Fatigue crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel

The crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel was studied by rotating bending fatigue test with smooth cylindrical specimens and notched specimens (theoretical stress concentration factor Kt=3). The fatigue limit and S- N curve of bearing steel were measured by up- and- down method and group method, respectively. The fractures of the specimens were observed by scanning electron microscopy. The results show that the cracking type of the smooth specimens is single source initiation. The crack source is surface defects and subsurface inclusion. The surface defects are surface roughness, persistent slip band and machining dent, while the subsurface inclusion is Al2O3- CaO- MgO- SiO2 composite inclusion. The fatigue limit of notched specimens is significantly decreased. The cracking type of the notched specimens is multi- source initiation. The notch sensitivity factor qf of bearing steel is 1. 18. The fatigue failure of the smooth specimens is transferred from the surface roughness with high stress amplitude to the persistent slip bands, the machining dents and the inclusions with low stress amplitude. The fatigue crack initiation life accounts for more than 94. 1% of the whole fatigue life.     

Deformation structure and subsurface fatigue crack generation in austenitic steels at low temperature

In order to progress in the understanding of fatigue crack generation for high-strength alloys, the subsurface fatigue crack initiation sites were characterized and the deformation structure was investigated for the solution-treated 24Cr-15Ni-4Mn-0.3N and 32Mn-7Cr-0.1N austenitic steels. High-cycle fatigue tests of those steels were carried out at 4, 77, and 293 K. Subsurface crack initiation was detected in the lower-peak stress and/or in the longer-life range at the three temperatures. The subsurface crack initiation sites were intergranularly formed. The localized deformation and/or strain concentration by dislocation arrays of the (111)–〈110〉 system assisted intergranular cracking due to incompatibility at grain boundaries. Dislocation movements were restricted to their slip planes. Even at the lower stress level, dislocations had generated in more than one slip system and piled up to a grain boundary. The peak cyclic stress was lowered with the increasing size of the subsurface crack initiation site. The dependence of the subsurface crack size on the peak cyclic stress was discussed.     

Improvement of Ti alloy fatigue properties by Pt ion plating

The effect of Pt ion plating on the high cycle axial fatigue life of Ti-6Al-2Sn-4Zr-2Mo specimens was studied at room temperature and 455 °C. Unlike other coatings, the plated material tested in this work showed an increase in fatigue strength when compared to uncoated material. The fatigue strength improvement was greater at 455 °C than at room temperature. Coated specimens cycled close to the fatigue limit at 455 °C demonstrated the highest improvement, which was associated with subsurface fatigue crack initiation. The uncoated specimens, tested under similar conditions, failed by cracking at the surface. Sectioning analysis showed no defects in the subsurface initiation sites. The initiation was related to groups of similarly aligned coarse α platelets. It is suggested that the Pt ion plating caused some surface hardening as well as oxidation resistance at elevated temperatures leading to the suppression of surface crack initiation.     

The effect of microstructure on the mode of monotonie fracture,fatigue crack initiation,and stage i crack growth in an Fe-21 Pct Cr-11 Pct Ni heat resisting steel

This paper describes a study carried out at room temperature on an Fe-21 pct Cr-11 pct Ni heat resisting alloy under tensile and fatigue deformation. Specific microstructures were developed by heat treating the as-received alloy at different temperatures and times. The surface condition of all specimens displayed surface grain boundary oxidation to a maximum depth of 0.16 mm. In addition, the microstructure of specimens in one batch (B) contained intergranular chromium carbides. The major conclusions drawn from this study are that different microstructures respond differently to monotonie and cyclic modes of deformation. In particular, the embrittling effect of intergranular chromium carbides observed during the monotonie mode of deformation was different from that found when deformation was cyclic. During cyclic deformation these chromium carbides assisted in reducing the damaging effects of the surface grain boundary oxidation. Also during cyclic deformation, the overall fatigue life was found to depend on the mode of both fatigue crack initiation and Stage I crack growth. Fatigue life was reduced when crack initiation and Stage I crack growth were intergranular while it was enhanced when crack initiation occurred at slip bands and subsequent Stage I crack growth was transgranular. It was observed that surface grain boundary oxidation is a most deleterious micro-structural feature especially under fatigue loading but, if this feature is unavoidable then the presence of intergranular chromium carbides is considered to be highly beneficial in increasing the overall fatigue resistance of the material. Formerly a Postgraduate Student, School of Materials Science and Engineering, University of New South Wales, Kensington, New South Wales 2033.     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

