This article provides information about the nonessential variable in welding procedure specification based on requirements of the ASME Code Section IX. The code defines it as " Nonessential variables are conditions in which a change, as described in the specific variables, is not considered to affect the mechanical properties of the joint".
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In other word, if you have a welding procedure specification (WPS), and you want to utilize this procedure in other job but you have to change some variable and these variables are categorized in the nonessential variable, then you do not need to re-qualify your WPS.
For example, you have WPS with shielded metal arc welding process with V groove joint design, and you want to utilize this WPS in another job with the same condition but with U groove joint design, then you do not need to re-qualify your WPS. What you need to do is to revise your WPS and put the number of previous procedure qualification record (PQR) number on the revised WPS.
It means you do not need to go to such ordeal with the preparation of new test specimen and to conduct mechanical property test. Because this change categorized as nonessential variable and is not affecting the welding mechanical properties because it is not essential variable.
Normally, joint design, filler metal diameter, the minor change in the electrical characteristics, welding positions, welding technique (some of them) are nonessential variable in welding procedure specification..
The ASME Code Section IX on the article II provides a unique table for each welding process and list the essential, nonessential variables and supplementary essential variables. Based on the code requirements the nonessential variables must be indicated in the welding procedure specification.
The most common mistake is that the manufacturers do not revise WPS's when there is a change in the nonessential variable. They justify that it is the nonessential so they do not need to do anything. This is not correct, actually they should produce a new WPS but without performing a new PQR. The should add the number of the previous PQR on the revised WPS.
It is the manufacturer quality control team and third party inspector task, to check and make sure that welding procedure specification is completely consistent with actual welding job that is in the progress on the shop floor i.e. joint design, filler wire type and diameter, inert gas type and flow rate, voltage, and amperage, preheat and inter-pass temperature, etc.