Isaratat Phung-On

Analysis of Eddy-Current Measurement System for Residual Stress Assessment in Stainless Steel Grade 304

The objective of this paper is an analysis of statistical discreteness and measurement capability of an eddy-current measurement system for residual stress assessment in stainless steel Grade 304 (SS304). Cylindrical specimens with 50 mm in diameter and 12 mm thickness were prepared to generate residual stress by Resistance Spot Welding at which the welding currents were set at 12, 14, and 16 kA. The eddy-current measurement system was including a probe with frequency range of 0.1 to 3 MHz and an eddy current flaw detector. They were performed by contacting the probe on the specimen. The measurements were performed particularly in the vicinity of heat affected zone (HAZ). In order to determine the results of the residual stress measurement, the calibration curves between static tensile stress and eddy current impedance at various frequencies were accomplished. The Measurement System Analysis (MSA) was utilized to evaluate the changed eddy-current probe impedance from residual stress. The results showed that using eddy current technique at 1 MHz for residual stress measurement was the most efficient. It can be achieved the Gauge Repeatability & Reproducibility %GR&R at 16.61479 and Number of Distinct Categories (NDC) at 8. As applied on actual butt welded joint, it could yield the uncertainty of ± 58 MPa at 95 % (UISO).

Investigation of Fracture Location of T22/T91 Dissimilar Welds

The aim of this investigation is to study the fracture location as well as mechanical properties of weldments from elevated temperature tensile test in T22/T91 dissimilar joining by GTAW process. Two configurations of welding procedures are used: ER90S-B9 and ER90S-B3. Postweld heat treatment (PWHT) is carried out at 760°C for 30 minutes. The results demonstrate that the higher tested temperature, the lower the strength of welded sample; nevertheless, the greater the ductility of welded sample, except ER90S-B9 welded samples tested at 650°C. The fracture of both configurations tested at 550°C and 750°C occurs at T22 BM and T91 HAZ, respectively. At these temperatures, the fracture surfaces mainly reveal dimple ductile mode corresponding to mechanical properties’ results. On the contrary, the failure tested at 650°C occurs at T22 HAZ (Soft zone) and T91 HAZ of ER90S-B9 and ER90S-B3 welded samples, respectively. The mechanical properties’ results at this temperature are relatively different. The combination of brittle and dimple mode at the edge of samples is observed on the ER90S-B9 welded samples corresponding to low ductility at this temperature. Whereas samples welded by ER90S-B3 show only dimple ductile mode. The results exhibit that the fracture location could be affected by the configuration of T22/T91 dissimilar welding.

Direct Observation of Liquation in Ni-Base Superalloy by Using Confocal Laser Scanning Microscopy

In-Situ direct observation was used to observe the liquation in an Inconel 617. High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM) was used to observe the phenomena. Experiment was perform at 1330 °C with heating rate 5 °C/s. Results showed that the liquidation started at 1319.2 °C (268.08 seconds) in which liquid formed around precipitate. At 1330.5 °C (272.05 seconds) the liquid film wet and penetrated along grain boundaries. Then after 277.05 seconds, sufficient liquid film formed and moved freely on the surface of the specimen across grain boundaries. After cooling to room temperature, the test specimen was analyzed for local chemical composition gradient of precipitates and liquid film. Precipitate morphology changed from polygonal shape to round shape using SEM and EPMA. The results from this observation could be used to explain liquation mechanism occurred in Inconel 617 during heating.

Effect of Heat Treatment on Properties of INCONEL 617 Investigated in Accelerated Creep Testing

An accelerated creep test was implemented to study the effects of heat treatment on the creep properties of INCONEL 617. Three heat treatment conditions were set on the test specimens, including 1) as-received, 2) aged as the simulation of service conditions at 650°C for 1,000 h and 3) solutionised. The accelerated creep test was performed at 700, 750, 800 and 900°C with applied stress at 80% and 90% of the yield stress (YS) at tested temperatures. The results were analysed for both time to rupture and % RA, which showed that there was a significant drop in time to rupture and % RA at 750°C. Subsequently, the % RA increased while the time to rupture gradually decreased. The Larson Miller curve for all heat treatment conditions could be plotted with an interest in the presence of an intersection zone. This indicated the possibility of failure behaviour changes from a creep mode to an overload mode at low applied loads to high applied loads, respectively.

