4.5 Impact Testing, MDMT Exemption, and Charpy Requirements

API 570 Piping Inspector Exam

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1 Chapter 1: Piping Inspection Planning & Circuits

1.1 Inspection Planning, History, and Safety 1.2 Piping Circuitization and Circuit Design 1.3 Condition Monitoring Locations (CMLs) & Coverage

2 Chapter 2: Inspection Intervals & Extent of Inspection

2.1 Piping Classifications (Classes 1, 2, 3, and 4) 2.2 Thickness Inspection Intervals (Half-Life & Visual Rules) 2.3 Owner-User QA and Roles (Inspector, Engineer, Examiner)

3 Chapter 3: Calculations & Remaining Life

3.1 Corrosion Rates (Short-Term, Long-Term, and Initial Estimates) 3.2 Remaining Life and Retirement Thickness Calculations 3.3 MAWP Calculations for In-Service Piping

4 Chapter 4: ASME Codes for Piping Design

4.1 Wall Thickness Calculations for Pipe under Pressure (ASME B31.3) 4.2 Flange Ratings & P-T Limits (ASME B16.5) 4.3 Pressure & Leak Testing Rules (Hydrostatic vs. Pneumatic) 4.4 Weld Joint Quality Factors (Ej) and Casting Quality Factors (Ec) 4.5 Impact Testing, MDMT Exemption, and Charpy Requirements 4.6 Thermal Expansion Calculations (ASME B31.3 Appendix C) 4.7 Minimum Required Thickness of Permanent Blanks

5 Chapter 5: Nondestructive Examination (NDE)

5.1 Visual, Penetrant, and Magnetic Testing (VT, PT, MT) per ASME Sec V 5.2 Radiographic and Ultrasonic Testing (RT, UT) per ASME Sec V 5.3 Examiner Selection, Calibrations, and Acceptance Standards 5.4 Leak Testing per ASME Section V Article 10

6 Chapter 6: Welding Procedures & Qualifications

6.1 Reviewing WPS and PQR per ASME Section IX 6.2 Welder Performance Qualification (WPQ) and Limits 6.3 Preheating & PWHT Rules and Controls 6.4 Welding Processes, Inspection, and Metallurgy (API RP 577)

7 Chapter 7: Valves, Components, Materials & PRDs

7.1 Piping Components and Valves (API RP 574) 7.2 Positive Material Identification (PMI) (API RP 578) 7.3 Pressure Relieving Devices (PRDs) (API RP 576)

8 Chapter 8: Damage Mechanisms & Repairs

8.1 Corrosion Under Insulation (CUI) and Ambient Mechanisms 8.2 Process-Driven Damage Mechanisms (API RP 571) 8.3 Temporary and Permanent Repair Methods (ASME PCC-2 & API 570) 8.4 Soil Corrosion, Galvanic Corrosion, and Boiler Water Condensate Corrosion 8.5 Piping Rerating and Alterations (API 570) 8.6 Inspection Records, Reports, and Documentation 8.7 Amine/Caustic SCC, HCl, Sour Water, and MIC (API RP 571)

4.5 Impact Testing, MDMT Exemption, and Charpy Requirements

Key Takeaways Explain This Topic AI

ASME B31.3 323.2.2 and Figure 323.2.2A define when impact testing may be waived based on material curve, thickness, and minimum design metal temperature (MDMT).

Each material in Table A-1 is assigned an impact-test exemption curve (A, B, C, or D); thicker sections require a warmer MDMT to remain exempt.

When impact testing is required, Charpy V-notch values in Table 323.3.5 typically require 20 ft-lb average and 15 ft-lb minimum per specimen for full-size specimens.

Design minimum temperature is the lowest metal temperature expected in service; MDMT is the lowest temperature at which the piping may operate without impact testing — do not interchange these on exams.

A material exempt at design conditions can still require impact testing after repair welding, thickness increases beyond curve limits, or metallurgy changes.

Last updated: July 2026

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:

Design minimum metal temperature falls below the exemption temperature for the material/thickness combination on the assigned curve.

Thickness exceeds limits on the exemption curve at the stated MDMT.

Material is not listed with an exemption curve or is explicitly required to be impact tested.

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):

Find the material's assigned curve in Table A-1 (e.g., ASTM A106 Gr. B carbon steel is typically Curve B ).

Enter the figure at the piping MDMT on the x-axis.

Read the maximum exempt thickness on the y-axis at that temperature for the assigned curve.

If actual nominal thickness ≤ exempt thickness , impact testing is not required (subject to other 323 rules).

If thickness is greater than the exempt limit, impact testing is required unless another exemption applies.

Table A-1 Curve Assignment — Representative Values

Material (examples) Typical Curve
ASTM A106 Gr. B, A53 Gr. B (certain) B
ASTM A333 Gr. 6 (normalized) B
ASTM A350 LF2 forgings B
Higher-strength low-alloy grades C or D per table note

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

Term Meaning

Design minimum temperature Lowest metal temperature the piping system is designed to encounter in service (normal and upset conditions per process/design basis).

MDMT (for exemption) The temperature used with Figure 323.2.2A to determine whether impact testing can be waived for a given thickness and curve.

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:

Criterion Typical Requirement
Average of 3 specimens ≥ 20 ft-lb (27 J)
Minimum single specimen ≥ 15 ft-lb (20 J)
Lateral expansion (alternate) Per table notes for certain materials/thickness

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:

MDMT on the piping spec sheet and U-1/data report alignment.

Curve assignment matches actual MTR grade (A106-B vs A106-C differ in stress, same curve letter may still apply — confirm Table A-1).

Wall thickness at valves and fittings — controlling section may be thicker than straight pipe.

Charpy reports in the quality file when exemption is not allowed.

After hot tap, encirclement sleeve, or major weld repair , impact requirements may be re-opened — do not assume original exemption transfers automatically.