Hydrogen Induced Cracking and Sulfide Stress Cracking Evaluation of Low Alloy Casing Pipes

• Author / Creator

  Ouhiba, Saoussen

• Casing pipes used in sour service are prone to two forms of failure: hydrogen induced cracking (HIC) and sulfide stress cracking (SSC). The objective of this thesis is to investigate the resistance of L80 ERW and Seamless steel pipe, with five different chemical compositions, to both HIC and SSC. Analysis included electron microprobe analysis (EMPA) of macro-segregation at the centerline of the ERW pipes and Scanning Electrron Microscopy (SEM) and Optical Microscopy (OM) characterization of the inclusions (i.e., size, composition, morphology and number) present in each steel. These inclusions characteristics were correlated with the HIC and SSC cracking parameters. The susceptibility of L80 steel to HIC was evaluated using HIC testing based on NACE TM0284. Three parameters including crack length ratio (CLR), crack thickness ratio (CTR) and crack sensitivity ratio (CSR) were measured to determine the HIC resistance of the different tested materials. HIC cracks were observed at the center of the HIC samples. Investigation of the fracture surfaces of opened HIC cracks using SEM and EDX revealed that manganese sulfide (MnS) inclusions are the main initiation sites of HIC cracks. The susceptibility of L80 steel to SSC was also determined using slow strain rate tests (SSRT) based on ASTM G129. Two parameters that are reduction in area ratio (RAR) and plastic strain to failure ratio (PSFR) were calculated to assess the resistance of the tested steels to SSC. Fractography was carried out to identify the mode of fracture and the origin of the SSC cracks. SSC cracks were found to initiate from corrosion pits formed at the SSRT sample surface and then propagate in a brittle transgranular manner. A qualitative and quantitative characterization of the inclusions present in L80 steel was performed. This study confirmed that MnS inclusions are the main initiation sites of HIC cracks. Al enriched inclusions were also found to play a role in the initiation and propagation of HIC cracks. However, their role is less significant than the role of MnS inclusions. For the SSRT tests, due to the SSRT sample orientation relative to the applied stress, clusters of Al enriched inclusions were found to be the main initiation sites of SSC cracks. EMPA X ray mapping and line scanning of the major chemical elements (Mn, Cr, Si and P) across the HIC cracks were conducted to detect and quantify the macro-segregation behavior of these elements. EMPA X-ray maps showed that there is an enrichment of the zone around the HIC cracks in the above mentioned alloying elements, especially in phosphorus. The EMPA X-ray maps results were confirmed by EMPA line scans results that showed that the concentration of P around the HIC crack is at least 6 times higher than the nominal concentration of P. However, no clear correlation was found between CLR and the segregation ratios of the different analyzed elements determined by EMPA, which suggests that there are other factors that play a role in propagation of HIC cracks.

• Subjects / Keywords

  • Cracking
  • Steel
  • Centerline segregation
  • Hydrogen
  • Failure
  • Low alloy

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3RN30N2V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.