高井 健一

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

Hydrogen effects of pure titanium have been investigated in order to clarify a main factor causing the hydrogen embrittlement. Though a lot of studies reported that the main factor of hydrogen embrittlement of pure titanium was hydride, there is no study to evaluate using specimen including dissolved hydrogen content as much as the content of hydride. In the present study, hydrogen-charged titanium specimens including same hydrogen content as hydride or dissolved hydrogen are prepared by heat treatment. These specimens are tested under various hydrogen contents, strain rates and temperatures. As a result, not only hydride but also dissolved hydrogen affects mechanical properties such as tensile stremgth and fracture strain of pure titanium, that is, dissolved hydrogen also causes hydrogen embrittlement.

To improve the susceptibility of hydrogen embrittlement, it is important to identify hydrogen trapping site, binding energies, and occupation ratios at various lattice defects in metals. Thermal desorption spectrometer heated from lower temperature (L-TDS) which we have developed enables us to analyze them, In the present study, the influence of carbon and carbide on peak temperatures of hydrogen desorption, binding energies and occupation ratios in iron was examined. The specimens including 6ppm, 0.005%, 0.01% and 0.1% carbon in iron were prepared. For dislocation trapping, the peak at 283K desorbed from dislocation decreases with increasing carbon content. The binding energy between dislocation and hydrogen decreases with increasing carbon content from 24.9kJ/mol to 17.7kJ/mol. The hydrogen occupation ratio at dislocations also decreases with increasing carbon content. These finding indicate that carbon in iron causes significant influence on trapping behavior, since interstitial atoms such as carbon occupy lattice defects instead of hydrogen.

High strength steel with lower susceptibility of delayed fracture has been required to be developed, while, generally, steel above 1200 MPa of tensile strength increases the susceptibility. In the present study, the susceptibility of tempered martensitic steel with 1450 MPa is tried to reduce through higher-temperature tempering by adding Si and surface-softening , using induction heating. The strength of surface-softened steel is 1150 MPa at the surface, 1500 MPa at the center and 1450 MPa in the average. The susceptibility reduces by adding Si-content of 1.88 mass %. This is because the Si adding reduces carbide precipitations along prior γ grain boundaries, which is observed by TEM, and prevents intergranular fracture. In addition, the Si adding reduces stress relaxation, i.e. stabilizes dislocation structure. Furthermore, the surface-softening enables us to reduce the susceptibility, since the hydrogen concentration at the surface of crack initiation is lower than that at the center.

Although the interaction between dislocation and hydrogen in bcc metal such as pure iron plays the essential role in hydrogen embrittlement, little is known about the interaction in fcc metal such as stainless steel. Hydrogen spectra evolved from SUS 316L and 304 stainless steels during elasticity and plastic deformation were detected using a quadrupole mass spectrometer. Hydrogen desorption increased rapidly when plastic deformation began for SUS 316L, since dislocations drag hydrogen to the specimen surface. In contrast, hydrogen desorption increased with applying strain for SUS 304, because of phase transformation from austenite into martensite with larger hydrogen increased with decreasing strain rate. This result indicates that dislocation can drag and transport more amount of hydrogen when dislocation velocity approaches hydrogen diffusion rate. These lead conclusion that the interaction between dislocation and hydrogen occurs in not only bcc but also fcc metal. In the present study, relation between interaction between dislocation and hydrogen and hydrogen embrittlement is considered.

The lattice defect formation behavior associated with hydrogen under elastic tensile stress of tempered martensitic steel was examined from the viewpoint of dislocation and vacancy. In addition, the relationship between hydrogen embrittlement and lattice defects associated with hydrogen was investigated. In the early stage of applying elastic stress, the amount of lattice defects decreased, and then increased gradually with time of applied stress. The most probable reason for the decrease in the amount of lattice defects is due to dislocation annihilation and rearrangement of dislocation structure. In contrast, the increase in the amount of lattice defects can be assigned to the increase in the amount of vacancy clusters. The vacancy clusters enhanced by hydrogen and elastic stress cause ductility loss of the steel, even though hydrogen is not present in tensile test. Therefore, not only hydrogen but also the vacancy clusters are concluded to be important factor causing hydrogen embrittlement.

