Key Engineering Materials Vols. 353-358

Abstract: With the development of high toughness pipeline, it is necessary to develop new welding wires for high-efficiency submerged-arc welding, to produce weld metals whose properties can match corresponding ones of the base metal, such as strength, toughness, H2S corrosion resistance, etc. In this paper, the weld wires with variations of elements have been designed, and the effects of alloy elements on the microstructures and low temperature impact toughness are discussed.

Abstract: The model of slub-like short fibers reinforced composites is suggested from the viewpoint of bamboo in the nature and patterns characteristic of simulated silk PET used in textile industry. The stress distributions in the enlarged-end fiber and in the matrix are analyzed. The axial stress in the fiber and matrix is found to increase and the interfacial shear stress decrease with the radius of the enlarged end.

Abstract: The effect of ageing processes on the toughness and the yield strength of the Li-containing Al-Zn-Mg-Cu alloys (Al-5.6%Zn-1.9%Mg-1.6%Cu-1.1%Li-0.24Cr) was investigated. The microstructure was observed by transmission electron microscopy, tensile test was performed at a rate of 1mm·s-1, fracture toughness was experimentally determined by the impact test, and the fracture modes had been assessed by image analysis of scanning electron microscopy micrographs. The strength of the Li-containing Al-Zn-Mg-Cu alloys treated at 120 °C is not more than 430MPa, which is very lower than that of 7075 alloys. The strength is comparable to that of 7075–T6 alloys after double-ageing or multi-ageing, however its ductility is lower than that of 7075-T6 alloys. The single-aged Li-containing alloys have high toughness. The multi-aged alloys become brittle. The fracture surface for the Li-containing Al-Zn-Mg-Cu alloys is intergranular, since the intergranular precipitation weakness the grain boundaries.

Abstract: In the present paper, the non-equilibrium grain-boundary segregation of P atom was studied in low alloy steels subjected to a low tensile stress at different temperatures. The AES (Auger electron spectroscopy) experiments and dynamic analyses were conducted to study on the non-equilibrium grain-boundary segregation of P atom. The research results show that non-equilibrium segregation of phosphorus occurred at the grain boundaries of the steels 2.25Cr1Mo and 12Cr1MoV, while the critical time reached about 0-1 hour at constant temperatures 773 and 813K. The relationship between the diffusion rate and the diffusion time for the complex and the phosphorus atom was investigated based on the experimental results. Eventually the diffusion coefficients of complex and P were calculated with using a proposed dynamic model.

Abstract: Cross-sectional nanoindentation (CSN) is a new method for measuring interface adhesion of thin films. The interfacial energy release rate (G), characterizing interfacial adhesion, is calculated from the material and geometrical parameters relevant to the test. Effects of residual stresses on G and crack tip phase angle Ψ, have been studied by finite element simulation in this study. The results show tensile residual stresses increase G and compressive stresses reduce it, and they have similar effects on the magnitude of Ψ.

Abstract: The effect of heat treatment on the microstructure and mechanical properties of aged hardening 2Cr16Ni2MoN stainless steel was examined. The steel contained undissolved (CrFeMo)23C6 carbides in a martensite at lower solution treated temperature. Solution treating at 1100°C resulted in coarsening of the packet and block structure of the lath martensite. The optimum solution treated temperature is 1040°C.Aging at 500°C resulted in peak strength due to the precipitation of extremely fine coherent (CrFeMo)2C carbides. The optimum combination of tensile strength and impact toughness was attained at the aging temperature of 560°C,was attributed to both the formation of fine M2C in the lath and a mixture of lath martensite with high density dislocations.

Abstract: The design of pressurized components at elevated temperature is limited by available mechanical properties that often do not span appropriate range of stress, time, strain and temperature. To improve the design process, a new approach based on short-time stress relaxation test (SRT) is introduced in steel tensile creep analysis. The experiments are implemented using SRT and tested for creep properties of 12Cr1MoVG steel at 560 °C. The results of creep curves agree well with the traditional constant load creep test data. The possibility to apply SRT approach is shown promising in the design of the pressurized component at elevated temperature.

Abstract: The compressive properties characterized as a function of the true stress-strain response of the ferritic oxide dispersion strengthened (ODS) alloys, fabricated by mechanical alloying, were examined at strain rates ranging from 0.01 to 0.5s-1. Based on the differential scanning calorimeter (DSC) analysis, the solidus temperature of the ferritic ODS alloys is 1446oC. Therefore, the compressive temperatures were chosen to be from 1050oC to 1300oC. Transmission electron microscopy (TEM) observation is indicative of the homogeneous distribution of the nano-yttria particles with the granularity of 100~200nm, which may exert a strong pinning effect on subgrains. The results indicate that flow stress of the ferritic ODS alloys enhances with decreasing compressive temperature and increasing strain rate. Investigation of the strain rate sensitivity exponent and apparent activation energy of the ferritic ODS alloys has been carried out in detail.

Abstract: The critical fracture stresses (σ* f(sp)) in various low alloy steels for a reactor pressure vessel(RPV) were evaluated by a small punch test (SP test) and a finite element analysis (FEA) in the cleavage temperature region(-150~-196 °C). The load-displacement curves and distances from the center to the fracture surface (Df) of the FEA results are in good agreement with the experimental results. The maximum principal stresses (SP fracture stresses, σf(SP)) were determined from the FE analysis, when the maximum load was applied to the SP test. The SP critical fracture stress, σ* f(sp) in various reactor pressure vessel (RPV) steels was found to have a linear relationship with the values obtained from the precracked specimens (σ* f(PCVN)). The σ* f(sp) shows a lower value than σ* f(PCVN) because the SP specimen had a lower triaxial stress condition. However, this result indicates that a small punch test could be a useful method to evaluate the cleavage fracture behavior of low alloy steels.

Abstract: A load-displacement curve from a small punch test includes several useful information that is related to standard test properties such as the tensile property, fracture toughness and ductilebrittle transition temperature. In this study, the empirical relationship between the material property factors in SP curves and the tensile curves has been investigated by comparing test results and finite element analysis results. SP and tensile tests and finite element analyses were performed for several Mn-Mo-Ni low alloy steels with different manufacturing processes. It was found that the yield loads (Py) in the SP curves, if they were adequately defined, were linearly related to the yield strength (σ0). The yield loads defined from the intersection point of two lines tangent to the elastic bending region and plastic bending region showed a better relation with the yield strength than those from the offset line. The slope of the SP curves from the simulation results had a close correlation with the hardening coefficient and strength constant as well.