Effects of partial recrystallization on high-cycle fatigue deformation and crack generation of a nitrogen-strengthened 32Mn-7Cr austenitic steel at liquid-nitrogen temperature

To achieve higher fatigue resistance against subsurface crack generation, both the refinement of grain structure and the introduction of mobile dislocations on various slip systems have been shown to be effective in the 32Mn-7Cr austenitic steel. A novel treatment which consisted of cold grooved rolling and partial recrystallization was introduced to modify the microstructure. High-cycle fatigue properties and fatigue-crack generation were investigated for both the solution-treated (ST) and the partially recrystallized (PR) materials at 77 K. The PR material displayed higher fatigue strength than the ST material, especially in the high-cycle regime. No subsurface crack generation was detected for the PR material; however, it appeared in the lower peak stress and/or in the longer-life range for the ST material. Intergranular facets formed a subsurface crack initiation site in the ST material. Since the dislocation structure that developed in the fatigued PR material assisted homogeneous and multidirectional plastic deformation, the localized deformation and/or the stress concentration at the grain boundaries by coplanar arrays were believed to be relieved. Therefore, intergranular cracking due to incompatibility at a grain boundary may disappear.     

Effects of Fine Particle Peening Conditions on the Rotational Bending Fatigue Strength of a Vacuum-Carburized Transformation-Induced Plasticity-Aided Martensitic Steel

The effects of fine particle peening conditions on the rotational bending fatigue strength of a vacuum-carburized transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.20 pct C, 1.49 pct Si, 1.50 pct Mn, 0.99 pct Cr, 0.02 pct Mo, and 0.05 pct Nb were investigated for the fabrication of automotive drivetrain parts. The maximum fatigue limit, resulting from high hardness and compressive residual stress in the surface-hardened layer caused by the severe plastic deformation and the strain-induced martensite transformation of the retained austenite during fine particle peening, was obtained by fine particle peening at an arc height of 0.21 mm (N). The high fatigue limit was also a result of the increased martensite fraction and the active plastic relaxation via the strain-induced martensite transformation during fatigue deformation, as well as preferential crack initiation on the surface or at the subsurface.     

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.     

GDL-1钢表面摩擦磨损及其干扰层对疲劳寿命的影响

 在不同外加拉压疲劳载荷条件下,通过对磨损层厚度的控制,研究了旋转磨损载荷作用对试样疲劳寿命的影响,并观察了摩擦磨损载荷作用下疲劳裂纹在表层和次表层的萌生、扩展过程。由于表面摩擦磨损时形成的高应变梯度干扰层的存在,改变了小裂纹的形态和扩展方向,从而对疲劳寿命有显著影响。     

The effect of frequency on the elevated temperature fatigue of a nickel-base superalloy

In this investigation, the effect of a frequency variation between 2 cpm and 6 × 10⁴ cpm on the 1400°F fatigue properties of wrought Udimet 700 was determined at a constant stress range of 85 ksi. It was found that a peak existed in the cyclic life vs frequency curve such that a) an increase in frequency from 2 to 600 cpm increased the fatigue life 100 times and b) an increase in frequency from 600 to 6 × 10⁴ cpm reduced the fatigue life sevenfold. The peak in the cyclic life vs frequency curve is the result of two competing processes: 1) there is a reduction in the effects of creep and oxidation with increased frequency that tends to increase the life and 2) there is an increase in the heterogeneity of deformation with increased frequency that tends to reduce the life. At low frequencies, crack initiation occurred at surface-connected grain boundaries. Crack propagation was initially intergranular and then proceeded noncrystallographically normal to the stress axis (Stage II mode). Crack initiation at high frequencies occurred at subsurface brittle phases located at grain boundaries or at the intersection of coherent annealing twin boundaries. Crack propagation was entirely transgranular, proceeding initially along twin boundaries or slip bands (Stage I mode) and then changing to the Stage II mode. The statistical nature of the fracture process, the significance of subsurface crack initiation, and the relation of these results to existing high temperature fatigue models are discussed.     

