Fukunaga Terasaki

Effects of Fe3C and Mo2C precipitation on hydrogen diffusivity and hydrogen embrittlement in iron alloys

Abstract The effects of carbide precipitation on the diffusion of hydrogen atoms and hydrogen embrittlement in Fe-0.25wt.%C and Fe-3.9wt.%Mo-0.20wt.%C alloys have been investigated by means of the electrochemical permeation technique and tensile tests during cathodic charging of hydrogen. Although the effective hydrogen diffusivity in the FeMoC alloy decreases to the minimum value by tempering at temperatures at which secondary hardening is most prominent, the activation energy for it is independent of tempering temperature in both alloys, and the calculated binding energies between hydrogen atoms and traps are in the range 22–28 kJ mol −1 (almost the same as the binding energy for dislocations). The calculated trap density exhibits a tempering temperature dependence similar to that of the strength. The ductility and the fracture mode, however, are markedly affected by microstructural factors such as the high dislocation density, the almost continuous precipitation of fine Mo 2 C particles on the grain boundaries or the discontinuous precipitation of coarse Mo 2 C particles and the formation of precipitate-free zones in the vicinity of the grain boundaries, in addition to the average density of reversible or irreversible hydrogen-trapping sites.

Effect of nickel on hydride formation and hydrogen embrittlement in NiCrFe alloys

Abstract Hydride formation and hydrogen embrittlement in NiCrFe alloys were investigated with particular emphasis on the effect of nickel content. In all the alloys containing 30–60 wt.% Ni, γ hydride formed during cathodic hydrogen charging but, in the 30 wt.% Ni alloy, the formation of β hydride was also observed. During the decomposition of the hydride after the cessation of charging, surface cracks appeared along both the grain boundaries and the interfaces of {111} microtwins which were induced by the volume expansion due to the hydride formation. In the lower nickel alloys the formation of microtwins and the subsequent surface cracks along these interfaces were more pronounced. This probably arose from the decrease in stacking fault energy with decreasing nickel content. The ductility loss caused by cathodic hydrogen charging was much enhanced by increasing the nickel content, and the fracture mode changed from transgranular to intergranular. This change was quite similar to that observed in surface cracks and is thought to be related to the frequency of microtwin formation. Aging at 673 K suppressed hydrogen embrittlement, changed the fracture mode from intergranular to transgranular and reduced the amount of absorbed hydrogen. This can be explained in terms of the site competition mechanism due to the segregation of carbon atoms to the grain boundaries.

Steel Plates for Pressure Vessels in Sour Environment Applications

It is well known that wet hydrogen sulphide (H2S) can cause embrittlement of steels, hydrogen induced cracking (HIC) and sulphide stress corrosion cracking (SSCC). Several fractures of pipelines handling sour crude oil or gas led to vigorous researches on these problems. As similar failures have also been experienced in petroleum refinery equipment, degradation of steel by hydrogen sulphide is now recognized as a serious environmental problem. The paper considers the mechanism and factors involved in HIC. This type of cracking occurs mainly in the parent steels. The susceptibility of steels to cracking is influenced strongly by inhomogeneities such as the shape and distribution of non-metallic inclusions, and segregation of alloying elements. These have a significant effect on HIC because they modify the microstructures in the segregated regions. With reference to environmental factors, these mainly concern the influence of H2S partial pressures, pH of the solutions and other phenomena relevant to the absorption of hydrogen by the steel. SSCC poses problems in weld zones. It can occur especially in heat affected zones (HAZ) with high hardnesses. Such cracking can be prevented by the control of hardness by a suitable selection of the chemical composition of the steel and the welding conditions. Nevertheless, countermeasures similar to those described for the prevention of HIC are necessary to prevent SSCC in HAZ even with relatively low hardness. Research on factors influencing HIC and SSCC has resulted in the development of steels which are highly resistant to wet H2S cracking. These steels have been supplied in plate form for pressure vessels. Experience has confirmed the good performance of welded constructions in aggressive service environments.

Some Remarks on the Notch Sensitivity in Creep Rupture Properties of Heat-Resisting Alloy A286

Synopsis: The notch sensitivity of creep rupture properties of alloy A286 were reported. The effect of notch forms, heat treatments, melting processes, and the short-time tensile properties on the notch sensitivity were investigated. Changes in hardness, microstructure, and the nucleation and propagation of microcracks at the notch root during creep rupture test were also investigated. Since the plastic flow was constrained under notch condition, the notch sensitivity of this alloy was much affected by microstructure, that is, the resistivity to the nucleation and propagation of microcracks was significant for the notch sensitivity. Reduced resistivity was greatly due to the presence of grain boundary precipitates such as G phase and ri phase, as mentioned in the previous paper (Tetsu-to-Hagane, 46 (1959) 9, p. 1029; zairyo-shiken, 10 ( 1961) 90, p. 70) From the above mentioned point of view, the effects of notch forms, heat treatments, melting processes, and also the short-time properties on the notch sensitivity were discussed according to the experimental results.