Corrosion fatigue of steel under cavitation erosion generated intermittently in 3% salt water (results of two-stage tests with and without cavitation erosion)

Two-stage fatigue tests with and without cavitation erosion were performed in 3% salt water. When corrosion fatigue with cavitation erosion (erosion fatigue) is changed to corrosion fatigue during a test, the fatigue life decreases remarkably and the total number of stress cycles to failure becomes smaller than that for tests involving erosion fatigue only. In the reverse case, when corrosion fatigue conditions are changed to erosion fatigue conditions, the fatigue life increases slightly compared with the life expected from the linear damage law (Miner's law). The variation in fatigue strength can be explained by the characteristics of crack propagation in each environment. From these results, it is presumed that the corrosion fatigue strength of the material exposed intermittently to cavitation erosion decreases remarkably.     

Relations between cavitation erosion resistance of materials and their fatigue strength under random loading

The paper contains results of tests on fatigue strength under uniaxial random loading and cavitation erosion resistance for three steels: 10HNAP, 18G2A and 15G2ANb. The obtained fatigue and cavitation characteristics were used for determination of relations between these two phenomena. From the analysis it appears that there is correlation between fatigue strength of the material under random loading and its cavitation erosion resistance. It has been shown that fatigue tests under random loading and tests on cavitation erosion of 10HNAP, 18G2A and 15G2ANb steels may be described with a mathematical model of the same type. It has been also found that there is a linear relation, in the dual logarithmic system, between cavitation erosion resistance of the steels tested and their fatigue strength under random tension—compression with zero mean value.     

Cavitation erosion resistance of high nitrogen stainless steel in comparison with low N content CrMnN stainless steel

Abstract

The cavitation erosion (CE) of a high nitrogen stainless steel (HNS) and a low nitrogen CrMnN stainless steel in both distilled water and 3%NaCl solution at 20±1°C was investigated by using a magnetostrictive induced cavitation facility. The evolution of CE with test time was analysed by morphology observation by SEM and roughness measurement after different CE intervals. The possible phase transformation of austenite to martensite due to cavitation was analysed by XRD, and cross-sectional microhardness after cavitation was also measured to evaluate the work hardening ability. The role of corrosion was analysed by polarisation curve. The test results indicated that HNS had a relatively higher CE resistance than CrMnN steel, which was mainly attributed to its higher work hardening ability, thicker wok hardening layer and lower stacking fault energy. Different from that of the HNS, many tiny cracks could be clearly seen in the cross-section of eroded CrMnN steel especially at the ferrite zones. The pure erosion dominated the whole cavitation damage process, and the synergistic effect between corrosion and erosion was relatively small for both steels. The CE behaviour of HNS was relatively more sensitive to the corrosion media than that of CrMnN steel. Therefore, it should be a little bit careful when HNS was used in corrosive media.     

钢齿轮表面合金涂层的显微结构和接触疲劳强度研究

研究了45钢齿轮轮齿表面Ni-P和Ni-Co合金涂层的显微结构和接触疲劳强度。采用电子显微镜、光学显微镜等对合金涂层进行了表面形貌、次表面组织、纵剖面界面组织、表面粗糙度、涂层的硬度和疲劳点蚀坑形貌特征的分析。接触疲劳试验结果表明,表面合金涂层能显著提高接触疲劳寿命和强度,使之与40Cr钢调质试件的相当。     

Formation and Development of Iridescent Rings Around Cavitation Erosion Pits

The erosion pits formed in an ultrasound vibration cavitation were compared to those corrosion pits formed in static water. Concentric iridescent rings around the erosion pit make it different from those corrosion pits. The distinct color and the α-FeOOH structure of the rings indicated that the iridescent rings were oxidization film formed under high temperature generated in the cavitation environment. According to the direct observation on the development of the iridescent rings under microscope, the formation of the iridescent rings was attributed to the repeated interactions between mechanical removal and high temperature oxidization during the cavitation erosion. This work also illustrated the synergistic effect of erosion-enhanced corrosion during the cavitation erosion.     

冲击形式产生的冲蚀、空蚀和腐蚀联合磨损试验研究

舰船等海洋运输装备在海水中行驶时,船体表面受到含沙海水的冲蚀磨损,舰船行驶速度变化引起的空蚀磨损,以及海水的腐蚀磨损,因此舰船表面材料的破坏形式是冲击式的冲蚀、空蚀和腐蚀联合磨损。为了研究联合磨损的影响因素,研制冲蚀、空蚀和腐蚀联合磨损试验台,并实验研究沙粒浓度、沙粒尺寸、含盐浓度以及冲蚀速度对45钢试件的联合磨损影响规律。实验结果表明:冲蚀、空蚀和腐蚀联合磨损程度强于冲蚀和空蚀交互磨损或单纯腐蚀磨损,在较低的沙粒浓度和较低的含盐浓度条件下,金属材料联合磨损质量损失与沙粒浓度、沙粒尺寸、含盐浓度和冲蚀速度均成正比关系。磨损面形貌分析表明:随着沙粒浓度、沙粒尺寸、含盐浓度、冲蚀速度的增加,试件表面的冲蚀沟和空蚀孔的磨痕深度和面积增大,而在人造海水中,试件表面不仅存在冲蚀沟和空蚀孔,还产生了腐蚀坑。     

