高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。     

Influence of H<Subscript>2</Subscript>S Corrosion on Rotating Bending Fatigue Properties of S135 Drill Pipe Steel

Fatigue properties of S135 drill pipe steel 27CrMo under different conditions were investigated by rotating bending fatigue test. The results show that the fatigue limit at 10⁶ cycles of the tested steel is 540 MPa. However, the fatigue cycles of the specimens after H₂S corrosion shows a sharp decline from 10⁶ to 10⁴, when the stress is also 540 MPa. SEM observation of fatigue fracture surfaces reveal that the fatigue fracture has the characteristics of ductile fracture before H₂S corrosion. After H₂S corrosion, its shows the characteristics of brittle fracture. The main effect of H₂S corrosion is that the hydrogen atoms gathered at grain boundaries, inclusions and defects, which promoted the process of fatigue crack initiation and propagation, reducing the fatigue properties.     

Ratcheting Behavior of a Titanium-Stabilized Interstitial Free Steel

Engineering stress-control ratcheting behavior of a titanium-stabilized interstitial free steel has been studied under different combinations of mean stress and stress amplitude at a stress rate of 250 MPa s^?1. Tests have been done up to 29.80 pct true ratcheting strain evolution in the specimens at three maximum stress levels. It is observed that this amount of ratcheting strain is more than the uniform tensile strain at a strain rate of 10^?3 s^?1 and evolves without showing tensile instability of the specimens. In the process of ratcheting strain evolution at constant maximum stresses, the effect of increasing stress amplitude is found to be more than that of increasing the mean stress component. Further, the constant maximum stress ratcheting test results reveal that the number of cycles (N) required for 29.80 pct. true ratcheting strain evolution exponentially increases with increase of stress ratio (R). Post-ratcheting tensile test results showing increase of strength and linear decrease in ductility with increasing R at different constant maximum stresses indicate that stress parameters used during ratcheting tests influence the size of the dislocation cell structure of the steel even with the same amount of ratcheting strain evolution. It is postulated that during ratcheting fatigue, damage becomes greater with the increase of R for any fixed amount of ratcheting strain evolution at constant maximum stress.     

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.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 × 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 x 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

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.     

Fatigue Behavior of Four High-Mn Twinning Induced Plasticity Effect Steels

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2·10⁶ cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.     

Gigacycle fatigue of ultra high density sintered alloy steels

Abstract

Sintered steel specimens with density levels of up to 7·6 g cm^?3 have been prepared from Cr–Mo and Mo prealloyed powders. The fatigue response has been studied using an ultrasonic resonance testing device that enabled testing up to 10⁹ cycles. It showed that the fatigue endurance strength can be drastically increased by raising the density and that the sintering conditions are effective, though less than the density. The existence of a true fatigue limit was disproved up to 10⁹ cycles for all materials tested, with sintered steels thus being similar to wrought ones. Cr–Mo steels was shown to be superior to Mo alloyed grades due to the markedly finer as sintered microstructure and higher sintering activity. Fatigue crack initiation was found to originate from pores at first at multiple sites, with microstructural orientation being dominant compared to the direction of stress; with progressive loading, some cracks join to form a propagating macrocrack from which the final failure then starts.     

Modeling creep and fatigue of copper alloys

This article reviews expressions to quantify the thermal creep and fatigue lifetime for four copper alloys: Cu-Ag-P, Cu-Cr-Zr, Cu-Ni-Be, and Cu-Al₂O₃. These property models are needed to simulate the mechanical behavior of structures with copper components, which are subjected to high heat-flux and fatigue loading conditions, such as molds for the continuous casting of steel and the first wall in a fusion reactor. Then, measurements of four-point bending fatigue tests were conducted on two-layered specimens of copper alloy and stainless steel, and thermal ratchetting behavior was observed at 250 °C. The test specimens were modeled with a two-dimensional elastic-plastic-creep finite-element model using the ABAQUS software. To match the measurements, a primary thermal-creep law was developed for Cu-0.28 pct Al₂O₃ for stress levels up to 500 MPa and strain rates from 10⁻⁸ to 10⁻² s⁻¹. Specifically, (s⁻¹)=1.43×10¹⁰ exp (−197,000/8.31 T(K)) (σ(MPa))^2.5 (t(s))^−0.9.     

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.     

Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.     

Microstructural Characterization and Fatigue Performance Evaluation of MIG‐Welded Ship Hull Steel

The effectiveness of MIG welding with Argo‐shield gas & ER70S‐6 electrode in joining LRS (Grade‐B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near‐endurance stress amplitude up to 10⁵ cycles. The crack growth rate (da/dN) maintained a non‐linear relationship with logarithm of stress intensity factor range (logΔK) for the near‐threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure.     