Life Assessment for Cr-Mo Steel Dissimilar Joints by Various Filler Metals Using Accelerated Creep Testing

Accelerated creep rupture tests were performed on T22/T91 dissimilar metal joints to determine the fracture location and rupture time of different weldments. Four configurations of deposited filler metal were tested using gas tungsten arc welding to estimate the service life for Cr-Mo steel dissimilar joints at elevated temperatures in power plants. Results indicated that failure in all configurations occurred in the tempered original microstructure and tempered austenite transformation products (martensite or bainite structure) as type IV cracking at the intercritical area of the heat-affected zone (ICHAZ) for both T22 and T91 sides rather than as a consequence of the different filler metals. Creep damage occurred with the formation of precipitations and microvoids. The correlation between applied stress and the Larson-Miller parameter (PLM) was determined to predict the service life of each material configuration. Calculated time-to-failure based on the PLM and test results for both temperature and applied stress parameters gave a reasonable fit. The dissimilar joints exhibited lower creep rupture compared to the base material indicating creep degradation of the weldment.

Nondestructive Evaluation for Duplex Stainless Steel Tube Using Multi-Frequencies Remote Field Testing

This paper presents the determination of Remote Field Testing (RFT) frequencies to accomplish the inspection of duplex stainless steel tubes grade ASME/ASTM SA 789. The tube specimen was 25.4 mm of outside diameter, and thickness of 1.65 mm with the different artificial flaws. A dual-pickup coils type of RFT probe was employed to inspect the specimen by inserting a probe within the tube. Optimum of testing frequency Range was determined based on an eddy current through transmission generation to produce different magnetic field density. RFT inspection frequency range for duplex stainless steel was consequently determined from 5 to 25 kHz which was different than those inspection frequencies of general ferromagnetic steel tube. In the experiment, calculated frequencies were then generated to the Eddy current (ET) and RFT probes for detecting the flaws of the tube specimen. The inspection signals were specifically shown in function of impedance plane to identify the flaw characters. The results showed that the RFT can be utilized to quantify the wall loss levels of duplex stainless-steel tube better than the ET. Especially, phase angle of inspection signals can be used to evaluate the different depths of the wall losses. Sensitivity of RFT showed the detection performance at minimum 20% of tube wall loss.

An Automatic Selection of Segment Width for the Single Sensor Differential Thermal Analysis (SS-DTA) Based on Piecewise Linear Approximation Technique

Single Sensor Differential Thermal Analysis (SS-DTA) is a novel non-destructive testing technique for studying and detecting the phase transformations and structural changes in materials. It uses only one temperature sensor to measure the temperature in a particular point of interest in the material during actual and simulated thermal processing of the material. SS-DTA compares the temperature recorded in a tested specimen against a reference thermal profile which can be generated either by analytical formulae or piecewise linear approximation. The main advantage of piecewise linear approximation over the analytical formulae is that it does not need the knowledge of tested material and processing conditions to optimally estimate the parameters of reference thermal history. On the other hand, in order to apply the piecewise linear approximation technique we must specify the segment width which is normally fixed at a nominal value of 1.5 seconds. We have recently found that this nominal value might not be an optimal choice for the segment width as it does not guarantee to give the best detectability of phase transformation. Therefore, in this research work we proposed a technique to automatically select an appropriate value of the segment width. The performance of proposed method has been evaluated by investigating the phase transformations of welded stainless steel SUS 321and SUS 304. It was found that the appropriate segment width could be ranging from 1.25-1.75 seconds and by using this selection technique, we could detect the differential temperature more accurately than when using the nominal value.