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Hydrogen Effect on Cumulation of Failure, Mechanical Properties, and Fracture Toughness of Ni-Cr Alloys
Influence of hydrogen pressure and internal hydrogen contents on short-term strength, plasticity, and plane-stress fracture toughness of 05Cr19Ni55 alloys at pressure up to 30 MPa was investigated. It was established that the crack resistance parameters Kc of alloys decrease with displacement rates decreasing similar to elongation (δ) and reduction of area (ψ) of smooth specimens. The maximum hydrogen influence is achieved at strain rate speeds less than 0.1 mm/min and hydrogen pressures above 15 MPa when δ and Kc of prehydrogenated samples are reduced by 3 times. The plane-strain conditions required for the evaluation of KІc were fulfilled on compact tension 05Cr19Ni55 alloy specimens with thickness above 20 mm under hydrogen pressure 30 MPa and preliminary dissolved hydrogen concentration above 20 wppm. Regardless of the test conditions, the value of the characteristics of plasticity (δ, ψ) and fracture toughness (Kс) of alloy (TO1) specimens oriented in the transverse direction (orientation TV) is significantly lower than that of specimens oriented in the longitudinal direction (LT). Alloy cleaning by vacuum arc remelting and optimization of heat treatment regime increase their hydrogen resistance.
Heat-resistant nickel alloys are widely used in energy and aerospace engineering in contact with high-pressure hydrogen-containing gas mixtures [1–5]. Therefore, one of the most important requirements for such alloys is their resistance to hydrogen degradation. However, it is known that precipitation-hardening alloys are sensitive to hydrogen embrittlement [1–5]. So, for correct estimation of their workability in hydrogen environment, it is important to determine the effect of the structure and parameters of the load on the mechanical properties.
In order to ensure the safety and reliability of materials for a steam and gas turbine, structural integrity and lifetime prediction are of great importance. Working structures and their elements (blades and discs) are subjected to the influence of various loads and hydrogen-containing gas. In order to ensure an adequate level of safety and optimal durability of such structural elements, experimental tests in gaseous hydrogen are required to determine the effect of various factors. By the fracture mechanics approaches, evaluation of static load durability of structures and critical crack (defect) size in technological or operational circumstances have been calculated as the values of critical stress intensity factor KIc (Kc) [1, 3–6]. Use of this option to evaluate workability details in gaseous hydrogen is limited to a number of factors caused by the methodological aspects of the determination in laboratory conditions and characteristics of initiation and growth of cracks in the presence of hydrogen [3–9]. The materials employed in hydrogen energy should have little sensitivity to cracks and cuts that have a high ability to relax tension in the areas of highest concentration (high plasticity), which does not archive the plane-strain state for research specimens and determines KIc by standard methods in air [5–8]. Therefore, most studies of the impact of hydrogen on mechanical behavior of structural steels and alloys were carried out with tests on short-term static tensile and low-cycle durability [1–3, 10, 11]. The influence of load conditions (speed for static tension, frequency and amplitude by low-cycle fatigue, temperature, pressure, and dissolved hydrogen content in advance) on strength, ductility, and durability of materials of different structural classes was established [1–4, 11]. However, literature data on regularities of influence of hydrogen on the static crack resistance are limited and controversial [1–5]. The issue of the effect of sample orientation on the sensitivity of materials to the action of hydrogen remains poorly understood. The influence of texture components on hydrogen-induced cracking of pipeline steel has been studied in [12, 13], and similar data on heat-resistant materials are limited and ambiguous [14, 15].
In what follows, we study the influence of high-pressure gaseous hydrogen on short-term strength, plasticity, and static crack resistance of a nickel-based alloy 05Cr19Ni55 with different modes of heat treatment and chemical composition variations.
2. Materials and Experimental Procedure
The alloy 05Cr19Ni55 from which parts used in energy, petrochemical engineering, and aerospace engineering products [16, 17] are manufactured for use in the hydrogen environment has been investigated. Chemical composition of investigated alloys is shown in Table 1. Alloying with niobium, vanadium, titanium, aluminum, and boron leads to the formation of carbides (Ni, Fe, Cr)23C6, borides Me3B2, and intermetallics Ni3(Al, Mo, Nb) in the amounts up to 10%, which substantially increases their high-temperature strength and significantly influences the sensitivity to the action of hydrogen [1–3, 9, 11]. Presented in Table 2 and in Figures 1 and 2 are the results obtained in the first version of the chemical composition (CC).
