CrMoW转子钢的高温超高周疲劳性能

使用自主搭建的高温超声疲劳实验系统在常温和600℃进行Cr Mo W转子钢1×1010周次超高周疲劳实验,研究了Cr Mo W转子钢在工作温度下的超高周疲劳性能。结果表明:在600℃和常温下转子钢的S-N曲线均呈连续下降型,试件在109周次后仍发生疲劳断裂。在常温实验条件下疲劳寿命小于1×107周次试件的裂纹多萌生在表面,而高于1×107周次的多以内部萌生为主。在高温实验环境下裂纹也在内部夹杂处萌生,且裂纹萌生方式对疲劳寿命没有决定性作用。鱼眼定量分析结果表明,Cr Mo W转子钢在600℃和常温下裂纹扩展的应力强度因子门槛值分别为3.4 MPa·m1/2和1.0 MPa·m1/2。     

热处理工艺对50CrVA高强弹簧钢超高周疲劳损伤机制的影响

研究了50CrVA高强弹簧钢在不同热处理状态下(淬火+中温回火和退火)的超高周疲劳破坏行为及其裂纹萌生机理。结果表明,50CrVA高强弹簧钢在107~109循环周次内发生疲劳破坏,两种热处理状态的S-N曲线下降形态不同,均未出现疲劳极限。热处理工艺改变50CrVA的微观组织,从而影响超高周疲劳阶段(寿命107周次)的疲劳破坏损伤机制:经淬火+中温回火处理材料破坏多起源于内部夹杂物,夹杂物周围存在的应力场与溶质原子发生弹性交互作用,吸引间隙原子向其周边扩散、富集,使间隙原子富集区材料性能下降,导致裂纹在内部夹杂物处萌生;退火热处理后材料微观组织对间隙原子向材料内部夹杂物扩散起到阻碍作用,所以超高周疲劳裂纹易于在材料表面萌生。     

汽轮机转子钢常温与600℃超高周疲劳行为研究

利用自主研发的高温超声疲劳实验系统,开展Cr Mo W转子钢常温及600℃下的超高周疲劳实验。为新型超超临界汽轮机转子提供高温超高周疲劳数据,实验结果表明高温会极大降低转子钢的疲劳强度。S-N曲线在常温及600℃下均呈现连续下降型,且600℃下S-N曲线在整个疲劳寿命周次内保持一定下降趋势。断口分析发现,常温下疲劳寿命107周次试件的疲劳裂纹以内部萌生为主,600℃下疲劳破坏的内部夹杂萌生方式与表面萌生方式均分布于整个疲劳寿命。夹杂物尺寸分析表明,高温降低疲劳裂纹内部萌生夹杂物的临界尺寸。     

合金元素Mo对材料超高周疲劳性能的影响

采用超声疲劳试验技术对42CrMo钢超高周疲劳性能进行了研究。将42CrMo钢超高周疲劳S-N曲线与已有文献中40Cr钢在相同试验方法和试验条件下得到的超高周疲劳S-N曲线进行比较,结果显示42CrMo钢超高周疲劳性能优于40Cr钢。通过分析推测,合金元素Mo增加了疲劳裂纹内部萌生的时间,进而提高了材料超高周疲劳的寿命。     

