Abstract
Flow forming is a cold-work process that reduces the wall thickness of a tubular by forming it over a mandrel. This enables efficient production of tubulars and can be used to increase the strength of the material via cold working. Though the flow forming process is straightforward, the alloy chemistry, microstructural cleanliness, cold reduction, residual stress orientation, and stress corrosion cracking susceptibility of the finished tubulars must be considered. Also, flow formers have reported issues related to the formation of defects at cold work levels well below the alloy’s ductility limits. Re-melted austenitic stainless steels, developed for use as drill collars, can alleviate problems caused by the higher inclusion content in air-melted grades and enable higher reduction ratios. These alloys are typically strengthened via a warm-work forging process that can put alloys at risk of forming chromium-rich precipitates at grain boundaries that reduce the material’s pitting resistance. Strengthening by flow forming solution-annealed-and-quenched material avoids this issue. However, residual stress orientation in the cold worked tubing must also be considered.
A case study is presented in which flow formed pressure housings made from a re-melted drill collar alloy cracked unexpectedly during field service. Residual stress measurements were made to confirm that the root cause of the cracks was stress corrosion cracking promoted by a tensile residual stress at the surface contacted by chloride-rich drilling fluids. Results from ASTM E827 testing were compared with C-ring and strip cutting methods to confirm the presence of tensile residual stress and demonstrate the usefulness of the simple residual stress orientation tests for screening flow-formed tubing.
