ASME B31.3 Table A-1M Material Analysis
Select Material
Allowable Stress & Safety Ratios
Allowable Stress vs. Temperature Chart
Material Properties
Mechanical Properties Overview
Applicable Notes and Explanations
Knowledge Bank – ASME B31.3 Table A-1M: Understanding Allowable Stress Table
The ASME B31.3 Process Piping Code is the bible for engineers designing and constructing industrial piping systems. It ensures safety, reliability, and integrity. At the heart of this code lies Table A-1M, a critical resource that provides the basic allowable stresses for various metals used in piping. Understanding every column of this table is fundamental for making correct and safe engineering decisions.
Who This Guide Is For
This knowledge bank is designed to be a practical guide for a wide range of professionals, including:
- Fresh Engineering Graduates entering the world of piping design.
- Working Professionals looking for a quick refresher on code specifics.
- QA/QC Engineers & Inspectors who verify material compliance.
- Design and Stress Engineers who use these values daily for calculations and material selection.
Table A-1M Parameters Explained
Line No.
Definition
A simple sequential number assigned to each row in the table for easy reference.
Purpose in ASME B31.3
It provides a unique identifier for a specific material under specific conditions, which is useful for communication and referencing in documents.
How to Interpret
There is no engineering significance to the number itself; it is purely for identification.
Nominal Composition
Definition
A general description of the material’s primary chemical makeup.
Purpose in ASME B31.3
It allows for quick grouping and identification of material families, such as “Carbon Steel,” “1.25Cr-0.5Mo,” or “18Cr-8Ni” (Stainless Steel 304).
How to Interpret
This provides a high-level understanding of the material’s properties. For example, the presence of Chromium (Cr) and Molybdenum (Mo) indicates suitability for high-temperature service, while Nickel (Ni) suggests good toughness at low temperatures.
Tips for Engineers
Use this column for initial, broad material screening before diving into specific grades and properties.
Product Form
Definition
The shape or form in which the material is supplied, such as pipe, tube, plate, forging, or casting.
Purpose in ASME B31.3
A material’s mechanical properties can vary significantly based on how it was manufactured (e.g., rolled plate vs. forged fitting). This column ensures you are using the correct stress values for the component type.
How to Interpret
Match the product form in the table to the component you are designing or evaluating. For example, use “Pipe” for pipe runs, “Forgings” for flanges, and “Plate” for heads or shells.
Tips for Engineers
Be careful: A fitting (e.g., an elbow) can be made from pipe (A106) or from a forging (A105). The allowable stress may differ. Always verify the manufacturing standard of the component.
Spec No. (Specification Number)
Definition
The standard (usually from ASTM or API) that governs the material’s manufacturing, chemical composition, and mechanical properties.
Purpose in ASME B31.3
This is the primary link between the design code (ASME B31.3) and the manufacturing standard. It ensures that the material used in construction has predictable and certified properties.
How to Interpret
This number tells you which document to consult for detailed material requirements. For example, A106 refers to ASTM A106.
A106 is the specification. All pipes supplied under this spec must meet the chemical and mechanical tests defined in the ASTM A106 document.Tips for Engineers
QA/QC engineers must check the Material Test Certificate (MTC) to confirm the material was manufactured and tested according to the specified Spec No.
Type / Grade
Definition
A sub-classification within a specification that often denotes different strength levels or chemical compositions.
Purpose in ASME B31.3
It allows for differentiation within a single specification. For example, different grades can have different tensile and yield strengths.
How to Interpret
A higher grade number or letter often (but not always) indicates higher strength.
Tips for Engineers
Specifying only “A106” is incomplete. You must specify the grade (e.g., “A106 Gr. B”) to ensure the correct material is procured and the correct allowable stress is used in calculations.
UNS No. (Unified Numbering System)
Definition
A standardized system for identifying specific metal alloys. It consists of a letter followed by five numbers.
Purpose in ASME B31.3
It provides a precise, unambiguous way to identify an alloy’s chemistry, eliminating confusion between different international or commercial naming conventions.
How to Interpret
The leading letter indicates the material family (e.g., K for carbon and alloy steels, S for stainless steels, N for nickel alloys).
K03006. This specific number is a chemical fingerprint for this grade of carbon steel.Tips for Engineers
The UNS number is the most reliable way to identify a material. When comparing materials from different standards, the UNS number can confirm if they are chemically equivalent.
P-No. (P-Number)
Definition
A number assigned by the ASME Boiler and Pressure Vessel Code (Section IX) to group base metals for welding procedure and performance qualification.
Purpose in ASME B31.3
It drastically reduces the number of welding procedures that need to be qualified. A procedure qualified for one material in a P-Group is generally accepted for other materials in the same group.
