Application of meso- and fracture mechanics to material affected by a network of thermal fatigue cracks

Within the concept of physical mesomechanics of materials and fracture mechanics the peculiarities of deformation and failure of heat resistant 25Cr1Mo1V steel with a network of thermal fatigue cracks are investigated. The basic regularities and typical characteristic stages of deformation process in specimens of 25CrMo1V steel damaged volumetrically by a network of cracks under localization of plastic strain are found and described numerically.     

Evaluation of crack growth behavior and probabilistic S–N characteristics of carburized Cr–Mn–Si steel with multiple failure modes

The unexpected failures of case-hardened steels in long life regime have been a critical issue in modern engineering design. In this study, the failure behavior of a carburized Cr–Mn–Si steel under very high cycle fatigue (VHCF) was investigated, and a model for evaluating the probabilistic S–N curve associated with multiple failure modes was developed. Results show that the carburized Cr–Mn–Si steel exhibits three failure modes including the surface flaw-induced failure, the interior inclusion-induced failure without the fine granular area (FGA) and the interior inclusion-induced failure with the FGA. As the predominant failure mode in the VHCF regime, the interior failure process can be divided into four stages: (i) the small crack growth around the inclusion, (ii) the stable macroscopic crack growth outside the FGA, (iii) the unstable crack growth outside the fish-eye and (iv) the momentary fracture outside the final crack growth zone. The threshold values are successively evaluated to be 2.33 MPa m^1/2, 4.13 MPa m^1/2, 18.51 MPa m^1/2 and 29.26 MPa m^1/2. The distribution characteristics of the test data in transition failure region can be well characterized by the mixed two-parameter Weibull distribution function. The developed probabilistic S–N curve model is in good agreement with the test data with multiple failure modes. Although the result is somewhat conservative in the VHCF regime, it is acceptable for safety considerations.     

Influence of processing parameters on hot workability and microstructural evolution in a carbon–manganese–silicon steel

In this work, the influence of processing variables such as strain, strain rate, temperature and cooling medium, on workability, microstructural evolution and mechanical properties of a carbon–manganese–silicon (C–Mn–Si) steel have been studied. Hot deformation of the C–Mn–Si steel has been carried out using compression testing over a domain 1223–1473 K and 0.001–10 s^− 1 where the steel is in austenitic phase field. The effect of cooling medium on the microstructural evolution has been studied by carrying out post-deformation cooling of the specimens in air and water media. Influence of the cooling medium on properties of the steel has been evaluated by comparing the hardness and Charpy impact test results. Based on the flow behavior analysis and microstructural examinations the optimum domain for the hot deformation of C–Mn–Si steel is found to be in the ranges of 1273–1350 K and 3–10 s^− 1. Flow instability in C–Mn–Si steel is manifested in the form of deformation bands in the microstructure. The signature of instability is not influenced by the phase transformation. The hardness of the material is dependent on the temperature of deformation and influenced by cooling medium. However, it does not show any correlation with deformation strain rate.     

Biocorrosion and Localization of Degradation Processes on the Steel Surface of a Gas Main

Materials Science - We establish basic regularities of influence of sulfate-reducing bacteria on the biocorrosion of 17G1S-U steel. The mechanisms of degradation of steel surfaces are generalized...     

A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel

To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel.     

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

Abstract

The effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more tortuous crack path. This result can be explained by the increasing energy absorption through crack deflections and tortuous crack paths by the Mn dispersoids.     

60Si2Mn钢壳体裂纹原因分析

目的查找某产品60Si2Mn钢关键零部件壳体,在冲压工序和超声波水浸探伤工序中,壳体底部存在裂纹疵病的原因。方法通过疵病部位宏观和微观观察、金相组织检查和验证试验等,检查壳体裂纹性质和原因。结论生产该壳体的原材料存在严重偏析现象,使壳体在热加工加热时,组织向奥氏体转变过程也产生差异,由于感应加热的时间相对较快,且感应加热时,钢材内外受感应磁场电流的不同大小的影响,内外组织变化产生不均匀性,导致在冲压过程中变形不一致,是壳体底部裂纹产生的原因。采取对壳体放慢加速度,局部回火改为整体退火等措施,能有效控制裂纹的产生。经过500发生产验证,结果未发现裂纹疵病产生。     

Effect of plastic deformation on magnetic fields around fatigue crack tips of carbon tool steel (JIS,SKS93)

Small cracks form in steel and then grow to cause failure. It is important to observe the plastic deformation around cracks in order to understand the fatigue level of all kinds of steel components. However, at present, it is not easy to observe the plastic deformation at the crack tip during crack growth. In this study, in order to find a new, non-destructive testing method which is related to the cracks, we investigated the relationship between magnetic fields and plastic deformation around the crack tip. A scanning Hall probe microscope (SHPM) equipped with three-dimensional GaAs film equipment was used to observe the magnetic fields around a crack tip and the plastic deformation induced by Vickers indentations in tool steel specimens (SKS93, JIS B 4404: 2006, equivalent to AISI W4 tool steel). We found that the magnetic fields around the crack tip changed during the crack growth, and also that the decrease in the magnetic fields depends on the plastic deformation size.     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

