Fish-eye Fractures Research Articles

Retained Austenite Reduction near Fracture Surface in Repeatedly Quenched SUJ2 Steel

High-carbon high-strength JIS-SUJ2 bearing steel is an alloy having the characteristics of high wear resistance and fatigue strength as a result of quenching. When this kind of high-strength material is exposed to long-lasting low stress application loadings, fisheye fractures can often be seen. In this research, we measured the retained austenite near the fracture surface in JIS-SUJ2. After the measurement, we confirmed the decrease of the retained austenite in the vertical surface under both fisheye fracture and ductile fracture.

Influence of Hydrogen on Steel Components for Clean Energy

The influence of hydrogen on the mechanical properties of four, medium-strength, commercial, quenched-and-temped steels has been studied using the linearly increasing stress test (LIST) combined with cathodic hydrogen charging. The relationship was established between the equivalent hydrogen pressure and the hydrogen charging overpotential during cathodic hydrogen charging, though the use of electrochemical permeation experiments and thermal desorption spectroscopy. The cathodic hydrogen charging conditions were equivalent to testing in gaseous hydrogen at hydrogen fugacities of over a thousand bar. Under these hydrogen-charging conditions, there was no effect of hydrogen up to the yield stress. There was an influence of hydrogen on the final fracture, which occurred at the same stress as for the steels tested in air. The influence of hydrogen was on the details of the final fracture. In some cases, brittle fractures initiated by hydrogen, or DHF: Decohesive hydrogen fracture, initiated the final fracture of the specimen, which was largely by ductile micro-void coalescence (MVC), but did include some brittle fisheye fractures. Each fisheye was surrounded by MVC. This corresponds to MF: Mixed fracture, wherein a hydrogen microfracture mechanism (i.e., that producing the fisheyes) competed with the ductile MVC fracture. The fisheyes were associated with alumina oxide inclusion, which indicated that these features would be less for a cleaner steel. There was no subcritical crack growth. There was essentially no influence of hydrogen on ductility for the hydrogen conditions studied. At applied stress amplitudes above the threshold stress, fatigue initiation, for low cycle fatigue, occurred at a lower number of cycles with increasing hydrogen fugacity and increasing stress amplitude. This was caused by a decrease in the fatigue initiation period, and by an increase in the crack growth rate. In the presence of hydrogen, there was flat transgranular fracture with vague striations with some intergranular fracture at lower stresses. Mechanical overload occurred when the fatigue crack reached the critical length. There was no significant influence of hydrogen on the final fracture.

Fatigue behavior of ultrafine grained medium Carbon steel processed by severe plastic deformation

The endurance limit of materials has been observed to be significantly increased in materials with an ultrafine grained microstructure [1, 2]. As this effect, however, has not yet been investigated in steels, fatigue experiments of an unalloyed medium carbon steel with a carbon content of 0.45 wt.-%, which was treated by high pressure torsion (HPT) [3-5] at elevated temperature were carried out. The treatments were applied to discs which had different initial carbide morphologies and showed an increase of hardness after HPT by a factor of 1.75 – 3.2 compared to the initial states, whereby the amount of increase depends on the initial carbide morphology. The maximum hardness achieved was 810 HV. The discs were cut into fatigue specimens in the form of bars of the size of 4 mm x 1 mm x 600 gm. Until a hardness of 500 HV the endurance limits correspond linearly with the hardness. This is no longer the case at higher hardness values, where inherent and process-initiated flaws lead to lower fatigue limits. The maximum endurance limit exceeded 1050 MPa in 4-point-micro-bending and at a load ratio of R = 0.1. Fractography revealed different crack initiation sites like pre cracks and shear bands [6, 7] resulting from HPT or fisheye fractures initiated from non-metallic inclusions.

