Cyclic Crack-growth Resistance Research Articles

Strength and Cyclic Crack-Growth Resistance of Thermally Deformed Alloys of the Al-Mg-Sc System

Strength and Cyclic Crack-Growth Resistance of Thermally Deformed Alloys of the Al-Mg-Sc System

Estimation of the resource of pipes of the main circulation pipeline of a NPP by the criterion of crack resistance with account of operational degradation of properties

The results of an experimental determination of the characteristics of cyclic crack growth resistance of specimens made of stainless steel 08Kh18N12T are given. Compact samples 20 mm thick cut from Du 550–30 pipes of the primary circuit of the main circulation pipeline (MCР) were studied under eccentric tension in the initial state and after 100 thousand hours of operation in the conditions of the Novo-Voronezh NPP. Using a new engineering approach based on measuring the specimen tightening and constructing a cyclic elastoplastic fracture diagram for a specimen with a crack in the coordinates “tightening – the stress intensity factor, calculated taking into account the plasticity correction”, a calculated life estimation is given for the pipe of Du 550–30 MCР NPP, made of 08Kh18N12T steel, according to the criterion of cyclic crack growth resistance, taking into account operational degradation of properties caused by the running time of 100 thousand hours. The life (survivability), expressed in a number of cycles of change in internal pressure, for a pipe with the most dangerous defects (surface semi-elliptical crack and combined defect “pitting + crack”) is determined. An analysis of the influence of the level of initial damage and the type of hazardous defect on the survivability of the pipes is made. The results can be used to analyze damage and predict the life of pipes made of ductile metals and alloys, in cooling systems of reactors, as well as main pipelines of hydrocarbon transport.

Cyclic Corrosion Crack Resistance of an Exploited Welded Joint of 17G1S Pipe Steel

We analyze the influence of numerous factors, such as an NS4 model soil medium, the load ratio (R = 0.1 and 0.9), and the potential of cathodic protection (–0.98 V) on the cyclic crack-growth resistance of the base metal and the metal of circular butt weld of 17G1S pipe steel in the intact state and after 30 years of the operation in a gas main. It is shown that the long-term operation of steel maximizes the negative influence of the soil medium on its cyclic crack-growth resistance under the combined action of the high load ratio and high potential of cathodic protection.

Strength and Corrosion-Fatigue Crack-Growth Resistance of Alloys of the Ti–Nb–Zr–Si System Intended for Biomedical Purposes

We study the influence of the chemical composition and the conditions of thermomechanical and thermal treatments on the structural strength determined by the characteristics of strength and cyclic crack-growth resistance, as well as on the corrosion and corrosion-fatigue properties of alloys of the Ti–Nb–Si and Ti–Nb–Zr–Si systems. These characteristics are compared with the characteristics determined for the extensively used Ti–Al–V alloy intended for biomedical applications. It is shown that, under the conditions of reliable control over the formation and development of mechanical defects (fatigue cracks) in the products made of Ti–18.7Nb–1.0Si alloy, this alloy may serve as a good alternative to Ti–6Al–4V alloy.

Influence of the Elevated Contents of Silicon and Manganese on the Operating Characteristics of High-Strength Wheel Steel

We study the temperature dependences of strength and plasticity, the cyclic crack-growth resistance after the thermal and force loading in the course of braking, and the wear and damage resistances of wheel steels with elevated contents of silicon and manganese and lowered contents of carbon and compare these characteristics with the earlier determined characteristics for standard 2 and T steels. It is shown that silicon and manganese promote the leveling of cyclic fracture toughness of steels in different zones of the wheel rim and the decrease in the high-temperature (above 500°С) plasticity (elongation), which reduces the susceptibility of steel to the formation of slid flats on the rolling surface of the wheel. However, the solid-solution hardening of these steels observed for relatively high (∼ 0.6%) carbon contents results in a low crack-growth resistance under cyclic loading in the initial state and under the influence of thermal and force factors under the conditions of braking, as well as in the low damage resistance under the conditions of contact fatigue. We recommend to reduce the carbon content in these steels down to 0.52–0.53%.

