Does ASTM A6 thickness tolerance affect WPS qualification range?

Yes. If your structural plate is at the lower tolerance limit, the actual weld thickness may fall below the minimum qualified by your WPS. For PQR-qualified procedures, the qualified thickness range is a function of the test coupon thickness, not the nominal specified thickness.

How much thickness variation does ASTM A6 allow?

ASTM A6 permits over-thickness up to roughly 10-25% over nominal depending on width and thickness, and under-thickness up to 0.01 to 0.03 inch on thinner plates. The specific tolerances are width- and thickness-dependent tables in A6 Section 12.

Does plate over-thickness affect minimum preheat requirements?

It can. AWS D1.1:2025 Table 5.11 preheat requirements step up at specific thickness thresholds. If actual plate thickness exceeds a nominal value and crosses a Table 5.11 step boundary, the higher preheat requirement applies to the actual plate, not the specified nominal.

Should I measure actual plate thickness before writing my WPS preheat requirements?

Best practice is to measure or conservatively account for over-tolerance when designing your WPS preheat section and verifying qualified thickness range. For critical connections with thick-to-thin combinations, actual measurements prevent surprises during CWI inspection.

Nov 4, 20267 min read

ASTM A6 Plate Tolerances and WPS Qualified Thickness Range

ASTM A6 allows significant over- and under-thickness variation in structural plate. Here's how those mill tolerances interact with your WPS qualified base metal thickness range under AWS D1.1.

WPS PQR AWS D1.1 Base Metal Fabrication

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

Structural fabricators know that AWS D1.1 welding procedure specifications must cover the base metal thickness being welded. What is less often discussed is that the structural plate being welded is not exactly the nominal thickness on the drawing—and the tolerance band allowed by ASTM A6 is wide enough to matter for WPS qualification ranges and preheat requirements.

This article covers how ASTM A6 mill tolerances interact with AWS D1.1 WPS qualified thickness ranges and preheat calculations, and what CWIs and QC managers should check before production begins.

What ASTM A6 Allows

ASTM A6/A6M is the general requirements standard for rolled structural steel bars, plates, shapes, and sheet piling. Section 12 covers permissible variations in thickness for plates and universal mill plate.

The tolerance table is a function of both the nominal thickness and the nominal plate width. Key points for structural fabricators:

Over-thickness is permitted up to roughly 10-25% over nominal on common structural plate widths. A 1/2 inch [12 mm] nominal plate, for example, may actually be as thick as 0.57 inches or more depending on width—a 14% variance. On wide plates used for base plates, gussets, and thick shear tabs, this is common.

Under-thickness tolerances are tighter and specified in absolute terms. For plates up to about 3/8 inch nominal, under-thickness tolerances range from 0.01 to 0.03 inch. For thicker plates, tolerances are expressed as a fraction of nominal. Under-thickness is less common in production but is not prohibited.

What this means practically: when you specify "3/4 inch A572 Grade 50 plate," the material arriving at your shop may be anywhere from approximately 0.72 to 0.84 inches depending on the mill's rolling practice and the width of the plate. Both extremes are conforming to ASTM A6.

WPS Qualified Thickness Range: Nominal vs Actual

AWS D1.1 Clause 6 (qualified WPS) bases the qualified base metal thickness range on the thickness of the PQR test coupon. For a prequalified WPS under Clause 5, the range is defined by Clause 5 requirements, which are expressed as minimum and maximum thicknesses for the qualified joint details.

In either case, the coverage is for the actual thickness being welded, not necessarily the nominal drawing dimension.

PQR-qualified WPS example:

Suppose your PQR test coupon was cut from a 3/4 inch nominal plate, actual measured thickness 0.76 inch. Your WPS qualified thickness range is derived from that 0.76 inch coupon per the AWS D1.1 Table 6.11 rules (qualified range is a multiple of the PQR coupon thickness, subject to maximums).

