Welding Procedure Specification (WPS)

Welding Procedure Specification (WPS)

Before any welding activity can commence, the welder and the QC Engineer has to be fully aware of the content of the welding procedure specification, WPS. This document is produced by the welding engineer in order to ensure the quality and correct design output of the weld. The WPS contains information regarding:

  • the type of weld to be performed
  • Base and filler metals
  • Weld position & joint tolerances
  • Preheat temperature, Interpass temperature and Post-weld heat treatment (PWHT)
  • Electrical characteristics
  • Welding technique
  • Gas usage
  • Electrode information; and
  • Voltage, Ampere and travel speed during welding

All sections on the WPS are related to a section in the ASME IX for code reference; for example QW-403 (base metals).

The WPS is always supported by a PQR, Procedure Qualification Record. This document is a quality document that supports the WPS design with an actual welded joint and corresponding material testing records (hardness test, tensile strength, NDT etc.).

Joints (QW-402)

Joint design, use of backing and maximum root gap is defined in this section.

Base metals (QW-403)

This section describes the base metal for which the WPS can be used. All base metals have been assigned a P number which is related to the metals chemical composition, weldability etc (see Table 1).  The WPS can be valid for a single metal or a range of metals. This section also describes the metal thicknesses and diameters covered by the WPS. Ferrous metals with specific impact test requirements have been assigned different group numbers within the P numbers.

Base Metal P numbers

Base Metal

Welding

Brazing

Steel and Steel Alloys

P-No. 1 through P-No. 11 incl. P-No. 5A, 5B and 5C

P-No. 101 through P-No. 103

Aluminum and Aluminum-base Alloys

P-No.21 through P-No. 25

P-No. 104 and P-No. 105

Copper and Copper-Base Alloys

P-No. 31 through P-No. 35

P-No. 107 and P-No. 108

Nickel and Nickel base Alloys

P-No. 41 through P-No. 47

P-No. 110 through P-No. 112

Titanium and Titanium base Alloys

P-No. 51 through P-No. 53

P-No. 115

Zirconium and zirconium base Alloys

P-No. 61 through P-No. 62

P-No. 117

Filler metals (QW-404)

All information regarding appropriate filler metal use is described in this section; Filler number, Spec number, size limitations, maximum deposit thickness and type of filler.

Position (QW-405)

This section defines if the welding position has any limitations and if the welding should progress in a certain direction (uphill/downhill).

Preheat & Interpass temperature (QW-406)

If preheat is needed to ensure the quality of the weld, it will be defined in this section. In addition, interpass temperature limitations (maximum allowable base metal temperature in between weld passes) and (if needed) preheat maintenance are specified here.

Post Weld heat Treatment (PWHT) (QW-407)

Post weld heat treatment is a post welding heating process used to ensure proper toughness of the weld. It is essential when welding Grade 91, 911, 92 or 122. The temperature and time is defined in this section. 

Gas (QW-408)

This section defines the type of shield gas used during welding and the flow rate. The reason for using shield gas is to protect the welding area from atmospheric gases; nitrogen, oxygen or vapour.

Electrical characteristics (QW-409)

Welding electrical characteristics, like volt, ampere, current AC/DC etc. and tungsten electrode and size are defined in this section. Three different current types exists; DCEN which generates deep, narrow penetration where electrode capacity is excellent; DCEP generates a shallow, wide penetration profile where electrode capacity is generally poor; AC (balanced) generates medium penetration with good electrode capacity.

Technique (QW-410)

If limitations/recommendations regarding welding technique exist then it is defined under this section. Stringer is the easiest welding technique, where the rod is dragged along the weld. Weaving involves small movements from side to side between the base metal sides and requires more skills to ensure a good quality weld. This section also defines if a single or multiple welding passes are to be used.

Electrode section (from this example WPS)

As can be seen in Figure 1, the designer (welding engineer) can decide to use different types of welding processes for the different weld passes.

wps

Figure 1: Electrode section describing welding process, electrical characteristics and travel speed.

This section also describes voltage, ampere and travel speed which are all related to the heat input during welding. Slower travel speed increases the heat input and amount of weld filler metal being welded. Higher travel speed decreases the heat input and minimizes filler metal laid in the weld bead. The weld heat input can be calculated as

Heat input [J/mm] = Voltage x Ampere x 60 / travel speed [mm/min]

The amount of heat inserted during welding will affect the material properties of the weld. The limits are designed by the welding engineer for a specific reason and should always be followed.

References

Qualification Standard for Welding and Brazing procedures, Welders, Brazers, and Welding and Brazing operators. ASME IX, July 2010.

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