ASME B31T-2018 — Standard Toughness Requirements for Piping

28 10 月, 2026
ASME Standards

❄️ ASME B31T-2018 is the standard in the ASME B31 Code for Pressure Piping devoted specifically to material toughness. It answers a question that is critical in low-temperature and heavy-wall service yet long lacked a unified answer: will this piping system fail by brittle fracture in low-temperature service, and if the risk exists, what impact testing demonstrates adequate resistance? The 2018 edition is a revision of B31T-2015, approved by ANSI on December 6, 2018, and issued on February 14, 2019.

The origin matters. In 2000 the B31 Materials Technical Committee determined that a standard set of toughness requirements was needed — one that could be adopted by reference by the various piping codes. At the time, requirements across the B31 code books varied widely, and some had no toughness requirements at all, meaning identical low-temperature piping could end up with very different levels of assurance depending on which code book applied. B31T was created to close that gap, providing a common technical basis for evaluating whether piping materials are suitable where brittle failure due to low-temperature service is a concern.

💡 Understand what toughness means: strength describes how much load a material can carry, toughness describes how much energy it absorbs before fracturing. At low temperature the ductile-to-brittle transition can turn a normally ductile steel brittle — it fractures with almost no plastic deformation, and a crack can propagate at hundreds of metres per second. Everything in B31T exists to identify and control that risk.

1. Structure and Scope of the Standard 📌

The standard has four sections supported by three mandatory and two nonmandatory appendices, forming a complete chain from “is testing required” to “how, and to what value”:

Section / Appendix	Content	Technical Role
Section 1	Introduction	States purpose and method of use
Section 2	Glossary	Unifies toughness and impact testing definitions
Section 3	Low-temperature ranges and requirements (Tables 3.1-1, 3.2-1)	Determines low-temperature service requirements by material group
Section 4	Impact testing methods and acceptance criteria (Tables 4.4.2-1, 4.5.1-1)	Test execution and minimum impact values
Appendix I (mandatory)	Temperature-thickness curves (Figures I-1, I-1M; Table I-1)	Determines exemption from impact testing
Appendix II (mandatory)	Stress ratio curves (Figures II-1, II-1M)	Permits reduced test temperature at low stress
Appendix III (mandatory)	Material groupings by T-Number (Table III-1)	Assigns material specifications to toughness groups
Appendix A (nonmandatory)	Flowchart of requirements (Figure A-1)	Visual decision path
Appendix B (nonmandatory)	Guidelines for establishing a T-Number group	Assigning new materials

2. Low-Temperature Ranges and Material Grouping 🔧

Section 3 is the entry point. Table 3.1-1 states low-temperature service requirements by material group, while Table 3.2-1 assigns specific material specifications — the various ASTM/ASME grades — to those groups. This two-step approach, first group then grade, lets the standard cover hundreds of materials without a separate entry for each.

On the temperature-thickness curves, materials fall into four groups, A through D (Figure I-1 in U.S. Customary units, Figure I-1M in SI, with Table I-1 giving the curves in tabular form). The grouping is essentially a ranking of toughness: Group A materials are permitted the widest service envelope at a given temperature and thickness, Group D the narrowest. As a rough engineering reading: fine-grain normalized steels and austenitic stainless steels are most tough, carbon-manganese steels next, and some high-strength low-alloy steels without grain refinement are comparatively least tough. The curves are read as follows: if the combination of thickness and minimum design temperature falls on the permitted side of the curve for that group, impact testing is exempt; if it falls on the non-permitted side, testing per Section 4 is required.

Group	Typical Material Types	Low-Temperature Envelope	Curve Character
Group A	Austenitic stainless steels; some fine-grain normalized steels	Widest; down to very low temperatures	Lowest position on the temperature-thickness chart
Group B	Fine-grain normalized carbon-manganese and low-temperature steels	Wide	Permitted temperature rises noticeably with thickness
Group C	General carbon and carbon-manganese steels	Moderate	Most common service falls here
Group D	Some high-strength low-alloy and quenched-and-tempered steels	Narrowest; most stringent verification	Most thickness-sensitive

⚠️ The most frequent error is the thickness value. The “thickness” on these curves is the governing thickness that must be assessed — the actual thickness at the joint — not the nominal pipe wall. At branch connections, flange hubs, and reinforcing pads, governing thickness can be considerably greater than the adjacent pipe wall, and using the wrong value can wrongly place the case on the exempt side.

3. Impact Testing Methods and Acceptance Criteria 🎯

Section 4 specifies how Charpy V-notch (CVN) impact testing is performed and accepted. The key technical points:

Specimen orientation and location: specimens are taken from specified positions in base metal, heat-affected zone, or weld metal; orientation (longitudinal versus transverse) affects absorbed energy and must follow the standard.
Test temperature: normally the minimum design metal temperature, or a temperature adjusted per the stress ratio curves of Appendix II.
Specimen count and criteria: a set of three specimens is normally required, judged on both an average and a minimum single value. Table 4.5.1-1 states the minimum required Charpy V-notch impact values; for carbon and low-alloy steels the usual order is an average of at least 18 J (13 ft·lbf) with no single value below the specified floor (commonly two-thirds of the average). Always read the actual values from Table 4.5.1-1.
Sub-size specimen correction: when material thickness or specimen size is under 10 mm (0.394 in), a full-size standard specimen cannot be machined, and Table 4.4.2-1 gives the test temperature reduction — testing colder to compensate for the lower absorbed energy of a smaller specimen.