If production plate shows up at 0.70 inch (at the lower ASTM A6 tolerance limit for 3/4 inch plate), that actual thickness may fall within or below your WPS qualified range depending on how the range was written. If it falls below the qualified minimum, you are welding outside the WPS—a nonconformance.

Prequalified WPS example:

Prequalified WPSs under Clause 5 state qualified thickness ranges in terms of the joint geometry, not a PQR coupon. The minimum and maximum material thickness associated with the prequalified joint must encompass the actual plate being welded. Over-thickness plate that pushes the actual weld cross-section beyond the stated Clause 5 maximum requires either a different prequalified joint configuration or qualification by test.

Where Over-Thickness Matters Most

Over-thickness within ASTM A6 tolerances is generally benign from a strength standpoint—more base metal is a conservative deviation. However, it can cause real problems in two areas:

1. Preheat Requirements

AWS D1.1:2025 Table 5.11 steps preheat requirements at specific plate thickness thresholds. The thresholds vary by steel category and welding process, but they are hard boundaries: a plate at the threshold or above requires the higher preheat.

If your drawing specifies 1 inch nominal plate and your WPS preheat was written based on 1 inch, but the actual delivered plate is 1.09 inch (within ASTM A6 tolerance for many widths), and that extra 0.09 inch pushes you across a Table 5.11 step—the actual plate requires the higher preheat.

The fabricator who measures nominal and sets preheat by the drawing, without checking actual mill thickness, may be applying insufficient preheat. A CWI reviewing the production weld has no way to know this unless actual plate thickness was measured and recorded.

Best practice: for connections near a Table 5.11 preheat threshold, measure actual plate thickness from the heat. Use the actual thickness to determine preheat requirements in the WPS. Document the actual thickness range in the WPS or project-specific procedure supplement if it differs from nominal. Rule library based on AWS D1.1:2025; verify against your governing edition.

2. Minimum Fillet Weld Size

AWS D1.1:2025 Table 7.7 sets minimum fillet weld sizes based on the thickness of the thinner part joined (for low-hydrogen processes). If actual plate thickness exceeds nominal by enough to cross a Table 7.7 step, the minimum fillet required by the code increases—even if the design specifies a smaller fillet that would be adequate for the nominal plate.

A shop welding a 5/16 inch minimum fillet based on 3/4 inch nominal plate, when the actual plate is 0.84 inch (over tolerance), may need a larger minimum fillet for the actual material per Table 7.7. This is subtle—the connection still has adequate strength—but it is a code compliance issue.

Under-Thickness: WPS Qualified Range and Joint Fit-Up

Under-thickness within ASTM A6 tolerance is less common but raises its own concerns:

WPS minimum thickness: If your WPS has a minimum qualified base metal thickness (common for PQR-qualified procedures on thin plate), under-tolerance material may fall below it. For typical structural connections this is usually not a problem because structural plate is thick relative to thin-walled qualification minimums. But for shops welding base plates or connection plates at the thinner end of the structural range, it is worth checking.

Joint fit-up: Under-thickness plate affects root opening dimensions in CJP groove welds and joint geometry for PJP connections. A prequalified joint dimension table specifies tolerances on root opening and groove geometry. Under-tolerance plate on one or both sides of a joint can widen or narrow the root opening outside the qualified range, requiring a CWI decision on whether the joint can be accepted or requires modification.

What CWIs Should Verify

Before production welding begins on a lot of structural plate, the CWI or QC manager should:

Review the MTR (mill test report) for the actual thickness recorded at the mill. Many MTRs include the actual measured thickness, which may differ from nominal.
Spot-measure plate thickness at the joint location, particularly for thick plate used in base plates, column flanges, and connection plates where over-tolerance is most common.
Cross-reference actual thickness against the WPS qualified range and confirm the preheat requirement using actual thickness and AWS D1.1:2025 Table 5.11.
Confirm minimum fillet size compliance using actual thinner-part thickness per Table 7.7.
Document actual plate thickness in the production traveler so there is a record if questions arise later.