Effect of stacking fault energy on work hardening behaviors in Fe–Mn–Si–C high manganese steels by varying silicon and carbon contents

To improve the low work hardening capacity of Hadfield steel at low stress, the effect of stacking fault energy (SFE) on the microstructures and the work hardening behaviors of the Fe–Mn–Si–C high manganese steels were investigated by varying the silicon and carbon contents. The work hardening rates of the Fe–17Mn–Si–C steels with lower SFE were higher than that of the Hadfield steel at the strain below 0.28. The reason was that the amount of deformation-induced ε-martensite or mechanical twins was higher in the Fe–17Mn–Si–C steels than in the Hadfield steel due to their earlier onset. The work hardening rate of the Fe–17Mn–Si–C steels increased with decreasing the SFE because the rate of the formation and the amounts of martensite and twins increased with lowering the SFE.     

Fracture Mechanism Analysis of the Heat-Resistant Steel 15Kh2MFA(II) After Laser Shock-Wave Processing

The surface strengthening mechanisms of the 15Kh2MFA(II) heat-resistant steel are analyzed after the laser shock-wave treatment in the air and epoxy resin. The regularities are established in the formation of the ordered surface relief of the steel after treatment. The optical and digital analysis of the surface is performed which allows determining the size of irregularities, taking into account the stochastic and cyclic nature of their formation. The effect of treatment of the 15Kh2MFA(II) heat-resistant steel on the regularities of its static failure is established.     

Conditions of invariance of corrosion crack resistance characteristics

Conclusions It has been established that for a number of materials the condition of invariance of fracture toughness depends not only upon the type of material but also upon its structure.For a high-strength steel with a martensitic structure the kinetics of subcritical crack growth and also the parameter K_Iscc are sensitive to the original austenitic grain size. Heat treatment for coarse grains has a favorable influence on the corrosion crack resistance of such a steel. The creation in coarse-grained steel of serrated austenitic grain boundaries leads to an additional increase in resistance to corrosion crack growth.The generally accepted criterion of invariance of fracture toughness Eq. (1) is unsuitable as a condition guaranteeing no change in corrosion crack resistance parameters. The value of the coefficient A_c in Eq. (2), which characterizes the condition of obtaining the parameter K_Iscc independent of sample thickness, is determined by the structure of the material and for the systems considered is more than 500.Corrosion cracks in steel with a martensitic structure may have a complex morphology dependent upon the subcritical crack growth mechanism, the size of the austenitic grains, and the form of their boundaries. In contrast to fine-grained and overheated steel, for which intergranular subcritical crack growth is characteristic, in steel with serrated grain boundaries the subcritical crack growth mechanism is more complex. It was also observed that in the center layers of thick samples there is primarily transgranular failure replaced by intergranular at the transition to the surface layers.To determine the effective stress intensity factor at the tip of a corrosion crack with a complex trajectory, a method based on determining the pliability of a sample with a crack propagating in a curved trajectory was found to be effective.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 17, No. 3, pp. 24–33, May–June, 1981.     

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel in 3.5‐wt% NaCl solution

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel are investigated in simulated seawater. Fractography is performed by using scanning electron microscopy (SEM). The macroscale fracture surface and microstructure of the failed specimen are acquired including the crack initiation, crack propagation, and pitting evolution. The maximum cyclic stress (S) versus number of cycles to failure (N) curves is derived by three‐parameter fatigue curve method. Fatigue life is predominantly controlled by the corrosion pitting‐induced crack initiation when tested in simulated seawater at lower stress levels. As the maximum cyclic stress is less than 185 MPa, the chloride ion erosion is the main influence factor, which affects the fatigue failure of the G20Mn5QT cast steel in simulated seawater.     

Phase transformations in zones of plastic deformation beneath the surface of impact fractures in steel N32T3

Impact tests are performed on steel N32T3 in the hardened and aged conditions. The mechanism of failure and martensitic transformation in zones of plastic deformation between the surface of fractures obtained over a wide temperature range is studied. It is shown that the failure mechanism for both hardened and aged steel in the stage of crack propagation depends weakly on test temperature. An increase in KCU and KCT impact strength for the steel with an increase in temperature is due to an increase in the work for crack generation and formation at the failure site of zone L whose microrelief is predominantly folded with elongated pits. Two zones of plastic deformation are detected by means of x-ray structural analysis beneath the surface of fractures in the hardened condition and one in the aged condition. It is established that within the limits of a highly deformed microzone for hardened steel the amount of martensite is constant in spite of presence of a deformation gradient, but in aged steel it decreases constantly. It is noted that for correct evaluation of the effect of phase transformations occurring in zones of plastic deformation on impact strength and failure mechanism for the steel it is necessary to consider local heating and the actual phase composition of the steel at the tip of a propagating crack.Translated from Problemy Prochnosti, No. 7, pp. 49–56, July, 1990.     

