For more detail, please download the PDF Presentation

Volumetric Control of Welding Parameters

It's a requirement of ISO 3834 that welding parameters are controlled and checked at suitable intervals.

The Gold Standard for welding parameter checks is to use dedicated automatic weld monitoring equipment, eg. TVC's range.

Whilst this type of automatic monitoring equipment is relatively inexpensive, and mandatory in many client welding standards for PQR / Production tests, it is useful to be able to supplement it - particularly for welder monitoring. Volumetric weld parameter checks can be easily applied to SMAW and GMAW/FCAW welding, and is ideal for bespoke fabrication where full automation is currently impractical.

Advantages

• Minimal equipment required;
• Steel rule for SMAW
• Steel rule and stopwatch for GMAW/FCAW
• Quicker than conventional parameter monitoring – no set-up
• Can be carried out by anyone, including the welder, with minimal training
• Can supplement traditional methods; doesn’t need to replace them

SMAW

Welders using SMAW (111) electrodes typically have a single control to alter the welding current, either on the welding set, or via a remote. Adjustment of the welding current is usually made by "what feels right”, according to the electrode diameter, material thickness, and welding position. It is rare for welders to preset a specific welding current.

The welding arc voltage is determined by a combination of the arc length, welding current, and electrode diameter.

For any combination of welding current, arc voltage, and electrode diameter, there is a corresponding burn-off rate of metal from the electrode. If the welding current is increased, the burn-off rate will increase. If the arc length is increased, there will be an increase in voltage and possible small decrease in welding current, but it requires a certain amount of power (volts x amps) to melt and transfer a unit volume of the electrode. An increase in arc length may appear to result in a slight increase in this power requirement, but some of this extra power is lost in additional radiation from the welding arc.

Therefore, regardless of the apparent welding power, the length of weld bead deposited for a given length of electrode consumed, is inversely proportional to the arc energy / heat input. For SMAW welding, the run-out-length (ROL) is the crucial parameter for control.

For this to be useful, we need to know the length of the electrode actually consumed, as this can vary. If we know the original electrode length, the unconsumed stub length provides this variable.

GMAW/FCAW

For the GMAW/MIG/MAG processes (131, 135) and FCAW processes (132, 133, 136, 138), similar principles apply, but the consumable is spooled wire, so we don’t have a fixed length of consumable. However, if we know the wire feed speed (WFS) and the welding travel speed (S) accurately, we can derive relationships between WFS and TS for a specific consumable wire.

Theory

The standard equation for arc energy per unit length is

AE = V * A * T / L

Where:

AE is arc energy per unit length (usually referred to as just "arc energy”)
V is arc volts
A is welding current [amps]
T is arc time in seconds
L is the weld length (ROL)

As L/T is the travel speed, the equation AE = V * A / S is another way of expressing this.

The heat input per unit length is given by:

HI = AE * Pf

Where:

HI is the heat input per unit length (nearly always referred to as just "heat input”).
Pf is the arc efficiency, typically 1.0 for SAW, 0.6 for GTAW/PAW, and 0.8 for other processes (though it can be determined more accurately for specific consumables)

In the USA or where ASME / AWS standards are being used, the arc efficiency (Pf) is often ignored (ie. Pf=1), so HI = AE * 1.0 = V * A * T / L

SMAW ROL vs Stub Length

An approximate arc energy per unit length (AE) is given by:

AE = (ElectrodeLength – StubLength) / (ROL * ElectrodeFactor) * ElectrodeDiameter²

Where:

ROL is run out length (or bead length)
The ElectrodeFactor is typically around 17 for arc energy in kJ/mm and all dimensions in mm, but this can be refined by analysis of PQR data. It can vary between about 16-19 for most basic electrodes in carbon manganese steel and duplex stainless steel. It is likely that there is little variation with other types of stainless steels and nickel alloys, but expected that there would be significant differences with lower melting point alloys of aluminium, titanium etc. 

If you multiply the ElectrodeFactor by π/4, it effectively becomes the volume of electrode consumed per unit of energy, typically 13.4 mm³/kJ  for steel. This means that…

Weld Bead Cross-Sectional Area = ElectrodeFactor x π/4 x ArcEnergy

The ElectrodeFactor then links two measures recognised by QW 409.1 of ASME IX.

NB. The ElectrodeFactor will vary slightly according to the % recovery of the electrodes, as well as their diameter, so should be determined by analysis of PQR data for accurate results.

