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.     

Endurance of structural materials in complex low-cycle loading

The results of examination of the relationships governing damage cumulation on complex low-cycle loading are presented. A phenomenological criterion of evaluating the kinetics of damages cumulation in the structure of the material is proposed for this purpose. The components of the deviator of additional stresses is determined on the basis of the mutlisurface theory of plastic yielding with Pisarenko-Lebedev loading surfaces. A method of determining the mean parameters of the model is described. The calculated endurance data are compared with the experimental values for VT14 alloy and 14Kh17N2 steel at T=293°K. Experiments were carried out on tubular specimens in the conditions of piecewise-polygonal trajectories of low-cycle deformation.Translated from Problemy Prochnosti, No. 3, pp 3–9, March, 1991.     

Loss of stability for the process of plastic deformation with a complex stressed state

Basic preconditions are presented for considering the stability of the plastic deformation process in evaluating the ultimate deformation and strength properties of structural materials in tubular specimens under conditions of a complex stressed state. Calculated dependences are provided for calculating these characteristics. Results of experimental studies are analyzed for heat-resistant steels 15Kh2MFA, 15Kh2NMFA, and VK-2 (KP-100) (the latter simulates radiation embrittled steel 15Kh2NMFA) with different principal stress ratios in the temperature range 20–350°C. Calculated values of ultimate stresses and strains are compared with experimental data. The possibility of using the results of experiments carried out with stepwise loading as a basis for calculating the endurance under cyclic loading conditions with a complex stressed state is substantiated.Translated from Problemy Prochnosti, No. 10, pp. 3–8, October, 1991.     

Strength in low-cycle loading of high-nickel alloy being irradiated

The results are given of an investigation of low-cycle fatigue of the austenite class 03Kh20N45M4BCh chromium alloy, realized in its application to the selection of materials for the manufacture of structural elements of the thermonuclear reactor discharge chamber. The loading regime consists of alternating elastoplastic strain by twisting a thin-walled tubular specimen with holdings of 200-sec duration. Radiation tests were carried out in a materials science channel of the WR-M (water-cooled and water-moderated) reactor of the Institute of Nuclear Research at the Academy of Sciences of the Ukrainian SSR in a flux of fast neutrons 4.5 ' 10¹³ neutron · cm² · sec. The effect of neutron irradiation on strain and strength properties of the investigated alloy are evaluated. Comparison is made of the data on the effect of irradiation on the low-cycle fatigue of some steels.Translated from Problemy Prochnosti, No. 2, pp. 46–50, February, 1990.     

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.     

Low-cycle fatigue of welded joints of 08Kh18N10T steel

We present systematized experimental data on low-cycle fatigue strength at 20 and 350°C for 08Kh18N10T steel and its welds, made by manual electric arc welding with EA-400/10U electrodes and argon-arc welding with a nonconsumable electrode and using Sv-04Kh19N11M3 filler wire. The data obtained are compared with the “Standards for Strength Analysis” PNAé G-7-002-86. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 104–109, January–February, 2007.     

Deformation resistance and limiting state of structural materials in multiaxial low-cycle deformation

The results are presented of investigations into the relationships governing low-cycle fatigue of VT14 alloy and 14Kh17N2 steel at T=290 K under the conditions of proportional and nonproportional rigid low-cycle loading. Symmetric and pulsed (nonzero) deformation cycles were examined. The limits of applicability of the determining equation in the form of a generalized diagram of low-cycle loading for the case of piecewise-broken flat trajectories were determined. The results show that strain asymmetry has a strong effect on the endurance (in the stage of macrocrack formation) because of the special features of the deformation resistance of structural materials in the presence of a mean strain and the effect of the properties of cyclic hardening and softening.Translated from Problemy Prochnosti, No. 10, pp. 11–16, October, 1990.     

Structural strength of steels of the austenitic and transition classes at low temperatures

The article investigates the structural strength of the metastable austenitic steel 03Kh16N9M2 and of the steel of the transition class (martensite + residual austenite) 05Kh14N5DM in the temperature range from 293 to 77°K for the purpose of determining their suitability for large components in cryogenics. It is shown that for their characteristics of impact toughness and low-cycle fatigue steel 05Kh 14N5DM containing up to 15% residual austenite cannot be recommended as material of cryogenic structures operating at 77°K.Translated from Probiemy Procnnosti, No. 5, pp. 43–50, May, 1993.     

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.     

Development of an austenitic chromium-manganese steel with high hydrogen resistance

We compare the results of studying the effect of hydrogen on the characteristics of short-time strength and low-cycle durability of austenitic stainless steels and age-hardening alloys. We show that austenitic stainless 06Kh20N16G6AF steel with solid-solution strengthening and the optimal content of nickel, manganese, and nitrogen possesses high mechanical characteristics and is least inclined to hydrogen degradation. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 95–100, September–October, 2005.     

