Influence of Hydrogen on the Fracture Resistance of Pre-Strained Steam Generator Steel 22K

Abstract

In the paper, experimental studies of the hydrogen and pre-strained effect on fracture resistance of steam generator steel 22K were carried out. Special cylindrical samples were loaded up to fracture under a uniaxial tensile test with different pre-strained degrees and hydrogen charged times of the material. Stress-strain curves «Si–ei» were plotted. The true strain «ei» in the local volume was determined using the method of optical-digital image correlation (ODIC). The results showed that the hydrogen influence is practically absent in the elastic area of strain. The fracture energy of steel 22K decreases under the hydrogen influence and pre-strain in all investigated cases. It is shown that during six months of air exposure, with the 0% pre-strained samples release almost all hydrogen. In pre-strained samples, the hydrogen concentration decreased by 1–3% compared to the initial values. This indicates that they have trapped hydrogen that cannot escape on its own. Hydrogen embrittlement (HE) indexes for 0% pre-strained samples at different levels of hydrogen-charging calculated by the strain and energy approaches are equal to each other. There is a difference in the values of the HE index depending on the hydrogen-charge time for pre-strained samples. This indicates that both strain and strength characteristics of the material, which are integrally taken into account in the energy approach, are sensitive to HE.

A New Energy Approach to Predicting Fracture Resistance in Metals

“…A technique to plot stress-strain curves based on the non-contact ODIC method was used to determine true strains in the stress-strain homogeneous region of a sample. Additional information regarding the geometry, load modes, and materials of the modified Bridgman sample is described in detail in our previous paper [ 18 ]. Modified Bridgman cylindrical steel 22K samples were created to plot fracture diagrams ( Figure 4 a) using a quantity of three samples for different research parameters.…”

“…The tests were carried out for thermal energy steel of type 22K, for which the yield strength was and the ultimate strength was . In our previous work [ 18 ], the stress–strain curves of the modified Bridgman samples of this steel were plotted at different levels of prestrain and hydrogenation time using the ODIC method. In this article, we considered four load modes: The tensile load to Pmax = 12,340 N. The sample was preloaded to a force of 5200 N that resulted in stresses equal to the yield point in the sample, then unloaded, and after 2 min, it was reloaded.…”

“…Calculations were performed for the modified Bridgman sample made of steel 22K ( Figure 2 a), which was tested experimentally in other work [ 18 ]. Taking into account the symmetry of the sample, a three-dimensional calculation model ( Figure 2 b) was built for half of the sample relative to the longitudinal plane of symmetry ( Figure 2 c).…”

“…With the introduction of the ODIC method into the experimental and FEM into the theoretical mechanics of materials, it became possible to precisely study the local strain, not only on the surface but also on the internal volumes of the material. Experimental studies [ 8 , 18 ] showed that the damage accumulation and fracture of cylindrical steel samples under static tension begin in the internal volumes of the material and not from the surface. This is due to the inhomogeneity of the stress-strain state in the zone of neck formation [ 19 , 20 ].…”