1. Overview — Why Exemptions Exist

Impact testing under UG-84 of ASME BPVC Section VIII Division 1 is not universally mandatory. The code recognises that many carbon and low alloy steel pressure vessels — operating in moderate temperature ranges, at lower stress ratios, or from inherently tougher heat-treated materials — pose no meaningful brittle fracture risk at their intended service conditions. Imposing Charpy V-notch testing on every vessel regardless of these factors would be both technically unjustified and commercially burdensome.

Accordingly, ASME Section VIII Division 1 (2025 Edition) provides a structured hierarchy of exemption pathways — each grounded in the engineering logic that material toughness at the Minimum Design Metal Temperature (MDMT) is either inherently assured, or can be demonstrated without physical testing. These exemptions are primarily contained in:

For a thorough understanding of what happens when impact testing IS required — the procedures, specimen dimensions, test temperatures, and acceptance criteria — refer to our detailed guide on UG-84 Charpy Impact Test Requirements as per ASME Section VIII Division 1 (2025 Edition).

2. Exemption 1 — UG-20(f): The General Exemption

The first — and most straightforward — exemption is found in UG-20(f), which establishes the general rule that impact testing per UG-84 is NOT mandatory for pressure vessels meeting a specific set of concurrent conditions. This is the starting point of any impact test exemption evaluation.

Conditions for UG-20(f) Exemption — All Must Be Met:

3. Exemption 2 — UCS-66 Impact Test Exemption Curves

Figure UCS-66 (Figure UCS-66M in SI units) is the central tool for establishing impact test exemptions for carbon and low alloy steel vessels. It consists of four curves — A, B, C, and D — each representing a family of materials with progressively increasing inherent toughness. The curves plot Minimum Design Metal Temperature (MDMT) on the vertical axis against Governing Thickness on the horizontal axis. Understanding the ductile-to-brittle transition temperature (DBTT) that underlies this framework is explained in our Charpy Impact Test — Principle, Procedure & Results guide.

The governing rule from UCS-66(a) is: If a combination of minimum design metal temperature and governing thickness falls on or above the applicable curve assigned to the subject material, impact testing is NOT required by the rules of this Division. If the combination falls BELOW the curve, impact testing IS required.

Reading Figure UCS-66 — Step-by-Step Procedure

The procedure for using Figure UCS-66 to determine whether impact testing is required is summarised in the Figure UCS-66.2 decision diagram, which provides the following structured approach:

4. How to Determine Governing Thickness — UCS-66(a)(1)

The governing thickness is the single most critical input to Figure UCS-66. Getting it wrong — either overstating or understating it — directly leads to incorrect exemption conclusions. UCS-66(a)(1) provides exhaustive definitions for every common vessel configuration.

5. Material Curve Assignments — Which Material Goes Where?

The curve assignment for each material in Figure UCS-66 is determined by the Notes to that Figure. Understanding how materials are grouped — including the significance of P-Numbers and Group Numbers in distinguishing P-No. 1 from P-No. 3 through P-No. 5 steels — is essential for correct curve assignment. The following tables consolidate the key material assignments from the 2025 Edition. Always verify against the current edition of the code for the complete listing.

6. Exemption 3 — Figure UCS-66.1: MDMT Reduction Without Impact Testing

Figure UCS-66.1 provides one of the most powerful — and frequently misunderstood — exemption tools in the code. It allows the minimum design metal temperature established from Figure UCS-66 to be reduced further without impact testing, by recognising that a vessel operating at less than its maximum design stress is inherently safer against brittle fracture at lower temperatures.

The physical logic is straightforward: brittle fracture is a stress-driven phenomenon. A vessel whose wall is significantly thicker than required for the applied loading carries a lower stress intensity at any given pressure — and therefore the material can be used at temperatures colder than those implied by the Figure UCS-66 curve without increased fracture risk.

The Coincident Ratio — How It Works

MDMT Reduction Values from Figure UCS-66.1

The following reference values are extracted from Figure UCS-66.1. For reductions up to and including 40°F (22°C), the code also permits a simplified formula: Reduction in MDMT = (1 − Ratio) × 100°F [or 56°C].

7. Exemption 4 — UCS-68.2: PWHT Credit (30°F / 17°C Reduction)

UCS-68.2 provides an additional exemption credit when Post Weld Heat Treatment (PWHT) is voluntarily applied. When this PWHT route is used and impact testing is eventually required, note that supplementary essential variables per ASME Section IX QW-250 become mandatory for WPS qualification to a pressure-retaining weld where it is not otherwise required by the Division. PWHT reduces residual stresses in welds and thereby improves resistance to brittle fracture, warranting a further reduction in the impact test exemption temperature.

Important Conditions and Limitations:

• The 30°F (17°C) reduction applies to P-No. 1 materials only.
• PWHT must be applied when it is NOT otherwise mandated by the code — it is the voluntary application of PWHT specifically for toughness credit.
• Per UCS-68.2(b), the resulting impact testing exemption temperature from this provision may be colder than −55°F (−48°C) when the PWHT exemption in UCS-68.1(c) is also applicable — meaning PWHT credit can extend the window of exemption below the usual −55°F floor.
• This credit can be combined with the Figure UCS-66.1 stress ratio reduction, allowing engineers to achieve very cold MDMTs without impact testing through a combination of: (a) material curve assignment, (b) stress ratio credit, and (c) PWHT credit.
• The MDMT adjustment determined using Figure UCS-66.1 may also be used for impact tested welding procedures or production welds per UCS-66(i)(5).
• The MDMT for any component shall not be colder than −155°F (−105°C) regardless of all applied credits — per UCS-66(i)(6).