TV: transverse orientation; LT: longitudinal orientation; OM: open melting; VAR: vacuum arc remelting; d: grain size; t: thickness of the specimen. βψ = ψhydr/ψair; βКС = Kc hydr./KIc(Jc) air.
The orientation of specimens, method of melting, heat treatment regimes, grain sizes, thickness of the specimens for fracture toughness testing, the mechanical properties of alloy in the air and in hydrogen under the pressure of 30 MPa after preliminary hydrogenation (623 K, 30 MPa H2, 10 h), and coefficients of influence of hydrogen on reduction of area and KIc are shown in Table 2.
Static tensile tests were carried out on standard fivefold cylindrical specimens with a diameter of working part 5 mm using the strain-rate range V = 0.01–100 mm/min. Stress intensity factor under static loading Kc was computed either for the maximum force FC in the “F-V” linear diagram or for the force FQ determined by using 5% secant for nonlinear diagrams. Rectangular compact specimens were tested for eccentric tension under the pressures up to 30 MPa at a strain rate of 0.01–100 mm/min. The values of Kc are calculated from the Srawley–Gross formula . Plastic properties of the alloy in the air are so high (see Table 2) that plane-strain conditions on samples with a thickness of 20–25 mm are not implemented. Therefore, plane-stress fracture toughness was determined by the method of J-integral . Despite some reservations, this method is often used to evaluate static fracture toughness of plastic materials and for small thicknesses of parts of structures [1, 6, 7]. For tests, we have used the 50x48x20 mm specimens with previously groved at 293 K fatigue cracks.
Specimens were hydrogenated in a working chamber at 623 K and hydrogen pressure 30 MPa during 1–10 h. The maximum hydrogen content in the specimens varied from 21 wppm to 27 wppm depending on the structural state of the alloy (see Table 2). Hydrogen concentration (CH) was determined with a LECO TCH 600 instrument  with precise 0.1 wppm (3 specimens for point). To determine the indicated mechanical characteristics in hydrogen, the working chambers were preliminarily evacuated, blown-out with hydrogen, again evacuated, and filled up with hydrogen to a given pressure. All tests were performed at room temperature.
3. Results and Discussion
3.1. Influence of Loading Rate on the Cumulation of Failure and Properties in Hydrogen
The behavior of the rate dependences of hydrogen degradation is determined by the kinetics of penetration of hydrogen into the material and changes in the mechanisms of deformation. As the rate of short-term tension decreases, the influence of hydrogen first increases, and then the properties become stable (see Figure 3). Under a constant hydrogen pressure and content, the rate interval and the degree of maximum embrittlement are determined by the chemical composition and the structural state of the material. Substantial reductions in ductility characteristics of the alloy with CC I (see Table 1) after HT III (see Table 2) were found in the range of rates 0.1–10 mm/min and with CC II after HT II in the range 0.1–1 mm/min (see Figure 3, curves 1, 4 and 3, 6).
The maximum embrittlement of hydrogenated material in hydrogen under a pressure of 30 MPa is realized at a rate less than 0.1 mm/min (0.67·10−4·s−1), which is three orders of magnitude smaller than the regulated rate of determination of the hydrogen resistance of steels with a body-centered cubic structure equal to 10 mm/min [4–7, 11, 12]. This ratio of the rates correlates with the ratio of the diffusion coefficients of hydrogen in alloys with face-centered cubic and body-centered cubic lattices given in [1, 4–7, 11, 21, 22], which testifies to the penetration of hydrogen into the metal in the process of static tension.
Similarly, the level of Kc decreases with decreasing dK/dt to the minimum value, defined of material structural condition (see Figure 1). The widest rate range and greatest sensitivity to the action of hydrogen typical for longitudinal samples after open melting HT1 with a coarse-grained structure (see Figure 1, curve 1). Maximum embrittlement of these specimens and specimens with grain size 36 microns are achieved by loading rate 0.18 MPa·m1/2/s (see Figure 1, curves 1 and 8), a grain size of 21 microns, and the largest fracture toughness in the air −0.054 MPa·m1/2/s (see Figure 1). At the same time, if in air with increasing thickness of the sample, parameter Kc slightly increases, then in hydrogen, it decreases (see Table 2, positions 5, 6, 7, and 8); i.e., the embrittlement effect of hydrogen on the fracture toughness of the alloy increases.
