In this article we discuss special welding procedure specification (WPS) variables of Gas tungsten arc welding (GTAW), According to the American Society of Mechanical Engineering (ASME) Boiler Pressure vessel code (BPVC), There are three types of welding variables that are used in welding procedure specification (WPS), that is given below:
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| GTAW special variables |
1. Essential variables.
2. Non-essential variables.
3. Supplementary variables.
Every welding process has different variables, that is Essential variables, Non-essential variables, and Supplementary essential variables, Every Welding process has different variables and maybe different content. Here given below special process variable Content of Welding procedure specification (WPS) variables that are used in gas tungsten arc welding.
1.1 Joint.
1.2 Base metal.
1.3 Filler metal.
1.4 Positions.
1.5 Preheat.
1.6 PWHT (Post weld heat treatment).
1.7 Electrical characteristics / Gas.
1.8 Technique.
Quick links for more details:
WPS-Base metal thickness limit
WQT-Welder qualification Thickness limit
3LPE Coating
SMAW Welding
Visual inspection of a weld joint
1. Essential variables:
Essential variables are those variables, when they will be changed they need to be requalification of welding procedure specification (WPS). Every welding process has different welding variables. There are two special terms in essential variables, that is...
* Hard facing weld metal overlay or hard facing overlay (HFO)
Hard‐Facing Weld Metal Overlay refers to the weld metal deposits made, using a variety of welding processes, to avoid the effects of wear and abrasion. The requirements specified apply regardless of which hard‐facing weld metal overlay process is used.
The size of the test coupons, and limits of the procedure qualification, required examinations and tests, and test specimens shall be as specified. Welding variables shall be as specified for the application process.
Where Spray Fuse welding methods of hard‐facing weld metal overlay are to be used, the coupons for these welding methods shall be prepared and welding variables applied to the procedure.
If the weld metal deposit is to be used under a hard‐facing weld metal overlay process, the base metal with an assigned P‐Number, and a chemical analysis nominally matching the weld metal deposit chemical analysis may be substituted to qualify the procedure qualification record (PQR).
* Corrosion resistance weld metal or Corrosion resistance overlay (CRO)
The size of the test coupons, limits of qualification, required examinations and tests, and test specimens shall be as specified. Essential variables shall be as specified for the applicable welding process.
Here a take an example of a special process of essential variables for gas tungsten arc welding (GTAW), when it will be changed then It will need the requalification of WPS.
1.1 Joints:-
Hard Facing-overlay
(a) Less than < Finished weld metal thickness "t".
Corrosion Resistance-overlay (CRO)
(a) Less than < Finished weld metal thickness Qualified "t".
* A decrease in the distance between the approximate weld metal interface and the final surface of the production corrosion‐resistant or hard‐facing weld metal overlay, below the minimum thickness qualified. There is no limitation on the maximum thickness for corrosion-resistant or hard‐facing weld metal overlay that may be used in production.
1.2 Base metal:-
Hard Facing-overlay
(a) Change in P Number.
(a) Change in Base metal thickness Qualified "T"
Corrosion Resistance-overlay
(a) Change in P Number.
(a) Change in Base metal thickness Qualified "T".
* For the test coupons less than 38 mm thick, the addition of a cooling medium (water, flowing gas, etc.) to the backside of the weld. Qualification on test coupons less than 38 mm thick with a cooling medium on the backside of the weld qualified base metal thickness equal to or greater than test coupon thickness with and without coolant.
* A change in the joint configuration.
1.3 Filler metal:-
Hard Facing-overlay
(a) Change in classification
(b) Addition or deletion of filler metal.
(c) Change in filler metal product form.
Corrosion Resistance-overlay
(a) Change in "A" number.
(b) Addition or deletion of filler metal.
(c) Change in filler metal product form.
* A change in the filler metal classification within a special filler assignments (SFA) specification, or for a filler metal not covered by a special filler assignment (SFA) specification or a filler metal with a “G” suffix within a special filler assignments (SFA) specification, a change in the trade designation of the filler metal. conforms to analysis for any bare wire given in any special filler assignments (SFA) specification or American welding society (AWS) Classification shall be considered as having the same filler metal-F‐Number as that bare wire.
* A change in the composition of the deposited weld metal from one A‐Number and any other A‐Number, or to an analysis not listed in the table. A change in the UNS number for each American welding society (AWS) classification of A‐No. 8 or A‐No. 9 analysis of each nonferrous alloy, shall require separate welding procedure specification (WPS). A‐Numbers may be determined.
1.4 Positions:-
Hard Facing-overlay
(a) Addition in positions.
Corrosion Resistance-overlay
(a) Addition in position.
The additions of the other welding positions than already qualified welding position qualified. Except as specified below,
(a) The qualification in the horizontal, vertical, or overhead weld position shall also qualify for the flat positions of the qualifications. The qualifications of the welder in the horizontal fixed positions, 5G, shall qualify for the flat, vertical, and overhead positions. And if the welder qualifies in the 6G position that means, it shall qualify for all positions.
(b) The organizations that do production welding in particular orientations may make the tests for the procedure qualification in this particular orientation. Such qualifications are valid only for the positions actually tested, except that an angular deviation of ±15 deg is permitted in the inclination of the weld metal axis and the rotation of the weld metal face as defined. The test specimen or data shall be taken from the weld test coupon in each special orientation.
