Welding Variables- Essential variable, Non- Essential Variables & Supplementary Essential Variables


What are Welding Variables?

It is important to remember that the requirements of different welding codes and standards (E.g. ASME Section IX, AWS D1.1, or ISO 15614-1) have been adjusted to meet specific service requirements. Each welding code and standard such as ASME Section IX or AWS D1.1 or ISO 15614-1 have given & considered:

  1. essential,
  2. nonessential, and
  3. supplementary essential variables differently.

Hence, to properly develop welding documents such as WPS, PQR, or WPQ, shall be reviewed by competent qualified Welding Engineer or Welding Inspector. These qualified and responsible person shall review and decide the relevant & applicable welding variables such as essential, non-essential and supplementary essential variables for each qualification. A righteous review will ensure the documents comply to the code requirements to maintain the welding quality on job.

A qualified PQR & WPS specifying these variables within the qualified range is mandatory for a good welding quality and integrity. This is hence essential to perform this task with proper due diligence and utmost care.

Watch this YouTube video for full classroom training on Welding variables, their types, and practical Importance.

Watch the online video and learn:

  • What is the welding variable?
  • Types of welding variables?
  • Procedure and Performance variables?
  • What are essential, supplementary, Non-essential, and special variables?
  • Importance of welding variables?

What is Essential Variable in Welding?

Essential welding variables are listed in the ASME Section IX, clause QG-105.1, QW-251.2, and QW-401.1.

For Essential Welding variables, a change in the essential variable is considered to affect the mechanical properties (E.g. tensile strength, yield strength & ductility) of the weldment and shall require the requalification of the WPS. Hence, the WPS must be requalified, if any of the essential variable is changed. Examples of Essential variables are P-Number for base metal, welding process or PWHT, etc.

Examples of Essential Variables in Welding for WPS

The examples of some essential welding variables as per ASME Section IX are:

  1. Base Metal P. Number
  2. Base Metal Thickness qualified range
  3. Decrease in preheat temperature more than 100 Degre Fahrenheit
  4. Change in PWHT

What are Supplementary Essential Variables in Welding?

Supplementary Essential welding variables are listed in the ASME Section IX, clause- QG-105.3 & QW-251.2, QW-401.3

Supplementary essential variable affects the weld toughness properties (CVN). So, the supplementary essential variables shall be considered as essential variables for procedure qualification (WPS-PQR) when CVN toughness is applicable for the welding. For example- a pressure vessel fabrication has toughness requirements for its weld, then supplementary essential variables must be considered too.

If there is no requirements for weld toughness, supplementary essential variables are non-essential variable only. For example- a weld operating at elevated temperature does not requires weld toughness as material is already having good ductility a high temperature. Examples of Supplementary essential variables are for example- Group Number for base metal, thickness limit qualified in the PQR.

Examples of Supplementary Essential Variables in welding for WPS

The examples of some Supplementary-essential welding variables as per ASME Section IX are:

  1. Groove design
  2. Removal of backing
  3. Change in welding rod diameter
  4. Change in welding position
  5. Change in welding progression
  6. Change in polarity
  7. Change in welding technique
  8. Method of cleaning

What are Nonessential Variables in Welding?

Non-Essential welding variables are listed in the ASME Section IX, Clause- QG-105.3 & QW-251.3, QW-401.4

Nonessential welding variables refers to those conditions in which a change can be done without the need to requalify the existing WPS. The reason- changes in these variables does not affects the mechanical properties of the weld.

Although a change in the nonessential variable doesn’t require prequalification of the WPS but as per ASME, these variables shall be specified in the welding procedure specification (WPS). Examples of non-essential welding variables are- joint design, filler wire/ electrode diameter for example.

Examples of Supplementary Essential Variables in welding for WPS

The examples of non-essential welding variables as per ASME Section IX are:

  1. Material Group Number
  2. Weld metal Thickness qualified limit
  3. Change in welding rod classification
  4. Increase in interpass temperature
  5. PWHT (Time & temperature change)
  6. Welding Heat Input
  7. Welding current & voltage

What are Special Process Variables?

Special welding variables are listed in the ASME Section IX, Clause- QG-105.4 & QW-251.4

Special process variables are conditions that apply only to special processes (Such as corrosion resistance overlay & hard-facing) that are described in the ASME Section IX. Special processes are such as weld overlay applications. These processes are used for special purposes instead of joining applications.

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