Papers by Author: Guo Dong Zhang

Abstract: The stress corrosion cracking (SCC) sensitivity of aged Z3CN20.09M stainless steel (SS) was investigated by scanning electrochemical microscopy (SECM), nanoindenter and slow strain rate tensiometer (SSRT). The results indicated that the hardness and modulus of the samples increased with increasing aging time at 475 °C, which increased about 80% and 15% respectively after aging for 1000h. After immersed in NaCl+HCl solution (pH=2) for 7 h, the pitting corrosion point appeared of specimen aged for 1000h. While it was not found even for 24 h of as-received (AR) and aged for 500 h specimens at the same condition. The SCC sensitivity was increased about one time of the specimen aged for 1000 h tested in 300 °C high temperature water. The combined effect associated the embrittlement of mechanical properties and the decrease of corrosion resistance is regarded as the main reason for the increasing of SCC sensitivity.

Abstract: A large number of inclusion defects were found during the metal supervision process of 15Cr1Mo1V steel main steam pipeline of the Russian-made thermal power unit. In this paper, the tracking supervision research of 15Cr1Mo1V steel pipe elbow with inclusion defects is carried out. The variation law of strength, toughness, metallographic structure and creep rupture strength with inclusion defects at different operating time is studied. Type and composition of inclusions are analysed and creep fatigue crack evaluation of the most serious inclusion defects discovered is performed. The results show that with the increase of service time, the room temperature and high temperature strength of the material gradually decreased, the impact toughness deteriorated, the ductile-brittle transition temperature increased and greater than room temperature, the metallurgical organization aging grade rose from 3 to 4, creep rupture strength decreased, creep aging was increasingly serious, and creep residual life reduced. The main inclusions are plastic MnS、SiO₂ and severe inclusion levels up to 3. Longitudinal inclusions are mostly long-chain features, and the ends are sharp but no sharp cracks are found at the sharp ends; the transverse inclusions are granular. Creep fatigue crack evaluation show that there was no obvious growth of material inclusion defects with the increase of service time, it is necessary to strengthen the supervision and inspection of brittle inclusions in the follow-up operation.

Abstract: An typical mode of a structural integrity failure in dissimilar steel welded joints. This paper aims at studying crack tip stress of a steam generator dissimilar welded joint under residual stress field with the method of interaction integral and XFEM. Firstly, the corresponding weak form is obtained where the initial stress field is involved, which is the key step for the XFEM. Then, the interaction integral is applying to calculate the stress intensity factor. In addition, two simple benchmark problems are simulated in order to verify the precision of this numerical method. Finally, this numerical method is applying to calculate the crack tip SIF of the addressed problem. This study finds that the stress intensity factor increases firstly then decreases with the deepening of the crack. The main preponderance of this method concerns avoiding mesh update by take advantage of XFEM when simulating crack propagation, which could avoid double counting. In addition, our obtained results will contribute to the safe assessment of the nuclear power plant steam generator.

Abstract: The fretting wear behavior of Inconel 690 for steam generator tube in a nuclear power plant against Inconel 600 (Cr plating) for anti-vibration bar was investigated in a plain cylinder/plane contact configuration under ambient conditions. The fretting wear tests were conducted under various applied normal loads of 20-100 N, slip amplitudes of 20-100 μm and frequency of 2 Hz. Observation of worn surface and corresponding chemical composition analysis were performed to clarify the fretting wear mechanism. It is found that the friction coefficient value increases firstly and then tends to stabilize with decrease in normal loads and increase in slip amplitudes. In addition, the fretting regime is identified to be gross slip regime, indicating that the fretting damage mechanism of Inconel 690 tube against Inconel 600 (Cr plating) bar is wear. The corresponding fretting wear mechanisms are dominant by delamination wear, abrasive wear and friction oxidation.

Abstract: Damage of metals due to the influence of hydrogen is quite frequent and leads to dangerous failures. The characteristics of the hydrogen embrittlemnt of the 65Mn steel were evaluated with small punch test. With the increment of the amount of the hydrogen absorbed into the alloy at room temperature, the strength and the toughness of the material reduce. From the small punch experimental results, it is found the total impact energy, the fracture strain and the fracture stress decrease with the increment of the cathodic hydrogen charging time. The fracture surfaces change from the typical ductile fracture with big voids to the typical intergranular brittle fracture mode after hydrogen absorbed in the specimens with higher charging current density.

Abstract: In order to confirm the structural integrity of pressurizer surge line affected by thermal stratification and thermal shock, the thermo-mechanical fatigue (TMF) behavior of the material used for surge line was investigated based on the real situation in the pressurized water reactor (PWR). Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) cycling in air under a mechanical strain control mode. For the sake of comparison, low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling. The Nano Hardness Tester was used to test the nano hardness of the sample on the cut section surface. The results are shown that there is no significant difference between the IP, OP and IF lives in the investigated temperature ranges. The fracture analysis reveals that the crack initiation and propagation occurred in a transgranular mode under OP, IP and IF cycling condition, and a harden layer occurrence may be the cause of the crack initiation.

Abstract: At the present time, as the steam conditions and capability of the supercritical power unit increasing, the unit reliability is an important factor for the unit efficiency. High temperature, thick walled pipes are widely used in power plants and chemical plants. In this paper, life of the welded joint was predicted by the methods of skeletal point (SP) rupture stress rupture stress, which was calculated by finite element method (FEM). For the life prediction of welded component, the continuum damage mechanics was employed too. The life prediction of the welded joint by SP rupture stress was compared with the life prediction by the method of continuum damage mechanics (CDM). The research results showed that the two predicted methods were consistent. So, it can be concluded that the SP rupture stress can be used for predicting life of the high temperature welded component. The SP rupture stress method was used conveniently for the structure of power plant or other high temperature components.

Abstract: By the finite element analysis software ABAQUS and the function of coupling process between heat and stress, the welding residual stress of Cr5Mo and 20 steel joint was analyzed. In addition the heat treatment of dissimilar steel welded joint was simulated. The residual stress distributions of dissimilar steel welding and heat treatment after welding were obtained. The comparison of welding residual stress between the homogenous steel and dissimilar steel was carried out. The results indicate that the welding residual stress of the same steel is lower than that of dissimilar steel welded joint obviously. Because of the difference of thermal expansion coefficient for base metal and welding microstructure, the relatively higher residual stress is produced due to the bigger thermal expansion coefficient of base metal. The highest annular residual stress is in welding line root of internal wall, while the highest axial residual stress is in welding line surface of outer wall. The welding residual stress of dissimilar welded joint is reduced obviously after heat treatment. The research results provide the possibility for optimizing the welding procedure and improving the reliability of dissimilar steel welding joint.