Multiplaz-3500 Evaluation, Part 11: Testing the Welding Torch (continued)
DISCLAIMER!
Let me emphasize that I will not be able to tell you whether the Multiplaz-3500, or any other piece of equipment, will be a good investment for you. Only you can decide that. My intent is to provide as much factual information as I can about the Multiplaz-3500 so that others in our company can make an informed decision about that. The company has no objection to my sharing the information with you as long as I leave their name out of it and make it clear that I am not endorsing any particular product.
DISCLAIMER!
The testing of the welding torch, started in Part 09 of this Evaluation, was continued by testing the welding of mild steel and stainless steel with a range of alcohol concentrations. The Multiplaz User Manual recommends welding with (50% water / 50% alcohol) for mild steel (and under some conditions stainless steel) and with (40% water / 60% alcohol) for stainless steel under most conditions. We used distilled water and isopropyl alcohol in all of the welding tests, but varied the alcohol concentration from 50% to 70%. The variation made little difference in the appearance of the surface of the mild steel, but with stainless steel the surface oxidation was noticeably more with 50% alcohol than with 60% alcohol. There was no obvious difference between 60% and 70%, but with the 70% alcohol concentration there were occasional times when the excess alcohol vapor in the plasma would catch fire and give a flame in front of the plasma. This usually occurred when welding in a corner or in a tight fillet where the vapors were reflected back toward the plasma.
We also looked more carefully at the surface “oxide” on both the mild steel and the stainless steel. As mentioned before, the surface oxide on the mild steel looks similar to the surface oxide produced when using an oxy-acetylene torch with a neutral flame. Mild steel is perhaps the most “forgiving” of all of the weldable metals with regard to shielding from the oxygen in the air. This is presumably because the melting points of the iron oxides are almost the same as pure iron, while the densities of the oxides are less. Hence, the molten oxides tend to float to the surface and help shield the base metal underneath from further oxidation. This makes it possible in many cases to “weld through rust” and still get decent fusion of the base metal below the rust. One very obvious result of the presence of the various oxides is that the puddle does not wet the surfaces easily and does not flow smoothly. Hence, the surface of the weld looks “ragged” in both the horizontal and vertical directions. Our tests indicate that, at least for mild steel, the tensile strength and ductility are not seriously reduced. But the welds lack visual appeal. Look at any of the Multiplaz welding videos listed in Part 10 of this Evaluation. The final weld is never shown (for good reason), but you can see the flow (or lack thereof) during the view of the welding puddle.
For stainless steel, the situation is different. The melting point of 18-8 stainless steel is actually slightly less than that of mild steel, but the oxides of chromium and nickel have much higher melting points and so the oxides that form tend to be porous solids that float to the surface and form a hard “crust”. This is aggravated by the low thermal conductivity of the solid base metal that tends to foster overheating of the weld puddle. This is particularly true for a constricted plasma torch with its highly focused plasma (such as the Multiplaz-3500 welding torch) compared to the more diffuse plasma of a conventional TIG torch, for example. (In commercial applications of constricted plasma welding torches, of which there are many but all of which tend to be fully automated, this problem is overcome by close control of the rate of motion and of the composition of the shielding gas; sometimes also by water spray cooling of the base metal.) For the 304 SS samples welded in our tests with the Multiplaz-3500 welding torch and 60% alcohol, the “oxide” layer was perhaps 0.5 mm to 1 mm (0.020” to 0.040”) thick on a piece of 3/8” x 2” x 4” stock. The weld metal below this layer appeared to be solid and no separation was obvious between the weld metal and the base metal when the weld was cut at right angles to the weld in several places. Interestingly, when I went over the “oxide” layer later with a TIG torch using pure argon, the “oxide” layer could be eliminated (reduced?) by remelting a surface puddle along the top of the weld. Sections of the reworked weld, cut as before, showed no evidence of porosity at the surface or below. It was also obvious that the more diffuse plasma of the TIG torch made for a somewhat larger, but much cooler puddle (as judged by the complete lack of “sparkles” boiling out of the puddle) that wet the surface better and flowed much more evenly.
My conclusion from all of this is that the constricted plasma torch with its highly focused plasma has, like most things, advantages and disadvantages, depending upon how you use it. On metals with medium to high thermal conductivity of the base metal and low sensitivity to surface oxidation (e.g., mild steel), the highly focused heat and the deep penetration of the constricted plasma can be used to advantage. On metals with lower thermal conductivity (e.g., stainless steel), overheating of the puddle and large amounts of surface oxidation are more difficult to avoid. Crystallization in the weld in stainless steel is a problem regardless of the welding heat source, but is probably worse with the constricted plasma torch due to the more concentrated heating in the puddle. Using a filler rod would probably help, since most filler rods are formulated with materials to reduce oxidation and promote ductility.
Let me also say that the conditions chosen for our tests were deliberate and intended to be the most revealing of flaws and problems in the welds. Hence,no filler rod or flux was used and the bend test that we used (90 degrees) is much more stringent than what is normal.
to be continued
larry lee