How to Interpret
Materials with the same P-No. have similar weldability characteristics.
P-No. 1. This groups it with other common carbon steels like A53 Gr. B and API 5L Gr. B, meaning a welding procedure qualified on any of these can often be used for the others.Tips for Engineers
This is critical for welding engineers and QA/QC. When reviewing a Welding Procedure Specification (WPS), the P-No. listed must match the materials being joined.
Minimum / Maximum Temperature
Definition
These columns define the safe operating temperature range for which the listed allowable stresses are applicable.
- Minimum Temperature (°C): The lowest temperature a material can be used at without requiring additional impact testing to ensure it doesn’t become brittle. A letter (e.g., ‘B’) refers to a curve in Fig. 323.2.2A of the code.
- Maximum Use Temperature (°C): The highest temperature at which the material can be used based on the allowable stress values in this table. Above this temperature, other phenomena like creep become the controlling factor.
Purpose in ASME B31.3
To prevent brittle fracture at low temperatures and excessive strength loss or creep damage at high temperatures.
- Min Temp is often ‘B’, which generally corresponds to -29°C (-20°F).
- Max Use Temp is typically around 593°C, but the allowable stress drops significantly at higher temperatures. Design is often limited to around 427°C (800°F) due to graphitization concerns (see Notes).
Tips for Engineers
Never select a material whose temperature range does not comfortably envelop the design conditions of your system. Always check the “Notes” column for additional temperature-related restrictions.
Min. Tensile & Yield Strength
Definition
- Min. Tensile Strength (MPa): The maximum stress a material can withstand before it starts to fracture. It represents the ultimate strength of the material.
- Min. Yield Strength (MPa): The stress at which the material begins to deform permanently (plastically). Below this point, the material will return to its original shape if the load is removed.
Purpose in ASME B31.3
These are fundamental mechanical properties derived from the material specification (e.g., ASTM). They are the basis from which the allowable stress is calculated, incorporating safety factors.
- Min. Tensile Strength: 414 MPa
- Min. Yield Strength: 241 MPa
Tips for Engineers
QA/QC should verify these values against the MTC. For design, a high yield strength is often desirable as it means a higher allowable stress, potentially allowing for thinner pipe walls.
Allowable Stress (S)
Definition
The maximum stress that a material is permitted to be subjected to under design conditions. It is a de-rated value of the material’s inherent strength, calculated to provide a margin of safety.
Purpose in ASME B31.3
This ‘S’ value is the core input for pressure design calculations (like pipe wall thickness) and stress analysis. It ensures the piping system operates well below its failure points.
How It’s Derived
The allowable stress ‘S’ at a given temperature is generally the lowest of:
- 1/3 of the specified minimum tensile strength at room temperature.
- 1/3 of the tensile strength at design temperature.
- 2/3 of the specified minimum yield strength at room temperature.
- 2/3 of the yield strength at design temperature.
- Other factors for high temperatures (creep) or specific materials.
This built-in safety factor is why you use ‘S’ in your formulas, not the raw tensile or yield strength.
Stress Ratios
Definition
- Ratio (Tensile/Stress): The material’s minimum tensile strength divided by its allowable stress at a given temperature.
- Ratio (Yield/Stress): The material’s minimum yield strength divided by its allowable stress at a given temperature.
Purpose in ASME B31.3
These ratios are not explicitly in the code table but are often calculated by engineers to quickly visualize the built-in safety factors against different failure modes.
- The tensile ratio shows the safety margin against outright rupture (fracture). A value of 3.0 means the allowable stress is one-third of the ultimate strength.
- The yield ratio shows the safety margin against permanent deformation. A value of 1.5 means the allowable stress is two-thirds of the yield strength.
How to Interpret
Higher ratios mean a larger safety margin. As temperature increases, allowable stress decreases, and these ratios will get smaller, indicating a reduced safety margin at elevated temperatures.
Interactive Table Demonstration: ASTM A106 Grade B
Hover over the cells in the example row below to see a detailed explanation of each value for this common carbon steel pipe material.
