Inverter conversion

SMAW strategies when using an inverter welding machine

Many welders who are used to working with an engine-driven welding machine in the field are surprised when they arrive in classrooms and are faced with newer inverter technology. Many times those who do a lot of shielded metal arc welding (SMAW) in the field have older-style machines with basically an amperage and a voltage dial. Newer inverter machines make them feel unsure of what the buttons on the machine do or how they can customize the arc because the technology doesn’t work precisely the way their own welding machines do. This article is a short primer on what makes inverter technology different and how welders can use the technology for pipe SMAW.

Dial Differences

Every machine has a typical heat setting, whether it’s an old “drooper-style” diesel engine-driven machine or a new inverter machine. For the purposes of comparison, an old Lincoln SA 200 machine has two dials; one provides control for the gearing of the motor, whether it be a 100-160-amp or 160-180-amp engine. That provides your heat range. The second dial deals with voltage.

The voltage control, which is a dial that goes from 1 to 10, keeps your puddle liquid as required. For any work you do out of position, you want that voltage to be as low as possible so that the puddle freezes as fast as possible.

With engine-driven power sources, the operator controls the depth of penetration himself. To create a longer or shorter arc length, the welder places the electrode farther away or closer to the puddle. Inverter technology changes this in a big way.

The new machines basically give the operator more control at the machine. The settings on inverter machines are similar to those on engine-driven power sources. The heat setting dial offers you the same range. It is the second dial that is different. This dial now gives you your “crisp,” “soft,” or “dig” percentage – these three terms are used by different manufacturers to mean essentially the same thing. Dig is the term used on Miller machines, but the purpose of that dial is basically the same on each machine. We’ll use the term dig for the purposes of this article.

This dial determines not only your voltage, but also the amount of arc force that you are applying to the weld. Therefore, if you have it in a high dig mode, that setting is basically going to drive that amperage into the piece. This eliminates sticking or snuffing out the electrode; when the arc length is shortened, the inverter will adjust and lower the voltage to eliminate this problem.

Pipe Welding With an Inverter

Pipe welding offers a good example of how SMAW is affected by this. With pipe you have an open root, so you need to have a certain amount of reinforcement or penetration on the inside of that pipe. Therefore, you would likely use a cellulose rod, 6010 or 6011, for the root weld. Setting the machine at 90 amps and a high dig at this point is going to increase the amount of arc force or penetration that you achieve.

Then, as you progress out, that setting changes. After your root pass, you would probably change to a 7018 low-hydrogen electrode and use around 100 to 105 amps and a lower dig. As you weave back and forth as you come up the pipe, the metal stacks properly with this setting; in other words, the puddle solidifies quickly and you won’t have to fight gravity so much. At the same time, the electrode is run hot enough that you can burn out any slag and remove any porosity.

A Schedule 80 pipe (a pipe that is roughly 3/8 to ½ in.) would likely call for six fill passes, although the exact number is welder-specific. Six is the rule of thumb. Laying fewer passes, you have more chance of incomplete fusion or leaving slag trapped. Laying more means you are running slower, which can lead to heat-affected zones. Speed is the biggest concern.

You may change your setting with each pass. Some welders adjust their settings with each pass to avoid too much heat buildup in the pipe. The decision goes hand in hand with the amount of heat you’re putting in. The only time you would turn your dig up again is if you were unsatisfied with the pass you had just completed and wanted to burn out the slag that was left in the pass.

About 99 per cent of the time when working on pipe, you travel up the pipe to complete a weld. In rare cases, when the pipe is not going to be under considerable pressure (for example, a mainline pipe), you can run a weld downhill. This is much faster because you are working with gravity.

However, it requires a different setup and different electrodes. The setting on your machine is important, though. You can’t run downhill if your voltage is set too high because gravity works against you, and the weld isn’t going to go where you want it to.

When you arrive at your cap, lower the amperage a little and go to a soft setting, or even less dig, less voltage. Basically, that will freeze the weld puddle quickly and cleanly.

Welders who are new to inverter technology fight with a machine because the dig isn’t set properly for them. These same operators’ welds test perfectly with an X-ray day in and day out in the field. It’s a matter of getting used to the technology.

The advantage that many of these new machines have is that their digital displays always let you know where your settings are. Once you understand the settings, they will always be accurate and will remove some of the guesswork involved in a pass.

Inverters offer customizable buttons and options as well, all of which need to be understood prior to testing or using. Arc characteristics mentioned previously are a good starting point. However, knowing the volt/amp curve of your specific machine is crucial to reduce confusion and frustration.