SHORT CRACK FATIGUE BEHAVIOUR IN A MEDIUM CARBON STEEL

The initiation stage and short crack behaviour in torsional fatigue of a 0.4% C steel was investigated by a replication technique. The fatigue cracks initiated and propagated in the ferrite phase which is located at the prior austenite grain boundaries in the form of long allotriomorphs. At this stage of crack development it is proposed that crack growth rate depends on the extent and intensity of plasticity at the tip of the crack. Crack growth per cycle is correspondingly proportional to the strength of the slip band. The ferrite-pearlite boundaries are strong barriers to crack propagation, which is manifested by a deceleration of growth and possible arrest. On raising the stress level the previously non-propagating cracks may continue to grow by branching or joining with other cracks in the ferrite phase. This process is repeated until the stress fields of one or more dominant cracks attain a critical value to sustain continued growth that leads to failure.     

渗碳层耐磨性的比较分析

对Q195普通低碳钢分别进行热浸镀铝加共渗处理和渗碳处理,并对两种处理件进行耐磨性试验.结果表明,经热浸镀铝加共渗的处理件,由于表面氧化膜的作用,以及合金层韧性和硬度的较好配合,其耐磨性在相同试验条件下要明显优于渗碳件.     

ESTIMATION OF TORSIONAL FATIGUE STRENGTH OF MEDIUM CARBON STEEL BARS WITH A CIRCUMFERENTIAL CRACK BY THE CYCLIC RESISTANCE-CURVE METHOD

Near-threshold fatigue crack propagation tests were performed on circumferentially precracked round bars of a medium carbon steel under torsional loading. The crack propagation rate decreased with crack extension, because of the shear contact of crack faces. The crack propagation rate without the influence of crack-surface contact was determined by extrapolating to zero crack extension the relationship between the crack propagation rate and crack extension. The applied stress intensity factor range was divided into two parts: one was the effective value responsible for crack growth and the other was the value corresponding to crack-tip shielding. The resistance-curve method was used to predict the fatigue limit for crack initiation and fracture. The R -curve was constructed using the experimentally determined threshold value of the stress intensity range, which was the sum of the threshold effective stress intensity range and the threshold shielding stress intensity range. The threshold effective stress intensity range was constant. The R -curve was independent of the precrack length and specimen dimensions. The predicted values agreed well with the experimental results.     

Q195钢热浸镀铝加共渗处理合金层与渗碳层耐磨性的比较分析

对Q195普通低碳钢分别进行热浸镀铝加共渗处理和渗碳处理,并对两种处理件进行耐磨性试验。结果表明,经热浸镀铝加共渗的处理件,由于表面氧化膜的作用,以及合金层韧性和硬度的较好配合,其耐磨性在相同试验条件下要明显优于渗碳件。     

激光熔敷Ti5Si3/NiTi2复合材料涂层的组织与耐磨性

以Ti-Si-Ni合金粉末为原料，利用激光熔敷技术在BT9钛合金表面制备出了以金属间化合物Ti5Si3为增强相，以NiTi2为基体的Ti5Si3/NiTi2金属间化合物快速凝固耐磨复合材料涂层，Ti5Si3/NiTi2合材料涂层的硬度高，组织均匀，致密，与基材之间为完全冶金结合，在干滑动磨损试验条件下具有很好的耐磨性。     

热喷涂层的梯度液相烧结

将金属陶瓷粉末与具有不同熔点的自熔合金粉末混合喷涂于金属基材表面,形成低熔点相的固液相变点温度从基材界面向涂层表面逐步升高的梯度涂层,研究了在1000℃烧结后烧结层的显微组织结构和力学性能。结果表明,在烧结时出现液相的区域内喷涂层所固有的层状结构消失,在界面上有大约20μm宽的扩散区并呈冶金结合,在表层存在明显的固－液相分界线,在显微组织中氧化物有球化趋势。梯度液相烧结可使涂层稳定成型所需的最佳控制参数区间变大,涂层的力学性能满足使用要求。     

A STUDY OF SHORT CRACK GROWTH IN TORSIONAL LOW CYCLE FATIGUE FOR A MEDIUM CARBON STEEL

A study of short crack growth for a medium carbon steel in low cycle torsional fatigue has been carried out in order to measure crack length and derive growth laws. Comparison with a previous analysis of uniaxial tests in low cycle fatigue for the same material provides a tentative basis for formulating an equivalent stress approach to describe uniquely the propagation of short cracks under multiaxial stress.     

