A component carrying no service load at all can crack. That fact surprises owners more than any other feature of stress-corrosion cracking, and it follows directly from where the tensile stress usually comes from. Welding, cold forming, machining and assembly fit-up all leave stress locked into the part, at levels that can approach the material's yield strength, and that stress is sustained, tensile, and concentrated precisely where the metallurgy has been disturbed. It was never in the design calculation because it was never designed in.
Stress that was never specified
Design stress is calculated from pressure, weight, thermal expansion and the other load cases, and a component within its allowable stress is by that measure adequately designed. Residual stress sits outside that accounting entirely. It is a self-balancing internal stress field created by non-uniform plastic deformation or non-uniform cooling: part of the section ends up in tension and part in compression, with no external load applied. For SCC the tensile portion is all that matters.
Welding is the usual source
A weld cools from the melt while restrained by cold metal around it, and the contraction it cannot accomplish freely becomes tensile residual stress in the weld and the heat-affected zone, commonly approaching yield. That the same operation also alters the microstructure — sensitising an austenitic stainless steel, hardening a heat-affected zone — means both vulnerable conditions arrive together. This is why SCC appears at or beside welds far more reliably than in undisturbed base metal.
Cold forming, machining and straightening
Rolled heads, formed elbows, bent tubing, expanded tube ends and cut threads all involve plastic deformation that leaves residual stress behind. Machining and grinding produce a thin, heavily worked surface layer whose stress state depends on how aggressively it was cut. Cold straightening of a distorted component after fabrication is a particularly quiet contributor, because it is often done on the shop floor without appearing in any record.
These sources share an awkward property: the deformation is intentional and the stress is a by-product nobody set out to create. A forming operation performed exactly as specified can still leave the finished part above the SCC threshold, so the presence of residual stress is not, on its own, evidence that anything was done wrong.
Assembly and fit-up
Stress can also be imposed at installation rather than in the shop. Forcing misaligned flanges together, pulling piping into position to meet a nozzle, over-torqued fasteners, and rigid supports that fight thermal growth all impose sustained tensile stress on the assembled part. This category is distinctive because the stress arrives after every fabrication record has closed, and because it usually leaves geometric evidence — measurable misalignment, gap or distortion — that survives the failure.
What this does to the investigation
If the tensile leg came from fabrication, the relevant question shifts from how the equipment was operated to how it was made and installed. Weld procedure specifications and procedure qualification records, heat-treatment charts, forming and straightening records, non-destructive examination reports and installation dimensional checks become the documents in dispute. Their absence is itself informative, particularly where a code or purchase specification called for stress relief and no record of it exists.
Measuring residual stress after the fact
Residual stress can be estimated on a recovered component. X-ray diffraction reads lattice spacing in a shallow surface layer; hole-drilling relieves a small volume and measures the resulting strain; sectioning and distortion measurement indicate the direction and rough magnitude of a locked-in field. Hardness mapping and metallography identify cold work and heat-affected microstructure even where the stress itself has since been relieved. Each method has a limited depth of interrogation and its own uncertainty, and results are read against the fabrication history rather than in isolation.
Stress relief, and what it is meant to accomplish
Post-weld heat treatment and solution annealing reduce residual stress by permitting local yielding at temperature, and for austenitic stainless steel solution annealing also reverses sensitisation, which ASTM A262 practices can be used to check. Shot peening works differently: it imposes a compressive surface layer that offsets tension rather than removing it, and its benefit is confined to that layer. Whether any of this was specified, performed and documented is a discrete factual question with a paper answer.
Testing with stress deliberately applied
The standardised SCC tests are built around applying a controlled stress to a specimen, which makes them directly relevant here. ASTM G30 uses a U-bend, G38 a C-ring, G39 a bent beam and G49 direct tension, while G44 sets out alternate-immersion exposure and G36 the boiling magnesium chloride environment. Specimens can be cut from the actual fabrication, including weld and heat-affected zone material, so that the condition tested matches the condition that failed.
Where the residual-stress argument runs into trouble
The stress that drove the cracking has usually been relieved by the cracking itself, so what is measured afterwards is not what was there before. Fabrication records are frequently incomplete, and a shop practice can often be inferred but rarely proven from documents alone. The defensible form of the opinion identifies the plausible sources, states which are supported by physical evidence and which by inference, and says plainly what the measurements can and cannot resolve.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.