腐蚀条件下高强钢超高周疲劳性能及损伤机理研究进展

在疲劳载荷作用下，材料发生裂纹萌生、扩展直至断裂的周次在107以上的过程被称为超高周疲劳。部分高强钢机械部件需在恶劣环境中服役，服役期间承受的疲劳载荷周次高达108～1011，高强钢在腐蚀环境中的超高周疲劳问题成为影响结构可靠性、安全性的关键问题，是航空航天、汽车、高铁等领域亟待解决的难点。得益于金属材料在传统疲劳问题上的总结积累与先进试验手段的助力，诸多学者开发出多种新的试验方法，有针对性地对高强钢在腐蚀条件下的超高周疲劳问题展开研究。目前，关于腐蚀条件下高强钢超高周疲劳性能退化规律及损伤机理、腐蚀条件下裂纹萌生竞争机制及裂纹初期扩展行为、氢对高强钢超高周疲劳性能及颗粒亮面形成机制的影响等核心问题的认识愈发清晰，逐渐从对试验现象的描述与归纳深入到对损伤机理的探索与推演，而且部分研究成果已经逐步在工程实践中得到应用。本文首先从S-N曲线等角度简述了高强钢无腐蚀条件下的超高周疲劳损伤特征;然后总结了典型腐蚀介质对高强钢超高周疲劳性能的影响，并指出氢脆断裂为高强钢在腐蚀环境中的断裂机理之一;随后详述了氢对高强钢超高周疲劳性能及颗粒亮面形成机制的影响研究进展;最后对几个关键问题进行了展望。     

低周疲劳下16MnR微孔的裂纹萌生与扩展

采用宏观与微观结合的方法,在疲劳试验机上进行疲劳试验,在光学显微下观察裂纹的起裂与扩展.研究了压力容器用钢16MnR在低周疲劳下微孔(40～200μm)的裂纹萌生与扩展规律.研究表明裂纹的萌生机制:滑移带启裂和疏松带启裂,前者由剪应力起主要作用,后者由正应力起主要作用.而滑移带的局部性和裂纹开叉是低周疲劳下微裂纹的两大典型现象.微观缺陷尺寸、应力水平对疲劳寿命有显著影响,当应力水平较低时,微孔尺寸对寿命的影响明显.而当应力水平较高时(超过屈服极限),孔径对寿命的影响不敏感.在同一应力水平下,微缺陷尺寸存在临界值dt,当d＞dt时,疲劳寿命下降很多.     

微合金化渗碳齿轮钢的接触疲劳性能

通过滚动接触疲劳试验方法,研究了两种渗碳齿轮钢的接触疲劳性能.结果发现,渗碳齿轮钢接触疲劳试样失效方式主要为渗碳层的点蚀和剥落.氧含量较低的Nb微合金化齿轮钢(含0.04%的Nb)中氧化物夹杂数量少、尺寸小,接触疲劳裂纹起裂较难;同时,Nb微合金化齿轮钢渗碳层晶粒尺寸小、硬度高,提高了疲劳裂纹萌生及扩展阻力,导致Nb微合金化后,齿轮钢的接触疲劳寿命大幅度提高.     

基于内部失效机理预测评估渗碳Cr-Ni齿轮钢的超高周疲劳强度

对渗碳Cr-Ni齿轮钢进行应力比为0和0.3的室温超高周疲劳实验,观测试样中诱发裂纹萌生的夹杂和疲劳断口形貌,以全面评估渗碳Cr-Ni齿轮钢疲劳性能。将疲劳失效模式分为有细颗粒区(Fine granular area,FGA)的内部疲劳失效和有表面光滑区(Surface smooth area, SSA)的表面疲劳失效,并阐明了渗碳Cr-Ni齿轮钢的超高周内部疲劳破坏机制。基于累积损伤和位错能量法并结合细颗粒区形成机理和夹杂的最大评估尺寸,分别构建了两种渗碳Cr-Ni齿轮钢内部疲劳强度的预测模型。利用FGA尺寸与夹杂尺寸的比值和夹杂应力强度因子及应力比之间的关系,修正所提出的两种疲劳强度预测模型并给出了最大夹杂尺寸下的l_FGA-S-N曲线。结果表明,基于累积损伤法和位错能量法分别构建的疲劳强度预测模型都可用于预测评估渗碳Cr-Ni齿轮钢在多种应力比下的内部疲劳强度,基于位错能量法的强度预测模型精度较高。     

18Ni马氏体时效钢低周疲劳性能的影响因素分析

本文主要对具有优异综合性能和加工特性的18Ni马氏体时效超高强度钢的低周疲劳性能影响因素进行了分析研究,系统分析了18Ni马氏体时效钢合金元素对其力学性能的影响机理,并从晶粒度、材料微观组织、低周疲劳试验条件、疲劳试样加工状态以及非金属夹杂物等方面综述了18Ni马氏体时效钢低周疲劳性能的影响机制。     

