Key Engineering Materials Vols. 297-300

Abstract: In the case of inner pressurized pipe, the hoop stress mainly causes the failure of pipe. For evaluating the hoop tensile properties of pipes, a ring test is used although it has an inevitable shortcoming of disturbance by the bending. Arsene suggested a central piece-inserted ring test for reducing the bending. At high temperatures, the friction between the specimen and the central piece exerts a serious influence on the results, so an effect on friction should be considered in the analysis. In this study, for the evaluation of the hoop properties using the central piece-inserted ring model of Arsene, we performed the ring tensile tests at several temperatures. From the ring tensile tests and numerical analysis the load-displacement conversion relationship of ring specimen (LCRR) was determined. We could obtain the hoop tensile properties by means of applying the LCRR to the results of the ring tensile test. From the result, it was observed that the strength decreased as the temperature increased, and the elongation dropped a little around 400°C.

Abstract: The structures connected by welding are fractured often in Heat Affected Zone (HAZ) when cracks grow under high-temperature and high-pressure conditions. In this study, the creep crack growth tests of P92 and P122 steels were performed and the creep fracture characteristics were evaluated. Degradation rate by cavities was investigated and the influence of degradation rate on crack growth rate was evaluated. Crack growth rate of new material was compared with that of the degraded material to confirm the influence of the degradation rate by cavities on the crack growth rate. From these results, the crack growth rates in welded joints were determined. The crack growth rate of the step test specimen was much faster than that of new material under the same test conditions of step temperature, while it was slower than that of new material under the same step loading conditions. It implies that crack growth rate of the welded specimen is faster when degradation rate by cavities is high even under the same temperature and stress conditions. HAZ and weld metal have faster cavity nucleation and growth rates than base metal. Therefore, the HAZ and weld metal specimens showed shorter crack initiation time and the initial crack growth rate (da/dt)c of the HAZ and weld metal was faster than the base metal. If (da/dt)c is fast, fracture life will be reduced.

Abstract: The crack growth behavior in a 304 stainless steel has been investigated at 538°C in air environment. Compact tension specimens were subjected to fatigue, creep and creep-fatigue loading. The combined effects on crack growth rates of load level and hold time have been examined. Stress intensity factors are found to correlate crack growth rates reasonably well for fatigue crack growth. Creep crack growth rates are found to correlate with stress intensity factor and C*(t). Crack growth rates under hold time cycles are successfully correlated with C*(t)avg under various load levels and hold times. Crack growth under creep-fatigue loading has been simulated by elastic-plastic-steady state creep finite element analyses. The results of analysis show that fatigue loading interrupts stress relaxation around the crack tip during hold time and causes stress reinstatement, thereby giving rise to accelerated crack growth compared with crack growth under static loading. Analysis of hold time crack growth based on the cyclic stress-strain response yields crack closure during unloading, and creep deformation during hold time tends to lower the closure load.

Abstract: The effect of heat treatment on the morphologies of precipitates and grain boundaries of Inconel 690 alloy was studied. When the specimens were slowly cooled from the solutionizing temperature, typical discontinuous coarsening of Cr-rich carbides in grain boundaries was observed. As the cooling rate decreased, Cr-rich carbides grew and wider Cr depletion zone was created between the carbide precipitates, which resulted in wavy grain boundaries. The driving force of the wavy grain boundary was explained by the coherent strain energy.

Abstract: Ti-48Al-2Mn-2Nb alloy was produced by “centrifugal spray deposition” (CSD), and then hot isostatic pressing (HIP) was employed to remove the porosity formed by CSD. The effects of CSD and HIP processing on the mechanical properties and microstructure of the TiAl alloy were investigated. The results show that the CSD and HIP processing can both improve the strength, plasticity of the TiAl alloy, and the tensile elongation values of the CSD or HIP samples are around 3%, which are better than those of as-cast TiAl alloys in room temperature. Especially, they show more excellent compressive properties at ambient temperature with a compressive ratio of 33.8% and compressive strength of 2210MPa for the CSD samples, and a compressive ratio of 37.8% and compressive strength of 2348MPa for the HIP samples. The CSD processing also improves the fracture toughness of TiAl alloy, which is much higher than that of the HIP processing, while the HIP processing seems to be beneficial the ductility and plasticity as having a duplex structure. The effects of CSD and HIP processing on microstructure and properties of TiAl alloys are discussed to understand the deformation and fracture process of the alloy.

Abstract: In order to simplify production processes and to lower production cost of thicker coldrolled IF steel sheets for deep drawing applications, a new warm-rolled IF steel sheet was developed in our lab through hot-rolling in high-temperature ferrite range. In this paper, effect of processing parameters on properties, microstructures and precipitate morphology of warm-rolled IF steel sheets was investigated. It is found that firstly, good deep drawing properties and favorable textures were achieved as steels were warm-rolled in good lubricant condition. Secondly, most precipitates in steels were TiS, TiC, TiN and Ti4C2S2.

Abstract: The Al sheet used for pressure can manufacture (or called Al piece) was prepared by different melt-treatment techniques and using the lower grade commercial purity raw Al (Al99.5). And the influence laws of melt-treatment techniques on the metallurgical quality and mechanical properties of Al piece were investigated. It is found that high-efficient melt-treatment technique improves the metallurgical quality of Al piece remarkably, that is, the rate of removing inclusions and the extent of lowering porosity amount to about 66.6% and 87.0%, respectively, and as-cast mean grain diameter decrease to a great extent, and the coarse and long needle/flake Fe-rich impurity phases transform into complex compounds of tiny, sphere/short stick form, which makes fracture mechanism of the material be transformed into the congregation of transgranular micro-hole, thus raising mechanical properties of this material remarkably. Its metallurgical quality and mechanical properties have reached or exceeded the level of present products. It is feasible that Al piece is made by using the lower grade commercial purity raw Al. The viewpoint that the purification is the fundamental and key process, i.e., the basic of modification and grain refinement processes, has been also further tested and verified.

Abstract: 20 vol.% aluminum borate whisker (Al18B4O33w) reinforced AC4CH alloy composite was fabricated by squeeze casting method. The matrix is controlled its impurities, which make metal alloys, especially AC4CH alloy that is made by restraining 0.2% Iron and aluminum to make a matrix material. The perform of composite materials was made from Al18B4O33w with 5% regulated three kinds of inorganic binders, SiO2, Al2O3 and TiO2 sol, respectively and then sintered at 1373K. The composites were obtained by squeeze cast infiltration of the molten alloy to the perform. Then it was squeeze casting into the matrix. The status of adhesions of whisker perform was Observed by SEM. Micro-Vickers hardness and tensile properties at the room temperature were estimated. Al18B4O33w/ AC4CH composites containing TiO2 sol has excellent properties on tensile properties and microvickers hardness compared with the other composites without inorganic binder, containing SiO2 and Al2O3 sol inorganic binder.