GMAW/FCAW Travel Speed vs Wire Feed Speed

This method relies on knowing the wire feed speed accurately. This is easily achieved using MIG sets with digital wire feed speed settings. If these are not available, the WFS control will need to be calibrated.

The general form of the volumetric relationship for continuously-fed wire welding is:

AE = f(WFS) / TravelSpeed

The function f could be defined in many ways, but the simplest is:

f(WFS) = WireConstant + WFS * WireFactor

The WireConstant is necessary because the linear relationship often doesn’t quite pass through zero. This is the relationship used by WIMS.

The relationship could also be a slight curve:

f(WFS) = WireFactor * WFS^WirePower

(WireFactor would have a different value from the linear example)

Any function of the wire feed speed could be used, and would be determined from a regression analysis of existing procedure qualification data. It could also be refined to take account of welding position or transfer mode.

It is important to note that a change in transfer mode (eg. from spray to dip/short-circuit) could have a significant effect on the correlation.

The wims.org.uk website and the WIMS App(s) use the simple linear method, but these can be extended if needed.

There are some MIG sets available that do not control the wire feed speed directly and allow the user to preset the welding current. In this instance, if the MIG set also allows the arc voltage to be preset, direct calculation using Volts * Amps / Travel Speed is more appropriate.

Standards Support

It is often overlooked that national and international standards do support volumetric methods:

In BS 4515-1 (2009) welding parameters essential variables are:

"j2 Arc voltage : Any change exceeding +/- 10%
j3 Wire feed speed (SAW, MAG, FCAW) or welding current : Any change exceeding +/- 10%
j4 Travel speed : Any change exceeding +/- 10%
j5 Calculated value of heat input : No separate restriction”

In EN ISO 15614‑1:2017+A1:2019, 8.4.7 it states:

"For process 111, the heat input may also be measured by the run out length per unit length of electrode.”

And in ASME IX:

"QW-409.1 An increase in heat input, or an increase in volume of weld metal deposited per unit length of weld, for each process recorded on the PQR. For arc welding, the increase shall be determined by (a), (b), or (c) for nonwaveform controlled welding, or by (b) or (c) for waveform controlled welding.
…
(b) Volume of weld metal measured by
(1) an increase in bead size (width × thickness), or
(2) a decrease in length of weld bead per unit length of electrode”

Implementation – SMAW

There are 3 main ways of using the ROL/Stub Length Arc Energy approximations:

Charts

Tables

Apps & Spreadsheets

There are many ways these simple calculators can be implemented on phones, tablets, and laptops.

At the simplest level, spreadsheets can be developed that will run on any device. Dedicated Apps for Android or iOS can be created, or online cloud applications that will run on any web browser.

Implementation – GMAW/FCAW

Once again, the approximations can be used in three ways:

Charts

Not as useful as the SMAW charts during welder qualifications, because the welder cannot easily check their travel speed, but still possible, and easy to use with an assistant.

However, presenting the acceptable travel speed ranges related to wire feed speed is an improvement on normal WPS ranges, particularly where large WFS ranges are allowed (eg. where SWIS is used, or WPSs cover multiple positions)

Tables

The Arc Energy can be read from the intersection of the wire feed speed and travel speed. Of course this does mean that some method for calculating the travel speed has to be provided.

This is the trickiest bit for MIG/MAG processes. Obviously, calculators (or possibly phones) can be used. It’s also possible to provide simple tables, such as this one.

Apps / Spreadsheets

There are many ways these simple calculators can be implemented on phones, tablets, and laptops.

At the simplest level, spreadsheets can be developed that will run on any device. Dedicated Apps for Android or iOS can be created, or online cloud applications that will run on any web browser

Accuracy

There are two ways of considering accuracy.

As a method of welding parameter control in its own right, the volumetric approach is as accurate as the ability to measure lengths with a steel rule and measure arc-time with a stopwatch. This has minimal calibration risks.

As a method of parameter monitoring with the aim of measuring the arc energy, as an alternative to electrical measurements, it typically estimates arc energies per unit length within about 10% of the actual calculation value. However, if the Volts * Amps * Time measurements differ significantly from an arc monitoring system total energy figure, the deviation increases. For a detailed analysis look at pages 34-52 of this PDF.

It’s important to remember that the absolute accuracy is not as important as whether we can detect welders working outside of the approved range. Acceptance criteria can be tweaked for critical applications: