Strahlverschleiß von Stahl mit niedrigem Kohlenstoffgehalt: Untersuchungen zur Verschleißpartikelgeometrie

Erosion of low‐carbon steel by solid particle impingement: aspects of wear debris geometry Image analysis software was used to analyse the geometry of debris formed during the erosion of low‐carbon steel by impinging solid particles. Depending on the two‐dimensional aspect ratio (ratio between debris height and debris width), three different debris types could be distinguished. The most frequent type observed was a platelet‐type debris as suggested by the Bellman‐Levy (1981) model. This wear debris shape type covered about 60 % of all acquired debris. Plain micro‐machining according to Finnie’s (1959) suggestion played a negligible role only, but other processes, namely ploughing as suggested by Winter and Hutchings (1974), were more important. The statistically estimated mean debris size was about 14 μm. About 92 % of all wear debris had sizes smaller than the target material grain size. This result supports the figure that multi‐step removal modes – the formation and detachment of lip or platelet from crater rims ‐ were responsible for material removal.     

The microstructure,mechanical, and fatigue behaviours of MAG welded G20Mn5 cast steel

The microstructure, mechanical strength, and fatigue response of metal active gas (MAG) butt‐welded G20Mn5 cast steel was thoroughly investigated for exploring the service safety and reliability of new‐generation railway bogie frames. The fatigue properties of the matrix and welded joints were determined by both low‐ and high‐cycle service regimes. On the basis of nanoindentation testing, the fatigue crack growth (FCG) was derived by correlating with cyclic plastic response of microdomain materials across the MAG joint. The results show that the MAG induces considerable changes in microstructures and hardness of the G20Mn5 matrix and resultantly produces an overmatching welded joint but show comparatively low‐ and high‐cycle fatigue properties to as‐received material. The calculated threshold FCG range based on the Murakami model indicates that the maximum 1.5‐mm defect might be the cracking site subjected to fatigue loading from the structural integrity viewpoint.     

Impact of wear debris on striated muscle‐ a comparative in‐vivo study with titanium and stainless steel

Wear debris and corrosion products of metal implants induce biological events that may have severe consequences for skeletal muscle microcirculation. We therefore studied in vivo leukocyte‐endothelial cell interaction and leukocyte transmigration in skeletal muscle after confrontation with characterised titanium and stainless steel fretting corrosion particles, and compared these results with those of the bulk materials. Using the hamster dorsal skinfold chamber preparation and intravital microscopy, we could demonstrate in 30 animals that stainless steel induces a more pronounced inflammatory answer in contrast to the implant material titanium. However we were not able to show a general benefit of bulk vs. debris. Overall the study suggests that not only the bulk properties of orthopaedic implants but also the microcirculatory implications of inevitable wear debris may play a role in determining biocompatibility and ultimately longlivety of an implant. The skinfold chamber is a feasible and versatile model for observation of the dynamic process of microvascular response after foreign‐body implantation, and offers much perspective. With a minimum of adverse host reaction, our results indicate that titanium still represents the gold standard in metallic implant material, even in the case of generated wear debris, which shows a comparatively low inflammatory potential.     

Verschleißbeständige MMC mit in situ Karbiden

Wear Resistent MMC with in situ Carbides Metal matrix composites for wear resistance applications are made up of hard particles embedded in a softer metal matrix. The hard particles contribute to a significant increase of wear resistance while the metal matrix ensures strucural support and ductility. The efficiency of hard particles depends on their volume fraction and distribution in the matrix as well as on their size and hardness. On the one hand the variety of efficient hard particles is limited and on the other hand their costs are considerably higher than those of the matrix powders. For this reason it was the intention of this work to expand the variety of efficient and cost‐effective hard particles. A new powder metallurgical process is presented, not based on the typical addition of hard phase powders. Instead, commercially available ferro alloy powders, with and without additional graphite, are mixed with different steel powders and are hot isostatically pressed (hiped) to full density. During the HIP process a steel matrix / carbide composite is formed by carbon diffusion into the ferro alloy particles. The results of these experiments show an interesting and cost‐effectice potential for applications with high tribological but low mechanical and thermocyclic demands.     

Microanalysis of debris formed during electrical discharge machining (EDM)

The present investigation attempts to focus on the influence of EDM energy input on the structure and composition of the debris that forms during machining. An indigenously prepared ZrB₂-Cu composite and mild steel plate were used as tool and workpiece (WP) material for the present study. The machining was carried out in a kerosene medium. The composition analysis of different debris particles was investigated by SEM/EDS. At low EDM energy input the SEM micrographs of WP debris showed formation of smaller size particles with few satellites. The SEM micrographs of high EDM energy input showed formation of hollow spheres, with dents, larger population of satellites, surface cracks and burnt core structures.     

Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement

Shot peening is a dynamic cold‐working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly used as a surface treatment technique within the aerospace industry during manufacturing to improve fatigue performance of structural components. The compressive residual stress induced during shot peening results in fatigue crack growth retardation, improving the performance of shot‐peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Because of the relatively soft nature of AA7050‐T7451, shot peening can result in cracking of the constituent precipitate particles, creating an initial damage state. The aim of this paper is to understand the balance and fundamentals of these competing phenomena through a comparative study throughout the fatigue lifecycle of baseline versus shot‐peened AA7050‐T7451. Microstructure and surface topology characterization and comparison of the baseline and shot‐peened AA7050‐T7451 has been performed using scanning electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and optical profilometry techniques. A residual stress analysis through interrupted fatigue of the baseline and shot‐peened AA7050‐T7451 was completed using a combination of X‐ray diffraction and nanoindentation. The fatigue life performance of the baseline versus shot‐peened material has been evaluated, including crack initiation and propagation. Subsurface particles crack upon shot peening but did not incubate into the matrix during fatigue loading, presumably due to the compressive residual stress field. In the baseline samples, the particles were initially intact, but upon fatigue loading, crack nucleation was observed in the particles, and these cracks incubated into the matrix. In damage tolerant analysis, an initial defect size is needed for lifetime assessment, which is often difficult to determine, leading to overly conservative evaluations. This work provides a critical assessment of the mechanism for shot peening enhancement for fatigue performance and quantifies how incubation of a short crack is inhibited from an initially cracked particle into the matrix within a residual stress field.     

The role of debris-induced cantilever effects in cyclic fatigue of micron-scale silicon films

The presence of debris, surface roughness, or other asperities on crack faces creates a geometry that is reminiscent of a lever (the crack face) on a fulcrum (the debris or surface asperity). This ‘cantilever effect’ is intuitive and is routinely forwarded as a mechanism for crack advance when compressive loads are applied. Recently this cantilever effect has also been linked to fatigue of micron‐scale silicon films. Finite‐element modelling was used in this study to evaluate different wedging configurations that would be likely to occur within micromachined silicon thin films. The results of the model clearly show that wedges or other asperities in the wake of a crack do not increase the magnitude of the stress intensity factor during compression.     

High cycle fatigue mechanisms of aluminum self‐piercing riveted joints

A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.     

Geometrical features of wear debris

Image analysis software was used to analyse the geometry of debris formed during the erosion of low-carbon steel by impinging solid particles. Depending on the two-dimensional aspect ratio (ratio between debris height and debris width), three different debris types could be distinguished. The most frequent type observed was a platelet-type debris as suggested by the Bellman-Levy (1981) model. This wear debris shape type covered about 60% of all acquired debris. Plain micro-machining according to Finnie’s (1959) suggestion played a negligible role only, but other processes, namely ploughing as suggested by Winter and Hutchings (1974), were more important. The statistically estimated mean debris size was about 14 μ m. About 92% of all wear debris had sizes smaller than the target material grain size. This result supports the figure that ‘secondary’ removal modes—lip or platelet, respectively, detachment from crater rims—were responsible for material removal.     

Erratum to “An analysis of inclusion defects on strip steel surface induced by polyurethane roller [Eng. Failure Anal. 18(3) (2011) 1122–1127]”

In a cold rolling mill, inclusions were inspected on the strip steel surface after multiple cold rolling process in summer season, which caused a large amount of inferior strips. To clarify the reason of the surface defects, experimental research and in situ investigation have been performed. The micro-structure of inclusions was investigated under scanning electron microscope (SEM), and micropits were observed on the strip surface with vermiform particles trapped inside. A large content of carbon was tested in the micro-pit by energy dispersive X-ray analysis (EDAX). Adherents on polyurethane (PU) roller surface and debris in the PU roller bracket were examined by SEM, EDAX and Fourier transform infrared spectroscopy (FTIR) respectively. The vermiform particles were also observed in the SEM analysis of the debris, and the FTIR results of the debris showed good agreement with the PU standard spectrum. It was proved that inclusions were caused by the wear debris from PU roller. Replacing the PU roller in time and enhancing the crystallinity of PU roller were effective methods to eliminate such kind of inclusions.     

