Bent-Beam and U-Bend Stress Corrosion Testing for Fitting Alloys

Why SCC Testing Needs Two Distinct Decisions

Stress-corrosion cracking (SCC) is not a single test. For chloride SCC screening of stainless steels and related alloys, ASTM G36 provides the exposure environment: boiling magnesium chloride solution held at 155.0 °C ± 1.0 °C (311.0 °F ± 1.8 °F) at one atmosphere pressure. ASTM G30 provides one specimen configuration: the U-bend. The two documents work together, but they address separate variables. As a buyer, you must specify both the environment and the loading configuration to get a defensible test plan.

What ASTM G36 Covers

ASTM G36 is a Standard Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution. It is an accelerated method for ranking the relative degree of SCC susceptibility for stainless steels and related alloys in aqueous chloride-containing environments. The practice is applicable to wrought, cast, and welded stainless steels and related alloys. It can detect the effects of composition, heat treatment, surface finish, microstructure, and stress on susceptibility to chloride SCC.

The practice is concerned primarily with the test solution. It may be used with a variety of SCC test specimens, surface finishes, and methods of applying stress. The standard also describes a suggested test apparatus capable of maintaining solution concentration and temperature within prescribed limits for extended periods.

Important cautions from the standard:

• Materials that normally provide acceptable resistance in hot chloride service may crack in this boiling magnesium chloride test. The test is severe and intended for ranking, not for predicting service life.
• The test may not be relevant to SCC in polythionic acid or caustic environments.
• Boiling magnesium chloride may cause pitting of many stainless alloys. This can create confusion between real SCC failures and mechanical failures induced by corrosion-reduced net cross sections. Careful examination is recommended.

What ASTM G30 Covers

ASTM G30 is a Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens. The U-bend specimen is generally a rectangular strip bent 180° around a predetermined radius and maintained in a constant-strain condition. Bends slightly less than or greater than 180° are sometimes used.

Key characteristics:

• The U-bend specimen may be used for any metal alloy sufficiently ductile to be formed into the U-shape without mechanically cracking.
• It is most easily made from strip or sheet, but can be machined from plate, bar, castings, or weldments. Wire specimens may also be used.
• U-bend specimens usually contain both elastic and plastic strain. In some cases (very thin sheet or small diameter wire) it is possible to form a U-bend and produce only elastic strain.
• Because of the large amounts of elastic and plastic strain, the U-bend provides one of the most severe tests available for smooth (as opposed to notched or precracked) SCC specimens.
• Stress conditions are not usually known, and a wide range of stresses exist in a single stressed specimen. Therefore, the U-bend is unsuitable for studying the effects of different applied stresses on SCC or for studying variables that have only a minor effect on cracking.

For studies where elastic strain only is needed, ASTM G30 explicitly notes that bent-beam specimens (Practice G39) or direct tension specimens (Practice G49) are normally used.

The advantage of the U-bend is simplicity and economy. It is most useful for detecting large differences between:

• Different metals in the same environment
• One metal in different metallurgical conditions in the same environment
• One metal in several environments

What Buyers Should Verify Before Accepting SCC Test Results

The first rule: do not translate a laboratory screening result directly into service life. Residual stress, fabrication history, actual chemistry, temperature, and geometry differ from a controlled specimen. A test that shows a material cracking in boiling magnesium chloride does not necessarily mean it will fail in your process; conversely, a material that passes may still be unsuitable for a specific service. Use ASTM G36/G30 as a ranking tool, not as a life-prediction model.

When reviewing a supplier's SCC test report, check:

1. Environment specification: Did the lab use ASTM G36 with boiling magnesium chloride at 155.0 °C ± 1.0 °C? Was concentration maintained?
2. Specimen configuration: Did the report identify whether U-bend, bent-beam, or another specimen was used? Was it made in accordance with ASTM G30?
3. Material condition: What alloy grade, heat treatment, surface finish, and product form were tested? If you are buying wrought fittings, the test should reflect wrought material, not a cast lab coupon.
4. Pitting vs. SCC: Was the failure mode confirmed metallographically? If the report only says "cracked," ask whether pitting was considered.
5. Reference to standards: Does the test report cite the specific ASTM designation and year, such as ASTM G36-24 and ASTM G30-22?

RFQ Inputs for SCC Testing of Fitting Alloys

When you request SCC testing as part of a fitting order, specify:

• Alloy grade and product form (wrought, cast, welded)
• Heat treatment condition (e.g., solution annealed, as fabricated)
• Surface finish (e.g., pickled, passivated, machined)
• Specimen type: U-bend (constant strain, severe, includes plastic strain) or bent-beam (elastic strain only)
• Environment: boiling magnesium chloride per ASTM G36, or another environment if relevant (but note G36 is specifically for this environment)
• Acceptance criteria: number of specimens, exposure duration, maximum allowable crack length, and whether pitting is acceptable
• Documentation: a full test report with photographs, metallographic examination, and the standards used

By separating environment and loading configuration in your RFQ, you avoid the common mistake of treating "SCC test" as one fixed procedure.

Inspection and Documentation Points

For EPC engineers and inspectors, these points matter during pre-shipment inspection:

• Confirm that the test laboratory is qualified and that the test method is the latest active version of the standard.
• Review the raw data for temperature control. ASTM G36 requires 155.0 °C ± 1.0 °C. Large deviations invalidate the test.
• Check that the U-bend was properly formed and held in constant strain during exposure. G30 describes several methods of maintaining stress; the report should show which was used.
• Request photomicrographs of any cracks to distinguish SCC from pitting-induced mechanical failure.
• If the test is performed on a fitting, verify the sample location and orientation relative to the fitting's manufacturing process.

Common Risks and How to Avoid Them

1. Over-interpretation: A single boiling MgCl2 test does not prove fitness for service. Use it as a comparative screening result.
2. Wrong specimen choice: Using a U-bend to evaluate the effect of applied stress is not valid because the stress distribution cannot be controlled.
3. Ignoring environment chemistry: If your service is polythionic acid or caustic, ASTM G36 is not the relevant screen.
4. Pit confusion: Many stainless alloys pit in boiling magnesium chloride. Without metallographic examination, pitting can be misidentified as SCC.
5. Missing standard references: A test report that says "SCC tested" without citing ASTM G36/G30 is not verifiable.

For more on alloy selection and fitting manufacturing, explore our pipe fittings and flanges product range at /en/products. If you need to specify SCC testing on an order, send your requirements through our /en/inquiry and we can help you review the test plan and documentation.

Sources

• https://store.astm.org/g0036-24.html
• https://store.astm.org/g0030-22.html