Mechanical Behaviors and Failure Analysis of S38C Steel with Gradient Structure Fabricated by Induction Heating and Quenching

This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small-scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high-strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively.     

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature. Formerly with Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson AFB, OH.     

The development of two texture variants and their effect on the mechanical behavior of a high strength P/M aluminum alloy,X7091

The influence of texture on the deformation behavior and monotonic and cyclic properties of two recrystallized P/MX7091-T651 plates was investigated. Thermal mechanical treatments were used to produced two different textures which varied in intensity by a factor of four. The two plates had similar grain and precipitate structures. The deformation behavior and mechanical properties were correlated with grain orientation and grain boundary misorientation. Differences in fracture surface roughness and crack deflection frequencies were observed for the two texture variants during fatigue crack propagation studies. Deformation behavior, crack closure, and crack deflection affected the fatigue crack growth rates. A small but measurable improvement in tensile strength, fatigue strength, and fatigue crack growth resistance was obtained in the sharply textured material when compared with the weakly textured counterpart.     

Exoelectron emission from torsional fatigue damage in hardened steels

Exoelectron emission associated with surface fatigue damage in two types of steel was measured by means of an ultraviolet laser scanning system. The localized emission from damage areas in quenched and tempered 1541 steel increased systematically with continued fatigue cycling. Plots of emissionvs cycles were in good agreement with earlier results for 1018 steel. Much of this emission resulted from the accumulation of plastic deformation during the crack initiation stage. In the case of relatively brittle 1144 steel, the dominant source of exoelectrons was a fatigue crack with little associated plasticity. However, shot peening of the 1144 steels completely changed the character of the surface layers. While the core material still cracked in a rather brittle manner, the shot peened layer accumulated a broad distribution of plastic deformation as revealed by the systematic development of exoelectron emission. These results indicate that the accumulation of surface fatigue deformation during the crack initiation stage is a systematic and perhaps universal process when viewed on the appropriate microscale.     

Distortion of the depression on the rear of the blank in broaching

When using continuous-cast billet in pipe rolling, it is expedient to roll the billet prior to broaching, so as to improve the structure and properties of the steel. To that end, a three-high screw-rolling mill for billet reduction has been installed at the TPA-80 pipe-rolling unit at OAO Sinarskii Trubnyi Zavod. Introduction of the new process was associated with increase in the rejection rate on account of indentations in the pipe. These indentations may be attributed to the depression formed at the rear of the billet in reduction and the subsequent formation of metallic semicircles on broaching; the semicircles break away and enter the deformation zone. The sources of the indentations on the rolled pipe are investigated. The broaching of the billet on a screw rolling mill is mathematically simulated. Recommendations are made with a view to reducing the surface defects on pipe produced by the TPA-80 unit with a reduction mill.     

Effect of surface roughness on low-cycle fatigue behavior of type 304 stainless steel

The effects of surface roughness on the low-cycle fatigue life of Type 304 stainless steel at 593°C in air have been investigated. It is observed that, at a strain rate of 4 × 10^?3 s^?1 and a total strain range of 1 pct, the fatigue life (N _f cycles) decreases with an increase in surface roughness. Information on crack growthvs strain cycles has been generated, as a function of surface roughness, by the measurement of striation spacing on fractured surfaces of specimens tested to failure. Crack propagation follows the Ina ∞N (wherea is the crack length afterN strain cycles) relation for longer specimen fatigue lives (N_f > 2700 cycles) and departs from Ina ∞N for shorter fatigue lives. A quantitative estimate is made of the number of cycles N_o(r) to generate a crack length equal to 0.1 mm (≈ 1 grain diam). The initial surface roughness significantly affects only the initiation component of specimen life time. The effect of roughness on crack initiation is described byN ₀ (R) = 1012R^?0.21, whereR is the surface roughness (root-mean-square value) in microns.     