Automatic restraint and visual detection of fatigue crack growth by applying an alumina paste

A paste comprising fine alumina particles was applied on surfaces of steel plate specimens and a welded joint specimen, and the effects of the paste on restraint and visual detection of fatigue crack growth were experimentally investigated by performing fatigue tests with in situ observations by a c harge c oupled d evice (CCD) microscope and through various kinds of fractographic observations and elemental analyses using an scanning electron microscope (SEM) and an electron probe microanalyser (EPMA). As a result, the crack growth rate in the plate specimen was drastically retarded by the wedge effect of the alumina particles, and a 311% increase in failure life was produced on an average. The restraint of crack opening displacement (COD) value by the wedge materials was estimated by elastic finite element (FE) analyses, and the results were compared with the experimental data. Similar crack growth restraint effects were also observed on the welded joint specimen, producing a 117% increase in failure life. In the fatigue tests of the specimens on which the alumina paste was applied, a remarkable black colour developed in the white alumina paste along the paths of crack propagation, facilitating the visual detection of the cracks. An analysis using an X‐ray diffractometer showed that the black matter in the paste consists of fine debris derived from the base metal.     

Tribological properties of ultra‐high molecular weight polyethylene (UHMWPE) filled with copper micro‐powder

This paper investigates the tribological properties of ultra‐high molecular weight polyethylene (UHMWPE) filled with copper micro‐powder (CMP). The fabrication and testing procedures implemented to characterize strength and wear properties of the composite are discussed. The effect of copper micro‐powder concentration on tensile strength, elongation at break, impact resistance, coefficient of friction, and wear resistance of the composite is investigated. Results show that copper micro‐powder concentration of 1 wt% yields the optimal combination of wear resistance and tensile strength of the composite. A morphological analysis based on scanning electron microscope (SEM) images of the copper micro‐powder‐ultra‐high molecular weight polyethylene specimens is also discussed. The presence of ridges and plaques on the specimens, analyzed after the sliding wear tests, is attributed to fatigue and adhesion mechanisms. Investigations performed by using an electron probe micro‐analyzer provide evidence that the detachment of copper micro‐powder particles from the matrix during sliding wear tests creates a lubricating layer that drastically decreases the coefficient of friction of the composite and improves its wear resistance properties.     

TiO_2对挤压铸造Al_2O_(3p)/钢基复合材料组织与力学性能的影响

在预制坯中加入TiO_2粉末,利用挤压铸造法制备Al_2O_3颗粒增强1065钢基复合材料,研究TiO_2对复合材料组织与力学性能的影响。结果表明:TiO_2使基体与Al_2O_3的结合界面形成了TiO_2、Al_2TiO_5界面层;添加TiO_2的复合材料硬度和三点弯曲强度分别为39.0HRC,743.94MPa,比未添加TiO_2的复合材料分别提高了10.0%,26.4%;断口扫描表明,添加TiO_2的复合材料界面结合良好无裂纹,Al_2O_3颗粒表现为穿晶断裂。说明加入的TiO_2改善了Al_2O_(3p)/钢基复合材料界面结合强度,提高了复合材料力学性能。     

Electron microscope observations of impact crater debris amongst contaminating participates on materials surfaces exposed in space in low-Earth orbit

Debris particles extracted from a small sampling region on the leading edge of the Long Duration Exposure Facility (LDEF) spacecraft have been examined by analytical transmission electron microscopy and the elemental frequency observed by energy-dispersive X-ray spectrometry and compared with upper atmosphere (Earth) particle elemental frequency and the average elemental compositions of interplanetary dust particles. A much broader elemental distribution was observed for the exposed spacecraft surface debris milieu. Numerous metal microfragment analyses, particularly aluminium and stainless steel, were compared with scanning electron microscope observations of impact crater features, and the corresponding elemental spectra on selected LDEF aluminium tray clamps and stainless steel bolts. The compositions and melt features for these impact craters and ejecta have been shown to be consistent with microcrystalline debris fragments in the case of aluminium, and these observations suggest an ever changing debris milieu on exposed surfaces for space craft and space system materials.     

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr‐ODS steel as well as of a ferritic‐martensitic 9Cr‐ODS steel under alternating load conditions was investigated. In general, strain‐controlled low‐cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano‐sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non‐ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550°C. At 650°C, however, the higher inelastic strain energies of 12Cr‐ODS steel result in significant lower lifetimes compared with those of the 9Cr‐ODS steel. The strong degradation of the cyclic properties of the 12Cr‐ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650°C than at 550°C. Finally, the obtained results depict that the 9Cr‐ODS steel offers higher number of cycles to failure at 650°C, compared with other novel ODS steels described in literature.     