3.2. Influence of Hydrogen Pressure on Fracture Toughness
The dependences of the fracture toughness of prehydrogenated specimens on the hydrogen pressure consist of two regions. In the first region (low pressures), the pressure abruptly drops, and in the second, the negative action of hydrogen becomes stable (see Figure 2). This means that there exists a pressure under which degradation of the material with hydrogen reaches its maximum. In our case, this effect is found for pressures more than 15 MPa. Note that the properties of nonhydrogenated specimens of the alloy 05Cr19Ni55 do not reach the minimum values in the pressure range 0–35 MPa .
In hydrogen under the pressure 30 MPa, Kc of the alloy (CC I after HT II and HT III) of the thickness 20 mm decreases to 68 MPa·m1/2 and 57 MPa·m1/2 at the hydrogen concentration of 19 and 18 wppm, respectively, and does not change with further hydrogenation (see Figure 4, curves 5 and 8). Under these conditions, the diagrams “load–displacement” are linear with a sharp maximum of the load and fracture surface of specimens covered cleavage facet, typical for brittle fracture. Kc value reaches the critical value K1c that satisfies the condition l, b ≥ 2, 5 (/)2, where l is the crack length and b is the specimen thickness [18, 19]. Thus, hydrogen initiates the destruction of the mechanism of normal separation across the whole crack front, causing formation of the plane-strain state in the specimen. Conditions of the plane-strain state on all modifications of the alloy are performed by the specimen thickness 20 mm with the exception of positions 5 and 7 in Table 2. KIcH alloy with CC I after VAR, HT II (position 5) is equal to 64 MPa·m1/2 and achieved on specimens of thickness 25 mm (see Figure 4, curve 6). For a specimen thickness of 10 mm, plane-strain conditions for the alloy (CC I after HT III) did not perform even after the hydrogenation to the CH = 21 wppm (see Figure 4, curve 7). Thus, the maximum effect of hydrogen on fracture toughness of specimens with lower thickness is achieved by more intensive modes of action (hydrogen pressure or content) or slower loading rates (see Figure 1), when conditions are provided to allow sufficient hydrogen to enter the crack tip. At the same time, if in air with increasing thickness of the sample, parameter Kc slightly increases, then in hydrogen, it decreases (see Table 2, positions 5, 6 and 7, 8); i.e., the embrittlement effect of hydrogen on the fracture toughness of the alloy increases.
3.3. Influence of Metallurgical Factors on the Alloy Properties
Strength, plasticity, and fracture toughness of the alloy 05Cr19Ni55, melted in vacuum and open electric furnaces, were compared. For the same heat treatment, the alloy after vacuum arc remelting has much higher plasticity than after open melting (position 2, 3 and 4, and 5 in Table 2). Significantly higher are all crack resistance parameters: K1c(Jc) and Kc on air and K1c prehydrogenated specimens in hydrogen (see Figure 5). The resulting open smelting fine-grained structure under the same conditions absorbs the largest number of hydrogen (position 4 in Table 2). To establish the reasons for increasing the properties of the vacuum alloy was determined gases content (by vacuum melting) and impurities (spectral method). Also conducted chemical and X-ray analysis of phases and studied the structure. It was found that open and vacuum melting alloy containing an identical amount of nitrogen have the same lattice parameters of γ- and γ′-phases.
Chemical composition and quantity of intermetallics and carbide phases did not differ. Grain sizes after HT were practically the same (see Table 2, positions 2, 3 and 4 and 5). Difference between metal open and vacuum melting was found in the analysis of phosphorus (decreases by 2 times), sulfur (decreases by 1.5 times), and gases. According to the gas analysis, specimens after vacuum arc remelting contained less (2 times) amount of gases than after open melting, mainly due to hydrogen (3–8 times) and oxygen (2 times) content. Thus, the high content of sulfur, phosphorus, hydrogen, and oxygen affects the characteristics of the nickel alloy 05Cr19Ni55 in air and hydrogen and increases the hydrogen embrittlement.
The relationship between mechanical properties of the alloy and texture that appeared as a result of rolling was investigated. In air, the value of the characteristics of plasticity (δ, ψ) of cylindrical specimens of the alloy (TO1) oriented in the transverse direction (orientation TV) is significantly lower than that of the specimens oriented in the longitudinal direction (LT) (see Table 2, positions 1 and 2). Transversely oriented specimens are more sensitive to hydrogen than longitudinal: δ decreases in 4 and 3.1 times, ψ decreasesin 2.7 and 2.25 times.
Resistance to fracture of the alloy also depends on the orientation of the specimens.
