pulse frequency was brought to 500 PPS, the travel speed continued to increase instead of decreasing.
Fig. 6: Welding stainless steel demands good fit-up. Because molten metal
contracts as it cools, excess filler rod (to fill gaps) increases contraction and
promotes distortion. Further, adding more filler requires more total heat input
and slows travel speed, tripling distortion problems.
While test B and B1 did not measure total heat input, note that because total heat input is function of amperage applied over time, faster travel speeds necessarily decrease heat input. Thus, while improved productivity might not be of primary importance, reduced warping and compensating for warping are major issues in nearly every stainless steel tube and pipe applic ation.
The good news with high-speed pulsed GTAW is that you have a high probability of improving results simply by using the factory default settings for pulsing, following the above guidelines and adhering to standard best practices for welding stainless steel (Fig. 7). However, to determine which parameters optimize performance in any particular application, the authors strongly recommend conducting non-destructive and destructive tests to evaluate weld bead profile, weld integrity, travel speed and heat input.
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