A Method for the Evaluation of Fatigue Life under Nonproportional Low-Cycle Loading

We consider a method for the evaluation of fatigue life under multiaxial nonproportional low-cycle loading based on the concept of equivalent strains. The expression for the equivalent strain range is a function only of the strain path and contains a constant depending on the additional hardening of the material under nonproportional loading. We propose a new parameter of the material based on the work of plastic strains in a cycle. This parameter is universal when applied to materials with both low and high degrees of additional hardening. It is in good agreement with the results of testing of 08Kh18N10T stainless steel and VT9 titanium alloy under nonproportional low-cycle loading.     

Factors of estimation of stress states,durability, and resource of critical threaded connections

Factors influencing the estimation of stress-strain states and characteristics of the durability and resource of critical threaded connections used in various fields of mechanical engineering is studied taking into account the conditions of their exploitation. It is shown that the durability and the resource of such threaded connections are defined using their static and cyclic damages under the action of a difficult combination of static, high-cycle, and low-cycle operational loadings. Both calculation and experimental research methods can be used for the estimation of local stresses and strains in thread turns and their redistributions in the loading process. Approaches based on the use of nonlinear deformation rules (including the elastoplastic area), regularities of accumulation of defects, and equations of nonlinear mechanics of the failure have to be the basis for the technique of the specified calculations. Questions of the damageability of elements of critical threaded connections taking into account their possible deformation in local zones (zones of the stress concentration) in the elastoplastic area are associated with the performance of the corresponding calculations of cyclic durability for areas of both multicycle and low-cycle fatigue.     

Low-Cycle Durability of Steels in High-Pressure Hydrogen

We deduce the dependences of low-cycle durability of 11 corrosion-resistant steels from basic structural classes on the pressure of gaseous hydrogen within the range 0.1–35 MPa at 293°K. It is discovered that the highest serviceability in hydrogen is exhibited by homogeneous austenitic steels and steels with intermetallic hardening. The durability of Ni–Al–Ti steels treated for maximum heat resistance in hydrogen is proportional to the content of Ti and Ti//Al ratio and depends on the conditions of treatment. Steels in the state of substitutional solution are not affected by the medium. The materials from the transient class and steels with unstable austenite are characterized by medium sensitivities to hydrogen. High-strength martensite-aging and ferritic steels exhibit catastrophic embrittlement in hydrogen.     

Evaluation of the structure and properties of steel 35KhN2MFA under static and dynamic loading

We investigated the effect of the structure of heat-treated steel 35KhN2MFA on the mechanical properties under static and dynamic loading; the conditions of heat treatment were: hardening from 740–1100°C, isothermal quenching from 360°C, tempering at 300–550°C, heat cycling. It was established that a higher hardening temperature entails greater endurance: from 5700 cycles (T_h=740°C) to 10,100 cycles (T_h=1100°C), yet the required strength (_u=1740 MPa) is retained, and ductility and impact toughness are somewhat reduced. Isothermally quenched and high-tempered specimens have high endurance (N=12,000–13,500 cycles) but a low level of strength (_u=1330 MPa). Specimens subjected to heat cycling under the conditions hardening from 860°C and tempering at 650°C have the best complex of mechanical properties under static and cyclic loading.Translated from Problemy Prochnosti, No. 1, pp. 17–20, January, 1992.     

Structure of the plastic zone at the crack tip and the endurance of steel during low-cycle loading

From microhardness, metallographic, and also layered and sight x-ray analyses, the mechanisms controlling changes in the phase composition, structure, and size of the plastic zone at the crack tip during low-cycle loading of steels 12Kh18N10T and Kh11N10M2T have been established. These steels had various initial structures due to directional changes in their strength and ductility. It was shown that with increase in the maximum uniform elongation, there is an increase in the amount of intense structural changes in the plastic zone, an increase in the number of load cycles to failure, and a decrease in the rate of stable crack growth. These mechanical effects can be explained by the positive influence of the martensitic transformation and of dislocation mobility on the energy intensity of failure activation in the plastic zone. In particular, dislocation mobility leads to a partial relaxation of microdistortion in the crystallographic lattice of the matrix phase.Translated from Problemy Prochnosti, No. 8, pp. 23–31, August, 1993.     