碳/碳复合材料MoSi2涂层的防氧化研究

研究了碳/碳复合材料MoSi₂防氧化涂层的性能.结果表明,MoSi₂涂层系统具有1500℃长时间防氧化作用,在1500℃时,带涂层的碳/碳复合材料长时间氧化失重速率稳定在2.43×10-5g/m2·s,242小时的氧化失重为0.57%,该质量损失表现为涂层系统自身蒸发损耗;同时,MoSi₂涂层具有良好的自愈性和抗热震性.     

FATIGUE LIFE DISTRIBUTION AND GROWTH OF CORROSION PITS IN A MEDIUM CARBON STEEL IN 3%NaCl SOLUTION

Abstract— Statistical fatigue tests have been conducted on a structural medium carbon steel, S45C, in room air and in 3%NaCl solution, using five cantilever-type rotary bending fatigue testing machines which were specially manufactured for the purpose of the present study. Fatigue life distribution was examined at three and five stress levels in air and in 3%NaCl solution, respectively, and twenty specimens were allocated to each stress level. In room air, it was found that fatigue life distributions followed the three-parameter Weibull distribution, which were closely related to fracture morphology. In 3%NaCl solution, they also followed the Weibull distribution, but the scatter in fatigue life was smaller in comparison to that in air. It is suggested that the decrease in the scatter of fatigue life may be attributed to a smaller fraction of crack initiation life in 3%NaCl solution. The growth of corrosion pits was investigated using a laser microscope. The distribution of corrosion pit depths followed the log-normal distribution, and the corrosion pit depths increased with increasing time or the number of cycles. It was found that the growth of corrosion pits was accelerated by stress cycling and the depths increased with increasing stress level. Based on these results, a growth law of corrosion pits, including the effect of stress cycling, is proposed.     

化学镀Ni－P－B_4C复合镀层的耐磨性

本文通过比较化学镀Ｎｉ－Ｐ－Ｂ＿４Ｃ复合镀层、Ｎｉ－Ｐ－ＳｉＣ复合镀层、Ｎｉ－Ｐ合金层和电镀铬层的性能，发现Ｎｉ－Ｐ－Ｂ＿４Ｃ复合镀层是其中最理想的抗磨材料。试验证明：由于Ｂ＿４Ｃ微粒的硬度高于ＳｉＣ微粒，并且Ｂ＿４Ｃ本身又具有较高的抗显微切削能力，所以Ｎｉ－Ｐ－Ｂ＿４Ｃ复合镀层的耐磨性显著高于Ｎｉ－Ｐ－ＳｉＣ复合镀层和Ｎｉ－Ｐ合金层。由于电镀硬铬层的硬度随磨擦热的升高而迅速下降，所以其耐磨性远不及Ｎｉ－ＰＢ＿４Ｃ复合镀层。     

FAST索网疲劳评估及高疲劳性能钢索研制

500 m口径射电望远镜(FAST)采用柔性索网作为主要支承结构,变位观测过程中索网将长期承受高达500MPa的疲劳载荷,约为目前相关标准规范规定值的两倍。索网结构将是该望远镜系统的关键易损部件,故该文主要开展如下工作：1) 根据望远镜的科学目标,估算未来30年望远镜的观测轨迹,评估望远镜运行观测对索网结构疲劳性能的要求;2) 结合环氧涂层技术和改进挤压锚固工艺研制高疲劳性能钢索结构,并通过实验验证该种钢索结构可以满足该望远镜对索网结构的技术要求。     

THERMAL FATIGUE OF A 12% CHROMIUM MARTENSITIC STAINLESS STEEL

Abstract— The cyclic mechanical behaviour during thermal fatigue of a martensitic stainless steel (MANET I) is investigated and correlated with its microstructure. The tests were performed in air by allowing the sample to serve as its own heater and converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The effects of different thermal cycling ranges and the imposition of several hold-time conditions on the mechanical behaviour and the accompanying microstructural changes in the specimen were evaluated. A continuous softening is the characteristic feature observed in all thermal fatigue tests. Higher mean temperatures, larger temperature changes and the application of temperature-hold periods produce an accelerated softening process. All temperature hold-time conditions influence the mechanical behaviour, reducing the number of cycles to failure. The initial tempered martensite lath structure evolves to a mixed structure of expanded laths and subgrains or a fully subgrain structure, depending on the mean temperature, the temperature change and the temperature at which the hold period has been applied. Tests with temperature holds at higher temperatures show more deleterious effects than tests with hold-times at lower temperatures.     