This is not an exotic inspection step—measuring actual plate thickness is straightforward with calipers or an ultrasonic thickness gauge for in-process stock. The habit of using nominal without verification is what creates the exposure.

Writing WPS Preheat Ranges to Account for Tolerance

A practical approach for shops dealing with structural plate near Table 5.11 preheat boundaries is to write the WPS preheat section conservatively:

If your nominal plate is 3/4 inch but could arrive up to 0.84 inch, write the WPS preheat based on 0.84 inch, not 0.75 inch. The preheat will be slightly more conservative than necessary on nominal plate but will never be insufficient on over-tolerance plate.

This approach also avoids the production slowdown of re-checking preheat tables every time a heat of plate arrives slightly heavy. Build the tolerance into the WPS once and move on.

For projects with multiple steel grades and thicknesses, maintaining a WPS preheat matrix by actual measured thickness range is more reliable than by nominal drawing dimension alone. The preheat and interpass temperature on a WPS article covers how to structure that documentation.

Impact on Prequalified WPS Clause 5 Joint Geometry

AWS D1.1 Clause 5 prequalified joint details list specific root openings, groove angles, and tolerances. These are expressed as dimensions, not percentages—a groove detail that specifies a 1/4 inch root opening with ±1/16 inch tolerance means that tolerance applies to the actual joint, regardless of nominal plate thickness.

Over-tolerance plate does not directly change the groove dimensions, but it can:

Change the required root face dimension for a given joint configuration
Push the joint's effective throat or required groove depth outside the prequalified range
Require a larger weld access hole per the dimensional requirements in AWS D1.1 for access holes in flanges

For standard beam connections at the structural scale, these effects are typically small. For complex CJP groove welds in heavy column splices or transfer connections, confirming that the actual joint geometry—accounting for actual plate thickness—falls within the prequalified limits is a step worth taking during joint fit-up inspection.

A Note on Mill Cert Verification

The mill test report (MTR) for structural plate includes the heat number, actual chemical composition, and mechanical test results. It typically also includes the actual plate dimensions from the mill run. Reviewing the MTR for actual thickness—not just for yield strength and chemistry—is a small additional step that closes the tolerance loop.

For more on mill cert review as part of WPS base metal compliance, see mill certificate and MTR review for WPS base metal compliance and prequalified base metals and Table 5.6 eligibility.

If you manage multiple WPS procedures across different steel grades and thickness ranges, tracking actual qualified ranges—not just nominal—in a searchable procedure library reduces audit risk and prevents the kind of quiet tolerance exposure described above. See wpswelding.com/pricing for WPS management tools built for structural fab shops.

FAQ

Frequently asked.

Does ASTM A6 thickness tolerance affect WPS qualification range?: Yes. If your structural plate is at the lower tolerance limit, the actual weld thickness may fall below the minimum qualified by your WPS. For PQR-qualified procedures, the qualified thickness range is a function of the test coupon thickness, not the nominal specified thickness.
How much thickness variation does ASTM A6 allow?: ASTM A6 permits over-thickness up to roughly 10-25% over nominal depending on width and thickness, and under-thickness up to 0.01 to 0.03 inch on thinner plates. The specific tolerances are width- and thickness-dependent tables in A6 Section 12.
Does plate over-thickness affect minimum preheat requirements?: It can. AWS D1.1:2025 Table 5.11 preheat requirements step up at specific thickness thresholds. If actual plate thickness exceeds a nominal value and crosses a Table 5.11 step boundary, the higher preheat requirement applies to the actual plate, not the specified nominal.
Should I measure actual plate thickness before writing my WPS preheat requirements?: Best practice is to measure or conservatively account for over-tolerance when designing your WPS preheat section and verifying qualified thickness range. For critical connections with thick-to-thin combinations, actual measurements prevent surprises during CWI inspection.