Low-cycle fatigue of metal of rolled sheets with initial anisotropy of mechanical properties

The results are presented of examination of the relationships governing low-cycle deformation and failure of the metal of rolled sheets (03Kh16N9M2 steel) with initial anisotropy of the mechanical properties. The results show that the cyclic characteristics of this material depend on the direction in which the specimens are cut out. The given dependence is nonmonotonic. The lowest endurance is recorded for the specimens cut out in the Z-direction normal to the plane of rolled stock. With an increase of the angle , between the Z axis and the direction in which the specimen is cut out, the endurance increases, and at 45° it is almost independent of angle and corresponds to the values obtained for specimens cut out in the rolling direction. The results of evaluation of the cyclic endurance of specimens obtained on the basis of strain approaches and energy criterion are presented.Translated from Problemy Prochnosti, No. 6, pp. 12–16, June, 1991.     

High-temperature fatigue: Example of creep lifetime prediction for grade 2 alloy 800 at 550°C

Summary Experimental data on the material characteristics of structures subjected to thermal and mechanical cycling are needed for designing structural parts for creep and creep-fatigue interaction. Moreover, high-temperature low-cycle fatigue data are not sufficient to predict the fatigue creep lifetime.In order to check the reliability of steam generators, tests on pipe materials are conducted under cyclic thermal loading. The tests have been performed on an iron-nickel chromium alloy (alloy 800) in grade I condition. Isothermal low-cycle fatigue tests have been conducted at 350°C. For creep-fatigue tests, the strain was held constant during a variable relaxation period. To be representative of the service conditions, the duration of the hold time was equal to 168 hours with a total strain amplitude of 1.5%. Two damage processes are examined: fatigue and creep. The data are combined in three different models given by Cailletaud, Chaboche and Bul-Quoc. These models are used to predict fatigue-creep lifetime for very long hold periods. They were verified with the hold times of 1 week. It appears that the quality of the fatigue-creep lifetime predictions depends strongly on the quality of the creep relaxation phenomenon model. Comparison between prediction and experimental data Is quite satisfactory.Translated from Published in Problemy Prochnosti, No. 4, pp. 36–42, April, 1994.     

Energy method of longevity calculation under an irregular loading. Report 2. Longevity under programmed block loading

A method of calculating longevity to macrocrack development under a programmed loading in the form of a block repeated many times to failure is proposed. Relationships are derived for calculation of longevity under blocks with stepwise variation in stress amplitude, and also blocks assigned by the probability-distribution density of the stress amplitudes and tables of the differential frequency of repetition of peak and average stresses. Consideration of variation in the material's endurance limit under an irregular cyclic loading is demonstrated. It is shown that a specific type of relationships can be derived for the material under investigation with allowance for its kinetic cyclic-strain diagrams. An example of the longevity calculation, which is compared with experimental data for the block loading of steel 40Kh specimens, is cited.Translated from Problemy Prochnosti, No. 8, pp. 3–11, August, 1995.     

Autoclave Studies of the Durability of Specimens Simulating Pipe–Collector Joints in Steam Generators of Nuclear Power Plants

We performed studies of welded-pressed (by explosion) joints of pipes made of 08Kh18N10T and 08Kh14MF steels with nozzles made of 10GN2MFA steel in order to simulate the operation of PGV-1000 steam generators. We established that the fatigue limit based on 1·10⁵ cycles is greater for the joints of pipes made of 08Kh14MF steel in tests in air. On the autoclave tests in reactor boron-treated water in the primary contour of a nuclear power plant at a temperature of 350°C, the endurance of the joints of pipes made of 08Kh14MF steel was 39% less that that for the joints made of 08Kh18N10T steel. All specimens failed at the lower ends of joints that were in the water phase of the working medium rather than in the steam one, which corresponds to the service conditions of the output cold collector of a steam generator. We concluded that cracks are initiated due to fretting corrosion and grow by the mechanism of corrosion-cyclic cracking of the steels of pipes.     

Influence of electrolytic hydrogenation on cyclic crack resistance of chromium-nickel steels

Within the framework of linear fracture mechanics, we estimate the resistance of chromium-nickel 06Kh17N13M3, 07Kh16N6, and 10Kh26N5M steels to hydrogen embrittlement after various thermal treatments. We established that electrolytic hydrogenation leads to a considerable increase in the growth rate of fatigue cracks in steels with maximum in a low-amplitude range (by approximately a factor of 5–25 as compared with tests in air). The optimal mode of thermal treatment of 10Kh26N5M steel corresponds to quenching at 1293 K and annealing at 623 K. This mode ensures high plastic characteristics and maximum resistance to fatigue fracture in air in the course of electrolytic saturation with hydrogen, and long-term holding in a chloride-containing environment. We recommend this steel for the manufacture of critical parts of oil separators of marine ships. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 4, pp. 63–67, July–August, 1999.