6.2 Welder Performance Qualification (WPQ) and Limits

6.2 Welder Performance Qualification (WPQ) and Limits

While a Welding Procedure Specification (WPS) establishes the metallurgical viability of a welding process, the Welder Performance Qualification (WPQ) demonstrates the physical skill of the welder or welding operator to deposit sound, defect-free weld metal using that procedure. As an API 570 inspector, you must verify that each welder performing work on in-service piping systems is qualified for the specific process, position, diameter, thickness, and filler metal used. If a welder works outside their qualified limits, the integrity of the weldment is compromised, violating ASME Section IX and API 570 requirements.

Purpose of Welder Performance Qualification

The primary objective of a WPQ is to determine if a welder can manipulate the electrode or torch to create a sound weld. Unlike a PQR, which uses tensile and bend tests to verify mechanical properties, a WPQ is evaluated using visual inspection and either guided-bend tests or volumetric nondestructive examination (radiography).

The essential variables for welder qualification (QW-350) differ significantly from those for procedure qualification. Because welder qualification is a test of manual skill, variables like joint design, cleaning, or preheat are non-essential for the welder. Instead, essential variables for the welder focus on the physical factors that affect deposition quality.

Welder Essential Variables and Limits

A welder's qualification is limited by several essential variables, including:

• Welding Process: Welder qualification is process-specific. A welder qualified in Shielded Metal Arc Welding (SMAW) is not qualified to perform Gas Tungsten Arc Welding (GTAW) or Gas Metal Arc Welding (GMAW). Each process requires separate qualification.
• Joint Backing: The deletion of backing is an essential variable. If a welder qualifies on a joint with backing (e.g., a metal backing strip or a fillet weld), they are not qualified to weld a joint without backing (open root). However, qualifying without backing automatically qualifies the welder for joints with backing.
• Vertical Weld Progression: For vertical position welding (3G, 5G, 6G), the direction of progression is an essential variable. A welder who qualifies using uphill progression (welding from bottom to top) is not qualified for downhill progression (welding from top to bottom) and vice versa. Uphill welding generally provides deeper penetration, whereas downhill welding is faster but more prone to slag inclusions if not executed correctly.

Base Metals and P-Number Limits

Base metal P-numbers are highly restrictive for procedure qualification (WPS), but they are much broader for welder qualification. Under ASME Section IX QW-303, a welder who qualifies by welding a coupon of any P-number from P-1 through P-11, P-43, or P-45 is qualified to weld all of these P-numbers in production, provided the filler metal used is within their qualified F-number range. This reflects the code's recognition that the welder's manual skill is not significantly affected by base metal chemistry within these groups.

Filler Metals and F-Number Limits

Filler metal classification is highly restrictive. A welder's ability to manipulate an electrode is directly tied to the slag characteristics and arc behavior of the filler metal group (F-number).

• A welder qualified with an F-4 electrode (e.g., low-hydrogen E7018) is qualified to weld with F-1, F-2, F-3, and F-4 electrodes with backing.
• A welder qualified with an F-3 electrode (e.g., cellulosic E6010) is qualified to weld with F-1, F-2, and F-3 electrodes, but is NOT qualified to weld with F-4 electrodes.
• A welder qualified with an F-6 filler metal (solid or cored wire) is only qualified for F-6.

Pipe Diameter Limits

When welding pipe, the diameter of the test coupon restricts the production pipe diameters a welder can weld. This is because smaller diameter pipes require much tighter torch manipulation and faster hand-position changes.

For example, a welder who passes a performance test on a 2.0-inch OD pipe coupon is qualified to weld production pipe from 1.0-inch OD up to unlimited diameter. If they need to weld a 0.75-inch instrument line, they must perform another qualification test on a coupon smaller than 1.0-inch OD.

Weld Deposit Thickness Limits

The thickness of the weld metal deposited by the welder in the test coupon (t) limits the maximum deposit thickness they can place in production:

• A welder is qualified to deposit a maximum weld metal thickness of 2t in production, where t is the thickness of the weld metal they deposited in the test coupon.
• Unlimited Thickness Exception: If the welder deposits a minimum of 1/2 in (13 mm) of weld metal using at least 3 layers in the test coupon, they are qualified to weld unlimited deposit thickness in production.

Welding Positions and Qualification Matrix

ASME Section IX defines six standard positions for pipe and plate welding. Test coupon positions qualify a welder for specific production positions.

The 6G position is the most common test configuration for refinery and chemical plant pipe welders because passing a single 6G test qualifies the welder for all production positions, including groove and fillet welds.

Worked Example: Welder WPQ Limits Evaluation

An inspector is reviewing a welder's WPQ to verify if they can weld a production joint on a P-1 piping system. The production weld details are:

• Process: SMAW
• Joint: 4-inch Nominal Pipe Size (4.5 in. outside diameter), 0.337-inch wall thickness, single-V groove, open root (no backing).
• Filler Metal: E7018 (F-4) low-hydrogen electrode.
• Position: Horizontal fixed position (5G), welding uphill.

The welder's WPQ record shows:

• Test Process: SMAW
• Test Joint: Groove weld on 2-inch Nominal Pipe Size (2.375 in. outside diameter), 0.250-inch wall thickness, with a metal backing strip.
• Filler Metal: E7018 (F-4).
• Test Position: 6G (Inclined pipe), welding uphill.

Evaluation:

1. Process: The welder tested in SMAW and is welding in SMAW. Pass.
2. Backing: The welder tested with backing (metal backing strip), but the production weld is open root (without backing). Deletion of backing is an essential variable (QW-353). A welder qualified with backing is not qualified to weld without backing. Fail.
3. F-Number: The welder tested with F-4 (E7018) and the production uses F-4 (E7018). Pass.
4. Diameter: The welder tested on a 2.375 in. OD pipe. Testing on 1 in. to 2-7/8 in. OD qualifies the welder from 1.0 in. OD to unlimited (QW-452.3). Since 4.5 in. is ≥ 1.0 in., the diameter is qualified. Pass.
5. Weld Deposit Thickness: The welder deposited 0.250 in. in the test coupon. The maximum qualified deposit thickness is 2t = 2 x 0.250 = 0.500 in. Since the production weld thickness is 0.337 in. (which is ≤ 0.500 in.), the thickness is qualified. Pass.
6. Position: The welder tested in the 6G position, which qualifies the welder for all groove and fillet welding positions (including 5G). Pass.
7. Progression: The welder tested uphill and is welding uphill. Pass.

Conclusion: The welder is not qualified for this specific production weld because the production joint is open root, and the welder only qualified with backing. The welder must perform a new WPQ test without backing, or a backing ring/strip must be added to the production joint.

Maintenance of Qualification (6-Month Rule)

A welder's qualification remains active indefinitely unless:

1. The 6-Month Continuity Rule is Violated: The welder has not welded using that specific welding process (e.g., SMAW) for a period of 6 months or more. If this occurs, the qualification expires for that process.
2. Ability is Questioned: There is a specific reason to question the welder's ability to make welds that meet the specification.

To maintain qualification, the welder's employer must maintain a continuity log showing the welder has used each process at least once every six months. If qualification expires due to inactivity, the welder must weld a new test coupon (or have a production weld radiographed) to renew their qualification.