Bovornchok Poopat

The Effect of Ultrasonic Vibration on Properties of Weld Metal

Weld metal mechanical properties and weldability of materials are closely related to the microstructure of the weld metal. A significant amount of research has been studied to improve microstructure of weldments such as weld pool stirring by using magnetic arc oscillation and arc pulsation. In this work, the effect of ultrasonic vibration was used to modify weld metal solidification to improve microstructure of the weld metal. Microstructure and mechanical properties of carbon steel weld metal (ER70S-G filler metal) were studied. Filler metal was melted by using Gas Tungsten Arc Welding (GTAW) in a water-cooled copper mold. Ultrasonic vibration with a frequency of 20 kHz was applied during solidification of the weld metal. Microstructure and mechanical properties of weld metal were compared with those of conventional weld metal (no ultrasonic vibration assistance). Scanning Electron Microscopy (SEM) was also used to determine microstructure and phases at high magnification. The results showed that ultrasonic vibration applied during solidification promoted grain refinement in the weld metal. Mechanical properties of weld metal were improved significantly, microstructure analysis correlated well with the mechanical test results.

Feasibility Study of Acoustic Emission Monitoring of Hot Cracking in GTAW Weld

Acoustic emission testing can be used to detect the energy emitted from material fracture and the advantage of this method is the real time monitoring, however the weld metal discontinuities are normally inspected by using conventional NDT methods such as Penetrant Testing (PT), Magnetic particle Testing (MT), Ultrasonic Testing (UT) and Radiographic Testing (RT) after the completion of welding. The weld defect must be repaired, which involves the cost and consumes a lot of time as well as reduce the reliability of manufactures. This paper presents the application of acoustic emission (AE) technique for monitoring and detecting the discontinuities during welding. In this study, gas tungsten arc welding (GTAW) was selected as test process. Carbon steel plate and autogenous welding technique were used to simulate the hot crack. The data acquisition (DAQ) and AE sensor were used to capture the acoustic signal generated during welding. The AE signals were amplified and filtered by using preamplifier. Then, signals were modified by wavelet transforms (WT) technique and classified by Fast Fourier Transform (FFT) technique. The results showed the possibility to use AE technique for monitoring and detecting the low signal amplitude generated from crack by using frequency domain. The advantage of this research is to propose the technique for monitoring the weld metal discontinuities during welding.

Investigation into the influence of post-weld heat treatment on the microstructure and hardness of Inconel X-750

This work describes a post-weld heat treatment for a precipitation-hardened nickel alloy. Inconel X-750 is a nickel-based superalloy for gas tungsten arc welding processes. The materials were heat-treated in two steps: solution and aging. The post-weld heat treatment variables examined in this study included post-weld heat treatment temperatures of 705°C, 775°C, and 845°C and post-weld heat treatment time of 2–24 h in 2-h increments. The resulting materials were examined using the full factorial design of experiments to determine the resulting material hardness and observed with optical microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy in the fusion zone and heat-affected zone. The results show that a longer post-weld heat treatment time corresponds to larger γ′ precipitates and a smaller amount of Cr23C6 at the grain boundaries, which can decrease the overall hardness. The post-weld heat treatment analysis indicates that an increase in the amount of γ′ results in better mechanical properties for particles with octagonal shapes and a small size. A factorial analysis, which was conducted on the relationship between the post-weld heat treatment temperature and time to the hardness of the fusion zone, had a 95% confidence level.

Effect of Post Weld Heat Treatment on Fusion and Heat Affected Zone Microstructure and Mechanical Properties of Inconel X-750 Welds

The Inconel X-750 indicates good hot corrosion resistance, high stability and strength at high temperatures and for this reason the alloy is used in manufacturing of gas turbine hot components. The objective of this research was study the effect of post weld heat treatment (PWHT) on fusion zone and heat affected zone microstructure and mechanical properties of Inconel X-750 weld. After welding, samples were solutionized at 1500 0C. Various aging temperature and times were studied. The results show that aging temperature and time during PWHT can greatly affect microstructure and hardness in fusion zone and heat affected zone. As high aging temperature was used, the grain size also increased and M23C6 at the grain boundary decreased. This can result in decreased of hardness. Moreover excessive aging temperature can result in increasing MC carbide intensity in parent phase (austenite). It can also be observed that M23C6 at the grain boundary decreased due to high aging temperature. This resulted in decreasing of hardness of weld metal and heat affected zone. Experimental results showed that the aging temperature 705 0C aging time of 24 hours provided smaller grain size, suitable size and intensity of MC carbide resulting in higher hardness both in weld metal and HAZ.

Modelling and optimisation of the post-weld heat treatment of γ′ precipitation and hardness on Inconel X-750 using the response surface methodology

This work describes a post-weld heat treatment (PWHT) for Inconel X-750. The following PWHT variables were examined: the solution temperature, PWHT temperature, and PWHT time. In this research, the application of the response surface methodology and Box-Behnken design in a mathematical model was investigated and optimised. The resulting materials were examined using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) in the fusion zone (FZ). The experimental results reveal that using a full quadratic model with the proposed mathematical model of the γ′ precipitation size and γ′ hardness in the FZ, the obtained correlation of the hardness and γ′ precipitate size showed a reasonable linear relationship.

