Degradation of steel in gaseous hydrogen with inhibiting admixtures

We have investigated the possibility to protect metal constructional elements of power-generating equipment against hydrogen embrittlement by introducing O₂ and CO + H₂ O admixtures into the working volume. We have shown their positive influence on the mechanical properties of 03Kh12N10MT steel and KhN60K16MBYu alloy under short-term and long-term static tension within the temperature range 293–1073 K in gaseous hydrogen with a pressure of 35 MPa. However, we have not detected an inhibiting effect of the admixtures mentioned above in the case of preliminary high-temperature hydrogenation of unloaded smooth and notched specimens made of 05KhN23MTR steel and KhN60K16MBYu alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 71–75, May–June, 2007.     

Effect of Hydrogen on the Mechanical Properties of Welded Joints of 03Kh12N10MT Steel and KhN55MBYu Alloy

We have studied the hydrogen degradation of welded joints of 03Kh12N10MT steel and KhN55MBYu alloy, used in the aerospace industry. We have established that the highest level of strength, plasticity, and low-cycle durability in gaseous hydrogen under a pressure of 35 MPa is provided by argon-arc welding for joining homogeneous materials and by electron-beam welding for dissimilar materials.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 55–61, November–December, 2004.     

Antifriction properties of heat-resistant alloys in hydrogen

We investigate the influence of hydrogen on friction properties of heat-resistant KhN55MBYu alloy and establish the main regularities of variation in the sliding friction coefficient in the process of operation of a shaft-bush couple depending on the temperature and pressure of hydrogen as well as on the rate and type of loading. Some recommendations concerning the efficient use of certain plasma and self-lubricating coatings for the operation of valve hydrogen engines are given. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 2, pp. 108–112, March-April, 2000.     

Distinctive features of hydrogen degradation of heat-resistant alloys based on nickel

We present the comparative data on the resistance to hydrogen degradation of three oodifications of dispersion-hardened heat-resistant KhN60MV alloy (in deformed, as-cast, and powder forms) under the conditions static tension in gaseous hydrogen under a pressure of 35 MPa in the temperature range 293–1093°K. We show that the powder modification is preferable under pressure of 0–35 MPa in the temperature range 293–1093°K. It exhibits a high level of strength, the lowest sensitivity to hydrogen degradation, and the maximum stability of plastic properties. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 4, pp. 115–120, July–August, 1997.     

Mechanical properties of martensitic steels in gaseous hydrogen

We present the regularities of hydrogen degradation of 03Kh12N10MT, 15Kh12N2MFAV and 13Kh11N2V2MF steels under a pressure of 30 MPa within the temperature range of 293–673 K. The minimum values for plasticity, low-cycle fatigue, and static crack resistance, which do not decrease with an increase in pressure of hydrogen atmosphere and content of the absorbed hydrogen, are found. The difference between temperature dependences of the coefficients of influence of hydrogen on static and cyclic crack resistance of martensitic steels with various content of austenite is established. The main fractographic features of the influence of hydrogen on the micromechanism of fracture of steels under different types of loading and temperatures are determined.     

Temperature dependences of the mechanical properties of austenitic and martensitic steels in hydrogen

The influence of gaseous and preliminarily dissolved hydrogen on the characteristics of short-term static strength, crack resistance, and low-cycle durability of martensitic and austenitic steels is studied within the temperatures range 293–1073°K, under pressures of hydrogen varying within the range 0–35 MPa, for the strain rates of 0.01–100 mm/min, and the strain amplitudes of 0.8–1.6%. We determine the loading rates and the conditions of action of hydrogen leading the maximum possible hydrogen degradation of each material. The influence of the chemical composition and structural state on the degree of embrittlement and fractographic features of the fracture of steels in the presence of hydrogen is analyzed. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 53–64, September–October, 2007.     

