The fatigue limit of bearing steels – Part I: A pragmatic approach to predict very high cycle fatigue strength

Bearing steels and other high strength steels exhibit complex fatigue behavior in excess of 10⁷ cycles due to their sensitivity to defects like inclusions. Failure occurring in the very high cycle fatigue regime and the lack of an asymptote in the measured S–N data raise the questions as to the existence of fatigue limit and prediction of the fatigue strength of the high strength steel components. A series of two papers are written to discuss on the characteristics of the very high cycle fatigue and their implication for engineering applications. In the present paper (Part I) a deterministic defect model is developed to describe the fatigue crack growth from de-bonded hard inclusions. The model is shown to provide a unified prediction of fatigue behavior in different regimes, i.e. low cycle fatigue regime dictated by the tensile strength, high cycle fatigue regime obeying Basquin’s law and the very high cycle fatigue regime featured by the fish-eye and ODA (optically dark area) surrounding an interior fatigue-initiating inclusion on the fracture surface. The model predictions agree well with experiments. A combination of the deterministic model with a stochastic model that describes the inclusion size distribution allows prediction of fatigue strength and fatigue limit associated with certain reliability of a steel component. It is found that very high cycle fatigue, associated with interior inclusions, is attributed to the very slow crack propagation in vacuum condition, and that an asymptote for fatigue limit observed for mild steels also exists for high strength steels such as bearing steels, but extends beyond the very high cycle fatigue regime normally measured to-date. Monte Carlo simulation shows that such a fatigue limit asymptote becomes clearly visible in excess of 10¹² cycles, which is difficult to measure with today’s testing devices. Furthermore, the effects of steel cleanliness and specimen type and shape are studied by means of Monte Carlo simulations.     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

Development of a New Kind of High Strength Spring Steel

A new kind of high strength, high toughness and high plasticity spring steel has been developed. The strength, the reduction of area and the elongation of the steel are all higher than those of the steel 60Si2CrVA. The decarburization resistance and the sag resistance are also higher than those of the steel 60Si2CrVA. It has good hardenability, and is suitable for making springs with big cross section. The bogie springs made of this kind of steel have passed 2×106 cycles without broken under the conditions of maximum stress of 906 MPa and the minimum stress of 388 MPa.     

Crack growth process in GBF area of SUJ2 bearing steel in very high cycle fatigue regime

Abstract

The crack growth process in GBF of high strength bearing steel JIS SUJ2 is qualitatively investigated by low–high two-step variable amplitude loading (TSAL) tests. It is demonstrated that the GBF size reduces with the increase in applied stress amplitude, and that the GBF size at a given stress amplitude is a constant which is independent of inclusion size at the crack origin. The fatigue life at a given stress amplitude, which is proportional to ?8 power of the inclusion size, reduces with increasing inclusion size. It was shown by TSAL tests that GBF was formed at the earlier stage (approximate 10⁵ cycles corresponding to less than 1% total fatigue life in the regime N_f≧10⁷ cycles) and propagated hardly until near to the final life (more than 90% total fatigue life). After that the GBF crack will propagate rapidly to the ultimate size in a short time.     

SUJ2轴承钢超长寿命疲劳行为研究

采用超声疲劳试验机研究SUJ2轴承钢的超长寿命疲劳。结果表明:对于复合氧化物和TiCN裂纹源,裂纹从夹杂物与基体界面处萌生;铁、铬合金碳化物裂纹源则为夹杂物本身开裂。颗粒状亮面(GBF)相对尺寸正比于裂纹源处夹杂物边缘的应力强度因子范围1/ΔK2inc,对于本实验的SUJ2材料,当ΔKinc8MPa·m1/2时GBF不再形成。通过数据拟合得到了GBF内裂纹扩展规律:area~(1/2)_(GBF)/area~(1/2)_(inc)=(m_1+m_2N_f)~(m_0),证实了Paris公式可以用来描述GBF内的裂纹扩展。     

Factors influencing the GBF size of high strength steels in the very high cycle fatigue regime

The influences of major factors including applied stress amplitude, inclusion size and hydrogen content on granular-bright-facet (GBF) size of high strength steels in the very high cycle fatigue regime were studied in this article. It was found that the GBF size is determined by the applied stress amplitude and material hardness. If the applied stress amplitude is lower, the GBF size is larger. When a specimen containing bigger inclusions, the applied stress amplitude to form GBF can be reduced which results in the increase of GBF size. Hydrogen has different effects on the GBF size. The related reasons were discussed.     