Fatigue behavior of plasma radical nitrided SCM435 steel in super-long life regime

Fatigue behavior in a super-long life regime was investigated for plasma radical nitrided JIS SCM435 steels at room temperature in air. Specimens were nitrided at the 723 K and 823 K for 3 h and at 773 K for 1 h, 3 h, and 5 h. The fatigue strengths of the nitrided specimens were greater than that of the un-nitrided specimen. However, the fatigue strength and fracture mechanism of the nitrided specimens were markedly influenced by nitriding temperature and time. Both surface cracking and subsurface cracking, e.g. fish-eye fractures, were observed in the specimens subjected to the nitriding conditions. Hardened layers and compressive residual stress were formed on the specimen surfaces by nitriding, which led to an improvement of the fatigue strength and determined the initiation type of the fatigue cracking. Stress intensity factor at crack initiation was evaluated in fish-eye type fractures. As a result, the stress intensity factor range in the facet area indicated a constant value, approximately 3.5 MPa·m1/2, regardless of the fatigue life.

Effect of mean stress on fatigue properties of 1800 MPa-class spring steels

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude. However, the fatigue strength degradation was less than that expected from a modified-Goodman line, and the best fit line was obtained by connecting the fatigue limit at zero mean stress to true fracture strength instead of tensile strength. This research also reviewed application of a power low to the stress ratio effect evaluation. In these results, the difference between the spring and conventional steels was negligible.

1129 内部破壊を示すばね鋼の疲労特性に対する平均応力の影響(OS11-7 材料の疲労挙動と損傷評価-疲労損傷・寿命予測-)

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in tenns of stress amplitude. With these results, application of conventional methods to evaluate mean stress effects, such as modified-Goodman diagram, a power law of a stress ratio and so on, were discussed in case of fish-eye fracture of high-strength steel. As the result, the difference between the spring and conventional steels was negligible. This means that the conventional methods were applicable to the case of fish-eye fracture of high-strength steel within the scope of this research.

Fatigue Properties of Nitrided Ultrafine Ferrite-Cementite Steels under Rotating Bending Fatigue Testing

Fatigue tests under rotating bending were conducted on nitrided ultrafine ferrite-cementite steels. The ultrafine ferrite-cementite steels included carbon-increased and phosphorus-added versions to investigate the effects of grain growth suppression during nitriding. In the carbon-increased versions, grain growth was successfully suppressed both in the nitrided layer and in the core region. On the other hand, the low-carbon versions showed grain growth in the core region, even in the phosphorus-added types, although grain growth was suppressed in the nitrided layer. In the fatigue tests, many of the nitrided specimens revealed fish-eye fractures originating from inclusions located in or beneath the nitrided layer. In spite of the occurrence of fish-eye fractures, the fatigue strength of the carbon-increased versions was markedly improved due to nitriding, whereas it was a little improved in the low-carbon versions. The fatigue strength of the nitrided specimens was closely related to hardness at the fracture origin, even when fish-eye fractures occurred. This was why nitriding markedly improved the fatigue strength of the carbon-increased versions in which grain growth was successfully suppressed and high hardness was maintained beneath the nitrided layer.

Specimen size effects on gigacycle fatigue properties of high-strength steel under ultrasonic fatigue testing

Ultrasonic fatigue testing was conducted for high-strength steel, employing enlarged specimens with a straight section. The results showed good agreement with conventional servohydraulic fatigue test results, demonstrating the validity of ultrasonic fatigue testing using these enlarged specimens under conditions where fish-eye fractures occur. When the results were compared with those using conventional specimens, specimen size effects were clearly observed: enlarged specimens showed lower fatigue strength due to appearance of larger inclusions in the fish-eye fracture origin.

