Can Low-Stress Stamps Mark Pressure-Part Inner Surfaces?

The approved marking plan preserves positive identification without placing a sharp or uncontrolled impression on the steam-contacting face. An inner-surface low-stress stamp can remain an option, but only after the marking purpose, material, thickness, minimum design metal temperature, service, geometry, and inspection acceptance have been resolved.

Marking-method comparison

Before anything else, classify the proposed mark. Material identification, construction-code part marking, welder identification, and a fabricator's tracking mark do not automatically fall under the same provision. The supplier must identify the requirement that makes the mark necessary before selecting its location or method.

Approach	Best application	Pressure-boundary effect	Acceptance basis
Place the stamp on an accessible external or nonwetted surface	A suitable marking land remains visible and traceable	Avoids an impression on the process-contacting face	Verify that the location satisfies the applicable construction and traceability requirements
Use a low-stress stamp on the inner surface	No practical external edge or face is available	Creates a local indentation; a round-nose or interrupted-dot form reduces notch severity but does not remove it	Confirm the applicable code paragraph, material restrictions, thickness, temperature, service suitability, marking procedure, and Inspector acceptance
Use alternate identification without die stamping	Service conditions prohibit an impressed mark	Avoids the indentation	UG-77(b) requires the user to specify the alternate approach and the marking to permit positive identification upon delivery
Use paint or low-stress die marking for required plate identification	Plate marking governed by the cited material specification	Paint avoids an impression; a low-stress die still affects the surface locally	SA-20 13.2.2 addresses required markings on plate 1/4 in. [6 mm] and under, except as allowed by 13.4

Relocating the mark is the first choice when it satisfies identification and remains accessible. Alternate non-die identification is preferred when fatigue, corrosion, lining, cleanliness, or another service condition makes an indentation unacceptable. Use an inner low-stress stamp only when the mark is required, relocation is impractical, and the complete acceptance path is documented.

Recommended decision path

1. Define the mark. Record whether it is a material mark, code-required part mark, welder identification, or internal fabrication tracking. Cite the governing drawing, specification, quality procedure, or construction-code paragraph. Do not move on until its required content and permanence are known.
2. Challenge the location. Check the plate edge, outside face, an external marking land, and any area later removed from the pressure part. Confirm that relocation will not break traceability after cutting, forming, welding, machining, or assembly.
3. Evaluate alternate marking. When service conditions prohibit die stamping, apply the route described by UG-77(b): the user specifies a marking method that permits positive identification upon delivery. Establish how identification will remain linked to the part through fabrication.
4. Screen the inner-surface option. Compare the actual material, nominal thickness, minimum design metal temperature, service duty, and stamp type with the applicable requirements. For a proposed use of UW-37(f), all listed restrictions must be satisfied.
5. Obtain acceptance before marking. Submit the marked-up drawing, code basis, stamp geometry, proposed location, traceability method, and inspection points to the purchaser and Authorized Inspector when that Inspector controls code acceptance. Customer approval alone does not replace construction-code compliance.
6. Release the operation. Mark only after the approved procedure, correct stamp, part identity, orientation, and location have been checked at the workpiece.

Applicable code constraints

The cited UW-37(f) text permits use of a stamp provided the vessel part is not deformed and additional conditions are met. For ferrous materials, the cited conditions limit applicability to P-No. 1, Group Nos. 1 and 2; require a minimum nominal plate thickness of 3/16 in. (5 mm) or a minimum nominal pipe wall thickness of 0.154 in. (3.91 mm); and require a minimum design metal temperature no colder than -20°F (-29°C).

UW-37(f) check	Required project value	Disposition
Material classification	Material specification and assigned P-Number/group	For the cited ferrous-material route, accept only P-No. 1, Groups 1 or 2
Plate thickness	Nominal thickness at the marking location	At least 3/16 in. (5 mm)
Pipe wall, when applicable	Nominal wall at the marking location	At least 0.154 in. (3.91 mm)
Minimum design metal temperature	Value on the approved design documents	No colder than -20°F (-29°C)
Local deformation	Surface condition after marking	The vessel part must not be deformed

SA-20 13.2.2 addresses required markings for plates 1/4 in. [6 mm] and under. Except as allowed by 13.4, it identifies paint marking or steel die stamping with low-stress round-nose or interrupted-dot impressions. This provision governs the specified plate-marking context; it is not a general authorization for every fabrication, welder, or code-symbol mark.

UG-77(b) supplies a different route. Where service conditions prohibit die stamping for material identification, and the user specifies that restriction, the material manufacturer marks the required data by a method that permits positive identification upon delivery. Determine which paragraph governs the actual mark instead of combining the most convenient portions of separate provisions.