Effect of Helium Addition in Argon Shielding Gas on Metal Transfer Behavior in Gas Metal Arc Welding of Aluminum

Helium is widely used as mixing with argon for a shielding gas in GMAW process of Aluminum in order to improve weld quality and increase heat transfer to the weld pool. It has been known that helium could affect metal transfer behavior; however, its behavior has not been well understood. In this study, an analysis of the metal transfer behavior in the GMAW of aluminum was studied. The main objective is to study the effect of Helium on metal transfer in two main regions, short circuit (low welding current region) and spray transfer (high current region). The composition of 5 types of shielding gases were pure argon, 75%Ar + 25%He, 50%Ar + 50%He, 25%Ar + 75%He and pure helium. The welding parameters were fixed at 90A/17.0V, 100A/18.2V, 140A/24.6V and 180A/27.6V. Aluminum plates were welded bead-on-plate in a flat position. The metal transfer behavior was analyzed by using acoustic signals and arc voltage signals. For the result, at low welding current of 90A and 100A with pure argon, short-circuit transfer mode was observed. Adding helium in gas mixture gave no effect in metal transfer mode in low welding current regions but the metal transfer rate was slightly increased. At high welding currents of 140A and 180A with pure argon, spray transfer mode was observed and when increasing helium in gas mixture resulted in changing from spray transfer to combined mode of spray-globular. In these high welding currents, adding helium in gas mixture resulted in decreasing the metal transfer rate since helium gas tended to promote globular metal transfer. Acoustic signal and arc voltage signal can be used effectively in determining modes of metal transfer.

Visualization of gas metal arc welding on globular to spray transition current

ABSTRACT The transfer modes in gas metal arc welding have important effects on welding quality. However, present study of metal transfer modes is not yet fully understood. In this study, welding arcs was visualised using the optical emission spectroscopy technique. The carbon steel wire electrode was used for welding with 80% Ar + 20% CO2 shielding gas. The results showed that the globular to spray transition current was 330–350 A. During globular to spray transition, argon,CO2 and Fe plasma tended to gradually change from localising near the arc axis to a two-layer structure having 11,000 K in high-temperature region away from the arc axis and around 7000 K in low-temperature region near the arc axis.

Qualitative and quantitative analyses of arc characteristics in SMAW

This study was conducted to develop a quantitative evaluation system for arc characteristics such as arc stability and welding spatter generation related to shielded metal arc welding (SMAW) without human sensory evaluation. Factors that correspond to sensory evaluations by welders were investigated based on image processing. For the quantitative evaluation of arc stability, results show that the root mean square and the standard deviation of the arc center fluctuation, respectively, correspond to welders’ sensory evaluation at AC and DC discharges. For welding spatter generation, a method of counting white pixels in a binarized image evaluates the number and size of welding spatters which closely coincide with welders’ sensory evaluations.

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.

Welding Procedure Development for Welding of High Strength Carbon Steel Cladded with Austenitic Stainless Steel 316L by Using Overmatching Filler Metal

This work aims to develop welding procedure for small diameter longitudinal welded clad pipe made from clad plate. High strength carbon steel base metal bonded with 316L stainless steel clad layer was used in this study. The dissimilar materials at the weld joint and accessibility limitation of small diameter present difficulty in welding process selection to achieve weld soundness. The joint and welding se¬quence are designed to avoid solidification cracking. Nickel base over matching filler is used on the clad side. Typical joint configuration is double V groove weld without clad peel back to minimize the number of passes inside the pipe. Firstly, welding is done on the carbon steel side by using Shielded Metal Arc Welding (SMAW) and Submerged Arc Welding (SAW) with carbon steel electrodes. Then, welding on the clad side is done by using ERNiCrMo-3 filler metal. Two different procedures for the clad side are studied. The first procedure is to weld the clad side by using Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding pulse current (GMAW-pulse) and another procedure is to weld the clad side by using the SAW procedure. Hot cracking was observed in the case of SAW procedure at the clad weld centerline due to high heat input and high level of dilution. Mechanical properties and microstructure are evaluated. Clad weld by use of GTAW and GMAW-pulse could give sound weld metal. The tensile and yield strength of all weld metal were found to be greater than that of base metal and 100% shear failures were observed. Charpy impact energy of weld and HAZ at -10°C was found to be over 100 joules. Hardness of weld and HAZ area are surveyed over the weld cross section to determine local hardening. Additionally intergranular corrosion testing was carried out on the clad weld side and then bend testing was done. No crack was observed. Therefore, GTAW and GMAW-pulse clad weld procedure could give required properties according to clad line pipe standard, reduce cost of production and increase productivity compared to the peel back method.