What is essential variable?
Many variables and parameters affect welding quality such as such as such as joint details, base metals, filler metals, positions, preheat, PWHT. These parameters are known as WELDING VARIABLES (QG-105 & QW-250). All these variables are mentioned in WPS – Welding Procedure Specification so a welder or welding operator can develop good quality welding. In production, we have limitations, welding results can not be examined thoroughly. Some welding defects, can be seen visually by the Visual Welding inspector such as CSWIP 3.0 Welding Inspector. However strength, ductility and corrosion resistance can only be checked using destructive methods. Therefore the procedure (WPS) must be tested to prove that the procedure can produce a welding connection with the intended mechanical properties.
Variables in WPS are divided into 3, namely:
essential variables are variables that significantly affect the mechanical properties of weldment, for example affecting tensile strength, hardness and ductility – except for the mechanical properties of toughness. Examples of essential variables: P-Number, welding process, filler metal, electrode, preheat or post weld heat treatment (PWHT) and others. Any essential variable changes, require new welding procedures so they require re-qualification or we can say, we need to develop new WPS. Please also read QG-105.1 & QW-251.2, QW-401.1
One of the first factors to consider when making a weld is the base metal or metals to be joined. Some base metals are easily welded, some are more difficult, and others are downright ornery to weld. Base metals are grouped as ferrous or nonferrous metals. Ferrous metals can be further
classified as low-carbon, medium-carbon, high-carbon, high-strength low-alloy, and high-alloy steels, as well as ferritic stainless, martensitic stainless, austenitic stainless, duplex stainless, and precipitation-hardened stainless steels. The nonferrous metals include alloys of aluminum, copper, nickel, titanium, and many more. Some of the nonferrous metals are further classified as refractory or reactive metals. Welding standards usually group the base metals into families that have similar chemistry and weldability. ASME groups the various base metals by P- or S- numbers. AWS B2.1, Specification for Welding Procedure and Performance Qualification, uses M-numbers and
NAVSEA S9074-AR-GIB-010/248 uses S numbers to group the base metals. A change from one P-, M-, or S-number group to another affects the mechanical properties; therefore, base metals are
classified as essential variables.
Nonessential variables are variables that do not affect the mechanical properties of welding joints. Non-essential variables for example: joint design, method of back gouging or cleaning, and others. for more information, you can see ASME section IX para QG-105.3 & QW-251.3, QW-401.4
QW-402.1 A change in the type of groove [V-groove, U-groove, single-bevel, double-bevel, etc.).
A change in groove type, most of the times, will not affect the mechanical properties of the weld, so it is considered a nonessential variable for most of the processes (for example, it is a supplementary essential variable for PAW and an essential variable for EBW). Groove type can, however change mechanical properties of a weld by changing the A-number (chemical properties), especially when welding dissimilar metals. But it is non-essential for SMAW.
Supplementary essential variables
Supplementary essential variables are variables that affect the toughness of welding joints. If a qualification of welding procedure is required toughness testing or impact testing then all supplementary variables become “additional” essential variables , and if there is no need for toughness testing in a qualification procedure then all supplementary variables are not used or not applicable. You can get more information on ASME section IX para QG-105.3 & QW-251.2, QW-401.3
When we need a toughness testing? or when a toughness become critical properties?
Metal properties can changes. Metallurgies, scientist and engineers ( particularly mechanical engineers ) learn a lot about metal properties such as tensile strength, hardness and toughness. Some metal can be very ductile in each condition and suddenly change to brittle due to significant temperature changing. In general, when you operate an equipment ( such as piping or Pressure vessel ) at very low temperature, then they need metal or material that have good toughness to avoid brittle and catastrophic failure.
Here some examples. If you have a pressure vessel, and it does not require impact testing based on the requirement of ASME Code Section VIII, and then your supplementary essential variable in ASME Code Section IX must be considered nonessential variable. Contrary if your pressure vessel needs impact testing, then supplementary essential variable must be considered essential variable. For instance, you a have pressure vessel with 3 inch thickness and -55 degree Fahrenheit MDMT with SA 516 Gr 60 material. Based on ASME Code Section VIII, the impact testing is mandatory on the plate material at this MDMT, consecutively will be mandatory in the WPS qualification.