Greg Cain is the  VP Technical Services, Oxylance Inc. (14 years)

26 years as a Welder Diver

16 years on the AWS D3.6 Committee for Welding in Marine Construction and D3.6 B Sub Committee for Underwater Welding

From all of my (comments on John Roat's July 2010 column), some people may have the idea that I support Inverter Welding machines for Underwater Cutting. That is not true. I am writing this to clarify my reasons for supporting these types of machines. Inverters are not the answer to every welding or burning application. Not because they are un-safe for divers to use but because of their limited output or more importantly the DUTY CYCLE. They are actually some of the best machines available today for underwater wet welding but not for burning...

MACHINES FOR UW BURNING

If you are using tubular steel electrodes you should use a machine that is 100% Duty Cycle at 400 amps. In most older welding machines like the Lincoln bullet machines that are DC generators you have to have a 600 amp model in order to get 100% duty cycle at 400 amps. DC generators normally have lower duty cycles on the high end of their amp output. I just reviewed the Lincoln web site under their motor driven welding machines and all of their machines over 300 amps are AC generators with rectifiers. If you look at them their Big Red 500 is 60% duty cycle at 500 amps and 100% duty cycle at 450 amps.

DUTY CYCLE

When you exceed the duty cycle of a welding machine a number of things happen. As the armature heats up the output of the machine starts to go down. Depending on the length of the leads, the condition of the knife switch and all of the other places where cables are spliced together and how the ground is attached affects the burning rod. See Amps and Volts below.

WELDING LEADS

Most people think of the welding lead as the 300’ or 600 foot burning rig. When you are figuring the line loss in the burning rig you also have to consider that you have a lead that runs from the machine to the knife switch and you also have the ground. I have been on jobs where people would be using a ground cable that should have been thrown away and when I pointed out what bad shape the ground was in they would say “It’s just a ground, it doesn’t matter”. When you are burning, everything matters. The ground cable is part of the electrical circuit and should be just as good as the burning rig. When calculating the line loss you have to figure all of the lead in the circuit. I also do not like to see or hear of people using a Salt Water Ground. Now you are making all of the water around you part of the electrical circuit. For years people have told you to not get between the lead and the ground when wet welding or burning. When you use a salt water ground you are now surrounded by the ground.

KNIFE SWITCHES

I am sure that everyone has seen knife switches that have the bolts messed up and they will have a stack of washers on the bolts to get up to good threads. Usually when setting up the burning station people will just put the welding lead lug on top of the washers and then put the nut on. This creates a problem. Most of these bolts are stainless steel. SS has a very high resistance rating and the steel washers will rust and you are increasing the resistance because of the steel washers and the rust. ALWAYS remove the washers and put the copper lug on the welding lead against the copper base of the knife switch and then put the washers on top of the lug. Copper to Copper contact improves the flow of electricity because it reduces the resistance.

AMPS AND VOLTS UW BURNING

Amps and Volts are a function of the length of the arc when welding or burning. Arc Voltage is critical in both UW Welding and UW Burning. The relationship with Amps and Volts is very simple. When the welding or burning arc is very short the Arc Voltage is low and the Amperage is high. As you lengthen the arc the Arc Volts goes up and the Amperage goes down.

For UW Burning with Tubular Steel cutting rods the shorter you keep the arc the lower the arc volts will be and the higher the amps. The amperage is what makes the heat so;

Short Arc = Low Volts = High Amps = Maximum Heat.

The results are faster cuts, fewer hangers and more complete burning of the oxygen stream.

If you are long arcing the burning rod then you have;

Long Arc = High Arc Volts = Low Amperage = Reduced Heat = Inefficient Cutting.

How you clean the base metal prior to cutting affects the arc length. I don’t care what anyone says cleaning the base metal is critical to good burning. When you are trying to burn over any kind of calcareous growth or barnacles as the rod passes over the material the arc starts looking for a conductor and it starts to long arc. You can figure out pretty quick what happens next. For the rack operator or supervisor; You can tell a lot about how the diver is burning by watching the volt meter and listening to the noise in the radio. Those loud screaching noises on the radio happen when the diver is long arcing the rod.

If you are burning with a 400 amp machine and you have it set at say 350 amps you will get the 350 amps as long as you maintain a very short arc. When you long arc the rod you can drop the amperage that is crossing the arc down to as low as 200 amps. As you burn if you are using a machine that is not 100% duty cycle as you burn the armature heats up. The voltage becomes erratic and the arc becomes unstable. The results are as the amperage output of the machine drops then you will have to hold a longer arc in order to keep the rod burning. As you have to lengthen the arc to maintain it the Amps start to drop more. When this happens you put more load on the welding machine and the armature heats up more and your burning becomes more difficult and inefficient.