Triboengineering properties of austenitic manganese steels and cast irons under the conditions of sliding friction

We analyze the influence of parameters of the graphite phase on the ultimate strength in bending, deflection, and the intensity of fracture of manganese cast irons and steels. The least intensity of wear is exhibited by alloys with graphite inclusions 30–50 μm in length and a hardness of 250 HB. The decrease in the hardness of alloys, improvement of the parameters of the graphite phase, and increase in its amount (occupied area) to 6–10% lead to the disintegration of the metal matrix, decrease its strength characteristics, and promote the intensification of the fracture processes under the conditions of friction. It is shown that, unlike 40, 45, 45G2, and 20KhN hardened steels, the austenitic cold-worked manganese steel is capable of preserving the coating formed by the adsorbed layers of oil and graphite on the surface for 3–4 h under the conditions of shortage of oil. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 55–60, September–October, 2005.     

Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy

Compression properties of a refractory multi-component alloy, Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀, were determined in the temperature range of 296–1473 K and strain rate range of 10⁻¹–10⁻⁵ s⁻¹. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473 K for 24 h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter a = 340.4 pm. The grain size was in the range of 100–200 μm. During compression at a strain rate of έ = 10⁻³ s⁻¹, the alloy had the yield strength of 929 MPa at 296 K, 790 MPa at 673 K, 675 MPa at 873 K, 535 MPa at 1073 K, 295 MPa at 1273 K and 92 MPa at 1473 K. Continuous strain hardening and good ductility (ε ≥ 50%) were observed in the temperature range from 296 to 873 K. Deformation at T = 1073 K and έ ≥ 10⁻³ s⁻¹ was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473 K or a decrease in the strain rate to ~10⁻⁵ s⁻¹. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273 K and completed dynamic recrystallization was observed at 1473 K.     

Structure and properties of high-modulus alloys of the Ti-B system

We study the influence of alloying with Al, Zr, and Si on the structure and mechanical properties (including the modulus of elasticity) of cast and deformed alloys of the Ti-B system. It is shown that, by optimizing the compositions of titanium alloys subjected to combined silicoboride hardening and additionally alloyed with aluminum, it is possible to get the modulus of elasticity E as high as 160 MPa with a strength σ of 1500 MPa and a level of plasticity δ of 2–5% at room temperature. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 27–32, May–June, 2006.     

Effect of low-temperature deformation on the plasticity of aluminum alloys

We study low-temperature deformation of aluminum alloys and its influence on the characteristics of plasticity. We show that the admissible level of strains in the process of straightening after hardening thermal treatment increases with the plasticity of alloys in the temperature range 173–77°K and the plasticity margin remains constant. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences. L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 5, pp. 60–62, September–October, 1996.     

The high- and low-cycle fatigue behavior of Ni-contain steels and Ni-alloys in high pressure hydrogen

The effect hydrogen on short-term strength and plasticity, high- and low-cycle durability of 15Cr12Ni2MoNMoWNb martensitic steel, 10Cr15Ni27Ti3W2BMo austenitic dispersion-hardened steel, 04Kh16Ni56Nb5Mo5TiAl and 05Kh19Ni55Nb2Mo9Al Ni-base superalloys in range of pressures 0–30 MPa and temperatures 293–1073 K was investigated. In the case of 15Cr12Ni2MoNMoWNb steel and 04Kh16Ni56Nb5Mo5TiAl alloy the dependence of low-cycle durability (N) and characteristics of plasticity (δ and ψ) on the hydrogen pressure consists of two regions. In the first region (low pressures), the N, δ and ψ abruptly drops, and in the second, the negative action of hydrogen becomes stable or decrease negligibility. This means that there exists a pressure under which the degradation of this material with hydrogen reaches its limit. The additional effect of preliminary dissolved hydrogen on the properties of 15Cr12Ni2MoNMoWNb steel and 04Kh16Ni56Nb5Mo5TiAl alloy developed at hydrogen environment pressure least of 10 MPa. In the case of 10Cr15Ni27Ti3W2BMo steel and 05Kh19Ni55Nb2Mo9Al alloy the low-cycle durability N, characteristics plasticity δ and ψ decrease in whole hydrogen pressure range. Preliminary dissolved hydrogen leads to a considerable additional decrease in the properties of this alloy.     

