Resistance to hydrogen embrittlement of low alloy steels was evaluated based on their critical hydrogen content and critical stress. Constant load test (CLT), Slow Strain Rate Test (SSRT) and Conventional Strain Rate Test (CSRT) were carried out using JIS-SCM435 and V-added steels in six laboratories. It was confirmed that the same test results were obtained in different laboratories under the same test conditions. Furthermore, the relationships between diffusible hydrogen content and nominal fracture stress obtained by means of CLT and SSRT were similar to each other. In CSRT, the nominal fracture stress was higher than that in CLT and SSRT under the same absorbed hydrogen content in the specimens. In SSRT and CSRT, fracture surfaces showed Quasi-cleavage mode under small hydrogen content, while they showed Inter-granular fracture under large hydrogen content. In order to compare the three methods considering the concentration of hydrogen in stress field, locally accumulated hydrogen content under the same fracture stress is calculated. The locally accumulated hydrogen under the same applied stress, in other words, critical hydrogen content to hydrogen embrittlement, is the following order; SSRT < CLT < CSRT in JIS-SCM435, and CSRT < CLT ≒ SSRT in V-added steels.
To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations with respect to the lamellar interface were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load monotonically increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The load reduction with respect to the displacement was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.
To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The degree in the load reduction was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.
Thermal Desorption Analysis (TDA) was conducted at multiple research institutions on common specimens, after hydrogen charging with various methods. The specimens were SCM435 steel with/without pre-straining and Vanadium alloyed steel (V steel). Hydrogen charging methods were cathodic charging, soaking in FIP solution and exposure to cyclic corrosion test. All the TDA spectra (TDA curves) were separated systematically by Gaussian distribution function with unified parameters that are independent to method of hydrogen charging or research institute. They were basically composed of hydrogen trapped by cementite other than dislocation and grain boundary. The pre-strained SCM435 steel contains additional hydrogen trap sites of vacancy and vacancy cluster together with micro-void. The V steel contains further two types of trap sites, regarding two types of V carbide. The amount of trapped hydrogen was able to be analyzed quantitatively by Fermi-Dirac statistics considering entropy change as pre-exponential factor. It was shown to be controlled mostly by de-trapping process and should be changed according to the binding energy of trap site. This qualitative analysis was favorable for restoring the released hydrogen before TDA measurement.
Itiswell-knownthatpearliticsteelwireshaveahigherresistancetohydrogenembrittlementthantemperedmartensiticsteels.Itissignificant to clarify the effect of various mechanisms of the hardening on hydrogen embrittlement for the compatibility between high strength and resistance to hydrogen embrittlement. In this study, effects of drawing strain and aging temperature in pearlitic steel wires on hydrogen embrittlement properties were investigated. Absorbed hydrogen content after cyclic corrosion test (H E) was increased with added drawn strain and saturated with large amounts of drawn strain. Furthermore, the higher the aging temperature, the smaller H E is obtained. The critical diffusible hydrogen content (H C) in pearlitic steel wires aged at 450°C is higher than that aged at 250°C and as-drawn pearlitic steel wires. The reasons are considered to be a decrease in the dislocation density and suppression of crack propagation due to the short length of ferrite-cementite interface. Consequently, the pearlitic steel wires aged at 450°C are excellent in resistance to hydrogen embrittlement because H C is much higher than H E , although the tensile strength of pearlitic steel wires is decreased by aging at 450°C and above.
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