Sep 13, 2026Standards & Testing
Non-Destructive Testing of Straight Seam Steel Pipe: Methods Explained
How NDT of straight seam steel pipe works: appearance checks, magnetic particle and penetrant testing, radiographic testing and sequence.
Quick answer: NDT of straight seam steel pipe combines an appearance check, a surface method such as magnetic particle or penetrant testing, and radiographic or ultrasonic testing. The method follows the material and joint type: magnetic particle testing for ferromagnetic pipe, penetrant testing for non-ferromagnetic pipe, and radiographic or ultrasonic testing for butt joints. Testing is sequenced after visual inspection, with 100% radiographic inspection for defined joints such as casing girth welds.
Why NDT Comes After Visual Inspection
Non-destructive testing does not replace looking at the weld. Before any instrument-based test, the weld appearance must already meet the specified requirements. Visual inspection is the first filter: it confirms that joint geometry is acceptable and that no obvious surface condition would make a later test unreliable. Only joints that pass the visual stage are worth testing in detail.
Appearance Requirements Before Any Test
The weld should be well formed, with a width of about 2 mm on each side above the edge of the groove. For fillet welds, the leg height must meet the design requirement and the shape should transition smoothly. A badly formed joint cannot be evaluated fairly by surface or volumetric methods.
On the welded joint surface there must be no cracks, lack of fusion, pores, slag inclusions or spatter. For steel pipes designed for service below minus 29 C, and for stainless and alloy steel pipes with a high hardening tendency, undercut is not permitted on the weld surface. For other materials, undercut depth is normally limited to 0.5 mm, a continuous undercut should not exceed 100 mm, and total undercut on both sides of the weld should not exceed 10% of the total weld length.
The weld surface should not sit below the pipe surface, and reinforcement should not exceed 3 mm. Misalignment should not exceed 10% of the wall thickness and should never exceed 2 mm. These limits keep the joint within a range where later NDT results can be interpreted reliably.
Choosing the Right Surface NDT Method
For surface testing, the selection rule is based on magnetism. Ferromagnetic material steel pipes call for magnetic particle testing, while non-ferromagnetic material steel pipes call for penetrant testing. Each method only works on the material type it is designed for. Timing matters too: for welded joints prone to delayed cracking, surface NDT should be carried out after the joint has cooled for a certain period, so that late-forming cracks are not missed. For joints prone to reheat cracking, surface NDT should be carried out both after welding and after heat treatment.
Where surface NDT is applied
Typical applications include the outer surface of the pipe material; important butt welds, fillet welds, socket welds and bridging tee branch connections; detection after bending; the groove at welded joints after quenching; non-austenitic stainless steel grooves designed for service at or below minus 29 C; the weld root in double-sided weldments after back gouging; and repaired areas after flame cutting of welding jigs on alloy tubes with a hardening tendency.
Radiographic and Ultrasonic Testing
Radiographic and ultrasonic testing are mainly applied to the butt joints of straight seam steel pipes and the butt joints of butt welded pipe fittings. The method is selected according to the design file. For welded joints of titanium, aluminium and aluminium alloys, copper and copper alloys, and nickel and nickel alloys, radiographic testing is the specified method.
Timing matters here as well. For welds prone to delayed cracking, radiographic and ultrasonic testing should be performed after the weld has cooled for a certain period. Where the main pipe inside a casing has a girth weld, 100% radiographic inspection is required, and the joint is covered only after the test pressure is qualified. Welded joints covered by a reinforcement ring or support pad also require 100% radiographic inspection and should be covered only after passing the test.
Testing Sequence and Records
The correct order is visual inspection first, then surface NDT, and finally radiographic and ultrasonic testing. Where intermediate inspection of a weld is required, the joint should be tested after visual inspection, and radiographic and ultrasonic testing should follow surface NDT. A welded joint under inspection should only continue to be welded after it has been accepted, which prevents a defect from being buried under later passes.
Sequence is not optional: visual inspection comes first, surface NDT follows, and only then are radiographic and ultrasonic tests performed - a joint that fails an earlier step should not advance to the next one.
FAQ
Which NDT method is used for stainless steel pipe welds?
Penetrant testing is selected for non-ferromagnetic material, which covers austenitic stainless steel. For butt joints, radiographic or ultrasonic testing is chosen according to the design file, and specific non-ferrous materials such as titanium and nickel alloys call for radiographic testing.
When should delayed cracking be tested?
Surface non-destructive testing, and radiographic or ultrasonic testing, should be carried out after the weld has cooled for a certain period. Testing a joint too early can miss cracks that form as the weld cools.
Is 100% radiographic testing always required?
No. Full radiographic inspection applies to defined cases, such as a girth weld on the main pipe inside a casing, and welded joints covered by a reinforcement ring or support pad. Other joints follow the method and extent given in the design file.
What weld misalignment is acceptable?
Misalignment should not exceed 10% of the wall thickness and should never exceed 2 mm. Undercut and reinforcement limits are specified separately, so alignment alone does not make a joint acceptable.
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