Welding Parameter Development

Many welding equipment suppliers offer a series of pre-calculated weld programs for a variety of tube diameters, wall thicknesses and materials. Welders should always follow an equipment supplier's suggested procedures first, because they have usually performed a significant amount of qualifying and troubleshooting work to optimize electrode preparation for their equipment.

However, it is impossible for the equipment suppliers to have welding procedures for every welding application and there will always exist a trade off in maximum weld speed possible versus weld quality and repeatability. Where weld parameter specifications do not exist or the welder or engineer would like to change those settings to possibly improve or optimize their welding, the guidelines noted below give information on how to modify the welding parameters for a desired result.

Note: The "rules of thumb" noted below are general guidelines only and will not apply to every welding application and mix of parameters chosen. Although the welding parameters are often chosen and changed according to the specific needs of the application, there are some industry standards that have been developed as starting points. Experimentation and experience will determine the final weld parameters.

Arc Length

The arc gap setting is dependent on weld current, arc stability and tube concentricity/ovality. The objective of the welding engineer is to keep the electrode at a constant distance from the tube surface with sufficient gap to avoid stubbing out.

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 110 As a "rule of thumb" use a base arc gap of 0.010" and add to this half the penetration required (usually the tube wall thickness) expressed in thousandths of an inch. Thus if the tube wall is .030" then a good starting arc gap would be 0.010" + 0.015" = .025". For a wall thickness/penetration requirement of .154" the arc gap would be 0.010" + .070" = 0.080"

Weld Speed

The weld speed is dependent on flow rate of material to be welded, and wall thickness. The objective is to weld as fast as possible while still yielding a quality output.

As a starting point the tungsten surface speed should be 4 - 10 inches per minute with the faster welding speeds used for thinner wall materials and the slower welding speeds used for heavy wall thickness. As a good starting point, use 5 inches per minute.

Welding Current

The welding current is dependent on the material to be welded, wall thickness, weld speed, and the shield gas chosen. The objective is to achieve full penetration, defect free welds.

As a starting point use 1 ampere current per 0.001" wall thickness if the material is stainless steel. Thus for a 0.030" wall tubing the average weld current will be 30 amps in the first level.

Weld Current Levels

Orbital welding normally uses multiple levels of weld current to compensate for heat building up in the tube during the welding process. If the weld current used to initially penetrate the tubing was held at the same level for the complete weld, the weld penetration would increase as the weld progressed around the tube, producing too much penetration.

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 111

Figure15-8: A Typical Weld Program current Profile ( This weld profile shows a single level of weld time). Orbital welding normally uses a minimum of 4 levels of weld time with each level decreasing in weld amperage as the tube heats up during the welding process

Normally orbital welding uses a minimum of 4 levels of weld time with each level decreasing in weld amperage

Starting parameters: Set weld level 4 to be at 80% of weld level 1 amperages. Set weld level 2 and weld level 3 to gradually decrease the current from level 1 to level 4.

Figures 15-9 and 15-10 depict a typical weld program current profile for a 10 mm O.D. SS tube. It may be noted that in the weld program chosen by the welder, the time for each level is same (Impulse rate) and the average current decreases with each level.

Arc Pulsing

Arc pulsing involves using the welding power supply to rapidly alternate the weld current from a high (peak current) to a low (background current) value. This creates a seam of overlapping spot welds. This technique reduces the overall heat input to the base material and can also allow for increases in weld speed. This welding technique brings many benefits to the welding procedure, often

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 112 improving weld quality and repeatability. In some cases materials and weld joints with poor fit-up that are difficult to successfully weld with a non-pulsed arc can easily be welded with a pulsed arc technique. The result is improved weld quality and increased output.

Figure 15-9: Weld program data sheet of a typical weld in RAPP-6

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 113 In orbital welding, arc pulsing also offers another advantage due to the fact that the gravity pulls the weld puddle in different directions as the weld is created around the tube. When pulsing at peak current the base material(s) melt and flow together, at the lower background current the puddle can solidify before becoming liquid at the next peak current pulse. This diminishes the effect of gravity on the molten weld, minimizes the weld sagging at the 12 and 6 o clock positions, and reduces the molten weld puddle running/slumping downhill at the 3 and 9 o'clock positions and effectively alters the electrode to weld puddle distance. The arc pulsing technique thus becomes more advantageous as the wall thickness increases resulting in a larger weld puddle.

Arc Pulsing Parameters: Arc pulsing involves four welding parameters: peak current, background current, pulse width (duty cycle), and pulse frequency. Here

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008 starting parameters and the effect on the weld by changing each parameter.

The primary objective is to use the benefits of weld pulsation to improve weld quality and output.

Peak/Background Current Ratios: The peak to background current ratios basically provides a means for the welding current to pulse from one level to another.

Industry usage generally varies from 2:1 ratios to 5:1 ratios. A good starting point is to use 3:1 ratios, make the required weld and test other parameters to see if any benefit can be gained.

Pulse Frequency: The pulse frequency is dependent on spot overlap required. Good starting parameters are to attempt for a 75% spot overlap. Pulse rate for thin wall tube is often equal to the weld speed in ipm (5 ipm = 5 pps) {pps: pulse per second}

Pulse Width: The pulse width (the percentage of time spent on the peak current) is dependent on heat sensitivity of material and available current from power supply. Higher heat sensitivity requires lower pulse width % on peak current.

Standard pulse widths are often 20% to 50%. A good starting parameters would be to set a pulse width of 35%.

Welding Parameter Development Example for 1" Tube/.030" Tube Wall Thickness:

1. Arc Length/Gap = .010" + (0.5 x penetration required) Starting Parameters: .010" + (0.5 x .030") = .025"

2. Weld Speed = 5 ipm surface speed RPM = ipm/(3.1415 x dia.)

Starting Parameters: 5/(3.1415 x 1") = 1.59 RPM 3. Welding Current Levels

Level 1 = 1 amp per .001" of wall thickness for level 1 current Level 4 = 80% of Level 1 current

Levels 2 and 3 gradually decrease the current from Level 1 to Level 4 Starting Parameters:

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 115 Level 1 Peak Current = .030" wall thickness = 30 amps

Level 4 Peak Current = 30 amps x 80% = 24 amps Level 2 Peak Current = 28 amps

Level 3 Peak Current = 26 amps

Background Current will be 1/3rd of peak current. Pulse width/duty cycle is 35%

4. Tungsten Electrode Diameter & Tip Geometry - Use your equipment manufacturer's specifications or consult your pre-ground electrode supplier The above data gives starting parameters. On completion of the first test weld, the parameters will be modified to obtain the final result desired.

TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008

Nirbhay Gupta 116