不同温度环境下CrMoW转子钢超高周疲劳行为研究

利用旋转弯曲疲劳试验机,研究CrMoW转子钢在常温与600℃条件下的超高周疲劳特性。对试验数据采用三参数模型拟合,用扫描电子显微镜(SEM)对疲劳断口进行分析。研究结果发现,600℃下的S-N曲线呈现直线下降的趋势,不存在传统意义上的疲劳极限。高温会加速试样的氧化,促进裂纹的萌生与扩展,降低材料的疲劳寿命。断口分析表明:疲劳裂纹主要萌生于试样表面,很少发现裂纹萌生于内部的情形。在600℃下,裂纹萌生区普遍发现有夹杂物。对试验前后转子钢硬度值进行测量,没有发生明显变化。     

Very high cycle fatigue behaviors of Mn–Si–Cr series Bainite/Martensite dual phase steels

The present work studied the very high cycle fatigue (VHCF) behaviors of Mn–Si–Cr series Bainite/Martensite (B/M) dual phase high-strength low-alloy (HSLA) steels to ensure safe applications in railway and oil field. Four kinds of chemical compositions and seven types of B/M steels were designed and studied by ultrasonic fatigue test. The conception of apparent and intrinsic VHCF strength is addressed. Results show the effect of heat generation cannot be ignored in B/M steels in ultrasonic fatigue test, and higher is the intrinsic VHCF strength of materials, more remarkable is the negative effect of self-heating. More importantly, the VHCF property of B/M steels is less sensitive to the inclusion than that of tempered martensite steels, and the amount of retained austenite puts little effect on the VHCF property.     

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.     

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

The high cycle and Very-High-Cycle Fatigue (VHCF) properties of a structural steel with smooth and notched specimens were studied by employing a rotary bending machine with frequency of 52.5 Hz. For smooth specimens, VHCF failure did occur at fatigue cycles of 7.1 × 10⁸ with the related S–N curve of stepwise tendency. Scanning Electron Microscopy (SEM) was used for the observations of the fracture surfaces. It shows that for smooth specimens the crack origination is surface mode in the failure regime of less than 10⁷ cycles. While at VHCF regime, the material failed from the nonmetallic inclusion lies in the interior of material, leading to the formation of fisheye pattern. The dimensions of crack initiation region were measured and discussed with respect to the number of cycles to failure. The mechanism analysis by means of low temperature fracture technique shows that the nonmetallic inclusion in the interior of specimen tends to debond from surrounding matrix and form a crack. The crack propagates and results to the final failure. The stress intensity factor and fatigue strength were calculated to investigate the crack initiation properties. VHCF study on the notched specimens shows that the obtained S–N curve decreases continuously. SEM analysis reveals that multiple crack origins are dominant on specimen surface and that fatigue crack tends to initiate from the surface of the specimen. Based on the fatigue tests and observations, a model of crack initiation was used to describe the transition of fatigue initiation site from subsurface to surface for smooth and notched specimens. The model reveals the influences of load, grain size, inclusion size and surface notch on the crack initiation transition.     

Interior failure mechanism and life prediction of surface‐treated 17Cr‐Ni steel under high and very high cycle fatigue

The understanding of very high cycle fatigue (VHCF) mechanisms is critical to the development of life prediction approach. For this purpose, high cycle fatigue (HCF) and VHCF properties of a surface‐treated 17Cr‐Ni steel were investigated under axial loading with stress ratio of 0. This steel exhibits the constantly decreasing S‐N characteristics associated with the inclusion‐fisheye induced failure under the HCF and the inclusion‐FGA (fine granular area)‐fisheye induced failure under the VHCF. The cyclic pressing plays an important role in the FGA formation process, but the FGA still can be observed for the stress ratio of zero due to the slight crack closure effect. Two life modelling approaches associated with related failure mechanisms in the HCF and VHCF regimes are proposed based on the agreement between experimental and predicted results.     

