Tensile Testing and Yield Strength Determination
Tensile testing is the primary acceptance test for the mechanical properties of seamless steel pipe. Room temperature testing is carried out to ASTM A370, the umbrella method standard for mechanical testing of steel products, which refers to ASTM E8/E8M for tension testing; contracts written to ISO or EN practice use ISO 6892-1. The specimen is normally a longitudinal strip or round bar taken from the pipe body, and a full-section specimen is used for small diameters. Yield strength is determined either by the 0.2 percent offset method of ASTM E8/E8M or by the 0.5 percent total extension under load method used in API 5L, and the reportable values are yield strength, tensile strength and elongation. Elongation is measured over a 50 mm gauge length or a proportional gauge length of 5.65 times the square root of the original cross-sectional area, depending on the standard applied.
Impact Toughness Testing
Impact resistance is evaluated with the notched bar impact test to ASTM E23 or ISO 148-1. The standard specimen is 10 mm by 10 mm by 55 mm with a 45 degree V-notch 2 mm deep and a 0.25 mm root radius. A test set consists of three specimens, and the result is reported as absorbed energy in joules together with the test temperature, which may be 0 degrees C, -20 degrees C, -45 degrees C or lower depending on the product specification. Specification minimums are temperature specific: ASTM A333/A333M Grade 6 requires an average of 18 J minimum with no single value below 13 J at -45 degrees C, while API 5L line pipe impact requirements are set by the purchase order according to grade, wall thickness and design temperature. Shear-fracture appearance and lateral expansion are recorded alongside absorbed energy for critical applications.
Hardness Verification
Hardness testing is a fast, low-cost verification of heat treatment condition and is used as an acceptance criterion for tubes intended for sour or wet H2S service, where ISO 15156-2 limits carbon and low-alloy steels to 22 HRC maximum. Brinell testing to ASTM E10 is applied to heavier sections, Rockwell B or C testing to ASTM E18 to thin and intermediate walls, and Vickers or microindentation testing to ASTM E384 for weld zones, heat-affected zones and thin coated sections. ASTM A179/A179M heat exchanger tubes, for example, are limited to 72 HRB maximum. Hardness alone does not replace tensile testing because it cannot establish yield or tensile strength values.
Flattening, Flare and Hydrostatic Evaluation
Ductility of the finished pipe is checked by the flattening test, in which a ring cut from the tube end is flattened between parallel plates to a specified distance and examined for cracks; ASTM A1016/A1016M and ASTM A450/A450M define the test for heat exchanger and boiler tubes. API 5L requires flattening tests for seamless and welded pipe within defined diameter-to-thickness ratios, and a hydrostatic test at a pressure held for a specified time for PSL2 pipe. Nondestructive alternatives such as eddy current, ultrasonic or flux leakage inspection are permitted by ASTM A1016/A1016M in place of the hydrostatic test for certain tube sizes, and API 5L accepts full-body ultrasonic, radiographic or electromagnetic inspection in place of the hydrostatic test only when the purchase order states so.
From Test Data to an Evaluation Report
Evaluation follows a fixed sequence. First, record load, extension and absorbed energy data and plot the stress-strain curve or the absorbed energy against temperature. Second, extract yield strength, tensile strength and elongation from the curve or read the absorbed energy directly, together with hardness values at each test location. Third, compare the results with the specification minimums for the grade, heat treatment condition and wall thickness in the applicable standard. Fourth, confirm that the scatter within each test set is acceptable, that the test temperature was within tolerance and that the specimen orientation and location were as required. Trend analysis across heats and production lots is used to detect drift in the cold-drawing or heat treatment process before a non-conformance occurs. Statistical process control on yield strength and impact energy is more useful than single-pass or fail decisions, particularly when the same mill is supplying pipe to more than one specification.
Frequently Asked Questions
Q: Which standard governs tensile testing of seamless steel pipe?
A: ASTM A370 defines the methods and references ASTM E8/E8M for room temperature tension testing. ISO 6892-1 applies when testing is specified to ISO or EN practice, and GB/T 228.1 applies for orders placed against Chinese national standards.
Q: What is the difference between the 0.2 percent offset method and the 0.5 percent extension under load method?
A: The 0.2 percent offset method finds the stress at which permanent strain reaches 0.2 percent; the 0.5 percent extension under load method reads the stress at a total elongation of 0.5 percent of the gauge length. API 5L permits the second method, and results from the two methods differ slightly for the same material.
Q: How many impact specimens are needed and at what temperature?
A: A test set is three specimens tested at the temperature stated in the product specification or purchase order, for example -45 degrees C for ASTM A333 Grade 6. A supplementary set at a lower temperature can be agreed where the service temperature is below the standard test temperature.
Q: Can hardness testing replace tensile testing?
A: No. Hardness is an indicator of heat treatment and, for sour service, a hardness limit can be an acceptance criterion, but it cannot establish yield strength, tensile strength or elongation values.
Q: What does the flattening test prove?
A: It evaluates the ductility of the tube body and the soundness of the weld or the seamless wall by compressing a ring beyond the elastic range and checking for cracks or laminations after deformation.
Q: Why is testing done at more than one location on the pipe?
A: Mechanical properties vary with wall thickness, position relative to the weld and the amount of cold work, so specimens from different locations and orientations give a more reliable picture of the whole length than a single test.