Influence of the Modes of Heat Treatment on the Strength and Cyclic Crack-Growth Resistance of 65G Steel

To find the ways of increasing the brittle-fracture resistance of the metal in the heat-affected zone formed as a result of repair surfacing of the rolling surface of railroad wheels, we study the influence of various modes of heat treatment, including the traditional and modified (by the authors) Q-n-Ptreatments on the mechanical properties of 65G steel regarded as a model wheel steel. It is shown that, after the modified Q-n-P-treatment, the mechanical characteristics of steel increase to a greater extent than after the traditional treatment. However, according to the parameter of structural strength P = [ΔU · ΔKth · ΔKfc ], where ΔU is the ultimate strength, ΔKth is the fatigue threshold, and ΔKfc is the cyclic fracture toughness of the material, the mode of treatment proposed earlier, which leads to the relaxation of stresses of the second kind in the bulk of martensite and bainite lathes, proves to be more efficient.

Diagnostics of the Engineering State of Steam Pipelines of Thermal Power Plants by the Hardness and Crack Resistance of Steel

We propose a nondestructive method for the diagnostics of the current state of heat-resistant steels after long-term operation in the steam pipelines of thermal power plants (TPP). It is based on measuring the sizes of ferrite grains and the hardness of 15Kh1M1F heat-resistant steel directly on the pipe surface and on the use of the deduced relationship between these characteristics (of the Hall–Petch type). The plot of this relationship contains two rectilinear parts. The first part agrees with the physical ideas concerning the correlation between the grain size and the strength (hardness) of the material. The second part with a rapid decrease in the hardness of steel is connected not only with the subsequent changes in the microstructure of steel but also with the development of the in-service defects scattered in the volume of the metal. To determine the limiting state of degraded steel, we used one of the parameters of fracture mechanics, namely, the effective threshold amplitude of the stress intensity factor (SIF) ΔKth eff determined with regard for the effect of fatigue crack closure. The application of this parameter is substantiated by the detected inversion of the influence of hydrogen on ΔKth eff (from positive to negative) observed as the degree of in-service degradation of steel increases. The indicated inversion is caused by the transformation of the mechanism of fatigue crack growth in the subthreshold part of the fatigue crack growth diagram from transgranular (caused by microshears at the crack tip in each loading cycle) into a brittler mechanism accompanied by the formation of intergranular fragments of mode I fracture in the damaged areas. Therefore, the threshold of cyclic crack-growth resistance corresponding to this inversion is taken as the critical value $$ \varDelta {K}_{\mathrm{th}\;\mathrm{eff}}^c $$ below which the probability of brittle fracture of the exploited steel becomes much higher. By using the constructed correlation dependence between ΔKth eff and the hardness of the metal and the determined critical value $$ \varDelta {K}_{\mathrm{th}\;\mathrm{eff}}^c, $$ we estimate the critical value of hardness. Below this value, the hazard of uncontrolled brittle fracture noticeably increases due to the presence of defects scattered in the bulk of the metal.

Specific Features of the Deformation and Fracture of Low-Alloy Steels in Hydrogen-Containing Media: Influence of Hydrogen Concentration in the Metal

We establish basic regularities of the influence of hydrogen concentration in metals on the fracture strength of low-alloy pipe steels. In particular, we determine the characteristic value of hydrogen concentration for which the mechanism of its influence on the deformation of specimens by uniaxial quasistatic tension changes, namely, below this concentration, hydrogen enhances the plasticity of the material and, above this concentration, promotes its embrittlement. For the first time, we detected an ambiguous relationship between fatigue crack-growth rate and the volume hydrogen concentration in these steels under cyclic loading in hydrogen-containing media and established its value for which the cyclic crack growth resistance of steel increases. We revealed a correlation between the intensity of hydrogenation of steels and the strength of passive film at the crack tip, which gives us an instrument for the development of efficient barriers against hydrogen penetration into the materials.

