Papers by Author: Fu Zhen Xuan

Abstract: The stress corrosion sensitivities of 25Cr2Ni2MoV, 26NiCrMoV10-10 and 30Cr2Ni4MoV low-pressure rotor steels in simulated nuclear steam turbine operation condition were investigated by slow strain rate test (SSRT), and the stress corrosion cracking (SCC) mechanisms were studied by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Results revealed that the SCC sensitivity of 25Cr2Ni2MoV steel was highest in 3.5wt.%NaCl solution at 180°C, while the SCC sensitivity of 26NiCrMoV10-10 steel and 30Cr2Ni4MoV steel are similar. The SCC sensitivity of CrNiMoV steam turbine rotor steels could be decreased by the increase of Ni element and the decline of mechanical intensity. Cracks initiate from metal surface and then propagate to the inner metal, which showed a form of transgranular cracking.

Abstract: Axially push-pull cyclic tests of a low strength rotor steel were performed up to the very high cycle fatigue regime at ambient environment under ultrasonic frequency. Fatigue tests were interrupted at selected number of cycles for surface morphology observation and roughness measurement with the help of a 3D surface measurement system (Alicona InfiniteFocusSL). The fatigue extrusions and slip band developed on the specimen surface were recorded. The influence of stress level on the number and morphology of slip band was discussed. The surface roughness of fatigue specimens was found to be increased with the increasing of fatigue cycles. The fatigued specimens were finally cracked from surface or interior micro-defects after observation of fracture surface by scanning electron microscopy. The internal damage behavior consists of crack initiation and early propagation from micro-defect, crack growth within the fish eye, and fast crack growth. It is observed that there exists a competition between surface and internal fatigue damage in the very high cycle fatigue regime, i.e., surface damage is gradually developed with the increasing of fatigue cycles, while the critical interior micro-defect can be dominant for fatigue cracking.

Abstract: Fatigue crack initiation stage occupies a large proportion of total fatigue life in modern engineering materials and structures which are often designed under lower service loading conditions. In this paper, the fatigue crack initiation behavior from a micro-void in a small-scale specimen was studied in-situ in SEM. Surface morphologies were monitored in-situ and images were taken during interrupted tests at selected number of cycles, and displacement and strain map around the void was calculated based on digital image correlation (DIC) technique. The results indicated that the strain evolution near the micro-void could be divided into stages, before crack initiation. The strain increasing rate was fast in the early stage and slower in the second stage. A critical cyclic strain value for fatigue crack initiation from the micro-void was obtained around 9%, and was believed to be the dominant factor for early stage of fatigue damage.

Abstract: Based on the technology of the nanoindentation, the basic mechanical properties and cyclic deformation behavior of the austenite phase and ferrite phase in an austenitic-ferritic duplex stainless steel was studied. Firstly, from the displacement monotonic indentation experiment, the differences between the basic mechanical properties (e.g. hardness and elastic modulus) of austenite phase and ferrite phase are compared. Then, with the incremental load controlled loading-unloading conditions applied, the indentation depth evolution rule and the indentation creep behavior was studied. Finally, with the load controlled cyclic indentation experiments being conducted, the cyclic evolution rule of displacement into surface were observed, and the influence of different load level on the micro cyclic deformation behavior of each phase were discussed.

Abstract: The cyclic deformation behavior of the austenite and ferrite phase in an austenite-ferrite duplex stainless steel was studied by using the load-controlled cyclic nanoindentation approach. The results showed that the maximum penetration depth onto both austenite and ferrite phases increased continuously while the rate decreased gradually and finally reached to a constant during the repeated indentation. Both transient state and quasi-steady state were observed for the penetration depth per cycle on both of the austenite and ferrite phases with the increased cycles. By contrast, both the penetration depth and rate per cycle into the austenite phase were larger than those into the ferrite phase. This was ascribed to the stress-induced densification in the austenite and ferrite phases and strain-induced transformed martensite in the austenite phase.

Abstract: In order to determine the stress corrosion cracking susceptibility of NiCrMoV steel welded joint of low pressure nuclear steam turbine, long time immersion tests had been carried out in the form of cylindrical tensile specimens self-loaded to various applied load (0, 0.3YS, 0.6YS and 0.9YS) and exposed in the simulated environment with 180°C, 3.5% NaCl. Coarsen grain heat affected zone reacted preferentially as anode in welded joint, which was caused by the synthetic effects of micro-galvanic corrosion, much more precipitations of carbides and higher content of Cr element. Preferential local corrosion of coarsen grain heat affected zone with the formation of occluded corrosion cell accelerated corrosion rate. In addition, the dissolution rate also increased as the applied load increased.

Abstract: In this paper, the finite element method (FEM) based on GTN damage model was used to obtain ductile fracture toughness and investigate the establishment method of unified correlation of in-plane and out-of-plane constraints with ductile fracture toughness of steels. The unified constraint parameter A_p at different equivalent plastic strain (ε_p) isolines has been calculated and analyzed for SEN(B) specimens with a wide range of in-plane and out-of-plane constraints. The results show that the average A_p along the specimen thickness (A_pave) can well characterize a wide range of in-plane and out-of-plane constraints. The suitable ε_p isolines range for establishing the unified correlation between A_pave and ductile fracture toughness of the steel has been obtained. For the specimens with lower constraint, the higher ε_p values should be used. The results also show that the correlation line of J_C/J_ref-A_pave^1/2 is independent of the selections of the suitable ε_p isolines and the reference specimen. This may bring convenience for the establishment and application of the J_C/J_ref -A_pave^1/2correlation lines. Using ductile fracture toughness data of a small number of specimens with different constraints (such as three specimens with different a/W) together with FEM calculations of the parameter A_p, the correlation line of J_C/J_ref-A_pave^1/2can be established. The correlation line may be used in structural integrity assessments incorporating both in-plane and out-of-plane constraints.

Abstract: The creep crack growth rate in Cr-Mo-V steel has been numerically predicted for specimens with different constraints for a wide range of C* by using stress dependent creep model and ductility, and the simulated da/dt-C* curves were compared and analyzed with experimental data. The results show that the simulated da/dt-C* curves agree well with experimental data. At low and transition C* regions, the crack-tip constraint has obvious effect on CCG rates, while at high C* region it almost has no effect. With increasing constraint, the CCG rates and transition region size on da/dt-C* curves increase due to higher stress traxiality ahead of crack tip and stress-regime dependent creep ductility. If the extrapolation CCG rate data of standard high constraint CT specimen from high C* region (above the turning point 2) or from transition C* region are used in life assessments of the components with various constraints at low C* region (below the turning point 1), the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data for considering constraint effect should be obtained for a wide range of C* by long-term laboratory tests or numerical predictions using the stress dependent creep model and ductility.

Abstract: According to the all set theory, a fuzzy-random creep fracture model was presented in this work. To deal with the function, the following steps were taken. First, the steady state creep coefficient (A) and steady state creep exponent (n) were considered in the fuzzy-random variables, then the C*-integral was considered in a fuzzy-random variable. Finally, with the interval analysis, the result of the fuzzy-random creep fracture model was given.

Abstract: An average creep rate conversion model based on Schlottner-Seeley creep assessment procedure and creep damage equation has been developed by considering the relationship that two stages of stress relaxation are corresponding to the first and the second creep stage respectively and the effect of these two kinds of creep rate on relaxation, and stress relaxation is creep at various stresses. And an incremental calculation prediction methodology of stress relaxation performance was established. The predicted results are compared with the data of stress relaxation tests conducted on bolting steel 1Cr10NiMoW2VNbN used in ultra-supercritical turbines. Validation results indicate that the developed model has led to better consistent results with the measured data and thus can be recommended in stress relaxation behavior prediction of high temperature materials.