The value of static fracture toughness of compact specimens of 20 mm thickness with cracks oriented in the transverse orientation in air was Kc = 69 MPa·m1/2 and KIc(J) = 86 MPa·m1/2, respectively, which is approximately 20% smaller than that of specimens with cracks oriented in the longitudinal orientation (see Table 2, positions 1 and 2).
In hydrogen, characteristics of crack resistance transversely and longitudinal oriented specimens are 28 and 38.5 MPa·m1/2. Similar results were obtained in the study of hydrogen embrittlement of steels 4130 and 4310 and other materials [22–24].
The most likely reason for anisotropy of mechanical properties can be anisotropic structural boundaries in the alloy, i.e., the dependence of fate crack length, falling on the structural boundary, and on the orientation of the applied load. The accumulation of hydrogen at the grain boundaries makes it easy to crack, while at transverse-loading large areas such boundaries are under the influence of normal stresses .
The difference in the degree of hydrogen embrittlement of the TV and LT samples is also due, probably, to the formed rolling process by the dislocation texture, which is decisive in hydrogen cracking [12, 13]. In addition, it is known that the effective rate of hydrogen diffusion depends substantially on the orientation of the samples .
3.4. Influence of Heat Treatment Regimes and Chemical Composition on the Alloy Properties
Effect of dispersion structure of the alloy on strength, plasticity, and fracture toughness in air and hydrogen was studied. Quenching temperatures were 1253 K, 1283 K, 1323 K, and 1473 K, grain sizes is equal to 21, 36, 85, and 130 microns, respectively, and the average thickness of grain boundaries is 1, 2, 3, and 4 microns (see Figure 6).
The method of J-integral does not recommend to evaluate the materials static crack resistance in plane-strain conditions. In the absence of large local stresses during the short-term strength tests, arise the minimal embrittlement effect of hydrogen.
For the same aging regimes with decreasing grain size, parameters of plasticity and static crack resistance increased in air and hydrogen (see Table 2, Figure 2, positions 3, 5, 8). At the same time, strength characteristics differ insignificantly. The best combination of mechanical properties in air and greatest resistance to hydrogen embrittlement was observed on longitudinal samples after VAR and HT II (see Table 2, position 4, 5). In this case, the fine-grained structure with equally distributed over the body of grain particles of γ′-phase is obtained. Additional reduction of carbon, nitrogen (from 0.12 to 0.07 mass %), and total content of chromium, molybdenum, niobium, and iron (see Table 1, CC II) leads to the increasing of alloy hydrogen resistance and fracture toughness in air and hydrogen (Table 3).
Probably, this effect is due to a reduction from 3.1 to 2.0 mass % quantity of carbides and carbonitrides (see Table 3, Figure 7), which promote brittle fracture and hydrogen embrittlement of nickel alloys [1–3, 17–19]. It has been found that at low deformation rates (∼1 mm/min) and large hydrogen pressures (∼3 MPa), intergranular destruction and destruction along the boundaries of the carbide matrix occur with cracking of carbides (Figures 8(a) and 8(b)). The change in the nature of the fracture from transgranular to intergranular under the action of hydrogen is a characteristic feature of dispersion-hard austenitic steels and alloys due to the concentration at the grain boundaries of carbides, intermetallics, and hydrogen [1, 2, 4, 5, 24–27].
(1)With the decrease of strain rate from 100 to 0.1 mm/min, plasticity characteristics of the Ni alloy specimens in hydrogen are reduced in the interval rates 0.1–0.01 mm/min. Similarly, the level of Kc decreases with decreasing dK/dt to the minimum value, defined of material structural condition and cumulation of failure.(2)The values of hydrogen pressure and hydrogen content by which has achieved the maximizing effect on investigated alloys fracture toughness has been established. Compact specimens thickness increasing leads to the hydrogen embrittlement increases. With compact specimen thickness increasing, the hydrogen embrittlement increases.(3)After cumulation of failure, hydrogen has initiated the fracture by the mechanism of normal separation across the whole crack front, causing formation of the plane-strain state in the specimens.(4)The alloy properties depend on the orientation of the samples. Strength, plasticity, and fracture toughness in air and hydrogen in transversely oriented specimens are much lower than those in longitudinally oriented specimens.(5)Vacuum arc remelting, formation of fine structures of the thin grain boundaries, and a minimum number of carbides and carbonitrides have increased fracture toughness and hydrogen resistance of the alloy.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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