| Spec No. | Type / Grade | UNS No. | P-No. | Min Temp (°C) | Min Tensile (MPa) | Allowable Stress at 40°C (MPa) |
|---|---|---|---|---|---|---|
| A106 Spec No: This refers to the ASTM A106 standard for seamless carbon steel pipe, primarily for high-temperature service. | B Grade: ‘B’ is the most common grade of A106, offering a balance of strength and ductility. Grade A is weaker, and Grade C is stronger. | K03006 UNS No: This is the unique chemical identifier for A106 Gr. B, ensuring no ambiguity. | 1 P-No: As P-No. 1, it belongs to the main carbon steel group, simplifying welding procedure qualifications. | B Min Temp: The letter ‘B’ refers to a curve in the code, which for this material typically sets the minimum temperature at -29°C without impact testing. | 414 Min Tensile Strength: The pipe material is certified to withstand at least 414 MPa of tensile stress before fracturing. | 138 Allowable Stress: At low temperatures (up to ~200°C), the allowable stress is 138 MPa. This value is used in pipe wall thickness calculations (e.g., t = PD / 2(SE+PY)). |
Practical Engineering Use-Cases
Piping Stress Analysis
Stress engineers use the allowable stress ‘S’ from Table A-1M as the primary benchmark. In software like CAESAR II, the calculated stresses (from pressure, weight, thermal expansion) are compared against the allowable stress limits defined by the code, which are derived from the ‘S’ value. For example, the sustained stress (from pressure and weight) must be less than ‘Sh’ (the allowable stress at hot temperature).
Material Selection for a High-Temperature Steam Line
An engineer designing a 450°C steam line would consult Table A-1M. They would see that standard Carbon Steel (A106 Gr. B) has a significantly reduced allowable stress at that temperature and is prone to graphitization. They would then look for a material with a better high-temperature rating, like A335 P11 (1.25Cr-0.5Mo), which maintains a much higher allowable stress at 450°C, making it a safer and more economical choice.
Welding Procedure Qualification (WPS/PQR)
A welding engineer needs to join an A106 Gr. B pipe to an A234 WPB fitting. By checking Table A-1M, they see both have P-No. 1. This confirms they are in the same weldability group. A pre-qualified WPS for P-No. 1 to P-No. 1 materials can be used, saving significant time and cost compared to qualifying a new procedure from scratch.
QA/QC Checks During Inspection
An inspector receives a shipment of A106 Gr. B pipes with their Material Test Certificates (MTCs). They will use Table A-1M to verify the data. The MTC should report a minimum tensile strength of ≥ 414 MPa and a minimum yield strength of ≥ 241 MPa. If the MTC shows values below these, the material is rejected as it does not meet the code-referenced specification.
The Critical Role of Notes
The “Notes” column is one of the most important yet often overlooked parts of Table A-1M. These notes contain critical restrictions, conditions, or permissions that can completely change how a material is used.
Example: Note (2)
Official Text (Paraphrased): “The quality factors for castings Ec… and for longitudinal weld joints Ej… are basic factors…”
Plain Language Interpretation: This note is a reminder that if you are using welded pipe (like EFW) or cast components, you cannot use the full allowable stress value from the main table. You must multiply the ‘S’ value by a quality factor (Ej or Ec), which is less than 1.0 unless extensive examination is performed. This de-rates the allowable stress to account for the potential weaknesses in welds or castings.
Example: Note (57) for A106 Gr. B
Official Text (Paraphrased): “Conversion of carbides to graphite may occur after prolonged exposure to temperatures over 427°C (800°F).”
Plain Language Interpretation: This is a serious warning. Even though the table provides stress values for A106 Gr. B above 427°C, this note advises against long-term service at these temperatures. Graphitization is a metallurgical change that makes the steel brittle and weak. Therefore, for services operating continuously above 427°C, an engineer should select a more stable alloy steel (like Cr-Mo steels).
FAQs & Best Practices
How is the basic allowable stress ‘S’ calculated?
It’s calculated based on a fraction of the material’s certified minimum tensile and yield strengths, incorporating a safety factor. The code dictates it’s generally the lower of 1/3 of tensile strength or 2/3 of yield strength, with further reductions for high temperatures.
Can I use a material that is not listed in Table A-1M?
Yes, but it’s more complex. ASME B31.3 has specific rules for “unlisted materials.” You must have complete material specifications, and the allowable stress must be determined according to the rules in para. 302.3.2(f) of the code, which often requires more rigorous analysis and documentation.
Why does allowable stress decrease at higher temperatures?
Metals lose strength as they get hotter. The atoms vibrate more, making it easier for dislocations to move, which reduces both yield and tensile strength. At very high temperatures (the “creep range”), the material can slowly and permanently deform even under a stress lower than its yield strength.
What is the difference between Table A-1 and Table A-1M?
They contain the same fundamental data, but Table A-1 uses U.S. Customary units (psi for stress, °F for temperature), while Table A-1M uses SI / Metric units (MPa for stress, °C for temperature).
What should I do if a material has multiple rows in the table?
Read the surrounding columns carefully. The differences are usually due to size/thickness, class/condition, or specific notes. For example, a material’s properties might be different for thicknesses “≤ 25mm” versus “> 25mm”. You must select the row that matches your specific component’s attributes.