The effect of inclusion geometry according to forging ratio and metal flow direction on very high-cycle fatigue properties of steel bars

The objectives of the present paper are to clarify the effect of inclusion geometry according to the forging ratio and the metal flow direction on very high-cycle fatigue properties of steel bars and to propose an evaluation method for characteristic inclusion geometries observed at fracture origin. Specimens were made of steel plates rolled to three levels of thickness from steel bars of low alloy steel. Then the specimens were cut in both parallel and perpendicular directions to the metal flow of the steel plates. Fatigue tests were conducted in a long life regime by using an ultrasonic fatigue testing machine. The fatigue limit for 10 9 cycles decreased with the increase of the thickness of steel plate, i.e. a decrease of the forging ratio. The fatigue limit of specimens cut parallel to metal flow was higher than that of specimens cut perpendicularly to metal flow. Fish-eye fractures appeared on the fracture surface of most specimens. Especially, characteristic inclusions, which are clusters of oxides, sulfides and their combination, were mainly observed at the origin of fish-eye fractures in specimens cut perpendicularly from thick plates. In this paper, these characteristic inclusions were treated as the extended elliptical inclusion/crack from the point of view of fracture mechanics and the area parameter model. More careful inspection at the fracture origin revealed that the rough surface area called the optically dark area (ODA) appeared only around the edge of the short radius on the extended elliptical inclusion where the higher stress intensity factor is generated. This result suggests that the middle part of the extended elliptical inclusion dominates the formation of ODA and the very high-cycle fatigue property. Finally, based on the fracture mechanics approach, area L/ W=4 , which is the square root of the area of the surrounded part where length/width < 4 at the middle of the extended elliptical inclusion, is proposed as the evaluation parameter for area parameter model.

Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes

Gigacycle fatigue tests were conducted for several versions of JIS-SCM440 low-alloy and JIS-SUJ2 bearing steels using 20-kHz ultrasonic fatigue testing to elucidate the relationship of the inclusion size and type to fish-eye fracture properties. The total number of tested specimens was over 200. Most of the specimens revealed Al2O3 or (Cr, Fe)3C inclusion-originating types of fish-eye fractures, while TiN inclusions and the matrix also caused fish-eye fractures in some specimens. Based on these fatigue test results, 109-cycle fatigue limits were estimated according to inclusion size by resorting the obtained data points according to their inclusion sizes at the fish-eye fracture origin. The estimated fatigue limits revealed saturation when the inclusion sizes were smaller than 15 μm, while those fatigue limits depended on the inclusion sizes to the −1/6th power in the case of inclusions above 15 μm in size. The saturation of the 109-cycle fatigue limits was considered to be caused by the effects of the optically dark areas (ODAs). Moreover, the fatigue limits also depended on inclusion type. In comparing the Al2O3 and (Cr, Fe)3C inclusions, the key features causing the difference in the fatigue limits were likely to be bonding between the inclusion and the matrix, i.e., the (Cr, Fe)3C inclusions were tightly bonded to the matrix, unlike the Al2O3 inclusions, although both inclusions were of the hard type.

低合金鋼のギガサイクル疲労特性に及ぼす水素の影響

Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy steel in order to investigate the effects of hydrogen on fatigue properties of high-strength steel. 200 and 550°C tempered samples were prepared and an ultrasonic fatigue testing machine was used for the fatigue tests. The base steel without hydrogen charging showed only fish-eye fractures in case of the 200°C tempered version, while the 550°C tempered version showed surface and fish-eye fractures in short and long life regions, respectively. On the other hand, the hydrogen charged steel of both versions showed only fish-eye fracture and the fatigue strength largely dropped. These results meant that the hydrogen effects were larger on the fish-eye fracture than on the surface fracture. However, no extension of ODAs was observed in the hydrogen charged steels, so the degradation of fatigue strength due to hydrogen had nothing to do with ODAs. Moreover, in this research, it was confirmed that the ultrasonic fatigue testing machine was effective in evaluating gigacycle fatigue properties of the hydrogen charged steel. The ultrasonic fatigue testing machine enabled us to finish the gigacycle fatigue test before the specimen lost a large amount of hydrogen due to emission, and no frequency effects were observed in case of fish-eye fracture even in the hydrogen charged steel.

