Papers by Keyword: Weldment

Abstract: This paper discusses the modeling of gas-tungsten arc welding (GTAW) process being adopted for maraging steel plates used in aerospace applications. Primary objective of this work is to predict the temperature profile of heat affected zone in a maraging steel weld. Finite element modeling was done utilizing a gaussian heat flux distribution on a weldment. Model also incorporated a moving heat source along with provision for heat sinks in the form of copper backup plates. For validating the model, welding experiments were conducted on 2 mm and 8 mm thick 250 grade maraging steel plates. Thermal profiles were acquired at different locations of heat affected zone using thermocouples and compared with the predictions from the model.

Abstract: Materials are subjected to low temperatures either intentionally as in the case of cryogenic fuels or non-intentionally as in the case of aerospace environment and are observed to undergo changes in their properties. Microstructural changes are the premier indications of changes in the properties of materials. This paper investigates the effect of cryogenic temperature on the microstructure of low alloy steel weldments. The weldments were subjected to liquid nitrogen temperature of 77K and the microstructures were analyzed using optical microscope and transmission electron Microscope. A distinct change in the microstructure was observed which would reason out the changes in the mechanical properties of weldments.

Abstract: High-temperature martensitic steel P91, internationally marked GX12CrMoVNbN91, is the material used in the energy industry. Creep and high-temperature corrosion resistances are important properties that affect the application of this material at higher temperatures. Weldment reduces creep properties. This work deals with the quantification of this decrease in the case of material P91. The main focus is except the evaluation of creep test results given to the mathematical description of the weld creep strength reduction. Further metallographic analyses of weld joint after creep exposures were performed.

Abstract: 316L stainless steel is a common material in nuclear power equipments. Intergranular corrosion is a hidden and destructive mode of material failure. It is important to study the intergranular corrosion behavior of the 316L to ensure the safety of nuclear power equipments. A series of research has been conducted on properties of 316L including the microstructure of the base metal and the weldment and that under the condition of intergranular corrosion. Through the experiments, an intuitive understanding of the microstructure of the base metal and the weldment has been established. Moreover, the intergranular corrosion resistance can be reflected by the condition of grain separation on metallograph. In resistance of intergranular corrosion, by comparison, the heat affected zone was the worst and the weld joint performed quite well. Thus, the heat effect in welding process must be controlled to improve the intergranular corrosion resistance of the equipment.

Abstract: The influence of residual stress and sensitizing on the electrochemical and corrosion behavior of type SUS304 stainless steels weldment has been studied in the Freeze-Thaw condition. After immersion test at Freeze-Thaw cyclic condition the severe corrosion damage was detected in the vicinity of weld boundary more than isothermal condition. The corrosion damaged zone corresponds to the location in which there are high residual stress and sensitization. The influence of residual stress and sensitization was distinguished and its magnitude was evaluated by voltammetry method.

Abstract: In this paper, corrosion behaviors of 13MnNiMoR steel and its weld joint in the EO reactor service environment were studied. Metallographic analysis and electrochemistry corrosion tests in boiler water which are concerned on different zone metal of 13MnNiMoR steel weld joint have been carried out, and the effect of postweld heat treatment and the pH value of the working medium were discussed. The results indicated that the resistance of different zone metal in joint is tightly connected with their microstructure and heat-affected zone always shows the max corrosion current density. The results also showed that heat treatment could significantly improve the anticorrosion properties of 13MnNiMoR steel weld joint.

Abstract: The residual stress field in a compact tension specimen blank extracted from a non-stress-relieved thick section butt weld has been measured using neutron diffraction and the slitting method. Significant triaxial residual stresses were found in the specimen that is normally assumed to be stress free. Moreover the level of stress was sufficient to make a significant contribution to the crack driving force in creep crack growth tests. The benefits of using more than one measurement technique in such investigations are demonstrated.

Abstract: High residual tensile stress is an important factor contributing to stress corrosion cracking (SCC). Shot peening can impose compressive stresses on the surface of welded joints that negate the tensile stresses to enhance the SCC resistance of welded joints. In the present work, the distribution of residual stress caused by welding is measured by X-ray diffraction method. The maximum stress in the weld is close to the yield strength of AISI 304 stainless steel, and the stresses are negative at both ends of the weld and far from the weld. The X-ray method is also used to measure stress caused by shot-peening. The results show that the higher the peening coverage, the higher the residual compressive stresses in the surface of weldments. While under the same condition, the residual compressive stresses induced by glass beads shot-peening are larger than those by cast steel shots. Temperature and stress fields of welding are simulated by using ABAQUS codes. The 3-D solid elements are used in FEM. Temperature depending on material properties as well as the convection and radiation as boundary conditions are considered. The 3-D linear reduced-integration elements are used to simulate the shot peening process. The results of simulation have a good agreement with experimental data. All unpeened and peened weldments are immersed in boiling 42% magnesium chloride solution during SCC test. Unpeened specimens crack after immersion for 6 hours. The steel-peened specimens with 50% coverage crack after 310 hours, while the steel-peened specimens with 100% coverage crack for 3500 hours. However, steel-peened specimens with 200% coverage and glass-peened specimens with 50%, 100% and 200% coverage are tested for a total of 3500 hours without visible stress corrosion cracks in the peened surfaces. The experiment results indicate that shot peening is an effective method for protecting weldments against SCC and weldments peened by glass beads resist SCC better than those peened by steel shots.

Abstract: The difference in microstructures of the base metal (BM), weld metal (WM) and heat-affected zone (HAZ) in the weldment is one of the major reasons for the failure of the welded equipments, which can be essentially attributed to the non-homogeneous corrosion occurred electrochemically on the weldment. Therefore, it is necessary to explore the corrosion properties of weldment. In this paper, the electrochemical behavior of SPV50Q steel weldment was investigated. The polarization curves of BM, WM and HAZ in 5wt.%NaCl-0.5wt.%HAc solution containing H2S were measured by potentiodynamic polarization. Interface characterization was also conducted by electrochemical impedance spectroscopy (EIS). The results show the anodic curves are almost same regardless of various pH or H2S content, but the cathodic curves show some difference. Relatively large variation in corrosion current density (icorr) obtained by fitting technique exists among BM, WM and HAZ. icorr of WM and HAZ is larger than that of BM, and icorr of WM is maximum. According to EIS results, polarization resistance (Rp) increases in the orders of WM, HAZ and BM. It is concluded that WM and HAZ are less resistant to corrosion than BM, which can be correlated to the premature failure of the weldment serviced in H2S-containing environment such as sulfide stress corrosion cracking (SSCC) and /or stress oriented hydrogen-induced cracking (SOHIC) etc.

Abstract: There exists strong environmental and economic pressure to increase the thermal efficiency of fossil fuel power stations and this has led to a steady increase in operating temperature and pressure resulting in the world wide construction plans for ultra super-critical power plants. Consequently, in order to improve the thermal efficiency of power plant, there has been a strong drive to develop more advanced heat resistant steels with excellent creep, high temperature fatigue and thermal fatigue resistant properties as well as superior oxidation and corrosion resistant properties. In this study, the test material was P122 alloy which was developed for ultra super-critical power plant. To measure the fatigue crack growth rate in low #K range, fatigue tests were performed on the P122 alloy welds by #K decreasing method at three different microstructure (Base metal, HAZ, Weld metal) regions. Microstructure observation and micro-hardness tests performed for all three regions to find the relationship among the crack growth rate, microstructure and hardness. Fatigue tests were performed with compact tension specimens at 600°C, 650°C and 700°C at the loading frequency of 20Hz.