Effect of the anisotropy of forge-rolling and thermal treatment in hydrogen on the magnetostriction of K50F2 alloy

We have developed a procedure for predicting changes in the magnetostriction of ferromagnetic alloys based on measurements of the parameters of maximum alternating-current electric resistance. We have manufactured a facility for measurements of the elongation of specimens by an optical method. Finally, we have studied the effect of temperature, annealing time, hydrogen treatment, and the anisotropy of forge-rolling on the magnetostriction of K50F2 alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 122–124, September–October, 2007.     

High-temperature hydrogen resistance of stainless steels

At elevated temperatures, the influence of hydrogen on various mechanical characteristics of martensitic and disperse-hardened austenitic steels is different. The maraging steel has better characteristics of durability and plasticity and the critical values of static and cyclic crack resistance at temperatures of 450–600°K than the austenitic steel with intermetallic hardening. As a result of the intense temperature softening, its ultimate and yield strengths are much lower than for the austenitic steel. The austenitic steel has higher resistance in terms of the threshold value ∆K _th. At room temperature, the low-cycle fatigue limit proves to be most sensitive to the action of hydrogen, whereas at 673°K, the parameter K _fc for the maraging steel decreases.     

Mechanical properties of chromium-manganese austenite steels in a hydrogen environment

We study the influence of gaseous hydrogen on the mechanical properties of hardened and deformed specimens of 07Kh13G20AN4 and 03Kh13N9G19AM2 steels under a pressure of 35 MPa in the temperature range 293–773 K. We establish that hydrogen causes the strain martensite transformation of nitrogen-containing austenite stable in air. Formation of a polygonal dislocation substructure by means of preliminary mechanical or thermomechanical treatment significantly decreases the tendency of steels toward hydrogen degradation. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 5, pp. 75–78, September–October, 1999.     

Strength of Alloys Based on Iron,Nickel, and Titanium in High-Pressure Hydrogen

The influence of high-pressure hydrogen at room temperature on the static and fatigue properties of corrosion-resistant materials based on iron and nickel is compared for different types of loads. The sequence of characteristics can be arranged in the order of increase in the influence of hydrogen as follows: fatigue limit, ultimate strength of specimens with concentrators, relative elongation of smooth specimens, fracture pressure for a membrane under biaxial tension, percentage reduction of the area of smooth specimens, percentage reduction of the area of specimens with concentrators, and low-cycle durability. The dependences of the intensities of true stresses on the intensities of true strains (in uniaxial and biaxial tension) reveal the difference between the curves plotted in air and in hydrogen. The diagrams of hydrogen resistance taking into account the strain rates are presented. The sequence of investigated types of steel and alloys can be arranged in order of decrease in the low-cycle hydrogen resistance as follows: annealed stable austenitic and iron-nickel alloys, nitrided Cr-Ni-Mn steels, titanium alloys of the Ti-Al-Sn and Ti - Al -V systems, steels with nonstabilized austenite, high nickel alloys, maraging steels, and ferritic steels. The micromechanism of hydrogen fracture is characterized by the presence of a great number of microstructural fracture sites (bulk damage) and the completeness of local plastic relaxation. New procedures aimed at increasing the hydrogen resistance of steels and alloys are proposed, namely, the thermocyclic annealing with shot-term overheating, additional compression (with an optimal value of about 20%), and changing the temperature of aging.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 7–18, November–December, 2004.     

