This work investigates crack initiation and propagation from Hard Spots in TMCP steel surfaces that are exposed to wet sour environment. This paper evaluates the TMCP steel behavior and susceptibility to sulfide stress cracking (SSC) with hard spot when exposed to severe wet sour environments This investigation shall explore the effect of extreme sour conditions beyond ZONE III as per NACE MR0175 [1]. This research addresses concerns related to risks associated with TMCP steels when used in pipeline to transport wet sour oil or gas. One of these risks is the presence of hard spots that can be initiation points for SSC. These lines could be at high risk especially when exposed to high H2S partial pressure. This work shall offer pipe manufacturers and end users an understanding about risks associated with potential failures related to exposing TMCP steels with hard spots to high H2S partial pressure.
Hard spot cracking is a type of sulfide stress cracking (SSC), which is a common type of Hydrogen Embrittlement (HE). The embrittlement by SSC is attributed to the hydrogen atoms (H+), as corrosion byproduct, that permeate/diffuse through the metal with the presence of H2S. Then, when hydrogen atoms get entrapped at specific microstructural configurations, material ductility will be impaired and material will be embrittled [2]. Failure by SSC can lead to sudden catastrophic failures such as leaks, fires or explosions. The material susceptibility to HE or SSC shall be considered during project design, material selection and material qualification.
Hard spot cracking occurs in pipelines made of Thermo-Mechanical Control Process (TMCP) steel that are transporting wet sour hydrocarbons and exposed to contact with hydrogen sulfide (H2S). The use of TMCP steels to produce pipes is very common and it has resulted in significant improvement to the toughness, weldability and steel properties by forming a super fine grain structure through controlled rolling followed by accelerated cooling process. The controlled rolling process enhances toughness by refinement of the ferrite microstructure, where the subsequent accelerated cooling improves the productivity [3]. The use of TMCP steel has resulted in producing high strength welded pipes with low carbon equivalent and accordingly enhancing the welding properties and integrity.
