Hydrogen effects on mechanical performance of nodular cast iron

Abstract

The KBS-3 method for long-term disposal of spent nuclear fuel is designed with an external self-standing copper shell, which provides the most important barrier against corrosion and escape of radionuclides, and an internal nodular cast iron insert, which provides the load-bearing structure against external loads. The material intended for the load-bearing insert is ferritic nodular cast iron EN 1563 grade EN-GJS-400-15U. In this paper, hydrogen uptake and sensitivity to hydrogen-induced cracking of the cast iron were studied using tensile testing under continuous electrochemical charging in 1 N H2SO4 solution. Hydrogen uptake was measured by using the thermal desorption method. It was found that the hydrogen desorption profile manifests three distinct peaks at initial locations of 400, 500, and 700 K with a heating rate of 6 K/min. Plastic deformation results in a remarkable increase of the 400 K peak, which indicates hydrogen uptake during deformation. In the constant extension rate tests (CERT) and the constant load tests (CLT), electrochemical hydrogen charging reduced markedly the elongation to fracture and time to fracture, respectively. In CLT, hydrogen charging increased dramatically the creep rate at the applied load of about 0.7 yield stress. Ligaments between the graphite nodules exhibit brittle cleavage facets in the presence of hydrogen, while the ligaments show a characteristic ductile appearance of shear and small dimples when testing in air or distilled water. The obtained results are discussed in terms of the known mechanisms of hydrogen-induced cracking and the role of the graphite nodules in the embrittlement of ductile cast iron.

Hydrogen embrittlement of nodular cast iron

“…The as‐supplied state of the material was studied previously and it was found that the studied DCI showed two peaks, one around 500 K and the other one at 700 K. [ 4 ] These peaks correspond with the existing metallurgical hydrogen of about 2 wt ppm in the studied DCI. In tensile loading, a peak appeared at about 400 K and the 500 K peak shifted closer to 560 K with a large increase in its magnitude under electrochemical hydrogen charging.…”

“…The magnitude of the 400 K peak also markedly increased and the location shifted towards 450 K in tensile testing with hydrogen charging. [ 4 ]…”

“…The CLT and SSRT tests show that under continuous hydrogen charging the mechanical properties of DCI are markedly reduced compared with performance in the air showing similar behavior as in the previous study. [ 4 ] Fractography after CLT shows the separation of graphite nodules from the ferrite matrix with the faceted interface of the cavities and cleavage fracture in ferrite between the graphite nodules (Figure 11a,b). In the air without hydrogen (Figure 10), ductile fracture occurs forming small dimples in the ferrite ligaments typical for a ductile material such as DCI.…”

“…The CLTs were performed in two stages: preloading with 10 −4 s −1 strain rate up to load of 160 MPa (about 0.5 tensile strength of the studied DCI) as in the previous study. [ 4 ] After preloading, the load was kept constant for a predetermined time or until fracture. The hydrogen uptake of the material was then immediately measured by using TDS.…”

“…Recent studies of hydrogen effects on the mechanical performance of DCI have shown a remarkable sensitivity to hydrogen in the form of hydrogen embrittlement and hydrogen‐enhanced creep. [ 4 ] There are some possible internal sources of hydrogen in the canister in addition to the metallurgical hydrogen originating from manufacturing: residual water from the spent nuclear fuel assemblies, the amount of which left in each canister in accordance with the design case must be <600 g, [ 1,3 ] and hydrogen produced by (n, p) reactions and hydrides formed in the zirconium–alloy fuel assemblies. [ 5 ] The effects of hydrogen on all canister materials, including DCI, have to be known and taken into account in the safety analysis of spent nuclear fuel disposal.…”