Investigation of the Ring Area Formed Around Cavitation Erosion Pits on the Surface of Carbon Steel

The ring areas formed around micropits in cavitation erosion experiments were investigated. The corrosion behavior and vibratory cavitation erosion tests of mild carbon steel in tap and distilled water were carried out. Thus, the ring areas were densely formed in tap water and scarcely formed in distilled water in cavitation tests. The ring areas formed around micropits in cavitation and free cavitation tests have a similar shape. Moreover, SEM examinations showed that the corrosion products spread within the ring areas. Thus, the ring areas formed around micropits are the result of corrosion effect and are not the result of thermal effects due to bubble collapse.     

The role of sand particles on the rapid destruction of the cavitation zone of hydraulic turbines

Metal surfaces in the cavitation zone of hydraulic turbines working in sandy water are damaged rapidly. The damage is much more severe than that caused either by sand erosion or by cavitation alone. This paper presents an analysis of a worn surface of 18Cr-8Ni steel tested in a Venturi device at a hydroelectric power station in the flood season. Two types of damage were found. The first was a smooth erosion in the area of regular fluid flow. The second was in the regions where cavitation occurred. In the latter regions, the sand particles gain high velocities under the action of cavitation, impinge upon the metal surface and cause severe surface deformation and rapid damage. Embedded sand particles, cavities, cracks, cutting scars and fatigue patterns were found on the sponge-like worn surface.     

MICRO-COURSE OF CAVITATION EROSION

By cavitation tests and scanning electron microscope （SEM） microanalysis, the micro-appearance of cavitation samples is studied. It is the first time that the micro-appearance of metals is pursued successfully. According to the changing course of the micro-appearance of metals, the damaging course of cavitation erosion is determined. The destructive way of collapsing bubbles on the metal surface is known. Firstly cavitation pinholes appear on the metal surface, then cracks generate and grow under the action of collapsing bubbles. When cracks connect each other, small pieces are removed from pinhole wall and pinholes develop into cavitation pits. When the previous surface is removed completely, new pinholes are produced again on the new surface. A pinhole is the result of the powerful striking of a micro-liquid jet ejected by a large collapsed bubble near the surface. At some stages, cracks grow in the way of fatigue. The corrosion phenomenon is observed during the cavitation erosion. The cavitation pattern can be used to explain the cavitation pregnancy and the changing regulation of sample surface.     

表面粗糙度对304不锈钢空蚀腐蚀联合作用的影响

为探究不同表面粗糙度对腐蚀空蚀破坏的影响机制,利用超声空蚀实验装置,对不同表面粗糙度的304不锈钢试样在质量分数3.5%NaCl溶液中进行空蚀腐蚀联合作用实验。通过分析试样的质量损失、表面形貌、显微硬度和电化学性能变化,探究不同表面粗糙度对空蚀腐蚀表面形貌和性能的影响规律。结果表明：空蚀腐蚀后的304不锈钢试样表面出现蚀坑,且蚀坑主要发生在磨痕边缘,并呈现腐蚀和空蚀双重特征;随着表面粗糙度的增大,蚀坑的密度增加,试样质量损失更加严重,但粗糙度达到一定程度时,空蚀会降低表面粗糙度数值,起到平整作用;经过空蚀腐蚀作用后,试样表面均出现硬化现象,但随着表面粗糙度的增大,试样表面硬化现象变弱;空蚀腐蚀后,试样的腐蚀电流增大,腐蚀电位负移,且随着粗糙度增大,试样的腐蚀电位和腐蚀电流变化也增大,呈现耐腐蚀性能不断下降趋势。     

Effect of heat treatment on the structure,cavitation erosion and erosion–corrosion behavior of Fe-based amorphous coatings

In this study, high-velocity oxygen-fuel sprayed amorphous coatings have been heat treated at various temperatures to form microstructures with crystalline phases. The structure, micro-hardness, cavitation erosion resistance and erosion–corrosion resistance of these coatings are compared. Crystalline phases are discovered in the coatings after heat treatments at 650 °C and 750 °C. The coating heat treated at 750 °C exhibits the poorest cavitation erosion resistance in 3.5 wt% NaCl solution among all coatings due to the degraded corrosion resistance. However, the hardness of the crystallized coating can reach 1000 Hv and the erosion–corrosion resistance of the heat treated coating is better than the untreated one.     