高周波焼入れした炭素鋼Ｓ４０Ｃのギガサイクル疲労特性

Giga-cycle fatigue tests were conducted for two heats of induction hardened 0.40% C carbon steels. The fatigue testing types were rotating bending at 100 Hz and ultrasonic at 20 kHz. The fatigue test specimens were uniformly induction hardened from the surface to the center. The induction hardened 0.40% C carbon steels revealed fish-eye fractures whose origins were mostly a TiN inclusion in case of heat A and entirely an Al2O3 inclusion in case of heat B. In case of rotating bending fatigue tests of heat A, surface fractures also occurred at over 108 cycles, while that type of the surface fracture never occurred in case of the ultrasonic fatigue tests. When the ultrasonic fatigue tests were compared with the rotating bending, focusing on the results of fish-eye fracture, the ultrasonic type showed slightly higher fatigue strength in case of heat A in spite of good agreements in case of heat B. However, the difference in case of the heat A was so small as to be in scattering of the results of heat B. Moreover, effects of pre-treatments before the induction hardening were also investigated in heat B. In this case, the fatigue tests were additionally conducted for induction hardened specimens with a pre-treatment condition of only normalizing (N-IH), while other tests were of normalizing, quenching and tempering (Q-IH). As the result, difference between N-IH and Q-IH was negligible, showing no influence of the pre-treatment condition.

Inclusion-controlled fatigue properties of 1800 MPA-class spring steels

Fatigue tests were conducted on a series of 1800 MPa-class spring steels whose fatigue properties were inclusion controlled. The fatigue tests were conducted on billets and on hot-rolled bars, taking into account the elongation of the oxide-type composite inclusions that were deformed during hot rolling, i.e., controlled inclusions. Anisotropy of the fatigue properties due to the slender shape of the elongated inclusions was also discussed. Fatigue tests were then conducted for both the rolling direction (RD) and transverse direction (TD) in the case of the billets. The fatigue test results in the RD showed a slight difference between billets and bars. The inclusions that were deformed during hot rolling were sufficiently elongated, even for the billet specimens, and differences in the effective inclusion sizes between the billets and their hot-rolled bars for the RD were small. However, there were marked differences in fatigue strength between the RD and TD in the billet specimens: the fatigue strength was almost half in the TD due to the presence of fish-eye fractures originating in large and slender MnS inclusions. In these fatigue tests, the two types of deformable inclusions revealed remarkably different effects on fatigue strength: the deformable oxide-type inclusions never caused fish-eye fracture, although the MnS inclusions found in the billets were extremely detrimental to fatigue strength when stress was applied in the TD.

Gigacycle fatigue properties of a modified-ausformed Si-Mn steel and effects of microstructure

Fatigue tests were conducted for modified-ausformed, oil-quenched, and water-quenched versions of a Si-Mn steel (JIS-SMn443) with a chemical composition of 0.42C-0.2Si-1.52Mn (in mass pct). The Si-Mn Steel has an advantage with respect to recycling, since it contains no Cr or Mo, although the steel has disadvantages with respect to tempering resistance and hardenability. Although the tempering resistance was not improved by modified ausforming, the problem of hardenability was minimized, since modified ausforming created fine and uniform martensite. The gigacycle fatigue properties of the Si-Mn steel were improved by the modified ausforming. The low-strength modified-ausformed steel (AF1400) was free of fish-eye fractures and had a fatigue limit of 770 MPa at 109 cycles. In spite of the occurrence of fish-eye fractures, the high-strength modified-ausformed steel (AF2000) achieved a fatigue limit of 830 MPa at 1010 cycles, higher than the 710 MPa limit of the water-quenched steel (QT2000W). The fatigue limits of the oil-quenched steels (QT1400 and QT2000), which contained numerous and large nonuniform structures, could not be determined because of the temperature increase of specimens during the 20 kHz tests. The fracture surfaces revealed optically dark areas (ODAs) even in the modified-ausformed steel when the specimens failed at over 107 cycles, since the steel was partially recrystallized during modified ausforming. The gap of fatigue limit between the AF2000 and QT2000W steels was not entirely explained by the difference of the ODA sizes. Also, the sizes of the nonuniform structures were likely to play an important role in the gigacycle fatigue properties, since the sizes in AF2000 were smaller than those in QT2000W.