FATIGUE MICROCRACKS IN A LOW CARBON STEEL

Abstract— The behaviour of a low carbon steel has been studied, in particular the initiation, growth and coalescence of fatigue microcracks on the surfaces of smooth specimens via surface replicas and photomicrographs. From the study, quantitative information on the initiation period, growth and coalescence of cracks, statistical distributions of crack length and crack depth, density of cracks, distribution pattern and characteristics of the major crack, were obtained. Knowledge of these parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life assessment.     

碳钢材料的表面冲蚀疲劳研究

根据固体疲劳磨损理论并结合接触力学的有关分析结论,设计了一套确定塑性接触状态下材料表面粒子冲蚀疲劳特性参数的方法,同时依此方法确定碳钢材料的粒子冲蚀应变值,应变指数。     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW CARBON STEEL UNDER CORROSION FATIGUE CONDITIONS

The effect of an aqueous chloride environment upon the development and growth of short fatigue cracks from smooth specimen surfaces has been studied under fully reversed torsional fatigue loading conditions. Crack initiation and growth has been monitored using a plastic replication technique enabling a full history of cracking characteristics to be recorded. Corrosion fatigue conditions were achieved by complete immersion in a 0.6 M NaCl solution, of nominal pH value 6.0, with specimens corroding at the free corrosion potential. Variations to these conditions were obtained by the addition of concentrated hydrochloric acid enabling test solution pH values to be altered, typically pH values of 3.5 and 2.0 were obtained. Further information regarding the effects of the environment on the early stages of crack development were obtained by conducting two stage alternate immersion type testing conditions. Evaluation of these effects through previously established Elastic-Plastic Fracture Mechanics models shows that the environment plays a major role during the early stages of microstructure-dominated crack growth particularly as cracks approach major barriers to propagation and at decreasing levels of applied shear stress.     

化学镀Ni-P-B4C复合镀层的耐磨性

本文通过比较化学镀Ni-P-B₄C复合镀层、Ni-P-SiC复合镀层、Ni-P合金层和电镀铬层的性能,发现Ni-P-B₄C复合镀层是其中最理想的抗磨材料。试验证明:由于-B₄C微粒的硬度高于SiC微粒,并且-B₄C本身又具有较高的抗显微切削能力,所以Ni-P-B₄C复合镀层的耐磨性显着高于Ni-P-SiC复合镀层和Ni-P合金层。由于电镀硬铬层的硬度随磨擦热的升高而迅速下降,所以其耐磨性远不及Ni-P-B₄C复合镀层。      

LASER SURFACE TREATMENTS FOR PROMOTING CORROSION RESISTANCE OF A CARBON STEEL

The effect of laser treatment and laser alloying on corrosion resistance of 1045 steel has been studied. Various ways of surface alloying have been investigated by using continuous CO₂ laser beam: i) irradiation of chromium painted surfaces; ii) irradiation of Ni and Cr-electroplated surfaces; iii) direct injection of Ni and Cr powders into the melt pool. A high and uniform level of alloying in the surface layer can be achieved in the cases of electroplated surfaces and direct powder injection. It requires, however, an appropriate choice of irradiation conditions (such as beam power, beam traverse speed, beam defocusing and degree of overlapping) that provide remelted layer of a limited and fairly uniform thickness. It has been found that in order to achieve corrosion behavior of laser treated surfaces similar to that of austenite type 304 stainless steel chromium and nickel contents in the alloyed layer are to be higher than those of 304 steel.     

TORSIONAL FATIGUE OF A MEDIUM CARBON STEEL CONTAINING AN INITIAL SMALL SURFACE CRACK INTRODUCED BY TENSION–COMPRESSION FATIGUE: CRACK BRANCHING,NON-PROPAGATION AND FATIGUE LIMIT

In order to examine the threshold condition for the fatigue limit of materials containing a small crack under cyclic torsion, reversed torsional fatigue tests were carried out on 0.47% C steel specimens containing an initial small crack. Initial small semi-elliptical cracks ranging from 200 to 1000 μm in length were introduced by the preliminary tension–compression fatigue tests using specimens containing holes of 40 μm diameter. The threshold condition for the fatigue limit of the specimens containing artificial small defects under rotating bending and cyclic torsion are also reviewed. Crack growth behaviour from an initial crack was investigated. The torsional fatigue limit for a semi-elliptical small crack is determined by the threshold condition for non-propagation of Mode I branched cracks. The torsional fatigue limit of specimens containing an initial small crack can be successfully predicted by the extended application of the √area parameter model in combination with the σ_θmax criterion.