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OLAC (On-Line Accelerated Cooling)-based X65 Linepipe with low surface hardness for Severe Sour Environment

Picture for OLAC (On-Line Accelerated Cooling)-based X65 Linepipe with low surface hardness for Severe Sour Environment
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Product Number: 51321-16387-SG
Author: Junji Shimamura/Itaru Samusawa/Joe Kondo/Daichi Izumi/Nobuyuki Ishikawa
Publication Date: 2021
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TMCP (Thermo-mechanical controlled process) large diameter linepipes have been used since a long time for high-strength and severe sour application, but recently Sulfide Stress Cracking (SSC) caused by local hard zones has become a concern. In this paper, material design concept to prevent SSC was firstly introduced. In the author’s organization, Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining high strength with lean chemistry. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure is obtained, resulting in a stable low surface hardness at pipe inner surface. Developed OLAC-based Grade X65 LSAW Linepipe showed no SSC even for severe sour environment over than 1bar H2S while tested as per both four-point bend test and full ring test. The surface hardness limit in full ring SSC tests gave good agreement with that of 4PB SSC tests in 1bar and 16bar H2S partial pressure condition. The effect of H2S partial pressure condition on surface corrosion behavior and SSC crack propagation behavior is also discussed.

Key words: linepipe, sour gas, sulfide stress cracking, Thermo-mechanical controlled processing, Bainite, hardness, SSC crack propagation.

TMCP (Thermo-mechanical controlled process) large diameter linepipes have been used since a long time for high-strength and severe sour application, but recently Sulfide Stress Cracking (SSC) caused by local hard zones has become a concern. In this paper, material design concept to prevent SSC was firstly introduced. In the author’s organization, Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining high strength with lean chemistry. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure is obtained, resulting in a stable low surface hardness at pipe inner surface. Developed OLAC-based Grade X65 LSAW Linepipe showed no SSC even for severe sour environment over than 1bar H2S while tested as per both four-point bend test and full ring test. The surface hardness limit in full ring SSC tests gave good agreement with that of 4PB SSC tests in 1bar and 16bar H2S partial pressure condition. The effect of H2S partial pressure condition on surface corrosion behavior and SSC crack propagation behavior is also discussed.

Key words: linepipe, sour gas, sulfide stress cracking, Thermo-mechanical controlled processing, Bainite, hardness, SSC crack propagation.

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Picture for Sulfide Stress Cracking Test of TMCP Pipeline Steels in NACE MR0175 Region 3 Conditions
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Sulfide Stress Cracking Test of TMCP Pipeline Steels in NACE MR0175 Region 3 Conditions

Product Number: 51320-14446-SG
Author: Xin Yue, Weiji Huang, Andrew J. Wasson, Jamey A. Fenske, Timothy D. Anderson, Brian D. Newbury, Doug P. Fairchild
Publication Date: 2020
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Steel pipelines are sometimes subjected to demanding sour environments resulting from the presence of high H2S contents. Pipeline materials, therefore, must be resilient against sulfide stress cracking (SSC) which is caused by H2S. Beginning in the 1980s, thermo-mechanically controlled processed (TMCP) steels have been widely used for the manufacturing of large-diameter sour service pipelines. The failure of the Kashagan pipelines in 2013 raised concern regarding the use of TMCP steels in sour environments. These concerns arise from the potential for local hard zones (LHZs) to be produced on the surface of the line pipe during TMCP processes, ultimately leading to through-wall SSC failures. In the present study, several X60 - X65 TMCP steels (both with and without LHZs) have been tested under different Region 3 (R3) conditions in the NACE MR0175/ISO15156-2 pH-H2S partial pressure diagram. It can be concluded that the presence of LHZs increases TMCP steels’ sour cracking susceptibility; however, TMCP steels without LHZs pass the SSC tests at even the most severe R3 environments. Traditional HRC or HV10 testing are not able to detect LHZs, and so lower load HV 0.5 or HV 0.1 tests are necessary. For TMCP steels, the current R3 may be further divided into R3-a and R3-b sub-regions. The sour cracking severity of R3-a is less than that of R3-b. Additional actions, like enhanced mill qualification of the TMCP plate, should be considered to ensure that no LHZs exist in steels to be utilized in R3-b environments.  
Picture for Surface Hard Zone Phenomenon In TMCP Line Pipe For Sour Service: A State Of The Art Review
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Surface Hard Zone Phenomenon In TMCP Line Pipe For Sour Service: A State Of The Art Review

Product Number: 51321-16563-SG
Author: Ali Smith/ Carlo Spinelli
Publication Date: 2021
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Picture for Local Hard Zones On TMCP Large Diameter Line Pipes: What Do We Know?
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Local Hard Zones On TMCP Large Diameter Line Pipes: What Do We Know?

Product Number: 51322-17565-SG
Author: Hervé Marchebois, Bruce Cowe, Thierry Cassagne
Publication Date: 2022
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The current paper deals with a thorough analysis of these newly recognized LHZ with Scanning Electron Microscope (SEM) and Electron Back Scattering Diffraction (EBSD) investigations through the wall thickness of pipes. Internal diameter (ID) surface, intermediate zone and bulk metal microstructures showed an increase of a strong misorientation while approaching the ID. Thus, LHZ is characterized by the presence of lath and especially lower bainite type microstructures associated to high local hardness above the NACE MR 0175 / ISO 15156 limits for sour service applications.
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