8. Exemption 5 — Special Material & Service Exemptions

Beyond the primary curve and stress-ratio based exemptions, ASME Section VIII Division 1 (2025 Edition) provides several targeted exemptions for specific material types, product forms, or vessel service conditions. These are found throughout UCS-66 and are critical for engineers dealing with specialised applications.

8.1 High-Yield-Strength Materials — UCS-66(f)

Materials (other than bolting materials) with a specified minimum yield strength greater than 65 ksi (450 MPa) shall be impact tested. However, two special conditions provide exemptions:

8.2 Exempted Material Specifications — UCS-66(g) [Thin Pipe & Tube, P-No. 1]

For tubes or pipe of P-No. 1 materials, the following exemptions from impact testing are also permitted as a function of Specified Minimum Yield Strength (SMYS), for metal temperatures of −155°F (−105°C) and warmer:

Additionally, when the base metal is exempt from impact testing by this provision (or by Curve C or Curve D at −20°F/−29°C), the coldest MDMT assigned for welded components that do not meet UCS-67 requirements is −20°F (−29°C) per UCS-66(j).

8.3 Material Specification-Tested Materials — UCS-66 & UG-84.5(e)

Materials produced and impact tested in accordance with the material specification requirements listed in Figure UG-84.5-1, General Note (c) are considered to inherently satisfy the impact testing requirements of this Division. These materials include:

SA-320 • SA-333 • SA-334 • SA-350 • SA-352 • SA-420 • SA-437 • SA-540 (except materials produced under Table 2, Note 4) • SA-765 • Impact-tested SA/AS1548 (L impact designations)

These materials do NOT have to satisfy the energy values of Figure UG-84.5-1 as these specifications already impose qualification testing through their own requirements. See UCS-66(g) and UG-84.5(e).

8.4 Thin Plate Material Marking Exemption — UCS-66(e)

The material manufacturer’s identification marking required by the material specification shall not be stamped on plate material less than 1/4 in. (6 mm) in thickness unless the following requirements are met:

• Materials limited to P-No. 1, Group Nos. 1 and 2
• Minimum nominal plate thickness 3/16 in. (5 mm), or minimum nominal pipe wall thickness 0.154 in. (3.91 mm)
• Minimum design metal temperature no colder than −20°F (−29°C)

9. Exemption 6 — Bolting and Nuts: Dedicated Exemption Temperatures

Bolting and nut materials are not evaluated against Figure UCS-66 in the same way as vessel shells and heads. Instead, Figure UCS-66, General Note (c), provides dedicated impact test exemption temperatures for each bolting specification and grade combination. These temperatures represent the lowest MDMT at which bolting of that specification can be used without impact testing.

Nut Exemption Temperatures

10. Exemption 7 — UCS-66(b)(3): Very Low Temperature Exemption (Below −55°F / −48°C)

For services where the MDMT is colder than −55°F (−48°C) but no colder than −155°F (−105°C), a special low-ratio exemption exists under UCS-66(b)(3). This is a narrow, specific provision available only when the vessel is operating at a very low fraction of its design capacity.

A ratio of 0.35 or less means the vessel is operating at 35% or less of its maximum design stress capacity. This physical condition — extreme under-stress relative to design — provides sufficient assurance that the driving force for brittle fracture propagation is low enough that the inherent toughness of the un-impact-tested material is adequate. This provision is commonly applicable to hydrostatic testing conditions, batch process vessels with low operating pressures, or vessels with substantial corrosion allowances relative to their minimum required thickness.

11. Exemption 8 — UCS-66(g): Specification-Impact-Tested Materials

Materials that are produced and impact tested in accordance with the material specification requirements listed in Figure UG-84.5-1, General Note (c), are exempt from having to satisfy the energy values of that figure. These materials — SA-320, SA-333, SA-334, SA-350, SA-352, SA-420, SA-437, SA-540 (with noted exceptions), and SA-765 — are exempted at the minimum design metal temperatures that are no more than 5°F (3°C) colder than the test temperature required by the specification.

12. Exemption 9 — UCS-66(h): Metal Backing Strips

UCS-66(h) provides a specific exemption from impact testing for metal backing strips that remain in place. No impact testing is required for backing strips made of materials assigned to Curve A of Figure UCS-66 in thicknesses not exceeding 1/4 in. (6 mm) when the minimum design metal temperature is −20°F (−29°C) or warmer.

This exemption reflects the non-load-bearing nature of backing strips in most weld joint designs — they are primarily used as weld backing and do not carry significant structural tensile stress from pressure loading. Their contribution to the brittle fracture risk of the vessel is therefore minimal at these thickness and temperature levels.

13. Master Decision Flowchart — Exemption or Impact Testing Required?

The following step-by-step decision process consolidates all of the exemption pathways into a single logical sequence for evaluating any carbon or low alloy steel pressure vessel component under ASME Section VIII Division 1, 2025 Edition.

14. Complete Exemption Summary Table

15. Common Mistakes and Code Interpretation Pitfalls

This article is prepared for educational and reference purposes based on the ASME BPVC Section VIII Division 1 (2025 Edition). All code citations are direct references to the 2025 Edition. Always verify against the current edition of the code and consult qualified engineering professionals for design and compliance decisions. Code requirements are subject to change with each new edition.