Specific Features of the Fatigue Fracture of Welded Joints of 34KhN2MA Steel Formed by Electrodes with Different Phase Compositions

We study the cyclic crack resistance of the metal of welded joints of 34KhN2MA thick-plate steel obtained with the help of ferritic and austenitic electrodes. It is shown that the weld metal has a higher crack resistance than the base metal only within the second part of the kinetic diagrams of fatigue fracture. In the first and third parts, the characteristics of the weld metal are worse. The results of fractographic investigations demonstrate that the decrease in the fatigue crack-growth resistance in pure austenitic welds is explained by the presence of hot cracks in the zone between the near-root metal of the welded joint and the base metal. As for the mixed ferrite-austenitic welds, the decrease in the fatigue crack-growth resistance is explained by inadmissibly large slag inclusions creating additional stress concentration and deteriorating the characteristics of cyclic crack-growth resistance.

Influence of Alloying and Thermal Treatment on the Strength and Cyclic Crack Resistance of Welded Joints of Alloys of the Al–Cu–Mg System. Part 2

We study welded joints of annealed sheets 6 mm in thickness made of D16M (Al–Cu–Mg) aluminum alloy, which is an analog of the foreign 2024 alloy currently attributed to difficultly welded alloys. The joints are made by the impulsive-arc method with consumable electrode with the use of a Zv1201 (Al–6.3Cu–0.3Mn) filler wire and technological inserts made of 7056 (Al–9.5Zn–1.8Mg–1.6Cu), V96 (Al–8.5Zn–2.6Mg–2.3Cu), and V92 (Al–3.5Zn–4.2Mg–0.5Cu) alloys. We study the regularities of changes in the microstructure, hardness, strength, and cyclic crack growth resistance of the joints after welding and also in the course of subsequent artificial aging or quenching and artificial aging after welding. It is established that the strength of welded joints after thermal treatment does not exceed 68% of strength of the base metal but is comparable with it in the state after welding.

Fatigue Crack-Growth Resistance of Thermally Hardened 65G Steel Under Transverse Shear

We plot the kinetic diagrams of fatigue fracture of 65G steel under the conditions of transverse shear after different modes of heat treatment within the range of fatigue crack-growth rates covering six orders of magnitude with the use of the stress intensity factor for the evaluation of the stress-strain state in the vicinity of the crack tip. The main and auxiliary characteristics of cyclic crack-growth resistance are determined with regard for the friction of the lips of a fatigue crack. We construct the diagrams of structural strength and confirm the hypothesis that the fatigue thresholds increase with the yield strength of 65G steel under the conditions of shear.

Influence of the Long-Term Operation of Gas Pipelines on the Cyclic Crack-Growth Resistance of 17G1S Steel

We establish the kinetic regularities of crack propagation under low-, medium-, and high-amplitude cyclic tensile loads in 17G1S steel taken from the “Dashava–Minsk” gas main pipelines after 40 yr of operation and from the intact (original) pipes. The kinetic diagrams of fatigue fracture are plotted for various stress ratios of the loading cycle within the range of fatigue crack growth rates covering up to six orders of magnitude. The near-threshold regions of the kinetic diagrams are investigated for the material of different zones of the walls of the intact pipes and pipes after operation. The characteristics of fatigue crack-growth resistance are determined on the basis of the constructed plots. We reveal a significant decrease in the threshold value of the stress intensity factor of the material after operation as compared with the intact material caused by the degradation of its structure and the decrease in its fatigue-fracture resistance.

Influence of the Friction of Crack Faces on the Cyclic Crack-Growth Resistance of 65G Steel Under Mode II Loading

We study the fracture of 65G steel under cyclic mode-II loading of beam specimens with lateral cracks, experimentally determine the friction coefficient of crack faces for specimens made of steel with troostite structure, and plot the kinetic diagrams of fatigue fracture. The influence of the friction of crack faces on the cyclic crack-growth resistance of 65G steel under mode-II loading is established for troostite and sorbite structures depending on the growth rate of fatigue cracks.