空蚀破坏的微观过程研究

通过空蚀试验和扫描电镜分析对空蚀试样的破坏过程进行了研究,进行了微观点跟踪观察。结果表明：在金属表面产生空蚀针孔,在随后大量气泡溃灭作用下,针孔壁逐渐小块剥落,形成空蚀坑,当空蚀坑相互连接时,原始表面全部剥落,新的空蚀针孔和空蚀坑又重新形成,使金属表面层层剥落。确定单个气泡溃灭产生的微射流是在金属表面造成空蚀针孔的原因,而空蚀坑的形成则是疲劳裂纹萌生和扩展的结果。     

功率超声珩磨空化试验分析

功率超声珩磨加工过程中会发生空化现象,为探究空化对被加工材料表面的影响,进行超声珩磨空化正交试验并针对凹坑最大直径、表面侵蚀率、表面粗糙度三个指标进行试验分析,这三个指标可分别表征单泡溃灭强度,整体空化强度及空化对材料表面质量影响,结果表明：空化造成材料表面微小凹坑,影响凹坑最大直径的主要因素依次为距离和振幅,距离越小,振幅越大,则凹坑最大直径越大;影响表面侵蚀率的主要因素依次为试验时间和距离;而振幅则对材料表面粗糙度的影响最大,在距离5 mm、振幅65%、试验时间1/3 min条件下,试样表面粗糙度降低,表面质量提高。因此,在一定条件下,空化效应有助于改善超声珩磨中工件表面质量,试验分析结果对超声珩磨实际加工具有借鉴意义。     

A study of cavitation bubble collapse pressures and erosion part 3: the results in a venturi facility

Cavitation erosion generated in a venturi facility was studied by comparing the erosion loss with the distributions of cavitation bubble collapse pressures (impact loads). The erosion process in the venturi tests is similar to that in the vibratory tests, although its progression is very slow. That is, the surface first deforms and fractures as a result of fatigue with repeated bubble collapse pressures below the threshold pressure needed to form a pit impulsively. By comparing the distributions of impact loads measured using our method with the hypothetical stress-number of cycles curves for fatigue, it is found that the incubation period and the volume loss rate during the stable period follow Miner's law regardless of the venturi, vibratory and cavitation conditions and materials. Therefore we found that we are able to estimate cavitation damage in a flowing system in the same way as damage in the vibratory tests from Miner's law although the distributions of cavitation bubble collapse pressures are markedly different.     

Resistance to wear and cavitation erosion of bearing alloys

Sliding wear and vibratory cavitation erosion tests in paraffin oil were carried out on bearing alloys, i.e. tin-based and lead-based white metals, Cu-Pb alloy and leaded bronze. In lubricated wear under mild conditions the surface is worn smooth and a slight difference exists between the wear resistances of the four alloys. In cavitation erosion an eroded surface which is much rougher than the worn surface is formed. Cavitation erosion is affected strongly by the composition and crystal structure of the alloy and thus the erosion resistances of the four alloys differ greatly, the ranking of resistance being lead-based white metal < Cu-Pb alloy < tin-based white metal < leaded bronze. The surface damage, which is caused by the joint action of cavitation erosion and wear, was also investigated by rubbing the eroded surfaces which had been exposed to cavitation erosion for various times. This damage becomes larger with increasing cavitation damage. The resistance to this damage differs much more in the four alloys tested and tends to correlate with the results of the erosion tests rather than those of the wear tests. Therefore, it is clear that the cavitation erosion resistance should be considered in the selection of bearing materials.     

Bending fatigue characteristics of corroded wire ropes

The corrosion of a wire rope reduces its life expectancy. In this study, repeated bending tests were conducted using a bending fatigue tester by changing the tensile load and corrosion time of wire ropes, which were the same type as those used in elevators. The number of broken wires was studied, and a tensile test was conducted for cases in which the fracture was severe. The effect of corrosion fatigue on life expectancy was considered by comparing fracture strength values and observing fracture surfaces. The results indicate that an increase in accumulated corrosion fatigue, a greater tensile load, and repeated bending cycles may yield a rapid decrease in fracture strength and an increase in the number of broken wires. Therefore, it is concluded that corrosion fatigue is an important factor that decreases the life expectancy of wire rope.     

Dissimilar material welding and assessing reliability of super alloy for green and high efficiency thermal power plant

This paper studies the prediction of fatigue and corrosion fatigue lives using neural network and accelerated life methods for dissimilar material weld between Alloy617 and 12Cr steel. First, dissimilar material welding between Alloy617 and 12Cr steel was performed using buttering technology. The fatigue and corrosion fatigue strengths, and electrochemical corrosion susceptibility of dissimilar material weld were assessed. After that, on the basis of obtained data, fatigue life and corrosion fatigue life of dissimilar material weld were predicted using the neural network and accelerated life test methods. The predicted results showed good agreement with the actual fatigue and corrosion fatigue lives. Especially, the results of the neural network prediction were more accurate than those of the accelerated life method.     

Cavitation erosion of NiAl-bronze layers generated by friction surfacing

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material.     

日历腐蚀环境下LY12CZ铝合金力学性能研究

采用加速腐蚀当量折算谱模拟日历腐蚀环境，对飞机结构主体材料LY12CZ先做加速腐蚀后再进行疲劳试验，确定日历腐蚀环境对LY12CZ材料静强度和疲劳强度（S－N曲线）的影响。     

温度及压力对滑动轴承气蚀磨损影响的实验研究

采用超声振动原理建立了滑动轴承模拟气蚀实验装置 ,研究了温度及压力变化对滑动轴承气蚀磨损的影响。发现温度及压力的升高都会使滑动轴承的气蚀磨损增加。并对这一现象从理论上进行了分析