Due to its attractive combination of cost, mechanical properties and castability, use of Spheroidal Graphite Cast Iron (SGCI) has lately, to some extent, replaced steel for use in structural and mechanical components in subsea applications.
Subsea structures are typically protected by use of sacrificial anodes attached to the host structure. Under such conditions nascent hydrogen is generated on the surface of the protected material due to the cathode reaction, hence Hydrogen Induced Stress cracking (HISC) is a constant concern for subsea components subjected to tensile stress. In this work, the SGCI's resistance to Hydrogen Embrittlement (HE) has been examined by use of Slow Strain Rate Test (SSRT) and Stepwise Constant Load (SCL) test. Since structural steel is the main competing candidate material for such subsea applications, two grades of SGCI have been compared to two structural steel grades with similar mechanical strength.
The HISC tests have been supported by fractography characterization and microstructural examination by use of Optical Light Microscope (OLM) and Scanning Electron Microscope (SEM) in combination with use of Electron Back Scattering Diffraction (EBSD) for grain size distribution measurements.
Cast Iron with its ancient history, traced back to 6th century BCE1, has been used for centuries to anything from manhole covers & fire hydrants to bridges. However, the development of Spheroidal Graphite Cast Iron (SGCI) or Nodular Cast Iron, in the 1940's, with resulting improvement in mechanical properties such as ductility and fracture toughness, paved the way for further growth in industrial usage of cast iron.2 The material has been adopted by several industries such as automotive-, nuclear-, and wind turbine industry. During the last decade, SCGI has gained increased attention as construction material for subsea equipment in offshore oil & gas production, mainly competing with welded and bolted steel assemblies.
The attraction gained for SGCI use in subsea equipment is caused by its combination of cost, mechanical properties and castability. The good castability of the material allows for near net shape manufacturing of configurations initially consisting of assemblies joined together by fasteners or welds. Casting these initial assemblies into one part, results in a) part reduction, by combining multiple parts into one, b) elimination of fasteners and welds, c) reduced machining time, d) reduced assembly time, e) reduced waste and f) reduced transport/logistics. In an industry with extensive documentation requirements, simplification by reduction of parts and joints may contribute to significant cost reduction.