The factor that plays the essential role in hydrogen degradation has been examined for Inconel 625 and iron by means of tensile testing with interposed unloading and reloading with/without hydrogen charging. Aging at 30℃ or annealing at 200℃ was conducted at the unloaded stage in order to diffuse out hydrogen or to anneal out strain-induced defects. Hydrogen thermal desorption analysis was used to evaluate strain-induced defects that act as trapping sites of hydrogen. Fracture strain decreased in the initially hydrogen-charged specimens even though hydrogen was absent at the late stage of straining. Annealing at 200℃ at the unloaded stage almost completely recovered the decrease in fracture strain.

Hydrogen energy society has required for lower cost BCC metals with high-resistance to hydrogen environmental degradation instead of FCC metals with higher cost. However, BCC metals such as steels have higher degradation susceptibility than FCC metals, then the susceptibility must be improved. In the present study, the identification of lattice defect affecting the hydrogen environmental degradation has been investigated using pure iron emphasizing vacancy and dislocation, since the influence of other elements and inclusions is almost negligible.

Though various experiments of hydrogen in metal have been studied, there is very little understanding of hydrogen trapping states and degradation in aluminum. This is because the hydrogen dissolution enthalpy of aluminum is very high. In the present study, pure aluminum (99%) was charged with hydrogen by electrochemical method in various conditions. Large hydrogen concentrations were introduced in invariant condition. There were two peaks at 120℃ and 400℃ in hydrogen evolution profile. For the first peak (120℃), hydrogen is solute in lattice or trapped at vacancies or dislocations. For the second peak (400℃), hydrogen is precipitated as hydrogen molecule in blisters in aluminum. Hydrogen trapping states at first peak affects decrease in ductility. In contrast, the second peak does not affect the degradation. The failure strain of aluminum charged hydrogen decreases with strain rates.

High-strength steel has been required from the viewpoint of reduction in cost and environmental deterioration in recent years. However the probability of delayed fracture caused by hydrogen increases with steel strength. As universal mechanism of hydrogen embrittlement, hydrogen enhances the dislocation mobility and local plasticity and so stabilizes and increases vacancies. From these behaviors of hydrogen, high-strength steel with high resistance to delayed fracture was developed to suppression dislocation mobility and vacancy formation. Pre-strain and ageing method enable to improve the delayed fracture property of high-strength steel. The property of the delayed fracture was evaluated and the mechanism was examined.

The main role of hydrogen in hydrogen embrittlement has been investigated for fcc and bcc metals. Both fcc and bcc metals show that hydrogen contents of the strained metals with hydrogen are more than those of strained metals without hydrogen. It suggested when including hydrogen promote the formation of the trapping site. In SSRT test, when stress was relieved before the fracture strain, held at 30℃ and reloaded. In this case, fracture strain was not recovered. While held at 200℃, fracture strain was completely recovered. These showed that the vacancy cluster contributed to the ductility reduction.

Hydrogen embrittlement has been analyzed by using thermal desorption spectrometry (TDS). However, it is difficult to separate the states of trapped hydrogen at various lattice defects such as dislocation and vacancy. It is reported that peaks of hydrogen desorption are dependence on thickness of specimen, heating rate, binding energy and defect density. Thus, the objective of the present study is the separation of the hydrogen states in pure iron emphasized dislocation and vacancy using controlling thickness of specimen and heating rate.

Stainless steel has been expected for infrastructural materials for hydrogen energy society. However, metastable austenitic stainless steel is reported to deteriorate in high-pressure hydrogen. In the present study, to determine the cathodic charging condition of SUS 304, hydrogen absorption properties charged by high-pressure hydrogen and cathodic charging have been investigated. In addition, to clarify the mechanism for hydrogen degradation of SUS 304, the main factor affecting hydrogen degradation and recovery have been identified from the standpoint of hydrogen, vacancy, dislocation, and stress induced martensite.