Microstructure evolution and hydrogen embrittlement in super duplex stainless steels

Super duplex stainless steel has a wide range of applications in chemical transport and processing facilities, especially in subsea oil and gas pipelines. A desirable combination of corrosion resistance and mechanical properties can be delivered by a balanced duplex microstructure. However, the microstructure of steel can be altered during processing, which can result in degradation of mechanical properties and corrosion resistance. In offshore environment, cathodic protection is widely used to improve corrosion resistance of gas and oil transportation pipelines. However, the application of cathodic protection can trigger the evolution of atomic hydrogen, which can adversely affect the macroscopic mechanical properties. Solute hydrogen induces premature failure, which is known as hydrogen embrittlement. In this project, microstructure evolution in super duplex stainless steel was first investigated. A new Cr2N precipitation mechanism has been proposed that a nano size lamellar M23C6 facilitates Cr2N rods precipitation in super duplex stainless steel. To study Cr2N precipitates in super duplex stainless steel weldment, transmission Kikuchi diffraction (TKD) was used to measure the geometrically necessary dislocation distribution (GND) around Cr2N. The TKD-GND results suggest a high GND density can be measured in nano-sized regions adjacent to Cr2N. The effect of hydrogen charging on dislocation multiplication in super duplex stainless steel was investigated and it is found that dislocation density multiplies by about one order of magnitude in steels with under 5% pre-strain, but dislocation density remains the same in steel with pre-strain at 10% and above. EBSD was used to study the effect of hydrogen on crack propagation. Hydrogen assists crack propagation through ferrite but can be trapped by both ferrite and austenite. It is found that austenite traps cracks by emitting dislocations or forming secondary grain boundaries ahead of crack tips, while in ferrite grains, the grain boundaries can impede crack propagation. The above findings provide new insight into microstructure evolution and hydrogen induced failure in super duplex stainless steel.