Mn-Si-Cr系无碳化物贝氏体/马氏体复相高强钢的研究进展

无碳化物贝氏体/马氏体复相高强钢具有比同等强度马氏体钢更优异的韧性和塑性,被广泛应用到轨道交通、机械、建筑等领域。文章概述了低成本Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢近年来在合金化设计、工艺设计、微观组织、强韧化机理、强塑化机理、延迟断裂及疲劳性能等方面取得的研究成果。特别介绍了近年来笔者在BQP工艺处理CFB/M复相钢方面的工作进展,经过BQP处理之后,CFB/M复相钢显示了更优异的强度、塑性、韧性和疲劳性能的匹配。最后简单介绍了Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢在不同领域的应用情况,特别是其在重载高速铁路领域的应用现状和前景。     

EFFECT OF NON-METALLIC INCLUSIONS ON CORROSION FATIGUE RESISTANCE OF P/M DUPLEX STAINLESS STEELS

Abstract— The effect of non-metallic inclusions on high-cycle fatigue resistance of powder metallurgically (P/M) fabricated and hot isostatically pressed (HIP) duplex stainless steels (DSSs) was investigated with axial fatigue test specimens in a chloride and sulphate containing aqueous solution at room temperature. The inclusion content of the studied materials was analysed with bulk oxygen content measurements as well as with optical and digital inclusion analysis methods. Fatigue crack initiation was observed to take place at material defects, i.e., on contaminated prior powder-particle boundaries or, especially, at oxide inclusions. Localised corrosion was not noticed at the initiation sites. Material defects had an especially pronounced effect on fatigue properties, when the stress ratio was R =0, but their effect decreased, when the calculated stress intensity factor was reduced below a certain value. Moreover, the change of the stress ratio from R =0 to R =-1 decreased the difference in high-cycle corrosion fatigue properties between P/M-HIP DSSs with different inclusion contents.     

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

It is well‐known that the high cycle fatigue (HCF) strength of steel components is influenced by a lot of factors depending on both material, loading (including environment), specimen or component geometry (design), and manufacturing process. Based on a literature review of a lot of experimental data, a synthesis is proposed in this paper to discuss the effect of the structural and operational factors on the very high cycle fatigue (VHCF) characteristics of steels. HCF and VHCF regimes are distinguished in terms of failure mechanisms and S‐N curve shapes for high and low strength steels. Then, the effect of the microstructural and mechanical features on the VHCF resistance is debated as different parameters (microstructure, inclusion size type and depth, hydrogen, environment, maximum tensile strength, and residual stresses). Next, the influence of the loading conditions is addressed by taking into account both the frequency effect, the highly stressed volume, the loading type, and loading ratio. Finally, the influence of the testing techniques used in VHCF experiments is discussed.     

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10 V-2 Fe-3 Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10 V-2 Fe-3 Al alloy in very high cycle fatigue(VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/βdecohesion, occur when the stress ratio(R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10~7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from-0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.     

Very high cycle fatigue mechanism of carbide-free bainite/martensite steel micro-alloyed with Nb

In this study, the carbide-free bainite/martensite (CFB/M) steel was micro-alloyed with Nb (CFB/M-Nb). The very high cycle fatigue (VHCF) behaviors of this CFB/M-Nb steel were investigated by ultrasonic fatigue test. Especially, different microstructures of the CFB/M-Nb steel were deliberately designed to disclose the VHCF failure mechanism. In addition, the effect of hydrogen in the formation of the optical dark area (ODA) was analyzed, which was thought to control the formation of ODA by concentrating around interior inclusions. Results show the hydrogen cannot be considered to dominate the formation of ODA. Mostly important, “soft or coarsely soft structure induced fatigue crack” should be responsible for the primary VHCF failure mechanism of CFB/M-Nb steel.     

Very high cycle fatigue behavior of a low strength welded joint at moderate temperature

Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370 °C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed.