Papers by Author: Hong Xu

Abstract: The high temperature creep tests of standard specimen and double U-notched specimen of T92 steel were carried out under different stresses at 600°C and 650 °C. Then scanning electron microscopy was used to observe the fracture morphology. The results show that the notch weakens plasticity, and weakening with the notch acuity. Under multiaxial stress state, the failure mode of T92 steel gradually transfers from ductile dimple to brittle quasi-cleavage fracture. Compared with uniaxial stress state, the dimples under multiaxial stress state are smaller and shallower, within lots of carbides and second phase particles. The notch slows the connection, growth and gather of microvoids, and exhibits notch strengthening effect.

Abstract: A robust and efficient parameter identification method of the stress relaxation model based on Altenbach-Gorash-Naumenko creep equations is discussed. The differential evolution (DE) algorithm with a modified forward-Euler scheme is used in the identification procedure. Besides its good convergence properties and suitability for parallelization, initial guesses close to the solutions are not required for the DE algorithm. The parameter determination problem of the stress relaxation model is based on a very broad range specified for each parameter. The performance of the proposed DE algorithm is compared with a step-by-step model parameter determination technology and the genetic algorithm (GA). The model parameters of 12Cr-1Mo-1W-1/4V stainless steel bolting material at 550°C have been determined, and the creep and stress relaxation behaviors have been calculated. Results indicate that the optimum solutions can be obtained more easily by DE algorithm than others.

Abstract: In order to analyze the stress relaxation behavior under repeated loadings for 1Cr-0.5Mo-0.25V steel, a stress relaxation model based on creep equations has been developed. The model was implemented into the ANSYS finite element program in terms of user define material model. The calculated results were compared to the observed results of uniaxial reloading stress relaxation testing, which were performed by the National Research Institute for Metals of Japan (NRIM) for 1Cr-0.5Mo-0.25V stainless steel bolting material at 500°C. It was shown that the proposed model could be applied for the present data. The calculated residual stresses versus time curves were in good agreement with the observed for initial stress level of 297.1MPa at 500°C and for specific reloading time intervals of 24, 72, 240, and 720 hours.

Abstract: An analytical method based on a creep model is being developed to investigate the effect of retightening on stress relaxation behavior for high-temperature turbine and valve studs/bolts. In order to validate the approach, the calculated results are compared to the results of uniaxial reloading stress relaxation testing, which were performed by the National Research Institute for Metals of Japan (NRIM) for 12Cr-1Mo-1W-1/4V stainless steel bolting material at 550°C. It was shown that the proposed model based on Altenbach-Gorash-Naumenko creep model for the primary and steady state creep could be applied for the present data. The calculated residual stresses versus time curves were in good agreement with the measured for initial stress level of 273.6MPa at 550°C and for specific reloading time intervals of 24, 72, 240, and 720 hours.

Abstract: The rising international oil prices will cause the loss of national GDP and the establishment of strategic petroleum reserves (SPR) could avoid this loss as much as possible. The oil price-GDP elasticity coefficient is an important parameter in calculating strategic petroleum reserve, but since it is difficult to obtain, it is also hard to calculate. This paper provides the fitting formula of oil price-GDP elasticity coefficient based on the regression analyzing of literature data. China’s oil price-GDP elasticity coefficient in recent years has been analyzed and predictions for future trends in different situations have been made. Finally, the predicted oil price-GDP elasticity coefficient is used to calculate the size of China's strategic petroleum reserve and its earnings.

Abstract: One of the two challenges about Supercritical Water-Cooled Reactor is material, especially for the fuel cladding. High temperature creep tests of Ni-based alloy C276, one of the candidate materials for the fuel cladding, were carried out at 650°C~750°C, with stress 130MPa~430MPa. The effects of temperature and stress on creep were investigated, the change laws of steady state creep rate with stress and time to rupture were analysed, and creep damage factors were separately calculated based on Kachanov’s formula and Norton’s formula. The results indicate that there exist two types of primary creep characteristics in C276: Type I creep characteristic at lower stress level and Type II at higher stress level respectively. C276 shows excellent high temperature creep resistance, and Kachanov’s damage factors D vs normalized time are basically coincident at 650°C, 700°C and 750°C. The damage obtained by Norton’s formula starts at about 40% of lifetime, and the damage factors calculated by Kachanov’s formula are relatively conservative.

Abstract: The corrosion behaviors of ferritic/martensitic steel P92, austenitic stainless steel TP347HFG and HR3C have been investigated in supercritical water (SCW) at 550°C under 25MPa with 2ppm dissolved oxygen. After 600h exposue, all the samples formed a stable oxide layer, but of varying thickness and microstructure. A typical dual-layered oxide film on P92 and a single-layered structure on TP347HFG and HR3C were observed by SEM and EDS. Further XRD studies indicated the compositions of oxide layers weren’t independent to the type of the used steel. In comparison with ferritic/martensitic steel, austenitic steel showed a higher corrosion resistance.

Abstract: The creep experiments are performed under the various stresses at 595°C, 610°C, 640°C and 670°C for P92 steel which broadly used in USC plant pipeline. The specimens are taken from the extrados part of a new manufactured main steam elbow. A series of integrated creep curves are obtained, and the maximal test time last over 13000h. The relationship between the minimum creep rate and stress at different temperatures is explained by Norton formula. Both the 105 h creep rupture strength and stress of 1% creep strain in 105h of P92 steel at the four above temperatures are extrapolated with a list by using the Monkman-Grant formula, which are consistent with the reference experiment data at 600°C and 650°C. Meanwhile, damage during the creep process is discussed applying Norton creep damage formula and θ function method.

Abstract: The air cooling rate of the Low Carbon Mn-Si-Cr steel bar with different diameter after austenitizing at 910oC and 960oC was simulated by Formaster-F Phase transforming instrument and Gleeble-1500 thermal /mechanical simulating machine. Microstructure of the specimen was observed by OLYMPUS PME3 optical microscope and FEI QUANTA200F scanning electron microscope. The hardness and impact toughness of the steel was tested by HBRV-187.5 hardness tester and JCSJ300-I instrumented Charpy impact tester. The experimental result showed that with the amount of CFB in CFB+M mixed microstructure increasing the combination of strength and toughness of the steel was improved. The higher the austenitizing temperature of the steel, the wider the air-cooling rate range obtaining CFB+M mixed microstructure. However, the steel produces mixed grain after austenitizing at 960 oC. For obtaining fine prior austenite grain size, Ti and Nb alloying element need to be added.

Abstract: Softening, namely degradation, is an important characteristic for the components long-termly operating in high temperature environment. In this article, hardness is adopted to indicate this effect. In order to study the independent effect of high temperature and stress on the material properties, the zero-stress ageing experiments at 540 oC, 565 oC, 660 oC and the stress-accelerating softening experiments (i.e. creep experiment) at 540 oC, 565 oC with interruption were carried out in steel 30Cr1Mo1V, respectively. The stress range tested was from 240MPa to 320MPa. The Vicker’s hardness was measured. The relationship between Larson-Miller parameter and the Vicker’s hardness under zero-stress softening condition and under stress-accelerating softening condition is obtained. Based on the extraction of the effects of temperature and time, the stress softening effect formulae are established.