Inclusion-controlled high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel

Axial loading fatigue tests were carried out to study the influence of inclusion on high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel (AISI 11). The fatigue strength of 1538 MPa with endurance life of 10⁷ cycles were obtained by stair-case method. The fatigue specimens were also subjected to a constant maximum stress of 1650 MPa to investigate the relationship among inclusion origin size (10-30 μm), fish-eye size (70-130 μm) and fatigue life (10⁵-10⁷ cycles). The fatigue life was found to be dependent on the inclusion size and the crack propagating length. A compressive residual stress of 300-450 MPa turned out to be present at the specimen surface, and finally induced the interior failure mode. Further investigation into the correlation between stress intensity factors of inclusion origin and corresponding stages of fatigue crack growth and fatigue life revealed that the high cycle fatigue behavior was controlled by crack propagation. According to the fractographic investigation, two distinct zones were observed in fish-eye, representing Paris-Law and fast fatigue crack growth stage, respectively. Threshold stress intensity for crack propagation of 3.9 MPa√m was obtained from the well correlated line on the ΔK_I-log N_? graph. The fracture toughness can also be estimated by the mean value of stress intensity factor ranges for fish-eye.     

Very high cycle fatigue behaviour of 2000-MPa ultra-high-strength spring steel with bainite–martensite duplex microstructure

The very high cycle fatigue and fatigue crack growth (FCG) behaviours of 2000-MPa ultra-high-strength spring steel with different bainite–martensite duplex microstructures (designated as B-M1 and B-M2) obtained through isothermal quenching and fully martensite (designated as M) for comparison were studied in this paper by using ultrasonic fatigue testing and compact-tension specimens. It was found that for the B-M1 sample with well-controlled thin and uniformly distributed bainite, the fatigue crack threshold Δ K _th is higher and FCG rate da / dN at an early stage is lower than those of the M sample. Therefore, the former has rather longer fatigue life at high stress amplitude, though both have almost identical fatigue strength. However, the fatigue properties of bainite–martensite duplex microstructure are significantly deteriorated with the formation of large bainite. Furthermore, like that of the M sample, the S–N curves of the B-M1 and B-M2 samples also display continuous declining type and fish-eye marks were always observed on the fracture surface in the case of internal fractures, which were mainly induced by inclusion. A granular bright facet (GBF) was observed in the vicinity around the inclusion. For each of the three samples, the stress intensity factor range at the boundary of inclusion (Δ K_inc ) decreases with increasing the number of cycles to failure ( N _f), while the stress intensity factor range at the front of GBF(Δ K _GBF) is almost constant with N _f and equals to its Δ K _th. This indicates that Δ K _GBF might be the threshold value governing the beginning of stable crack propagation.     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...     

4Cr5MoSiV1，8407钢的热疲劳性能

采用自约束热疲劳试验方法,对比研究了相同热处理条件的4Cr5MoSiV1,8407钢的热疲劳特性,观察分析了疲劳裂纹形貌和深度,采用热疲劳损伤因子定量研究了二种钢的热疲劳过程。结果表明：两种钢的热疲劳裂纹萌生发生在100－200次之间。8407钢热疲劳裂纹的萌生较4Cr5MoSiV1钢均匀,细小;在1600次冷热循环前,二者的热疲劳损伤程度无明显差别,在1600次热循环后,前者的热疲劳损伤程度低于后者;在较低的回火温度条件下,8407钢的热疲劳抗力稍优于4Cr5MoSiV1;而在高温回火时,8407钢的热疲劳抗力属于4Cr5MoSiV1钢。分析了这二种钢的热疲劳机制,指出决定材料热疲劳裂纹抗力的是钢的热稳定性和钢的强度或硬度。     

Analysis of fatigue crack initiation and S-N response of model cast aluminium piston alloys

Fatigue crack initiation and S-N fatigue behaviour of hipped model Al7Si-Sr and Al0.7Si piston alloys have been investigated after overaging at 260 °C for 100 h to provide a practical simulation of in-service conditions. The results show that hipping did not affect the S-N behaviour of Al7Si-Sr. This is attributed to the lack of significant change in porosity distribution in this alloy because of its low porosity levels even in the unhipped state. However, hipping profoundly improved the fatigue performance of alloy Al0.7Si due to the significant reduction in porosity. In this investigation, it was observed that porosity was rendered impotent as a fatigue crack initiator in both hipped alloys. Instead, fatigue cracks were observed to originate mainly from intermetallic particles (particularly the Al₉FeNi phase) in both alloys and sometimes from oxide particles in Al0.7Si alloy. Fatigue cracking was also frequently observed at intermetallic clusters in hipped Al0.7Si. The observed scatter in fatigue life is discussed in terms of the size of fatigue crack initiating particles and the overall particle size distribution which follows a power law distribution function.     

Effect of inclusion/matrix interface cavities on internal-fracture-type rolling contact fatigue life

Flaking failure in rolling contact fatigue (RCF) of hardened bearing steels under well-controlled lubrication is known to originate from the non-metallic inclusion present in the material’s subsurface region. Many researchers stress the importance of the inclusion size as an important factor in RCF life. The RCF performance of steels has been improved through the development of a steelmaking technology that allows reducing the inclusion size. However, due to the limits in visualization of crack initiation and propagation during RCF testing, the issue regarding the number of factors involved in the process remains unsolved. In our prior research, we observed and analyzed cracks initiating from internal defects. The obtained results suggested that the inclusion/matrix interface condition might play an important role in RCF life. In this study, we investigated the effect of inclusion/matrix interface cavities on RCF life through close observation of fatigue cracks. Hot Isostatic Pressing (HIP) treatment was conducted in order to close the cavities and it was found that closing the inclusion/matrix cavities is a very valuable technique for improving RCF life of bearing steels.     

