综述了铁素体与铁素体异种金属焊缝(dissimilar metal welds,DMWs)接头界面组织及其影响。结果表明,在焊后热处理或运行温度下的铁素体钢DMWs接头的不均匀界面组织中,通常会形成脱碳层和增碳层。在铁素体钢DMWs焊接接头界面组织影响因素中,焊缝金属的化学成分有重要影响;焊后热处理规范(温度和时间)、工作温度下运行时间的影响较为突出;焊接工艺参数的影响亦不可忽略。异种钢接头界面处近缝区裂纹的产生,以及接头的蠕变强度随Larson Miller 参数增大而下降等不利影响,均为异种钢界面碳迁移行为所导致。焊缝成分控制法是接头界面组织控制或改善的必要条件,而脱碳层部位转移法能有效防止裂纹发生,亦是接头安全运行的重要工艺措施之一。 相似文献
The Fe−Ni−TiO2 nanocomposite coatings were electrodeposited by pulse frequency variation. The results showed that the nanocomposite with a very dense coating surface and a nanocrystalline structure was produced at higher frequencies. By increasing the pulse frequency from 10 to 500 Hz, the iron and TiO2 nanoparticles contentswere increased in expense of nickel content. XRD patterns showed that by increasing the frequency to 500 Hz, an enhancement ofBCC phase was observed and the grain size of deposits was reduced to 35 nm. The microhardness and the surface roughness were increased to 647 HV and 125 nm at 500 Hz due to the grain size reduction and higher incorporation of TiO2 nanoparticles into the Fe−Ni matrix (5.13 wt.%). Moreover, the friction coefficient and wear rate values were decreased by increasing the pulse frequency;while the saturation magnetization and coercivity values of the composite deposits were increased. 相似文献
Background: Despite increases in ecstasy (MDMA) use in the United States, little is known about characteristics linked with recent-onset ecstasy use, especially psychiatric symptoms and deviant behaviors. Aims: To test whether individuals with high levels of other drug use are more likely to be recent-onset ecstasy users; to test whether psychiatric symptoms in adults are associated with recent-onset ecstasy use; to explore the association between recent-onset ecstasy use and concomitant deviant behaviors in adolescents and adults. Methods: Data from the 2001 National Survey on Drug Use and Health. Findings: Recent-onset ecstasy use was significantly more likely to occur among adolescents and adults (18-34 years old) who engaged in deviant behaviors during the past year as compared with those who did not engage in deviant behaviors during the past year. Higher levels of deviancy indicated a higher likelihood of being a recent-onset ecstasy user, and associations were strongest with nonviolent deviant behaviors such as selling illegal drugs and stealing. Associations between deviant behaviors and recent-onset ecstasy use were similar in strength to associations between deviant behaviors and recent-onset cocaine and marijuana use, respectively. Adults who had past-year psychiatric symptoms (both depressive and panic symptoms) were twice as likely to be recent-onset ecstasy users as compared with those without past-year psychiatric symptoms. Greater levels of drug involvement increased the odds of being a recent-onset ecstasy user. Conclusion: Recent-onset ecstasy use seems to be associated with a range of other behavioral problems and may reflect one aspect of a larger problem behavior syndrome. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
The analysis of 124 curves obtained in short-term tensile tests demonstrate that they can be described by varying strain hardening and softening characteristics. Different stress–strain curves can be produced at invariable yield strength and ultimate strength and interrelated proportional variations of the above characteristics. To determine some specific stress–strain curve, it is necessary to take account of yield strength and ultimate strength as well as strain corresponding to the latter. The relations between yield strength, ultimate strength and hardening and their practically complete absence between these parameters and softening were statistically established. 相似文献
In the presented work some properties of a recently developed Si3N4/SiC micro/nanocomposite have been investigated. The material was tested using a pin on disc configuration. Under unlubricated sliding conditions using Si3N4 pin at 50 % humidity, the friction coefficient was in the range of 0,6 ‐ 0,7. The reduction of humidity resulted in a lower coefficient of friction, in vacuum the coefficient of friction had a value of about 0,6. The wear resistance in vacuum was significantly lower then that in air. The wear patterns on the Si3N4+SiC disc revealed that mechanical fracture was the wear controlling mechanism. Creep tests were realized in four point bending configuration in the temperature interval 1200‐1400 °C at stresses 50,100 and 150 MPa and the minimal creep deformation rate was established for each stress level. The activation energy, established from the minimal creep deformation had a value of about 360 kJ/mol and the stress exponent values were in the range of 0.8‐1.28. From the achieved stress exponents it can be assumed that under the studied load/temperature conditions the diffusion creep was the most probable creep controlling mechanism. 相似文献
A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved:
1. 1. the inclusion of creep relaxation
2. 2. the inclusion of external loads on components
3. 3. a more accurate method of calculating thermal stresses due to temperature transients
4. 4. the inclusion of high cycle fatigue terms.
The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred.
The site testing provided good practical experience and showed up problems which would not otherwise have been detected. 相似文献
The use of molecular-level materials modelling techniques in the development of advanced performance polymers is discussed, with particular emphasis upon bridging the large difference in the scales of dimensions between atomic structure and fabricated parts. The advantages and disadvantages of bulk quantitative structure-property relations and atomistic modelling are assessed, and the method of group interaction modelling is suggested as a means of bridging the dimensional scales.After a brief introduction to the concept of group interaction modelling, examples of modelling the engineering properties of polymers are presented which are difficult to model quantitatively by any other means. The important phase transitions from the crystal and glassy states of matter to those of rubber- and liquidlike states are shown quantitatively to be due to the same isoenergetic condition. The viscoelastic properties of a polymer are critical for many applications and expressions are derived for the loss and storage components of the complex modulus, with reference to failure initiation conditions. The effect of crosslinking in thermosets upon the glass transition temperature and viscoelastic properties is outlined. Finally, the scaling of time from atomic vibrations to the years involved in creep and ageing effects are discussed. 相似文献