✅ The stress ratio curves in Appendix II are an important economy tool: where actual working stress is low relative to yield strength, the driving force for brittle fracture is small, so impact testing may be performed at a lower temperature, or exemption may be extended to lower temperatures. Low-stress situations — long-span overhead lines, low pressure-class piping — therefore need not buy expensive high-toughness material merely to satisfy a uniform temperature threshold.

4. Decision Path and Engineering Practice 🛠️

The flowchart in Appendix A (Figure A-1) strings the whole decision into a clear path. In practice, proceed as follows:

Step 1: establish the minimum design metal temperature. This is not the same as the lowest ambient temperature — account for Joule-Thomson cooling across throttling, autorefrigeration during low-pressure letdown, and cryogenic fluid service.
Step 2: establish the governing thickness from the actual joint configuration, not simply the nominal wall.
Step 3: find the material group in Table 3.2-1, then check Table 3.1-1 and the temperature-thickness curves. If the combination falls on the permitted side, impact testing is exempt and the evaluation ends.
Step 4: where testing is required, sample per Section 4 and judge per Table 4.5.1-1, applying the sub-size correction of Table 4.4.2-1 and the low-stress correction of Appendix II as applicable.
Step 5: if the results fail, select a tougher material, increase thickness to lower the stress ratio, revise the design temperature, or perform a more detailed fracture mechanics assessment as permitted by the applicable code section.

For new or unlisted materials, Appendix B provides guidelines for establishing a T-Number group: chemical composition, microstructure, heat treatment condition, and measured impact test data are used to decide which toughness group the material belongs to. This gives domestically produced and imported specialty grades a compliant route into the system.

🚨 Brittle fracture incidents in low-temperature piping often occur during hydrostatic testing or the first cooldown, not in normal operation, because the combination of test medium temperature, ambient temperature, and stress level can be more severe than the design case. B31T requirements must therefore cover test conditions as well as operating conditions — a point easily missed on cold-climate projects.

5. Relationship to Other Code Sections and Practice Notes 📋

🔍 Reference relationship: B31T is itself a standard available for reference; whether it is mandatory depends on whether the specific code section (for example the low-temperature service chapter of B31.3, or the low-temperature requirements of B31.8) adopts it by reference. Always confirm the reference status and edition in the applicable code book.
📊 Relationship to code-specific exemptions: many code sections carry their own impact test exemption curves. Where both the code section and B31T apply, state explicitly which criteria govern; never take the more permissive of the two.
🔧 Welded joints: the heat-affected zone is often the least tough region, particularly in highly restrained heavy-wall joints. Sampling must cover weld metal and HAZ, not base metal alone.
⚖️ Material certificate review: require impact test reports stating test temperature, specimen size, individual and average absorbed energy, and sampling location and orientation, then check each item against B31T criteria.
📝 Record keeping: document the decision path — minimum design metal temperature, governing thickness, material group, curve or table used, conclusion — as part of the design file and for regulatory review.

6. Frequently Asked Questions ❓

❓ Q: How does B31T differ from the low-temperature rules built into individual code sections such as B31.3?
A: B31T is a unified toughness standard available for reference, created so that previously scattered and inconsistent code-specific rules converge on one method. Code-specific rules are the provisions within that code’s own scope. In a project, follow the referencing clause of the applicable code section; where it references B31T, B31T becomes the technical basis.

❓ Q: What exactly does “thickness” mean on the temperature-thickness curves?
A: It means the governing thickness requiring assessment, normally the actual thickness at the joint, including the effect of weld reinforcement and reinforcing components — not the nominal pipe wall. Thick sections dissipate heat poorly, are more highly restrained, and provide greater driving force for brittle fracture, which is why the permitted temperature rises with thickness. Using the wrong thickness is the most common cause of misjudgement.

❓ Q: Why must the test temperature be reduced for sub-size specimens?
A: Charpy absorbed energy correlates strongly with specimen ligament width. A sub-size specimen has a narrower ligament and therefore naturally records lower absorbed energy; judging it against full-size criteria would wrongly fail acceptable material. Table 4.4.2-1 compensates by lowering the test temperature so sub-size results can be compared against the full-size basis.

❓ Q: How do the stress ratio curves save cost?
A: Where actual working stress is low relative to yield strength, crack driving force is small and brittle fracture risk drops, so Appendix II permits a correspondingly lower impact test temperature requirement. On low-stress duties this avoids selecting high-toughness — often more expensive and harder to weld — material purely to clear a uniform temperature threshold.