WPS Verification / Essential Variable

Date 09-12-2019 02:18

I am looking for any information to show what variables should be verified when performing a WPS performance verification or set up for a GMAW weld. I have been led to believe that Volts, Amps and Travel speed are the foundation of any good weld and need to be verified as essential variables along with other variables. The heat input formula supports this belief

I had a discussion with another Inspector who has produced a series of WPS listing the amps range values followed by the word reference only..This implies that the amp settings are not of a concern as they are only a reference. In fact- It was indicated that it was not truly essential to check or verify the amp settings, and actually better to verify or check wire feed speed instead.

I can understand checking wire speed along with ALL essential values if that were the case - but I cannot understand how amps could be considered as a reference value only and checking wire feed speed could negate the need to check for proper amp setting.

I didn't have enough time to get any answers from the other inspector and am looking for any support for or against this or anyone who has encountered such a conversation.

By SWN1158 (***)

Date 09-12-2019 10:32

If the applicable code is D1.1, Table 3.7 lists the prequalified WPS variables, which includes process parameters... (9) a change in amperage, greater than a 10% increase or decrease, which is applicable to the GMAW process. BUT, (13) goes on to state “a change in wire feed speed, greater than a 10% increase or decrease, if not amperage controlled”, which is also applicable to the GMAW process.

By Lawrence (*****)

Date 09-12-2019 11:46

A grey area, but not really.

If your WPS were GMAW or FCAW   A CV process, The current varies as CTWD (stickout) changes right ? The welder does NOT set the current with CV processes.

So the WFS and Current have a relationship as we all know, but it is a thing that varies from welder to welder in production.

The essential values are stated and must be obeyed... That's the law so to speak.

But two welders may set a WFS exactly the same and have current values that are different (hopefully within the range)

I have seen some "compliant" WPS's that state a single current value.   The range is of course still applicable, just not published. So a statement like "for reference only" might have meaning here... Not that the value is not essential, but rather that it's going to vary a bit because of the nature of the process.

I use the following caveat in some of my GMAW WPS's. "Current is not set by the welder and is approximate" I think this says it a little better.

I haven't seen the WPS's in question and don't know the intent of the author... These are just a few words that might or might not help to get at the root meaning of the phrase.

By 803056

Date 09-12-2019 17:04

I am in agree with Lawrence's comments.

There are three constants when using a semi-automatic welding process such as FCAW and GMAW. Those constants are not truly constant, but are pretty close to being so. The three parameters are:
voltage - constant voltage power supply
wire feed speed - the WFS is constant whether the welder is welding or holding the gun pointing to the sky
electrode extension - some people use contact tip to work distance which is nothing more than the electrode extension ply arc length

Amperage can be approximated using Ohm's Law. A = V/resistance, where the voltage can be assumed to be constant and the resistance is the resistance of the welding circuit. The variable regarding resistance is the electrode extension. The electrode is a resistor whose resistance is a function of the length of the electrode extending beyond the contact tip. The rest of the resistance from the welding circuit can be considered to be a constant.

As the welder varies the electrode extension (or CTWD), the resistance changes as does the welding current. A small change in the electrode extension results in a small change in the welding current. A large change in electrode extension results in a large change in welding current.

The melt-off rate is dependent on amperage. the amperage is dependent on electrode extension. to be in balance the melt-off rate must be equal to the wire feed speed or the electrode with "burn-back" to the contact tip or stub out against the work piece. As the welder varies the electrode extension, the resistance varies and the amperage varies. The importance of using the correct and holding steady the electrode extension must be recognized.

The bottom line is that trying to set the parameters by setting amperage is a "fools game" unless the electrode extension is a known value and held constant. The arc voltage determines the arc length, but it is influenced by the shielding gas used.

It is unfortunate that too many welders and inspectors alike do not understand the relationship between electrode extension, wire feed speed. voltage, and amperage.

Best regards - Al