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。     

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。      

16MnR 钢的高应变超低周疲劳性能

从压力容器的实际工况和抗震安全性设计出发,研究了压力容器用钢16MnR 钢的高应变超低周疲劳性能。采用横向应变控制方法,在 MTS809拉扭复合疲劳试验机上测定了试验钢应变疲劳过程中的循环应力响应特征及循环应力与应变的关系等超低周疲劳行为;通过拟合 Basquin 公式和 Coffin-Manson 公式,获得了试验钢的高应变超低周疲劳寿命预测公式。断口扫描发现,超低周疲劳下,多处裂纹萌生于试件表面,然后裂纹共同向内扩展,直到最后快速断裂。     

超细晶工业纯钛在3.5%NaCl溶液中的高周疲劳

采用Instron-8801型电液伺服疲劳实验机对复合细化工艺制备的超细晶工业纯钛进行高周腐蚀疲劳实验,实验条件为水浴恒温(17.9±0.5)℃,3.5%NaCl(pH=8.1±0.2),加载频率(f)=25 Hz、应力比(R)=-1。利用得到的实验数据拟合实验加载应力幅(σ_a)与疲劳断裂循环周次(N_f)间的关系,绘制应力-寿命曲线(S-N曲线)。研究材料的腐蚀疲劳性能,并对腐蚀疲劳宏观断口形貌进行观察分析。结果表明：在3.5%NaCl溶液中,超细晶工业纯钛的腐蚀疲劳极限值(σ_-1)为403.1 MPa(44.35%)。与常规疲劳相似,随着加载应力的增加,材料的疲劳断裂循环周次急剧减小,并与常规超细晶工业纯钛和粗晶工业纯钛相比较,发现细晶强化极大地提高了材料的疲劳极限。腐蚀疲劳裂纹源萌生于材料表面,可以直观地观察到材料腐蚀疲劳裂纹的萌生、扩展、断裂典型区域的特征。断口表面较为平整光滑,观察发现有大量的疲劳辉纹和少量的二次裂纹。材料具有较强的疲劳裂纹扩展抵抗能力,耐腐蚀疲劳性能优良。     

钢的塑性形变

本文论述铁素体钢,铁素体珠光体钢,淬火回火钢的静载与动载作用下发生的塑性变形的微观理论,重点介绍位错与碳化物相互作用的位错缠结模型,疲劳裂纹萌生的粗大疲劳变形带的扩展模型和显微区域的循环硬化与软化模型。     

钴磷镀层表面热疲劳裂纹的萌生及扩展机理

在自约束型热疲劳试验机上对化学镀钴磷合金镀层进行了热疲劳试验,采用光学显微镜、扫描电子显微镜研究了在热应力作用下镀层表面热疲劳裂纹萌生、扩展的方式和形态。结果表明:200次冷热循环后,V型缺口处发生塑性变形,并且随冷热循环的进行,热疲劳裂纹由缺口底部萌生并沿着循环方向扩展。重点分析了钴磷合金镀层表面热疲劳裂纹的萌生及扩展机理。     

400 MPa级薄钢板的疲劳性能及断口分析

 利用液压伺服疲劳实验机研究了400 MPa级板带在对称循环应力下的高周疲劳性能,测量了S N曲线,确定了应力幅低于疲劳强度时疲劳寿命与应力幅之间的关系式;用扫描电镜观察该钢的疲劳断口,分析了疲劳断裂特点;在透射电镜下观察了断口附近的微观组织,并对其疲劳断裂机理进行了分析,结果表明：疲劳裂纹起源于表面驻留滑移带挤出脊、侵入沟或表面缺陷,裂纹源扩展最终导致断裂。     

不同烧结条件下Fe-Cu-Ni-Mo-C合金的弯曲疲劳性能及断口分析

以部分扩散预合金Fe-2Cu-2Ni-1Mo-1C粉末为原料,利用模壁润滑温压技术与3种烧结工艺制备合金材料,研究不同烧结工艺下合金的疲劳性能。结果表明:3组不同烧结工艺制备的试样在104~109循环周次下的应力幅值-循环周次(S-N)曲线均为一条连续下降的曲线,不存在传统疲劳概念上的疲劳极限,只存在条件疲劳极限;在107循环周次下,3组试样的条件疲劳极限分别为280、264和239 MPa。断口分析发现,3组试样的疲劳裂纹均萌生在试样表面的棱角处,且均表现为多源萌生;疲劳裂纹扩展以穿晶断裂为主,不同的组织对裂纹的扩展有明显的影响;扩展区存在典型的解理和疲劳辉纹形貌;断裂区有塑性韧窝出现。     

Q345R钢湿硫化氢预腐蚀低周疲劳行为

对Q345R钢在湿硫化氢环境中预腐蚀低周疲劳性能进行了研究,以硫化氢溶液的浓度和预腐蚀时间为环境因素.低周疲劳试验在MTS-809疲劳试验机上进行,对各试验组的结果数据进行回归分析,得到各试验组环境下Q345R钢的循环应力-应变曲线、应变-寿命曲线等低周疲劳特性.不同试验环境下的预腐蚀低周疲劳结果表明,材料循环应力应变响应特性不受环境因素影响,为循环硬化特性;预腐蚀时间因素对材料低周疲劳寿命的影响比硫化氢溶液的浓度因素显著.试样断口为典型的低周疲劳断裂形貌,裂纹扩展阶段为脆性准解理断裂特征.     

