首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 234 毫秒
1.
Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s−1. The steel with higher silicon content showed significantly lower metal loss rates under all conditions.

The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.  相似文献   


2.
High strain rate compression of titanium and some titanium alloys   总被引:1,自引:0,他引:1  
A modified Hopkinson bar was used to compress specimens of commercially-pure titanium, IMI 125 and titanium alloys, AMS 4911B and AMS 4916B, at natural strain rates of between 3 × 103 s−1 and 3 × 104 s−1. All three materials deformed in a viscous manner with a linear increase of flow stress with strain rate and a macroscopic viscosity of 4·5 kPa s, 5·7 kPa s and 9·2 kPa s, respectively. At strain rates above about 1 × 104 s−1 there was a decrease in macroscopic viscosities to 0·9 kPa s and 2·5 kPa s for the titanium and 4911B alloy respectively, which is attributed to changes in the dislocation drag mechanisms. The results for the 4916B alloy at the higher strain rates were too scattered to give a definite trend. It is suggested that the increase in the macroscopic viscosity with alloying is due to an increase in the dislocation drag coefficient by solution hardening and to a reduction in the density of mobile dislocations by dislocation pinning.

It is also proposed that the susceptibility of these materials to catastrophic shear failure which occurs without prior linear work-softening is due to the higher propensity for titanium and titanium alloys to shear failure as a consequence of their thermo-mechanical properties. The decrease of strain at which shear occurred with increasing strength is in accord with this suggestion.  相似文献   


3.
Steel specimens of type 1023 and heat treated 4130 steel of hardnesses, between 257 and 450 HV10, were compressed at strain rates between 3 × 103 and 3 × 104 s−1 using a modified Hopkinson bar. Both steels showed viscous type flow behaviour with a linear increase in flow stress with strain rate. For the 1023 steel there were two linear regions, one from 3 to 12 × 103 s−1 with a macroscopic viscosity of 4·7 kPa s and two from 14 to 27 × 103 s−1 with a viscosity of 0·7 kPa s. These results indicate some change in the controlling mechanism. For the 4130 steel the macroscopic viscosity decreased linearly with increasing hardness from 7·4 kPa s at 257 HV 10 to 2·2 kPa s at 450 HV 10. This was probably due to a decrease in the density of mobile dislocations with increased tempering temperatures. At lower hardness values (257 and 300 HV 10) and higher strain rates, a levelling off of the flow stress occurred but the data were too scattered to give a definite trend. The heat generated due to deformation of the 1023 steel was sufficient to produce steady state flow at strains above about 0·3. In the 4130 steel the thermo-mechanical properties were such that linear work softening was observed at a rate inversely proportional to the square root of the strain rate and independent of the hardness. This suggests that the work softening was a function of properties of the common ferrite matrix. The strain at which the work softening commenced decreased with increasing hardness, because of the more rapid heat generation at higher flow stresses. At higher hardness and higher strain rates 4130 steel failed by shear. Shear failure occurred at lower strain rates with increasing hardness and, for a given hardness, at lower strains with increasing strain rate. This behaviour was consistent with Recht's analysis of catastrophic shear failure.  相似文献   

4.
T.A. Libsch  P.C. Becker  S.K. Rhee 《Wear》1986,110(3-4):263-283
The unlubricated friction and wear behavior of toughened zirconias and toughened aluminas against hardened steel was studied using the Falex ring-and-block technique. Three experimental ZrO2-Y2O3 ceramics, two commercial ZrO2-MgO ceramics and two commercial Al2O3-ZrO2 ceramics were investigated. Each ceramic was tested at 500 rev min−1 (92 cm s−1) and 2000 rev min−1 (367 cm s−1) at normal loads in the range 2.3–40.8 kgf. The materials characteristics of the ceramics before test, the features of the tested samples, and the friction and wear data are presented and related. Under mild wear conditions, all the ceramics exhibited low wear, with the ZrO2-Y2O3 samples having the lowest. The wear of the toughened zirconias exhibited a strong sensitivity to sliding speed, while the toughened aluminas did not. Also, the lower toughness ceramics were susceptible to macroscale structural damage (cracking and chipping) even when the overall wear was low. Micro structural examination of a tested friction pair (ring, ceramic block and wear debris) has shown that the wear process is very complex, encompassing many mechanisms which are described. A generalized wear equation relating wear to load, sliding speed and sliding time is proposed.  相似文献   

5.
D. W. Wheeler  R. J. K. Wood 《Wear》2001,250(1-12):795-801
This paper describes an erosion study, which examines the effect of impact angle on the erosion behaviour of diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were 37–60 μm in thickness and were erosion tested using angular silica sand with a mean diameter of 194 μm at a particle velocity of 268 m s−1. The impact angles used were 30, 45, 60 and 90°. The results show that the damage features, termed “pin-holes” are generated at all angles, though the number of impacts required for pin-hole initiation is significantly increased at lower angles. This work provides useful information in attempting to explain the mechanism by which damage is generated during the high velocity sand erosion of CVD diamond.  相似文献   