Theoretical-experimental approach to the analysis of fatigue crack propagation in hydrogenated materials

We propose a theoretical-experimental model of fatigue crack growth in hydrogen-containing media. The model is based on the regularities of exhaustion of energy accumulated in a material under the conditions of cyclic fracture and the influence of hydrogen-containing media on the mechanical characteristics of the material. By using the deformation approach of fracture mechanics, we deduce analytic dependences for the determination of the conditions of elastoplastic deformation of the material near the crack tip. The effects of crack closure and loading ratio are taken into account. The interactions of various phenomena caused by hydrogen and their general influence on the changes in the fatigue-crack growth rate are evaluated. We also compare the computed values of the fatigue-crack growth rate with experimental values for two types of steel under different conditions.     

Ductile-to-Brittle Transition and Impact Fracture Behavior of 3Mn–Si–Ni Low Carbon Martensitic Steel

Strength of Materials - Impact behavior related to crack initiation and growth of low carbon martensitic 3Mn–Si–Ni steel was investigated by instrumented impact Charpy V-notch tests....     

The effect of thermo-mechanical loading on fracture-related parameters of austenitic steel

Failure analysis shows that 80–85% of emergency boiler shutdowns at power plants result from heating surface damage. When in service, the tubes are exposed to alternating thermo-mechanical loads, which trigger phase change affecting the individual service life. The service life sometimes differs significantly from its estimated value. This research is based on the hypothesis about the role and influence of internal structural stresses on the actual strength and long-term performance of tube products made of steels and alloys. The purpose of this work is to determine the limit state region of internal stresses of the first kind, in which microdamage will not lead to fracture. For that, we did a set of experiment studies to model the accelerated aging processes by thermal cycling and cold cyclic deformation. We chose a tube made of austenitic steel 10Cr13Mn12Si2Ni2Cu2Nb (Di59) as the object of research. The methods used were XRD analysis, microhardness testing, X-ray spectroscopy, and microstructure analysis. Due to redistributing properties, steel has no stable states: all its states are short-term and dependent on external disturbances. In accordance with the suggested hypothesis, the research findings make it possible to forecast the trends and direction of changes in the material properties. This allows us to evaluate the achievement of the limit state by the object and to use relaxations of internal stresses as a sign determining the conditions of crack initiation and propagation. The results are confirmed by the data obtained from the microstructure analysis of fractured tubes of a superheater from a functioning boiler.     

Fatigue crack growth behaviour of a Si–Mn steel with carbide-free lathy bainite

An investigation has been performed to examine the fatigue crack propagation (FCP) rate and fatigue threshold of an Si–Mn steel containing carbide-free lathy bainite. Compact tension specimens prepared from this steel were given four different heat treatments to produce four different austenite contents. The fatigue test was carried out at stress ratio of –1 in a room temperature ambient atmosphere. The results show that the FCP threshold of the steel increases with an increase in the volume fraction of carbon-saturated austenite. The crack growth behaviours show that the deformation strengthening ability of the austenite has a significant effect on the FCP in the threshold region. The effect of austenite on the FCP threshold is seven times that of the bainite.     

30CrMnSiA钢超塑性变形中的机理问题

利用扫描电镜和透射电镜研究了30CrMnSiA钢超塑性变形中组织结构变化。结果表明,变形中合金元素的扩散导致横向晶界的宽化,并且富集了Si、Cr、Mn元素。三角晶界上呈现的显微空洞宏观调节了晶粒的三维重排过程。未溶碳化物与晶格位错、晶界以及晶界位错之间有相互作用关系。扩散和位错运动微观调节了晶界滑动,并导致它的发展。     

Microstructural evolution of bainitic steel severely deformed by equal channel angular pressing

High Si bainitic steel has been received much of interest because of combined ultra high strength, good ductility along with high wear resistance. In this study a high Si bainitic steel (Fe-0.22C-2.0Si-3.0Mn) was used with a proper microstructure which could endure severe plastic deformation. In order to study the effect of severe plastic deformation on the microstructure and properties of bainitic steel, Equal Channel Angular Pressing was performed in two passes at room temperature. Optical, SEM and TEM microscopies were used to examine the microstructure of specimens before and after Equal Channel Angular Pressing processing. X-ray diffraction was used to measure retained austenite after austempering and Equal Channel Angular Pressing processing. It can be seen that retained austenite picks had removed after Equal Channel Angular Pressing which could attributed to the transformation of austenite to martensite during severe plastic deformation. Enhancement of hardness values by number of Equal Channel Angular Pressing confirms this idea.