DHO,
I think your trials are accurate. You will see negligable penetration profile differences until you your pulsation rate exceeds 500+ pps. Here is an excerpt from a nicely written article from the "Fabricator".
"High-frequency pulsed arc welding has a pulsing rate of more than 500 pulses per second. The high-frequency switching produces an increase in arc pressure.... Arc pressure and arc stiffness or stability are related, and, as the switching frequency nears 10 kilohertz, the arc pressure increases to nearly four times that of a steady direct current (DC) arc."
http://www2.thefabricator.com/Articles/Tube_and_Pipe_Article.cfm?ID=194
Now it's possible (especially if your powersource is a miller) that you may have high frequency pulsation capability that can be adjustable internally within the pulser itself, I have seen several optional pulsers that have a high and and low range. Check your owners manual.
thanks.
a senior E.E. at a welding equipment manufacturer said to me -
"The high frequency is the inverter operating frequency as far as any and all of the Xxxxxxx supplies go. It is in all cases that I'm aware of, close to 15,750 Hz (happens to be the same as the TV horizontal deflection frequency!!). The output choke does not completely attenuate this frequency, so that is why many people think of it as a separate "high frequency pulsation".
Xxxxxx Xxxxxx power supplies use a 150,000 Hz plus a filter to completely attenuate the ripple current at the high switching frequency. That's the only one I know doing such a filtering. Most other supplies (presently operating mainly at 50 kHz) leave 2 to 5 A peak to peak ripple.
There was a master's thesis work done at University of Tennessee - Knoxville about the pulsing effects. Merrick Engineering put together a special power supply with pulsing capability up to 20,000 Hz for this study. The conclusion was that you can occasionally hit the puddle resonance and achieve a higher penetration, narrower bead with the high frequencies available, but in practical terms it was no winner, because when the puddle size and viscosity changed (with the shape and temperature change) it immediately drifted out of the resonance.
In other words, my opinion is that the "high frequency pulsation" is just sales hype coming from a necessity to explain the ripple."
unfortunately i can not locate the thesis on web.
A couple of ideas for you.
1. GTAW penetration in stainless steel is strongly influenced by surface tension driven fluid flow, referred to in the literature as Marangoni or thermocapillary flow. In basic terms, the amount of sulfur (and other minor elements) in the particular "heat" (batch) of material you are welding can influence the depth and width of the weld pool. For a fixed set of welding conditions (amps, gap, travel speed), a steel with very low sulfur content, less than .002 wt%, may be very wide and shallow, while a steel of high sulfur content, .010 wt%, may be very deep and narrow. If you happen to be stuck with a heat of steel that is has low penetration characteristics, you will find as you change weld settings to increase penetration, such as increasing current, reducing travel speed, or decreasing arc length, the width of the weld pool with increase dramatically, while the depth will change little. So basically, pulsing may be of very little help to you, if you need a more depth of penetration, but not more width. See AWS welding journal reseach papers such as "Thermocapillary and Arc Phenomena in Stainless Steel Welding, BY S. W. PIERCE, P. BURGARDT AND D. L. OLSON, February 1999, for more information on this subject.
2. It is possible to affect the weld pool chemistry and increase penetration. Some contaminants such as machining oils, adhesives, etc, can increase penetration by adding sulfur compounds to the weld, but may cause other problems such as porosity. There is a flux that has been developed for stainless steels, just for the purpose of increasing penetration. One source I know of is the following: http://www.liburdi.com/web_pages/ows_catalyst.shtml, I think Hobart may offer a similar flux as well.
3. You may want to try some more "radical" pulse parameters. If you would like, email me and I'll send an EXEL spreadsheet that will let you play with the pulse settings. For instance, instead of 50/20 30% 32 pps, try 165/5 15% 25 pps, these both give you an average of 29 amps, but the very short duration blast at 165 amps may help with penetration. Also this web site has a pulse caluculator you may find helpful: http://www.pro-fusiononline.com/welding/pulseparams.asp
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