(c) For hard‐facing overlay and corrosion‐resistant weld metal overlay, qualification in the 3G, 5G, or 6G positions, where 5G or 6G pipe coupons include at least one vertical segment completed utilizing the up‐hill progression or a 3G plate coupon is completed utilizing the up‐hill progression, shall qualify for all positions. Chemical analysis, hardness, macro‐etch, and at least two of the bend tests, as required in Table, shall be removed from the vertical uphill overlaid segment
(d) A change from the vertical down to vertical up‐hill progression shall require requalification.
1.5 Preheat:-
Hard Facing-overlay
(a) Dec. > 100°F (55°C) preheat > Interpass.
Corrosion Resistance-overlay
Dec. > 100°F (55°C) preheat > Interpass
A decrease of more than 55°C (Degree centigrade) in the preheat temperature qualified or an increase in the maximum inter pass temperature recorded on the procedure qualification record (PQR). Minimum temperatures for the welding shall be specified in the welding procedure specification (WPS).
1.6 PWHT (Post weld heat treatment):-
Hard Facing-overlay
(a) Change in the post-heat weld treatment (PWHT).
Corrosion Resistance-overlay
(a) Change in the post-heat weld treatment (PWHT).
A change in post-weld heat treatment (PWHT) condition or an increase of 25% or more in total time at post-weld heat-treating (PWHT) temperature.
For the ferrous base metals other than P‐No. 7, P‐No. 8, and P‐No. 45, When the procedure qualification test (PQT) coupon receive, a post-weld heat treatment (PWHT) exceeding the upper transformations temperature and a solutions heat treatment for P-No. 10H materials, the maximum qualified base metal thickness, T, shall not exceed 1.1 times the thickness of the test coupon
A separate procedure qualification is required for each of the following:
(a) For weld corrosion‐resistant overlay of A‐No. 8 on all base materials, a change in post-weld heat treatment condition in QW-407.1, or when the total time at post-weld heat treatment (PWHT) encountered in fabrication exceeds 20 hr, an increase of 25% or more in total time at post-weld heat-treating temperature.
(b) For weld corrosion‐resistant overlay of A‐No. 9 on all base materials, a change in post-weld heat treatment (PWHT) condition, or an increase of 25% or more in total time at post-weld heat-treating temperature
(c) For all other weld corrosion‐resistant overlays on all base materials, a change in post-weld heat treatment (PWHT) condition in ASME Section IX.
1.7 Gas characteristics:-
Hard Facing-overlay
(a) Change in single, mixture, or percentage.
Corrosion Resistance-overlay
(a) Change in single, mixture, or percentage.
* A change from Alternating current (AC) to Direct current (DC), or vice versa; and in direct current (DC) welding, a change from the electrode negative (straight polarity) to electrode positive (reverse polarity), or vice versa.
* When an increase of more than 10 percent in the amperage used in the applications for the first layer.
1.8 Electrical characteristics:-
Hard Facing-overlay
(a) Change in current or polarity.
(b) 1st layer of heat input Greater than (>) 10 percent.
Corrosion Resistance-overlay
(a) Change in current or polarity.
(b) 1st layer of heat input Greater than (>) 10 percent.
1.9 Technique:-
Hard Facing-overlay
(a) Change to multiple layers to a single layer.
(b) Change in the number of electrodes.
Corrosion Resistance-overlay
(a) Change to multiple layers to a single layer.
(b) Change in the number of electrodes.
2. Non-Essential variables for hard-facing overlay (HFO) and corrosion resistance overlay (CRO).
Non-essential variables are those variables that make the changes not required to prequalification of welding procedure specification (WPS).
Note:- All the content which is mention in the Welding procedure specification (WPS) for the special process for shielded metal arc welding (SMAW), the same for all three variables. So.
2.1 Joints:-
* There is no hard facing-overlay and corrosion resistance-overlay in Joint.
2.2 Base metal:-
* There is no Hard Facing-overlay and Corrosion Resistance-overlay in Base metal.
2.3 Filler metal:-
Hard facing-overlay and Corrosion Resistance-overlay
(a) Change in the filler material.
2.4 Positions:-
* There is no Hard Facing-overlay and Corrosion Resistance-overlay in Post weld heat treatment.
2.5 Preheat:-
* There is no Hard Facing-overlay and Corrosion Resistance-overlay in Post weld heat treatment.
2.6 PWHT (Post weld heat treatment):-
* There is no Hard Facing-overlay and Corrosion Resistance-overlay in Post weld heat treatment.
1.7 Gas characteristics:-
(a) Change in flow rate.
1.8 Electrical characteristics:-
(a) Change in tungsten electrode.
2.9 Technique:-
(a) Change in String/weave.
(b) Change in an orifice, cup, and nozzle size.
(c) Change in method of cleaning.
(d) Addition or deletion of Peening.
(e) Change in Oxidation.
(f) Change in electrode spacing.
(g) Change in manual or automatic.
(f) Change in metal delivery.
3. Supplementary Essential variables.
* There is no special WPS variable in the supplementary essential variables.
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