Behind-armor debris from the impact of hypervelocity tungsten penetrators

Behind-armor debris is the main mechanism by which targets are destroyed by projectile impact. The behind-armor debris generated from the impact of tungsten heavy alloy (THA) penetrators with a length-to-diameter ratio (L/D) of 20 against 6061-T6 aluminum targets was characterized. Behind-armor debris characteristics described were the number of debris particles, their positions, and their size distribution. Experiments were performed against two nominal target thicknesses, 100 and 150 mm, and covered a velocity range from 1.7 to 2.6 km/s. Two methods of obtaining data were used—radiographs were taken of the behind-armor debris, and perforation patterns were generated on steel witness packs placed behind the aluminum target. Debris particles recovered from the witness packs were also studied. Results are discussed for the effect of changes in target thickness and impact velocity on behind-armor debris particle characteristics.     

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。     

Investigation of sliding wear surfaces in alumina using transmission electron microscopy

In this study, the subsurface microstructure of alumina wear surfaces and the microstructure of agglomerated debris generated from unlubricated sliding wear at room temperature have been investigated through transmission electron microscopy (TEM). Specimens were thinned through the use of a focused ion beam miller (FIB). TEM studies, including analysis of electron diffraction patterns from the agglomerated region of the specimen, revealed the presence of an aggregate of nano crystalline particles embedded in an amorphous phase, together with some larger alumina particles. These larger alumina particles appear at the base of pits in the alumina surface, whereas the finer material appears at the contact surface. The agglomerated debris was readily distinguished from the alumina substrate, which contained localised dislocation damage and microcracking. It is proposed that the wear process involves the removal of ‘large ’ alumina particles from thesurface by a combination of trans- and intergranular microcracking. These particles are then ground into very fine, nanometer-sized particles that react on the surface with moisture in the air to form an amorphous hydroxide film. These are then compacted to form a nanocrystalline structure within an amorphous matrix that may also be viewed as a grain boundary phase.     

Microstructural stability of ODS steels in cyclic loading

The remarkable microstructural stability of high chromium steels prepared by powder metallurgy and strengthened by dispersion of nanometric yttrium oxides in cyclic loading at high temperatures is reported. Contrary to the continuous cyclic softening and profound changes in the microstructure during fatigue of common high chromium steels, the addition of 0.3 wt% Y₂O₃ stabilizes the microstructure and significantly reduces cyclic softening of investigated steels. The evolution of microstructure as a result of fatigue loading at room temperature, 650 and 750 °C, was examined by means of transmission electron microscopy. Only minor changes in the microstructure were detected. The stability of oxide particles after high‐temperature exposure was confirmed by energy dispersion spectroscopy chemical analysis. The microstructural features are discussed in relation to the cyclic behaviour of the oxide dispersion strengthened steels. The analysis of the hysteresis loop indicates that oxide nanoclusters are intersected and dissolved in slip bands of ODS Eurofer steel. This process contributes to cyclic softening.     

Fabrication of in situ Al2O3 reinforced nanostructure 304 stainless steel matrix composite by self-propagating high temperature synthesis process

304 austenitic stainless steel reinforced by Al₂O₃ particles was prepared by microwave assisted self-propagating high temperature synthesis process using the Fe₂O₃Cr₂O₃NiOAlFe reaction system. Furthermore, effects of mechanical activation of the reactants and the addition of 21.2 wt.% extra Al to the chemical composition of the reactants on the chemical composition of the produced stainless steel was investigated. Atomic absorption spectroscopy analysis results indicated that by the addition of extra Al to the reactant mixture and using 30 minute mechanical activation, stainless steel containing 17.27 wt.% Cr and 7.73 wt.% Ni could be produced with its chemical composition very close to the chemical composition of 304 stainless steel. X-ray diffraction analysis showed that the stainless steel contains nanostructured austenite and ferrite phases. Also microstructural characterizations indicated that there is a uniform distribution of black particles in the steel matrix. Energy dispersive spectroscopy analysis showed that these particles are composed of Al and O elements while the matrix contains Fe, Cr and Ni elements. The presence of Al₂O₃ particles and nanostructure matrix improved the hardness and therefore the wear properties of the composite in comparison with the wrought 304 stainless steel plate.