Internal hydrogen can influence the fatigue life, crack growth rate, and crack morphology of austenitic stainless steel, but little is known about the effect of internal hydrogen on fatigue crack initiation sites. To determine the effect of internal hydrogen on the microstructural fatigue crack initiation sites, the location of small fatigue cracks was evaluated with respect to the microstructural features in notched middle tension M(T) 316L specimens both with and without pre-charged hydrogen. The notches of the M(T) specimens were electropolished prior to fatigue testing to facilitate post-test analysis. Fatigue tests were performed with the same constant load amplitude and an R-ratio of 0.1 for specimens with and without internal hydrogen. The fatigue tests were interrupted after a minimal amount of cracking was detected using the direct current potential difference (DCPD) technique. The microstructural locations of the small fatigue cracks were then evaluated with scanning electron microscopy imaging and electron backscatter diffraction (EBSD). Several small transgranular fatigue cracks initiated in the notches of specimens both with and without internal hydrogen. These transgranular cracks always intersected grain boundaries, twin boundaries, and/or triple points indicating that these microstructural features are the critical locations for crack initiation. The transgranular cracks did not propagate along the prominent slip traces. There was no discernible effect of hydrogen on the microstructural sites of fatigue crack initiation in 316L.
Austenitic stainless steel microstructures produced by directed energy deposition (DED) are analogous to those developed during welding, particularly high energy density welding. To better understand microstructural development during DED, theories of microstructural evolution, which have been established to contextualize weld microstructures, are applied in this study to microstructural development in DED austenitic stainless steels. Phenomenological welding models that describe the development of oxide inclusions, compositional microsegregation, ferrite, matrix austenite grains, and dislocation substructures are utilized to clarify microstructural evolution during deposition of austenitic stainless steels. Two different alloys, 304L and 316L, are compared to demonstrate the broad applicability of this framework for understanding microstructural development during the DED process. Despite differences in grain morphology and solidification mode for these two alloys (which can be attributed to compositional differences), similar tensile properties are achieved. It is the fine-scale compositional segregation and dislocation structures that ultimately determine the strength of these materials. The evolution of microsegregation and dislocation structures is shown to be dependent on the rapid solidification and thermomechanical history of the DED processing method and not the composition of the starting material.
Hydrogen additions to natural gas are being considered around the globe as a means to utilize existing infrastructure to distribute hydrogen. Hydrogen is known to enhance fatigue crack growth and reduce fracture resistance of structural steels used for pressure vessels, piping and pipelines. Most research has focused on high-pressure hydrogen environments for applications of storage (>100 MPa) and delivery (10-20 MPa) in the context of hydrogen fuel cell vehicles, which typically store hydrogen onboard at pressure of 70 MPa. In applications of blending hydrogen into natural gas, a wide range of hydrogen contents are being considered, typically in the range of 2-20%. In natural gas infrastructure, the pressure differs depending on location in the system (i.e., transmission systems are relatively high pressure compared to low-pressure distribution systems), thus the anticipated partial pressure of hydrogen can be less than an atmosphere or more than 10 MPa. In this report, it is shown that low partial pressure hydrogen has a very strong effect on fatigue and fracture behavior of infrastructure steels. While it is acknowledged that materials compatibility with hydrogen will be important for systems operating with high stresses, the effects of hydrogen do not seem to be a significant threat for systems operating at low pressure as in distribution infrastructure. In any case, system operators considering the addition of hydrogen to their network must carefully consider the structural performance of their system and the significant effects of hydrogen on structural integrity, as fatigue and fracture properties of all steels in the natural gas infrastructure will be degraded by hydrogen, even for partial pressure of hydrogen less than 0.1 MPa.