Welding procedure specification (WPS)
A WPS is a written qualified welding procedure prepared to provide direction for making production welds to Code requirements. The WPS or other documents may be used to provide direction to the welder or welding operator to assure compliance with the Code requirements. It contains all the necessary parameters such as joint details, base metals, filler metals, positions, preheat, PWHT, gas, electrical characteristic, technique etc. (including ranges). These parameters are known as WELDING VARIABLES (QG-105 & QW-250).
Essential Variables QG-105.1 & QW-251.2, QW-401.1
A change in essential variable is considered to affect the mechanical properties of the weldment and shall require requalification of the WPS. Hence, the WPS must be required qualification, if any the essential variable is changed.
Supplementary Essential Variables QG-105.3 & QW-251.2, QW-401.3
A change in the supplementary essential variable will affect the toughness properties of the joint, heat-affected zone, or base material. Hence supplementary essential variables become additional essential variables in situations where procedure qualification requires toughness testing. When procedure qualification does not require the addition of toughness testing, supplementary essential variables are not applicable.
Nonessential Variables QG-105.3 & QW-251.3, QW-401.4
Nonessential variables are those in which a change can be made without requalification of the existing WPS, since it is not considered to affect the mechanical properties of the joint. Though a change in the nonessential variable doesn’t require requalification of the WPS but still it should be properly addressed in the welding procedure specification (WPS).