Allowable Stress for ASTM A333 Grade 6 Pipe: Design Values and Rules

What Is Allowable Stress?

Allowable stress is the maximum stress that a material may carry in long-term service while keeping a safety margin against yielding and rupture. It is set by the design code, not by the material supplier, and it forms the basis of wall-thickness and pressure-rating calculations for piping and pressure vessels. For low-temperature carbon steel pipe such as ASTM A333 Grade 6, the allowable stress must be read from the code table for the design temperature.

Material Basics of A333 Grade 6

ASTM A333 Grade 6 is a killed-carbon steel seamless or welded pipe with a minimum yield strength of 240 MPa and a minimum tensile strength of 415 MPa. It is Charpy impact tested at -45 degrees Celsius, which qualifies it for cold service down to that temperature. The grade is widely used for low-temperature process piping, storage-tank connecting lines, heat exchangers and refrigeration systems.

Allowable Stress Values

In the ASME system, the basic allowable stress is the lesser of one third of the minimum tensile strength and two thirds of the minimum yield strength at room temperature. For A333 Grade 6, one third of 415 MPa is about 138 MPa and two thirds of 240 MPa is 160 MPa, so the tensile criterion governs and the tabulated value in ASME B31.3 Table A-1 and ASME Section II Part D is 20,000 psi, approximately 138 MPa. Chinese pressure piping and vessel codes, such as GB/T 20801 and GB 150, publish a comparable room-temperature allowable stress of about 135 MPa for the same material.

Temperature Effects and Low-Temperature Rules

Allowable stress is a function of temperature. At elevated temperatures the value decreases as strength drops, and the code tables must be read at the actual design temperature. For cold service, the important limit is not the allowable stress but the minimum design metal temperature. A333 Grade 6 is impact tested at -45 degrees Celsius; below that temperature the code requires a case-by-case evaluation, additional impact testing or a different material such as A333 Grade 3. In practical low-temperature design, the ambient allowable stress value is usually the governing one for carbon steel.

How to Apply the Value in Design

In ASME B31.3 the minimum required wall thickness for internal pressure is calculated with the basic allowable stress S from Table A-1, together with the design pressure, the outside diameter and the joint and casting quality factors. The designer selects the design temperature and reads the corresponding S value, then applies the code formula. For Chinese code design, the corresponding allowable stress table in GB/T 20801 is used in the same way. In both systems the pipe standard A333 or SA-333 must be named in the material specification so that the mill certificate matches the code-listed material.

Common Misconceptions

Allowable stress is not a material property printed on the mill certificate; it is a code-derived design value. The safety factor is not applied as a single multiplier of 0.4 as sometimes seen in simplified literature; the ASME value is derived from the tensile strength divided by 3 at room temperature. The allowable stress does not progressively decrease below -45 degrees Celsius; the governing limit at very low temperatures is the minimum design metal temperature and the impact-test requirement, not a falling allowable stress. Finally, the room-temperature value should not be used blindly at 300 degrees Celsius, where the code table already lists a lower number.

FAQ

What is the allowable stress for A333 Grade 6 pipe?

In ASME B31.3 Table A-1 the room-temperature allowable stress for SA-333 Grade 6 is 20,000 psi, about 138 MPa. Chinese codes list about 135 MPa.

How is the 138 MPa value derived?

It is one third of the minimum tensile strength of 415 MPa. The yield-strength criterion of two thirds of 240 MPa gives 160 MPa, so the tensile criterion governs.

Does allowable stress change with temperature?

Yes. Read the value from the code table at the design temperature. For carbon steel, elevated temperatures reduce the allowable stress; for cold service the ambient value usually governs.

Can A333 Grade 6 be used below -45 degrees Celsius?

Not as a standard catalogue item. The Charpy qualification is at -45 degrees Celsius, so colder service needs case evaluation or a higher-alloy grade such as A333 Grade 3.

What is the safety factor behind the ASME value?

At room temperature the basic allowable stress uses the minimum tensile strength divided by 3, with the two-thirds-of-yield criterion checked in parallel.

Why do ASME and GB values differ?

Chinese pressure codes apply their own safety factors and temperature tables, which yield about 135 MPa for the same material at ambient temperature.