无镍高氮不锈钢弯曲疲劳裂纹萌生与扩展行为

 采用四点弯曲疲劳试验研究了不同应力水平下无镍高氮不锈钢的疲劳行为,并对材料疲劳裂纹的微观形貌、萌生位置及扩展路径进行了分析。结果表明,试验钢疲劳为多裂纹起裂,随着应力水平的升高,裂纹总长度逐渐增加,当应力水平接近材料屈服极限时,裂纹长度趋于稳定;裂纹大多数在滑移带处萌生,裂纹在扩展过程中产生了扭曲、偏移和分叉现象;裂纹在晶内主要沿单滑移带或多滑移带交替扩展,穿过晶界或孪晶界时大多发生了偏转。     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

Inclusions,Stress Concentrations and Surface Condition in Bending Fatigue of an H13 Tool Steel

Combined effects of inclusions, surface condition and geometrical stress concentration on the fatigue strength of a tool steel have been studied. Bending fatigue testing was performed for milled tool components and smooth specimens produced from an experimental AISI H13 tool steel with varying additions of sulphur and oxygen. Inclusion distributions of the different test series were determined. Fracture surfaces after fatigue failures were studied in a scanning electron microscope. The sulphur content does not have a significant influence on the fatigue strength of the milled tool components in the range of 0.02 ‐ 0.09 wt%. A clear reduction in the fatigue strength is obtained for the smooth specimens when the steel becomes rich in inclusions. Here, the oxide type inclusions dominated as crack initiators and the effect of sulphides was smaller. The results demonstrate the reduced effect of inclusions on the fatigue strength in components with large stress concentrations and rough surfaces.     

15Cr14Co12Mo5Ni2齿轮钢的扭转疲劳特性及裂纹扩展行为

 通过扭转疲劳试验,研究了15Cr14Co12Mo5Ni2钢的扭转疲劳断裂的裂纹扩展行为和夹杂物尺寸与扭转疲劳寿命之间的关系。得到了钢的扭转疲劳极限强度和[τ-N]曲线,15Cr14Co12Mo5Ni2钢的扭转疲劳极限强度为350 MPa,扭转疲劳寿命分散度较大。通过断口观察,发现15Cr14Co12Mo5Ni2钢的疲劳破坏模式以表面破坏和近表面破坏为主,主要由氧化物夹杂引起。通过计算应力强度因子[ΔK]和裂纹扩展门槛值[ΔKth]分析15Cr14Co12Mo5Ni2钢的疲劳裂纹扩展的断裂力学条件,试验钢在断裂过程中受载荷情况为,II型载荷—I型载荷—II型载荷—I＋II型载荷,分别对应起裂源区、纤维区、疲劳裂纹扩展区和瞬断区;当有大裂纹产生时,则不会产生纤维区,受载荷情况则为：II型载荷—I＋II型载荷。通过公式推导和数据拟合得到夹杂物尺寸和15Cr14Co12Mo5Ni2钢扭转疲劳寿命的关系,发现随着夹杂物尺寸减小,钢的[τ-N]曲线向高寿命区移动。当引起裂纹萌生的夹杂物尺寸小于5 μm时,在350 MPa应力下,15Cr14Co12Mo5Ni2钢的扭转疲劳寿命超过107循环周次。     

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.     

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。     

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.     

CSP流程Ti微合金化高强钢的疲劳性能

采用升降法对CSP工艺生产的2mm厚Ti微合金化高强钢的疲劳性能进行研究.结果发现:高强钢的抗拉强度为830 MPa;疲劳强度为685 MPa,约为抗拉强度的0.83倍;伸长率为18.8%.绘制了高强钢的S-N曲线,并拟合出疲劳寿命与最大应力的关系.通过扫描电镜对疲劳断裂机理进行了分析.宏观疲劳断口可见明显的裂纹源区、扩展区和瞬断区形貌.疲劳裂纹起始于带钢表面微裂纹;疲劳扩展区存在微观疲劳辉纹、二次裂纹和宏观疲劳贝纹线;瞬断区出现撕裂棱,兼有韧窝存在.      

800 Mpa级含Ti高强钢铸坯断裂的原因

通过高温热塑性试验,借助光学显微镜、透射电镜对某钢厂生产的 800 MPa 级高强钢铸坯开裂原因进行了研究,并提出了改进措施.研究结果表明:铸坯开裂形式为"角部开裂";其主要原因是连铸后铸坯缓冷不充分,造成铸坯角部与其内部温度差异较大,在组织变化和热应力的双重作用下导致裂纹萌生,并进一步扩展为"角部开裂".连铸后采用扣罩缓冷措施可解决铸坯开裂问题.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Fatigue fracture mechanism maps for a type 304 stainless steel

Fatigue fracture mechanism maps at room temperature and 573 K for a type 304 stainless steel were constructed by correlating the crack propagation rate with information obtained on the fracture surface. Depending on the crack propagation rate, ranging from 1 × 10⁻⁶ to 1 × 10⁻¹¹ m/cycle, three types of fracture surfaces were observed. One was a striation region; the second was a “featureless” fracture region, which appeared rough under scanning electron microscope (SEM) observation; and the third was crystallographic fracture region, which appeared smooth under SEM observation. The area fractions and the indexes of the fracture surfaces were quantified and identified by the etch-pit method. From the results, crack initiation and propagation mechanisms were cleared and fatigue fracture mechanism maps were constructed. The maps may be useful for investigating the cause of the fatigue failure accident of structures made of type 304 steels.