1010-Cycles Fatigue Properties for a Series of SUP 7 Spring Steels

Giga-cycle fatigue tests were conducted for heats B and C of SUP 7 spring steels tempered at 430 and 500°C. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic fatigue testing machines were used at 30 or 100 Hz, 120 Hz, 600 or 800 Hz and 20 kHz, respectively. The heat B developed fish-eye fractures only with an origin of an Al2O3 inclusion, while Al2O3 and TiN inclusions and matrix cracks appeared at the origins in case of the heat C. The Al2O3 inclusions in the heat B were larger than those in the heat C. Hence, the heat C revealed higher fatigue limit than the heat B. In case of fish-eye fracture, S-N curves under 100 Hz rotating bending, and 600 or 800 Hz and 20 kHz uniaxial loading were coincident each other in both heats, while those under 120 Hz uniaxial loading were lower. The frequency effects were negligible in case of fish-eye fracture and the difference between 120 Hz uniaxial tests and others were explained by the difference of the control volumes.

A novel inclusion inspection method employing 20 kHz fatigue testing

In this report, we propose a novel inclusion inspection method based on the use of 20 kHz fatigue tests. In these fatigue tests, which take a little time, the most harmful inclusions or internal facets appear automatically at the origin of fish-eye fractures. The novel inclusion inspection employing fatigue tests was demonstrated, in two high-strength steels, to be superior to a conventional inspection by scanning of polished surfaces. Our novel inspection method was first demonstrated with a JIS-SUP12 spring steel that contained relatively large Al2O3 inclusions. The novel inspection method appeared to detect larger Al2O3 inclusions than the conventional inspection, since the inspection volume was 22 times more extensive than that of conventional inspections. On the other hand, there was little difference in the maximum Al2O3 inclusion sizes estimated for several volumes of steel between the novel and conventional inspections. Second an inclusion inspection was conducted on JIS-SCM440 low-alloy steel. For this steel, the discussion focuses on TiN inclusions. Fatigue tests appeared to be the only way of conducting inspections for TiN inclusions, since the inclusions could not be detected in the conventional inspection. In measuring the TiN inclusion sizes, however, etching of the fracture surfaces was found to be absolutely necessary, since the inclusion sizes on the fracture surfaces were significantly different from the true sizes. The difference seriously influenced the estimation of the maximum inclusion sizes. Even in the case of internal-facet-originated type fish-eye fractures etching was found to be necessary, since TiN inclusions occasionally appeared after etching.

高強度鋼のギガサイクル疲労における介在物寸法と種類の重要性

Fatigue tests were carried out up to 108 cycles for a series of 1800 MPa-class spring steels in order to investigate the relation between inclusions and fatigue properties. The fatigue tests were carried out both for billets and bars rolled from the billets and, in case of billets, the tests were both for RD and TD directions.Al2O3rich inclusions were undeformable and not lengthened in rolling. On the other hand, SiO2 rich inclusions were divided into small pieces and the effective sizes for the axial direction were little reduced in rolling although the inclusions were deformable. In turn the difference of fatigue strength between billets and bars was not so large in most cases. The TD direction fatigue tests showed fish-eye fractures with the origins of large and lengthened MnS inclusions and the fatigue strengths were about a half of those in the RD direction.The fatigue data plotted on a modified S-N diagram, in which stress amplitude were normalized by fatigue limits estimated by Murakami's equation, were divide into two groups. One consisted of large and lengthened MnS inclusions and large artificial notched defects. The other consisted of small defects such as Al2O3 and TiN inclusions and matrix cracks. The result showed that the effects of the lengthened MnS inclusions were almost equal to artificial notches and apparently different from relatively small defects. Moreover, the small defect group was also divided into Al2O3 inclusion group and others, suggesting the effect of properties of defects on the fatigue strengths.

