In-situ mechanical testing and digital image correlation of super duplex stainless steels to understand hydrogen embrittlement

Abstract

Super duplex stainless steel contains ferrite and austenite phases in its microstructure in equal amounts, providing superior mechanical and corrosion properties to most single-phase stainless steels. The microstructure has a continuous ferritic matrix with austenite islands embedded in the ferrite phase. Super duplex stainless steel is a workhorse material used in many industrial applications where the conditions are extreme in terms of corrosion and mechanical loading. Exposure to such extremes is often associated with the liberation of atomic hydrogen. The microstructure can readily absorb formed hydrogen. However, the presence of hydrogen in high-strength microstructures is detrimental; it causes a decrease in load-bearing capacity and ductility, a phenomenon called hydrogen embrittlement. There are failure reports of super duplex stainless steel used in subsea applications with cathodic protection against corrosion. However, analysis of these failed components has shown that only duplex microstructures with an austenite spacing larger than 50 μm fractured due to hydrogen. Critical components with finer microstructures show outstanding endurance, and no failure of such duplex stainless steel has been reported. It has remained unclear why finely-grain duplex microstructures show such an exceptional resistance to hydrogen-induced cracking and why duplex stainless steel with coarse microstructure shows high susceptibility to hydrogen embrittlement. This thesis aims to understand the hydrogen embrittlement of super duplex stainless steel with a small (10 μm) and large (30 μm) austenite spacing microstructure. An in-situ mechanical testing method was developed to study the effect of hydrogen absorption and mechanical strain on the susceptibility to hydrogen embrittlement. The testing method comprises a miniature-sized tensile specimen mounted on a micro-tensile tester, an electrochemical cell for in-situ hydrogen charging, and an optical microscope with an extended focal depth. The sample was continuously slowly strained (0.005 mm/min = 4.17ꞏ10-6 s-1) while the microstructure was imaged until fracture. The specimens were either electrochemically pre-hydrogen charged for up to 72 days and then tested or tested with simultaneous hydrogen charging using self-made electrochemical cells. The results were stress-strain curves and thousands of micrographs which all provide information about the deformation characteristics of materials. Then, these images were processed with digital image correlation software and strain maps were generated to understand local strain behavior. The results have shown that hydrogen absorption caused mechanical softening in the austenite phase, while hardening was observed in the ferrite phase. In addition, the finely-grained duplex microstructure, which has more resistance to hydrogen embrittlement, developed far fewer strain heterogeneities than the coarse one. The austenite grains in the coarse microstructure became more plastically than the austenitic grains in the finer microstructure. Likewise, the ferrite became less affected due to hydrogen absorption in the fine microstructure due to more hydrogen trapping at grain boundaries. It became understood that the magnitude and number of strain heterogeneities are the main reason for hydrogen embrittlement. It also became understood that as long as the austenite phase has the capacity for hydrogen absorption and mechanical straining, the entire microstructure is protected against brittle fracture. Super duplex stainless steel contains ferrite and austenite phases in its microstructure in equal amounts, providing superior mechanical and corrosion properties to most single-phase stainless steels. The microstructure has a continuous ferritic matrix with austenite islands embedded in the ferrite phase. Super duplex stainless steel is a workhorse material used in many industrial applications where the conditions are extreme in terms of corrosion and mechanical loading. Exposure to such extremes is often associated with the liberation of atomic hydrogen. The microstructure can readily absorb formed hydrogen. However, the presence of hydrogen in high-strength microstructures is detrimental; it causes a decrease in load-bearing capacity and ductility, a phenomenon called hydrogen embrittlement. There are failure reports of super duplex stainless steel used in subsea applications with cathodic protection against corrosion. However, analysis of these failed components has shown that only duplex microstructures with an austenite spacing larger than 50 μm fractured due to hydrogen. Critical components with finer microstructures show outstanding endurance, and no failure of such duplex stainless steel has been reported. It has remained unclear why finely-grain duplex microstructures show such an exceptional resistance to hydrogen-induced cracking and why duplex stainless steel with coarse microstructure shows high susceptibility to hydrogen embrittlement. This thesis aims to understand the hydrogen embrittlement of super duplex stainless steel with a small (10 μm) and large (30 μm) austenite spacing microstructure. An in-situ mechanical testing method was developed to study the effect of hydrogen absorption and mechanical strain on the susceptibility to hydrogen embrittlement. The testing method comprises a miniature-sized tensile specimen mounted on a micro-tensile tester, an electrochemical cell for in-situ hydrogen charging, and an optical microscope with an extended focal depth. The sample was continuously slowly strained (0.005 mm/min = 4.17ꞏ10-6 s-1) while the microstructure was imaged until fracture. The specimens were either electrochemically pre-hydrogen charged for up to 72 days and then tested or tested with simultaneous hydrogen charging using self-made electrochemical cells. The results were stress-strain curves and thousands of micrographs which all provide information about the deformation characteristics of materials. Then, these images were processed with digital image correlation software and strain maps were generated to understand local strain behavior. The results have shown that hydrogen absorption caused mechanical softening in the austenite phase, while hardening was observed in the ferrite phase. In addition, the finely-grained duplex microstructure, which has more resistance to hydrogen embrittlement, developed far fewer strain heterogeneities than the coarse one. The austenite grains in the coarse microstructure became more plastically than the austenitic grains in the finer microstructure. Likewise, the ferrite became less affected due to hydrogen absorption in the fine microstructure due to more hydrogen trapping at grain boundaries. It became understood that the magnitude and number of strain heterogeneities are the main reason for hydrogen embrittlement. It also became understood that as long as the austenite phase has the capacity for hydrogen absorption and mechanical straining, the entire microstructure is protected against brittle fracture.