A study of subsurface crack initiation produced by rolling contact fatigue

Results of subsurface crack initiation studies produced by pure rolling contact fatigue in 7075-T6 aluminum alloy are presented in this article. Microstructural changes and subsequent crack initiation below the contacting surface in cylindrical test specimens subjected to repeated rolling contact are illustrated. The rolling conditions are simulated in a three-dimensional elastic-plastic finite element model in order to estimate the plastic strains and residual stresses in the test material. The numerically estimated distribution of plastic strains in the model correlate well with the extent of microstructural changes observed in the test specimen. Results also indicate that a combination of plastic strains and low values of residual stresses is conducive to subsurface crack initiation and growth.     

Rolling-contact-fatigue behavior in backup roll of 5 ％ Cr forged steel

With a focus on the backup roll,a rolling-contact-fatigue experiment was performed on samples of 5％ Cr forged steel.The P-S-N fatigue curves were determined and the fatigue strength was calculated.The emergence of cracks on the test-sample surfaces was observed at different fatigue cycles.A micro-hardness tester was used to measure the hardness of the subsurface fatigue layer.The microstructures were analyzed at various magnifications with an optical microscope,scanning electron microscope,and transmission electron microscope.Based on these tests,the rolling-contact-fatigue mechanism of the large forged steel backup roll was also considered.The results showed that the contact-fatigue strength of the tested backup roll steel was 1 249 MPa;the surface fatigue crack lengthened continuously as the number of cycles increased and followed an S-shaped curve;the subsurface fatigue hardness reached its highest value at about 90 (HV) increment from the matrix hardness of 540 (HV) in the backup roll;the subsurface martensite/bainite microstructure was crushed and the dislocation density was greatly increased.Under alternating contact stresses,the surface/subsurface material was damaged and exhibited many microdefects.At the least,the surface fatigue layer on backup rolls should be fully removed before the microcracks enter a period of rapid propagation.     

Influence of thermomechanical processing on low cycle fatigue of prealloyed Ti-6AI-4V powder compacts

A wide range of microstructures was generated using various thermomechanical processing sequences in Ti-6A1-4V Rotating Electrode Process (REP) powder compacts of low contaminant content. Low cycle fatigue results were found to be superior to those in higher contaminant compacts tested in a previous program. All microstructural groups showed fatigue strengths equivalent to those found in wrought alloy, with the beta-annealed condition being lowest as expected. Alpha + beta work and solution treatment resulted in an excellent fatigue strength of 875 MN/m² (127 ksi) at 10⁵ cycles; 85 pct of the UTS. In the five conditions tested, the fatigue strength increased with increasing tensile strength, decreasing grain size, and increasing volume fraction of low aspect ratio primary alpha. Most crack initiation sites were observed at the specimen surface. Only alpha + beta worked and solution-treated material exhibited subsurface initiations, none of which was associated with any defect or with a lower fatigue life. Although compacts contained some tungsten particles, in no case were they associated with crack initiation sites, indicating that they were innocuous in the conditions evaluated.     

梯度结构硬质合金的疲劳断裂

采用三点抗弯的方法研究WC-6Co梯度结构硬质合金和均质WC-6Co硬质合金的疲劳行为,探讨疲劳断口形貌与破坏机制的关系。结果表明:梯度结构硬质合金的疲劳裂纹在亚表面萌生;梯度结构硬质合金表层Co相发生明显塑性变形,WC相以沿晶断裂为主;中间层Co相变形也很明显,WC相解理断裂增加;内层Co相塑性变形很少,WC、η相以解理断裂为主;均质硬质合金Co相塑性变形明显,WC以沿晶、解理断裂为主,各部位断口形貌接近;梯度结构硬质合金的疲劳极限比均质硬质合金高约100 MPa;梯度结构硬质合金中疲劳裂纹沿垂直于试样下表面、平行于Co相梯度的方向形核,而均质硬质合金的疲劳裂纹沿平行于试样外表面方向形核。在应力集中效应、循环应力的作用下,Co相的马氏体相变是裂纹在亚表面萌生的主要原因;马氏体相变使Co相成为裂纹形核的快速通道,裂纹沿Co相梯度方向形核。