00Crl8Nil0N奥氏体不锈钢200 - 600℃高温疲劳性能的研究

试验用00Cr18Ni10N钢（/%:0.018C,0.41Si,1.68Mn,18.18Cr,10.38Ni,0.16N）经1t EAF-AOD-ESR冶炼,锻成Φ30 mm材和1 050℃固溶处理,进行了00Cr18Ni10N钢200～600℃光滑（K_t=1）、缺口（K_t=3）的轴向高周疲劳性能研究,绘制了疲劳S-N曲线、计算了疲劳极限,并对疲劳试样的典型断口进行了SEM观察。结果表明,光滑试样的疲劳极限随试验温度升高而降低,缺口试样的疲劳极限对试验温度变化不敏感。当应力集中系数由K_t=1提高到K_t=3时,00Cr18Ni10N钢的200、400、600℃下10⁷周次条件疲劳极限分别从530、506、410 MPa,降低到323、370、392 MPa。表明在高温下00Cr18Ni10N钢存在疲劳极限对应力集中敏感,且应力集中敏感性随着试验温度的升高而降低。疲劳试样的断口形貌由疲劳源区、疲劳裂纹扩展区和瞬断区组成,疲劳起源于表面加工刀痕的不连续位置,呈韧性断裂特征。     

超声冲击对P355NL1钢焊接接头超高周疲劳性能影响

 通过超声疲劳试验探究超声冲击对P355NL1钢焊接接头超高周疲劳性能的影响。结果表明,由疲劳[S-N]曲线可知,在105～109寿命区间内,冲击态试样的疲劳性能要高于焊态试样,在1.0×108的疲劳寿命下,焊态试样的疲劳强度为139 MPa,冲击态的疲劳强度为217 MPa,冲击态疲劳强度相较于焊态提高了56%,这表明超声冲击可以明显提高焊接接头的疲劳强度。利用扫描电镜（SEM）观察断口形貌可以发现,裂纹源位于焊接接头焊根区表面。P355NL1钢焊接接头疲劳断裂为准解理断裂,超声冲击可以提高焊接接头的疲劳强度,但不会改变其疲劳失效机理。超声冲击可以降低焊根处应力集中,引入有益压应力和表面晶粒细化,从而提高焊接接头的疲劳强度。     

4Cr5MoSiV1热作模具钢700 ℃的低周疲劳行为

采用轴向应变幅控制的低周疲劳试验研究了总应变幅对4Cr5MoSiV1热作模具钢700 ℃低周疲劳行为的影响,包括循环应力响应行为、循环应力应变行为、循环迟滞回线和应变疲劳寿命行为等。结果表明:随着总应变幅从0.2%增大到0.6%,4Cr5MoSiV1钢在700 ℃时循环应力响应均表现为先循环硬化再循环软化的特性,并且应力幅最大值从220 MPa增大到308 MPa。同时,随着总应变幅的增大,4Cr5MoSiV1钢在700 ℃下的低周疲劳寿命由6750循环周次降低到210循环周次,且其过渡寿命约为1313循环周次。疲劳断口形貌分析结果显示,高温低周疲劳过程中裂纹主要萌生于试样表面处,且随着应变幅增大,裂纹源逐渐增多,疲劳条纹间距变宽,其断裂方式由韧性断裂转变为脆性断裂。透射电镜分析结果显示,循环软化可能与板条结构转变为胞状结构、基体发生位错湮灭、碳化物的析出和粗化有关。      

Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.     

Fatigue crack propagation in carburized X-2M steel

The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel (0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on the crack propagation rates. We define an effective stress intensity, K_e = K_a + K_i, where K_a is the applied stress intensity, K_i = σ_id _i ^1/2 , σ_i is the internal stress, and d_i is a characteristic distance associated with the depth of the internal stress field. In our work, a value of d_i = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses.     

Structure and properties of rapidly solidified 7075 P/M aluminum alloy modified with nickel and zirconium

Aluminum alloy 7075 was modified by additions of 1.1 wt pct nickel and 0.8 wt pct zirconium, rapidly solidified by ultrasonic gas atomization, canned, cold compacted, hot extruded, and evaluated in terms of structure and properties. Significant improvements in tensile strength (627 MPa YS and 680 MPa UTS) and crack growth rates were realized, along with a decrease in fracture toughness (23.7 MPa√m) while maintaining ductility (10 pct elong.) as compared to nominal I/M 7075 behavior. The stress for 10⁷ cycles fatigue life was greater than 275 MPa, which represents a 73 pct increase over that of I/M 7075. A variety of experiments was performed to evaluate effects on strength, ductility, and on structure. The variables were: powder size distribution, extrusion ratio, extrusion profile, different size fractions from the same lot of powder, and different locations of test bars in the several extrusions. Tensile properties, toughness, and fatigue properties were not importantly influenced by the location of test bars in the cross section or length of rectangular extruded bars. A comparison of mechanical properties from extruded bars prepared from ?53 μm powdersvs 53 to 250 μm powders showed a small loss of ductility and fatigue stress for 10⁷ cycles for the fine powder product. Higher extrusion ratios were beneficial for mechanical properties.