6.
The response of Al-Mg and Al-Cu alloys to multiple liquid impact in the velocity range 90 –148 m s−1 has been investigated. The variation in erosion rate with Al-Mg composition is discussed in terms of a change in the erosion mechanism from ductile rupture of small particles to the removal of whole grains by intercrystalline fracture. This transition occurs with both increased magnesium content and increased velocity of impact. In contrast, Al-4%Cu fails by a mixture of ductile rupture and transgranular fracture The latter mode exhibits river patterns typical of cleavage but occasionally fatigue-like striations are also visible.  相似文献   

7.
Starvation in ball bearings   总被引:2,自引:0,他引:2  
D. N. Olaru  M. D. Gafitanu 《Wear》1993,170(2):219-234
A complex analytical model for the starvation mechanism in ball-race contacts, in correlation with thermal effects, has been developed. From the theoretical results it was found that in high speed, mist-lubricated ball bearings, the reduction factor of the film thickness by starvation and thermal effects can be computed in correlation with the ball-race lateral oil meniscus thicknesses. Experimental investigations of 7206 C angular contact ball bearings, operating between 5000 and 35 000 min−1, are in good correlation with the theoretical results when only starvation and inlet shear heating have been included. When the ball bearing temperature increases, both the theoretical and experimental results show that starvation occurs at smaller ball-race lateral oil meniscus thicknesses, by decreasing the oil viscosity.  相似文献   

8.
An intermediate strain-rate mechanical testing technique was developed through proper modifications of a hydraulically driven loading frame (MTS 810) and a split Hopkinson pressure bar (SHPB). The modified MTS and SHPB were used to obtain valid stress–strain data for an epoxy syntactic foam at intermediate strain rates in the order from 10−1 to 102 s−1. Additionally, lower and higher strain-rate characterization of the foam material was conducted, such that the compressive stress–strain data of the syntactic epoxy foam were obtained at strain rates from 0.005 to 2150 s−1 without any gap in the intermediate strain-rate range. The syntactic epoxy foam exhibited nonlinear strain-rate dependency of failure strength.  相似文献   

9.
Y. I. Oka  M. Nishimura  K. Nagahashi  M. Matsumura 《Wear》2001,250(1-12):736-743
For the prediction of actual damage to plant component materials and for making the erosion mechanisms clear, it is important to control and to evaluate the particle impact conditions in a testing facility. A sand blast type erosion test rig, which can achieve the particle impact velocities up to 135 m s−1 and a wide range of impact angles has been constructed. The key factors in particle impact conditions of particle flux, impact velocity and impact angle were examined. The relative distance between particles and particle size was discussed, as the particle flux affected erosion rate of material. A new method was proposed to determine particle velocities in this facility. The theoretical velocity of the particle calculated by the equations of particle motion was compared with the experimental results. The divergence of particles from a geometrical angle was evaluated by measuring surface roughness of the specimens. Although some spread of the particles were observed surrounding the central damage area of the specimen surface, the greatest amount of damage was concentrated in the center. As a result, it was found that particle impact conditions were well controlled in this testing unit.  相似文献   

10.
J. R. Gomes  A. S. Miranda  J. M. Vieira  R. F. Silva   《Wear》2001,250(1-12):293-298
The superior high temperature resistance of silicon nitride (Si3N4) based ceramics makes them suitable for tribological applications above room temperature or in high speed unlubricated sliding. There are some published works on the wear behaviour of Si3N4/metal alloys. However, experimental data are shown in a form that is not of direct use for engineers involved in materials selection. In the present work, Si3N4 pins were tested against tool steel and grey cast iron on a pin-on-disc tribometer. Ceramics were produced by hot-pressing and tested without lubrication at variable temperature and sliding speed. SEM/EDS and XRD analysis were used for chemical and microstructural characterisation of worn surfaces and wear debris. At low speeds (0.05–0.5 m s−1) and room temperature, Si3N4 surfaces are polished-like due to a combination of humidity-assisted tribo-oxidation and abrasive action of very fine wear debris. At high sliding speeds (2–3.5 m s−1), as well as for temperatures in the range 400–600°C, an extensive coherent tribolayer mainly composed by iron oxides spreads over the ceramic surfaces. Polishing and protection by adherent tribolayers are the mechanisms responsible for observed severe and mild wear regimes, respectively. Wear maps are constructed showing the transition of wear regimes in Si3N4/iron alloys contacts determined by constant flash temperature curves. Equations for calculation of bulk and flash contact temperatures in tribocontacts between dissimilar materials are deduced.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号