The manufacturing of integral connections in OCTG includes cold end-sizing operations that may impact the SSC resistance of the cold-formed ends of high strength sour service pipes. The common process to recover this SSC resistance is thermal stress-relieving, but the latter raises concerns and is subjected to heterogeneous requirements from standards and from end-users.
This article discloses how the tensile properties behave after thermal stress-relieving with regards to the cold-forming history of the material (crimping vs. expanding of the outside diameter) and how this behavior correlates to the SSC resistance of pipe-ends sampled after industrial operations of end-sizing followed by induction stress-relieving operation.
The need for improved mechanical performance of integral threaded connections compels manufacturers of casing pipes for oil and gas wells to tighten the dimensional tolerances of pipe-ends, far beyond the specified range of the API Specification 5CT. The cold forming of the extremities of plain-ends - called end-sizing in this article - is an easy and essential way to solve that geometrical issue, but it shall be followed by thermal stress-relieving to recover the initial mechanical properties and the resistance to Sulfide Stress Cracking (SSC) of high strength sour service pipes.
The standards that apply explicitly to the end-sizing and the subsequent stress-relieving in the particular case of low alloy steels are not numerous and are hardly describing what is the state-of-the-art related to the quality control of this industrial practice.
ANSI-NACE MR0175-ISO 15156-Part 2 recognizes the process of end-sizing and defines the strain according to the deformation percentage of the outer fiber, regardless whether the pipe diameter is increased (box-expanding) or reduced (pin-nosing). It specifies a minimum temperature of 595 °C during thermal stress-relieving to properly recover the corrosion resistance, only if the strain level exceeds 5% in the case of low alloy steels showing hardness below 22 HRC, and in any case if the hardness is above 22 HRC.