4.5 Impact Testing, MDMT Exemption, and Charpy Requirements

4.5 Impact Testing, MDMT Exemption, and Charpy Requirements

Low-temperature brittle fracture is a catastrophic failure mode in piping. ASME B31.3 Section 323 establishes when Charpy V-notch impact testing is mandatory and when it may be exempt based on material, thickness, and temperature. API 570 inspectors must read Figure 323.2.2A, apply the correct exemption curve from Table A-1, and understand Table 323.3.5 acceptance criteria — while avoiding the classic exam trap of confusing design minimum temperature with MDMT.

323.2.2 — When Impact Testing Is Required

Impact testing confirms that steel retains adequate toughness at low metal temperatures. B31.3 requires impact tests unless the piping qualifies for an exemption under 323.2.2 and Figure 323.2.2A.

Impact testing is generally triggered when:

1. Design minimum metal temperature falls below the exemption temperature for the material/thickness combination on the assigned curve.
2. Thickness exceeds limits on the exemption curve at the stated MDMT.
3. Material is not listed with an exemption curve or is explicitly required to be impact tested.
4. Welding or metallurgical changes (e.g., repair welds on thick sections, material substitutions) invalidate prior exemptions.

Figure 323.2.2A — Reading the Exemption Curves

Figure 323.2.2A plots minimum design metal temperature (MDMT) on the horizontal axis versus nominal pipe wall thickness on the vertical axis. Four curves — A, B, C, and D — represent progressively tougher exemption limits (Curve A is the most restrictive; Curve D the most liberal for carbon and low-alloy steels).

How to use the figure (exam method):

1. Find the material's assigned curve in Table A-1 (e.g., ASTM A106 Gr. B carbon steel is typically Curve B).
2. Enter the figure at the piping MDMT on the x-axis.
3. Read the maximum exempt thickness on the y-axis at that temperature for the assigned curve.
4. If actual nominal thickness ≤ exempt thickness, impact testing is not required (subject to other 323 rules).
5. If thickness is greater than the exempt limit, impact testing is required unless another exemption applies.

Table A-1 Curve Assignment — Representative Values

Always use the curve letter printed in Table A-1 for the exact specification and grade — do not guess from chemistry alone.

Design Minimum Temperature vs MDMT — Critical Exam Trap

On many systems these temperatures are equal, which causes confusion. They are not interchangeable concepts:

• A line designed to −20°F must still show impact-test compliance if thickness exceeds the Curve B exempt limit at −20°F.
• Lowering MDMT on paper without engineering basis does not change actual low-temperature exposure — inspectors verify records, not just nameplate data.
• API 570 rerates and repairs may force re-evaluation when thickness drops (sometimes improving exemption) or when weld repairs on thick sections require retesting.

Trap: A question gives "design temperature −50°F" and asks about impact exemption. You must use the MDMT established per 323.2.2, which may include temperature reduction allowances from 323.2.2(g) — not casually substitute "design temperature" without checking the code path.

Charpy V-Notch Requirements — Table 323.3.5

When impact testing is required, specimens are typically full-size (10 mm × 10 mm) Charpy V-notch tests at or below the MDMT (or at a prescribed test temperature per 323.3.2).

Table 323.3.5 (standard full-size specimens) requires:

If sub-size specimens are used because material thickness is insufficient for full-size coupons, Table 323.3.5 provides reduced energy requirements proportional to specimen width — a common exam calculation.

Thickness forcing impact: For Curve B carbon steel at MDMT = −20°F, Figure 323.2.2A might allow exemption only up to roughly 0.50 in. nominal thickness (read the figure — values are graphical). A Schedule 80 NPS 6 wall (~0.432 in.) may be exempt, while Schedule 160 or heavy wall forgings may not. Always read thickness at the controlling component (pipe, flange neck, weld deposit).

Worked Example: Impact Exemption Determination

Given:

• Material: ASTM A106 Grade B seamless pipe, Curve B per Table A-1
• MDMT = −20°F per design basis
• Nominal wall thickness t_nom = 0.438 in. (NPS 8 Schedule 80)
• No unusual welding or material deviations

Step 1: Locate Curve B on Figure 323.2.2A at MDMT = −20°F.

Step 2: Read maximum exempt thickness at −20°F. For Curve B, the exempt thickness at −20°F is approximately 0.50 in. (verify on the figure in the exam booklet — interpolate between grid lines).

Step 3: Compare: 0.438 in. ≤ 0.50 in. → piping qualifies for impact-test exemption at this MDMT and thickness.

Step 4: If thickness were 0.625 in. at the same −20°F on Curve B, the point lies above the curve → impact testing required unless another 323.2.2 paragraph applies (e.g., stress ratio exemptions — know they exist but follow the question's stated path).

Step 5: If impact testing were required at −20°F, three Charpy specimens at the prescribed temperature must meet ≥ 20 ft-lb average and ≥ 15 ft-lb minimum.

When Thickness or Material Forces Testing — Summary Rules

• Increased thickness pushes the point upward on Figure 323.2.2A, often crossing the curve into the impact required region.
• Lower MDMT moves left on the x-axis, also tending to require impact testing for the same thickness.
• A333 low-temperature pipe is often purchased with guaranteed impact values at −50°F — distinguish material specification impact from B31.3 exemption logic.
• Repairs: Full-penetration repair welds in heavy sections may require welding procedure qualification with impact tests even when the original line was exempt.

API 570 Inspector Perspective

During inspection planning, verify:

1. MDMT on the piping spec sheet and U-1/data report alignment.
2. Curve assignment matches actual MTR grade (A106-B vs A106-C differ in stress, same curve letter may still apply — confirm Table A-1).
3. Wall thickness at valves and fittings — controlling section may be thicker than straight pipe.
4. Charpy reports in the quality file when exemption is not allowed.
5. After hot tap, encirclement sleeve, or major weld repair, impact requirements may be re-opened — do not assume original exemption transfers automatically.