Quan Duan

Effect of Welding Heat Input on Grain Size and Microstructure of 316L Stainless Steel Welded Joint

Influences of heat input on the microstructure and grain size of shielded metal arc welded 316L stainless steel joints were studied. Three heat input combinations were selected from the operating window of the shielded metal arc welding process and welded joints made using these combinations were subjected to microstructural evaluations so as to analyze the effect of thermal arc energy on the microstructure and grain size of these joints. The results of this investigation indicate that the microstructure of the weld zone and the fusion zone are austenite and a small amount of ferrite while the microstructure of the heat affected zone (HAZ) are austenite and a small amount of MC type carbides, and it can be seen that the amount of ferrite in the weld zone decreases with heat input. For the joints investigated in this study, the average grain size in the HAZ increases with heat input.

Acoustic Emission from Elevator Wire Ropes During Tensile Testing

The acoustic emission (AE) technique was used to monitor the tensile testing process for two kinds of elevator wire ropes in our work. The AE signals from wire breaks were obtained and analyzed by AE parameters and waveforms. The results showed that AE technique can be a useful tool to monitor wire break phenomenon of wire ropes and effectively capture information of wire break signal. The relationship between AE signal characteristics and wire breaks is investigated and it is found that the most effective acoustic signal discriminators are amplitude and absolute energy. Moreover, the wire break signal of two kinds of ropes is a type of burst signal and it is believed that the waveform and spectrum can be applied to analyze the AE wire break signals.

Acoustic emission detection of 316L stainless steel welded joints during intergranular corrosion

This study analyzes acoustic emission (AE) signals during the intergranular corrosion (IGC) process of 316L stainless steel welded joints under different welding currents in boiling nitric acid. IGC generates several AE signals with high AE activity. The AE technique could hardly distinguish IGC in stainless steel welded joints with different welding heat inputs. However, AE signals can effectively distinguish IGC characteristics in different corrosion stages. The IGC resistance of a heat-affected zone is lower than that of a weld zone. The initiation and rapid corrosion stages can be distinguished using AE results and microstructural analysis. Moreover, energy count rate and amplitude are considered to be ideal parameters for characterizing different IGC processes. Two types of signals are detected in the rapid corrosion stage. It can be concluded that grain boundary corrosion and grain separation are the AE sources of type 1 and type 2, respectively.

Reliability-based structural integrity assessment of Liquefied Natural Gas tank with hydrogen blistering defects by MCS method

Hydrogen blistering is one of the serious threats to safe operation of a Liquefied Natural Gas (LNG) tank, therefore safety analysis of hydrogen blistering defects is very important. In order to assess the reliability-based structural integrity of the LNG tank with defects of hydrogen blistering, the following steps were carried out. Firstly, Abaqus code, one of the Finite Element Method (FEM) software, was utilized to calculate 100 J-integral values of crack tip by defining directly. Secondly, the 100 J-integral values of crack tip were used as training data and testing data by Optimized Least Squares Support Vector Machine (OLS-SVM), Least Squares Support Vector Machine (LS-SVM) and Artificial Neural Networks (ANN) to get other 20000 J-integral values of crack tip. Finally, Monte-Carlo Simulation (MCS) was used to assess the reliability-based structural integrity analysis. The results showed that the hydrogen blistering defect with crack will propagate with about 14 percent chance in such a case. It also proved that MCS combined with FEM and SVM was an effective and prospective method for research and application of integrity assessment, which could overcome the data source problem.

Intergranular Corrosion of 316LN Stainless Steel Welded Joints

The intergranular corrosion characteristics of 316 LN austenitic stainless steel welded joints with different welding heat input in boiling nitric acid solution were investigated by metallography analysis and acoustic emission technology. The results show that the resistance to intergranular corrosion of welded joints deteriorated with the increasing welding heat input. The intergranular corrosion resistance of the weld zone is better than that of the heat affected zone and the base material. Five stages for the intergranular corrosion process can be differentiated according to the characteristics of the detected acoustic emission spectrum, such as oxidation, passivation, oxide film crack initiation, oxide film crack propagation and finally fast intergranular corrosion. In other words, in the acoustic emission spectrum,there exist an obvious energy peak by 20 k Hz representing the oxidation signal; an obvious energy peak by 40 k Hz of a burst-like signal representing the stage of passivation process; energy peaks in a range80 to 100 k Hz of burst-like signal representing the oxide film crack initiation; energy peaks by frequency of 60 k Hz representing the stage of crack propagation; and finally a peak by 50 k Hz of a burst-like signal with a slightly longer duration representing the stage of fast intergranular corrosion.

Comparative Study of Acoustic Emission Characteristics for Intergranular Corrosion of 316L Stainless Steel Parent Material and Weldment

The 316L stainless steel parent material and weldment specimens were made to carry out intergranular corrosion(IGC) test using the method of boiling nitric acid. During the corrosion experiment, the acoustic emission(AE) signals were collected. Through the comparative analysis of corrosion rate and metallographic structure, the results showed that the IGC of parent material and weldment can be divided into the preliminary corrosion stage and the rapid corrosion stage. The AE parameters and spectrum characteristics of the two corrosion stages of the parent material and weldment were analyzed. The results showed that: in preliminary and rapid corrosion stages, the AE signal amplitude and energy of weldment were higher than that of parent material; the spectrum characteristics of weldment was more abundant than that of parent material. Based on the results of the comparative analysis, the AE sources of parent material and weldment IGC and the possibilities of monitoring IGC using AE technique were analyzed.Copyright

Research on Intergranular Corrosion Behavior of Welded Joints of Nuclear Grade Stainless Steel

The intergranular corrosion (IGC) behaviors of welded joints of 316LN stainless steel with different welding heat input were investigated in this study. The boiling nitric acid method was chosen to provide the IGC environment. The corrosion rates of different specimens were studied and the micro-structures of each zone (base material, heat affected zone and weld zone) were analyzed in detail. The results show that welding heat input affects IGC resistance remarkably and low welding heat input can reduce the IGC tendency. The IGC test can be divided into three stages, i.e. the initial corrosion stage, stable corrosion stage and rapid corrosion stage. The IGC resistance of WZ for 316LN stainless steel is better than that of BM and HAZ due to the beneficial role of delta ferrite.

Acoustic Emission Response of 316LN Welded Joint during Intergranular Corrosion

The acoustic emission (AE) response of 316LN welded joint during intergranular corrosion (IGC) was investigated in this study. The boiling nitric acid method was chosen to provide the IGC environment and AE signals were detected by Macro-SAMOS AE testing system. The metallographic structures and AE results were studied and analyzed in detail. The results show that the IGC process of 316LN welded joint can generate many AE signals and the AE activity is high. The AE amplitude is below 35dB and mainly ranges from 25dB to 34dB. Two types of AE signals are obtained and both the frequency peaks are 50 kHz. It is believed that grain boundary corrosion, ferrite corrosion and separation of grains are the possible AE sources.

Microscopic Analysis of Intergranular Corrosion of 316L Stainless Steel

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.

Investigation on Acoustic Emission Characteristics from Corrosion of Conventional Materials of Primary Pipe in Nuclear Power Plants

304 stainless steel and 316L stainless steel are conventional materials of primary pipeline in nuclear power plants. The present work is to summarize the acoustic emission (AE) characteristics in the process of pitting corrosion of 304 stainless steel, intergranular corrosion of 316L stainless steel and weldments of 316L stainless steel. The work also discussed the current shortcomings and problems of research. At last we proposed the coming possible research topics and directions.