Improvement of gigacycle fatigue properties by modified ausforming in 1600 and 2000 MPA-class low-alloy steels

Fatigue tests were conducted for a modified ausformed and tempered JIS-SCM440 low-alloy (denoted as AF) steel with tensile strengths of 1600 and 2000 MPa, respectively, and were compared with normally quenched and tempered (QT) steel. The 1600 MPa-class modified ausformed steel (AF1600) developed no fish-eye fractures and a fatigue limit of 920 MPa, whereas normally quenched steel (QT1600) developed fish-eye fractures and showed a lower fatigue limit of 840 MPa. The 2000 MPa-class modified ausformed steel (AF2000) achieved a high fatigue limit of 1010 MPa, but developed fish-eye fractures. The fatigue limit of normally quenched steel (QT2000) was 840 MPa. Results of fatigue tests at 20 kHz up to 109 and 1010 cycles agreed with the fatigue limits for QT1600 and QT2000, respectively. The fracture sites of fish-eye fractures in QT1600 were Al2O3 and TiN inclusions, and those in AF2000 and QT2000 were Al2O3 and TiN inclusions and internal facets. Fracture surfaces of QT2000 that broke at over 107 cycles had large, clearly identifiable optically dark areas (ODAs). The AF2000 showed no fractures over 107 cycles, and the authors could not conclude whether the fracture surface of the specimens broken at around 5 × 106 cycles had ODAs. The fact that long-life fatigue failure over 107 cycles did not occur in AF2000 showed that ODAs did not form in this steel, since ODAs developed in proportion to cycle number and became large and clearly identifiable at over 107 cycles. Their incidence was quantitatively demonstrated by modified Stress amplitude vs Number of cycles to failure (S-N) diagram analysis. The modified S-N curve for AF2000, in which the specimens were assumed to have no ODAs, showed good agreement with that for QT2000, in which ODAs were assumed to be formed, especially at over 107 cycles.

超音波疲労試験を利用した介在物検査手法

This report proposes a new inclusion inspection method using ultra-sonic fatigue tests at 20 kHz. Fish-eye fractures occur in high-strength steels and the most harmful defect for fatigue in the risk volume of a specimen appears at the fracture site. In the other word, fatigue tests certainly specify the defect causing fatigue failure.The method was validated on the basis of fatigue test results for two low-alloy steels and a spring steel. Fatigue tests had a larger inspection volume and detected a larger inclusion than a conventional direct observation method on mirror surfaces. In case of a low-alloy steel in which about 10 μm Al2O3 inclusions existed together with smaller TiN inclusions, the TiN inclusion and matrix cracks also caused fish-eye fracture. Even in a such case, fatigue tests certainly specify the most harmful defect for fatigue. In case of Al2O3 inclusions, the inclusion size could be measured on the fracture surface. For TiN inclusions, however, the sizes were difficult to measure. For matrix cracks, the size could be measured in the spring steel but was difficult to measure in a low-alloy steel. Although distinction of inclusions based on ASTM standard and measurement of TiN inclusion sizes were basically possible in fatigue test, the procedures took time and were costly.

Fatigue Strength of Austempered Ductile Cast Iron under Thermal Environment.

Austempered ductile cast iron (ADI) is attracting much attention as one of new materials. Its mechanisms of crack initiation and propagation vary with the relation between the properties of graphite and matrix structures. In this study, we focused on the following two topics : First, we considered what effect the thermal environment has on high-cycle fatigue of ADI. As an example of thermal environment, when vehicle gears come into contact with each other for a long time, due to a material change in the contacting face as well as the thermal environment, fatigue strength will decrease. There is no literature discussing fatigue strength at the Ms transformation temperature below it. Therefore, high-cycle fatigue strength under as near the Ms transformation temperature environment was compared with that under room temperature environment and discussed. Plots of fish-eye fractures on the S-N curve show a wide scatter of life. Therefore, attention, was paid to fish-eye fractures and defects considered to be the starting point of crack development. It was clear that they show stress dependence from observations of their SEM micrographs. Thus, a modified S-N curve in the fish-eye fracture region could be obtained from an arrangement using the defects and fish-eye areas.