Effect of radiogenic <Superscript>3</Superscript>He and hydrogen on the mechanical properties and structure of 12Kh18N10T steel

We present some results of studying the influence of high-pressure hydrogen (80 MPa), radiogenic ³He (with concentrations up to 130 appm), and their joint action on the mechanical properties and structure of 12Kh18N10T steel in the temperature range from 293 to 873 K. We describe the procedure of tests of specimens containing ³He. It has been established that the joint action of hydrogen and ³He affects slightly the ultimate strength of the specimens. Saturation of steel with radiogenic ³He by the method of “tritium trick” increases its yield strength. Hardening of the steel caused by helium increases with temperature and ³He concentration and, at 873 K, is accompanied by substantial embrittlement. We also present results of the fractographic analysis of specimens tested under different conditions. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 47–52, September–November, 2007.     

Phase transition and mechanical properties of constraint-aged Ni–Mn–Ga–Ti magnetic shape memory alloy

Ni–Mn–Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni–Mn–Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni–Mn–Ga–Ti alloy and about 900 MPa in Ni–Mn–Ga–Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni–Mn–Ga–Ti alloy.     

Effect of iron additions on mechanical properties of Co3 Ti alloy

High temperature tensile tests were carried out on L1₂ type Co₃ Ti alloys, both undoped and doped with 1–4 at.-%Fe. There were anomalous increases of the 0·2% yield stress (yield strength) with increasing test temperature from 473 to 1073 K (or 1173 K, depending on the composition). The elongation and ultimate tensile stress (UTS) monotonically decreased with increasing temperature. The fracture surfaces of specimens showed a variety of fracture modes which were dependent on the test temperature and composition. There was a correlation between the ductility and the fracture mode: the more transgranular the fracture mode, the higher the ductility. It was found that Co₃ Ti with 2 at.-%Fe exhibited improved ductility and it exhibited the highest peak value of yield strength and peak temperature. The alloys were also hydrogen charged to investigate their hydrogen embrittlement behaviour. Room temperature tests indicated that the addition of 2 at.-%Fe decreased the hydrogen related embrittlement.

MST/3479     

Fatigue-Crack Propagation in Heat-Resistant Alloys under Thermomechanical Loading. Part 1. Experimental Method and Results of the Investigation of Crack-Propagation Rates

We propose a calculation and experimental method for studying the influence of cyclic thermomechanical loading, wherein the object under investigation is subjected to the simultaneous action of a cyclically varying mechanical load and temperature varying with time and resulting in the occurrence of thermal stresses, on fatigue-crack propagation in specimens of triangular cross section. We present results of the calculation of the stress-intensity factors in elastic formulation for an angular mode-I crack in the case of loading by pure bending and temperature varying with time. We studied crack-growth resistance of alloys KhN70VMTYu (Eacute;I617) and KhN73MBTYu (Eacute;I698) at a constant temperature and under thermomechanical loading. Kinetic fracture diagrams da/dN vs K_I have been constructed for the alloys at a constant temperature and for two regimes of thermal cycling within a near-threshold and mean-amplitude portions of the curve da/dN vs K_I .     

Microstructure and mechanical properties of Mg–Gd–Zr alloys with low gadolinium contents

Mg–xGd–0.6Zr (x = 2, 4, and 6% mass fraction) alloys were synthesized by semi-continuous casting process. The effects of gadolinium content and aging time on microstructures and mechanical properties of the Mg–xGd–0.6Zr alloys were investigated. The results show that the microstructures of the as-cast GKx (x = 2, 4, and 6%) alloys are typical grain structures and no Gd dendritic segregation. In as-cast Mg–6Gd–0.6Zr alloy, the second phases Mg_5.05Gd, Mg₂Gd, and Mg₃Gd will form due to non-equilibrium solidification during the casting process, and these second phases will disappear after hot-extrusion. The residual compressive stress exists in alloys after extrusion and increases with increasing Gd content. The existence of residual compressive stress contributes to the tensile strength. The elongation of all extruded alloys is over 30%, and the ultimate and yield tensile strength of the Mg–6Gd–0.6Zr alloy are 237 and 168 MPa, respectively. After isothermal aging for 10 h, the strength of extruded Mg–6Gd–0.6Zr alloys increases slightly, however, the elongation of alloys rarely decreases. The fracture mechanism of all studied alloys is ductile fracture.