Static Crack Resistance of Heat-Resistant KhN43MBTYu Nickel-Chromium Alloy in Gaseous Hydrogen
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STATIC CRACK RESISTANCE OF HEAT-RESISTANT KhN43MBTYu NICKEL-CHROMIUM ALLOY IN GASEOUS HYDROGEN O. I. Balyts’kyi,a,b,1 L. M. Ivas’kevych,a,2
UDC 620.197:669.788
b,3
and J. J. Eliasz
The effect of hydrogen at a pressure of up to 35 MPa and a content of up to 29 ppm on the strength, ductility, short-term and long-term static crack resistance of four KhN43MBTYu (EP-915VD) alloy modifications with different heat treatment modes and chemical composition has been studied. It has been found that the critical stress intensity factor K Ic in the presence of hydrogen, just as the ductility characteristics of smooth specimens, depends on the deformation rate, reaching minimum values at rates of less than 0.1 mm/min. The fracture toughness decreases under the action of hydrogen by a factor of 2.5, and the plane strain state occurs at a much smaller specimen thickness. An optimal combination of high strength, ductility, short- and long-term static crack resistance in air and hydrogen has been achieved in a fine-grained alloy with low carbon and sulfur content. Based on the results of long-term static crack resistance tests at the predetermined maximum fatigue test duration of 300 h, the invariant characteristics of crack resistance, the threshold values of stress intensity factor in hydrogen, have been determined, which vary from 23 to 48 MPa × m1 / 2 depending on the alloy heat treatment mode. Keywords: carbides, intermetallics, short-term and long-term static crack resistance, hydrogen brittleness, heatresistant nickel alloy. Introduction. The critical stress intensity factor (SIF) K Ic under short-term load and the threshold SIF value K ISCC (K IHST in hydrogen) under the long action of an aggressive environment are important characteristics of structural materials, which are used to assess the reliability and life of engineering structures if the part has technological or service-induced cracks [1–6]. The experimental determination of these parameters in gaseous hydrogen is complicated by the fact that to ensure self-similarity conditions in the case of ductile and slightly crack and notch sensitive hydrogen-resistant steels and alloys, large specimens must be used [1–5]. Therefore, the hydrogen brittleness of materials in gaseous hydrogen at high pressure was generally assessed from the results of short-term strength and low-cycle life tests [1, 2, 7–10]. The existing equipment [11] allows one to determine the crack resistance parameters and permissible operating conditions of materials. Three main parameters play an important role in hydrogen embrittlement: (i) hydrogen content and condition, (ii) the chemical composition and structural state of the alloy after heat treatment, and (iii) the value of stresses (induced by internal or external service loads) in the workpiece. Changing the chemical composition and structure of the alloy determines largely its strength, ductility and fracture toughness and hence affects hydrogen embrittlement susceptibility. Dopants change the phase composition and structure, affect the gra
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