AMPS AND VOLTS IN UNDERWATER WELDING

When performing wet welding the Arc Volt Meter is the most important tool you have. You need accurate arc volt readings to insure that the weld metal you are depositing is what it should be.

Again, Arc Voltage is a function of the length of arc.

Short Arc = Low Volts = High Amps = Maximum Heat Input.

With a short arc the deposited weld metal will have less porosity, more ductility and higher tinsel strength. The best wet welds are produced when the arc voltage is below 30 volts. It is even better if you can get it down in the 24 to 26 range.

Long Arc = High Arc Volts = Low Amperage = Reduced Heat Input = More Globular Transfer

Test have shown that when the Arc Volts get above 35 volts the weld metal will contain far more porosity due in part to a more globular transfer of weld metal. All Weld Metal Tinsel test results shows lower ductility and tinsel strength.

WELDING MACHINES FOR WET WELDING

Now this is where I recommend the Inverter welding machines. At both Taylor Diving and Sub Sea International we did a lot of testing to see if there was a discernable difference in welds made with different welding machines. The results of those test left several welding engineers scratching their heads and rethinking the qualification requirements for wet welds. During our testing we found that there were 2 machines that exceeded every other machine on the market at that time. They were the Miller Big Blue 400D and the Miller inverter XMT 304.

With the Big Blue diesel powered machine we achieved arc volts in the range of 28 to 34 volts with the average around 30 - 32 volts. With the XMT 304 we were able to maintain arc volts in the 22 to 28 volt range with the average at 24 - 25 volts. In both test we were running at an average amp setting of 165 amps. The mechanical results were both machines produced welds that exceeded the requirements of AWS D3.6 Specification for Underwater Welding Class “B” welds. In all of the testing we achieved less porosity and higher ductility with the XMT 304.

Now the Big Blue 400 is 100% duty cycle at 400 amps and the XMT 304 is 60% at 300 amps but it is 100% duty cycle at 200 to 250 (I do not remember the exact number).The arc stability was better and there was less deflection in the numbers on the Amp and Volt meters with the inverter.

CONCLUSION

Just because it is a welding machine doesn’t mean it is right for your particular application. Do your homework and use a machine that is correct for the project at hand. As I have said earlier, I never recommend the typical Inverter machine for underwater cutting applications. Now there are some electric welding machines that put out over 650 amps 60% duty cycle that do produce around 450 amps 100% duty cycle. I have cut underwater with these machines. Most of these machines are in the category of Multi Process machines and are best suited for fabrication shops where people are doing all types of welding. Years ago Miller built a series of these machines that were strictly for stick welding (SMAW).

Another thing that has to be considered when selecting an inverter or an electric type welding machine is the available electrical power at your job site. Many of these machines can be hooked up to Single phase 220 all the way up to 3 phase 480. You have to check the technical data on the particular machine to see what the output rating is for different input power. This link will take you to the technical sheet for the Miller XMT 304 (PDF). You will find that the XMT 304 when on 3 phase power is 300 amps 60% duty cycle and it drops down to 225 amps 60% duty cycle when hooked up to Single phase power. I DO NOT RECOMMEND using this machine for wet welding with SINGLE PHASE POWER. You can also visit the Miller web site for technical info on all of their products: http://www.millerwelds.com or visit Lincoln Electric at http://www.lincolnelectric.com.

I would hate to see inverter welding machines banned from use by divers for several reasons. One they are the best machines available for making really good wet welds. The second reason is that once you ban something it takes just as much time to get it re-accepted as it did to get it banned. With new materials, new techniques for manufacturing and advances in welding machine technology there is no doubt in my mind that in a few years there will be inverter welding machines with higher outputs and ever more safety protections built in. We may reach a point where the inverter machines are the best machine available for underwater cutting.

Greg Cain

Vice President

26 Years as a Welder Diver

On the board of the AWS D3.6 Sub Committee for Welding in Marine Construction

Oxylance Inc.

gcain@oxylance.com

(205) 322-9906 ext 222  

addendum: 20110504, from the Diver's Forum:

Says Greg,

I need to clarify one thing about the article I wrote about welding machines. Inverter machines that are DC out put only should be used for wet welding or burning. If the Inverter welding machine you are planning to use has a selector switch and can be switched from DC to AC DO NOT USE IT. There are some new machines available that have this feature and as sure as you use one of them someone will flip the selector on you.

Also if you are using a machine that has a selector switch for changing the polarity from Straight to Reverse, never make this switch with the machine under power. Always shut the machine down and switch the polarity.

Greg Cain