Confirm the adopted code edition, addenda, material specification edition, contractual supplements, and quality-control manual before release. Paragraph numbering and applicable wording must match the construction documents for the pressure part.

Inner-surface marking procedure

1. Verify part identity. Match the plate and its traceability record to the end-plug drawing and material documentation. Stop if heat, plate, or piece identity is uncertain.
2. Verify design inputs. Read the nominal thickness and minimum design metal temperature from approved documents. Confirm material classification from the applicable code data rather than inferring it from a trade name.
3. Select the location. Keep the mark away from weld toes, fusion boundaries, machined sealing faces, small ligaments, abrupt section changes, highly formed regions, and other geometric stress raisers. Confirm that later machining or welding will not remove the only traceable identity.
4. Select the stamp. Where low-stress marking is the accepted method, use the approved round-nose or interrupted-dot form. Segregate conventional sharp dies so they cannot be substituted at the workstation.
5. Control the impression. Apply only the force needed to create the required readable mark. The evidence provides no universal permissible impression depth; set the acceptance criterion through the approved marking procedure and engineering review. Reject double strikes, dragged impressions, sharp gouges, overlap, or visible distortion.
6. Inspect immediately. Verify the characters, location, orientation, traceability, and surrounding surface. Record any examination required by the approved inspection plan.
7. Protect traceability. Transfer the identity to the fabrication traveler, map, or other approved record before an operation can obscure the physical mark.

Stress, fatigue, and corrosion mechanisms

Every impressed character changes the local surface profile. A sharp die produces a smaller root radius and a higher local stress concentration than a controlled round-nose or interrupted-dot impression. Low-stress geometry reduces the notch effect; it does not make the impression mechanically invisible.

The design review must address cyclic pressure, thermal transients, vibration, and restraint loads at the selected location. A shallow mark in a low nominal-stress region and a mark beside a weld or section transition are not equivalent. If fatigue governs the component, use the actual stress range, local geometry, surface condition, and applicable fatigue assessment method. Do not accept the mark solely because static membrane stress is low.

Steam or water contact does not by itself create a blanket prohibition in the cited provisions. Service can still control the decision. An impression may retain deposits, disrupt protective films, or become an initiation site where the material-environment combination is susceptible to localized attack. Conversely, corrosion may make a shallow mark unreadable. Loss of visibility does not prove that the original notch has become harmless, and a physical stamp exposed to corrosion must not be the only traceability record.

Symptoms-versus-causes checks

Observed condition	Probable control failure	Required action
Sharp V-shaped character roots	A conventional die was used instead of the approved low-stress stamp	Hold the part and obtain engineering and inspection disposition
Bulging, bending, or visible surface displacement around the mark	Excessive impact or inadequate local section	Reject acceptance under the cited no-deformation condition pending formal disposition
Unreadable or incomplete characters	Insufficient control, poor access, scale, or an unsuitable surface	Do not restrike until the approved procedure defines the correction and new location
Duplicate or overlapping impressions	Part movement, a bounced tool, or an attempted correction	Map the affected area and submit it for evaluation
Mark lost after machining or corrosion	The process relied on the physical impression as the sole identity link	Recover identity only through the approved traceability system; do not recreate a mark from memory
Acceptable material mark but disputed fabrication mark	Different marking functions were treated as though one paragraph governed all of them	Reclassify the mark and cite the requirement applicable to that function

Inspection hold points and records

The inspection plan should place a hold point before the first inner-surface stamp. The review package needs the marking purpose, character content, exact location, material specification and classification, nominal thickness, minimum design metal temperature, service description, stamp form, and the governing code or project requirement.

After marking, inspect the impression under adequate lighting and magnification appropriate to the surface condition. Check for cracks, laps, gouges, excessive indentation, overlapping strikes, and surrounding distortion. When engineering or the inspection plan calls for a surface examination, select a method compatible with the material and surface condition; the cited information supplies no universal examination method or acceptance threshold.

Record the marked characters, part identity, location, date, procedure, operator, inspection result, and any purchaser or Authorized Inspector release required by the quality system. A photograph can supplement the record but does not replace controlled material traceability.

Final verification sequence

1. Match the completed mark to the approved characters and part identity.
2. Confirm the mark lies at the approved inner-surface location and outside the prohibited geometric features defined by the marking plan.
3. Confirm the die is the approved round-nose or interrupted-dot type where low-stress marking was specified.
4. Verify the recorded material classification, nominal thickness, and minimum design metal temperature against the governing requirement.
5. Confirm that no visible part deformation, sharp gouge, crack-like indication, double strike, or unapproved repair remains.
6. Complete the required inspection release and preserve the traceability record before the end plug advances to welding, machining, or assembly.

Frequently asked questions