Detrimental effect of nitride and aluminium oxide inclusions on fatigue life in rotating bending of bearing steels

Abstract

Rotating bending fatigue tests were performed on hardened AISI type 52100 bearing steel. Fracture surfaces after testing at a stress amplitude of 950 MPa showed that the Ti(C,N) inclusions which caused fatigue failure were significantly smaller than the corresponding alumina inclusions. The smallest crack initiating Ti(C,N) inclusion had a size of 3 μm and the smallest alumina inclusion was 17 μm. It was also shown that fatigue life was significantly shorter for a steel which showed cracked alumina inclusions on the fracture surfaces than for a steel which had non-cracked inclusions. Finite element calculations were performed to determine the driving forces of short cracks at Ti(C,N) and alumina inclusions. Two configurations were studied in each case, based on both non-cracked and cracked inclusions. The calculations incorporated heat treatment simulation and cyclic loading with successive growth of cracks. It was found that the Ti(C,N) configurations gave the highest driving forces for crack growth. The alumina configuration with a non-cracked inclusion gave the lowest driving force. It was concluded based both on experimental evidence and theoretical considerations that Ti(C,N) inclusions are more detrimental to fatigue life than alumina inclusions of the same size. It is their shape and thermal properties which make Ti(C,N) inclusions more detrimental than alumina inclusions. Internal cracking of alumina inclusions leads to reduced fatigue life.     

超长寿命疲劳破坏行为的可靠性研究

为了进行超长寿命疲劳破坏行为的可靠性研究,对GCr15钢超长寿命疲劳S-N试验数据进行了统计评估,采用Bansqun模型描述了其S-N曲线关系,建立了其概率S-N曲线模型,并采用SEV方法和Murakami方程分别预测了钢中最大夹杂尺寸和其相对应的疲劳强度。结果表明:其超长寿命疲劳S-N数据能较好地服从三参数威布尔分布,其概率S-N曲线形式都呈现持续下降的趋势;在验证了最大夹杂尺寸服从Gumbel分布的基础上,SEV方法预测的最大夹杂尺寸随着钢体积的增大而线性增加,Murakami方程预测的结果揭示了其概率S-N曲线呈现持续下降的原因。     

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr–Mo steel SCM435, which failed in the regime of N = 10⁵ to 5 × 10⁸ cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.     

Gigacycle fatigue properties of 1800 MPa class spring steels

Fatigue tests up to 10⁸ cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, σ _B, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface‐type fracture occurred at shorter lives below 10⁶ cycles, while the fish‐eye‐type fracture occurred at longer lives. The fatigue limit, σ _W, at 10⁸ cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. Al₂O₃ inclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish‐eye‐type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around Al₂O₃ and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 × 10^–6 Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 10⁸ cycles recovered to the estimated value of 920 MPa from the equation σ _w= 0.53 σ _B for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion‐controlled steel could achieve the higher fatigue limit estimated for the surface‐type fracture.     

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 μm in the rolled material and 80 μm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 10⁶ cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 10⁶ cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.     

STRATEGY FOR CHARACTERIZING FRACTURE TOUGHNESS IN THE DUCTILE TO BRITTLE TRANSITION REGIME

Abstract— A tentative method is proposed for treating the scatter and size effects of fracture toughness of ferratic steels in the ductile-to-brittle transition regime. The method is aimed at the determination of the probability function for initiation of fast growing cleavage cracks under the plane strain condition. The practical relevance of this method has been checked with toughness data sets obtained from various CT-specimen sizes and CCT-specimens of a pressure vessel steel tested at two different temperatures in the transition regime.     

FATIGUE CRACK INITIATION AND PROPAGATION IN A LOW-CARBON STEEL OF TWO DIFFERENT GRAIN SIZES

Rotary bending fatigue tests were carried out using both plain and notched specimens of a low-carbon steel with two different grain sizes (15 and 50 μm). The process of early crack development was observed by the replication method. The effect of grain size on crack development was studied. The main conclusions were as follows. (1) Fatigue resistance, in terms of the relative positions of the S-N curves, increases with decreasing grain size. This phenomenon is related to the number of cycles to propagate a crack to failure and the condition for the non-propagation of a fatigue crack. (2) The size of a non-propagating crack, which initiates below the fatigue limit, tends to become larger as grain size increases. (3) The difference in fatigue behaviour between small (15μm) and large (50μm) grain sized specimens is due both to a decrease in crack propagation rate and a smaller non-propagating crack limit in the finer grained material.