The Susceptibility of Spheroidal Graphite Cast Iron to Hydrogen Induced Stress Cracking

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. Key words: spheroidal graphite cast iron, nodular cast iron, ductile iron, hydrogen induced stress cracking, hydrogen stress cracking, hydrogen embrittlement, slow strain rate testing, stepwise constant load testing, incremental step load testing, oil & gas, subsea.