Cyclic Elasto-Plastic Fracture Diagram and Some Parameters of Cyclic Crack Growth Resistance for the Plastic Steels

Method of calculation and experimental estimation of crack growth resistance under cyclic elastoplastic deformation is proposed. This method is based on measuring of local plastic strain near the crack tip and plotting the cyclic elasto-plastic fracture diagram for a specimen with a crack. Analysis of two types of the cyclic elasto-plastic fracture diagrams and their parameters is made. Ex-perimental D-diagrams of cyclic elasto-plastic fracture for the plastic carbon steel are given.

Influence of Technological and Operational Factors on Fatigue Resistance of 10GN2MFA Steel*

The paper presents the results of low-cycle fatigue tests for 10GN2MFA steel obtained by martensitic melting and electroslag remelting in air and in heat transfer medium at both room and elevated temperatures, as well as cyclic crack growth resistance in high-temperature aqueous medium. It is shown that the method of steelmaking exerts a strong influence on the fatigue crack growth stage.

On the concept of selection of steels for high-strength railroad wheels

On the basis of the results of statistical analysis of the in-service defects of high-strength railroad wheels and the data of related laboratory investigations of wheel steels, it is shown that the major part of these defects are pockmarks and flats formed on the rolling surface. Therefore, it is necessary to change the concept of selection of steels for high-strength wheels in order to take into account not only the characteristics of strength and hardness (wear resistance) but also the characteristics of crack resistance under the conditions of contact fatigue. The new concept should be based on the approaches of the structural mechanics of fatigue fracture and aimed at the production of high-strength wheel steels with lowered carbon contents and higher characteristics of cyclic crack-growth resistance under the conditions of mode I and mode II fracture.

Influence of alloying by rare-earth metals on the mechanical properties of 17g1s pipe steel

The positive influence of microadmixtures of rare-earth metals on the strength, plasticity, impact toughness, and cyclic crack-growth resistance of 17G1S pipe steel is discovered. The decomposition of the impact toughness into the work of crack initiation $$ {a_i} $$ and the work of crack propagation $$ {a_p} $$ made it possible to discover the following specific features in the temperature dependences of $$ {a_p} $$ : at elevated testing temperatures guaranteeing the ductile fracture of steel, the positive effect of alloying by REM is preserved. At the same time, at lowered temperatures of brittle fracture, this effect becomes negative and the work $$ {a_p} $$ decreases. Distilled water somewhat decreases the cyclic crack-growth resistance of steel and the positive effect caused by the REM admixtures manifests itself only in the increase in the cyclic fracture toughness of steel.

In-service degradation of 20Kh13 steel for blades of steam turbines of thermal power plants

We compare 20Kh13 steel in the intact state and after 3∙105 h of operation in blades of the low-pressure cylinders of steam turbines by the mechanisms of fracture, structural, mechanical, corrosion, and stress corrosion properties. It is shown that, as a result of operation, steel hardens and becomes less plastic, its corrosion characteristics, and the parameters of fatigue and cyclic crack-growth resistance significantly deteriorate.

Modeling of the initiation of contact fatigue damages and estimation of the durability of elements of tribological conjunctions

We present a computational model for investigations of the fracture processes and estimation of the remaining life of solids under their cyclic contact interaction. This model is based on the step-by-step construction of the paths of crack propagation in the contact zone with regard for the operational parameters of tribounit and the characteristics of cyclic crack growth resistance of the materials of its elements. The model is applied for the contact interactions of rolling and fretting fatigue. We study the specific features and causes of the formation of such typical contact fatigue damages as pitting, spalling, dark-spot, and systemic cracking. Using the characteristics of cyclic crack growth resistance of rail steels for transverse shear and tension, we estimate their durability by the development of pitting. The main results concerning this problem for the last decade have been synthesized.

Influence of the mode of thermal treatment and load ratio on the cyclic crack-growth resistance of wheel steels

We study the microstructure, mechanical properties, and micromechanism of fatigue fracture of high-strength railway-wheel steel (0.63% C and 0.1% V). The necessity of elevation of the cyclic fracture toughness of this type of steel by isothermal hardening and tempering at 500°C is substantiated. It is shown that the cyclic crack-growth resistance of steel decreases for high loading amplitudes as the load ratio (simulating the influence of residual thermomechanical stresses) increases.