Steven L. McCracken

Effect of magnetic stirring on grain structure refinement Part 2 – Nickel alloy weld overlays

Abstract The formation of a columnar grain structure in high chromium nickel based alloy welds can be associated with cracking and poor resolution for ultrasonic non-destructive examination. The objective of this research was to characterise the effects of circular magnetic arc deflection (arc stirring) on grain structure of gas tungsten arc weld overlays made on Inconel 690 substrates with 52M filler wire. Welds and weld overlays were made at various arc stirring frequencies, and microstructures were analysed using optical and electron backscattered diffraction microscopy. Significant refinement of grain size occurred at a stirring frequency of 7 Hz. Ultrasonic non-destructive examination confirmed 100% improvement in signal/noise ratio in weld overlays made with magnetic stirring.

Weldability Studies of High-Cr, Ni-Base filler metals for power generation applications

The solidification behaviour and weld solidification cracking susceptibility of high-Cr, Ni-base filler metals that are widely used, or proposed for use, in the nuclear power industry have been investigated. Two heats of ERNiCrFe-13 (filler metal 52MSS), one heat of ERNiCrFe-7A (filler metal 52M), and one heat of a modified ERNiCr-3 (filler metal 82 with higher Cr content, designated here as filler metal 52i) have been tested using both the Transvarestraint test and the Cast Pin Tear test (CPTT). The solidification behaviour in these alloys has been studied by a newly developed procedure that accurately replicates the solidification process in fusion welds of Ni-base alloys and is based on the patented technique for Single Sensor Differential Thermal Analysis (SS DTA™). Results of the solidification studies showed that filler metal 52i has the widest solidification range, followed by the two heats of filler metal 52MSS, and filler metal 52M. The filler metal 52i also has the widest eutectic temperature range. The interdendritic eutectic constituent formed in weld metal of this filler metal and filler metal 52MSS is enriched in Nb and results from the eutectic reaction of γ + L → γ + NbC at the end of solidification. Both the CPTT and the Transvarestraint test provided the same ranking of solidification cracking susceptibility among these filler metals. Both heats of 52MSS and the heat of 52i were found to be more susceptible to solidification cracking than filler metal 52M. The slightly higher resistance to solidification cracking of filler metal 52i relative to the 52MSS filler metals is attributed to crack “healing” during the final stages of solidification. This is the result of the higher fraction of eutectic liquid of filler metal 52i, as confirmed by metallographic studies. The results of this study confirm the higher solidification cracking susceptibility of high-Cr, Ni-base filler metals that contain higher Nb levels to counteract ductility-dip cracking, relative to filler metals that are Nb-free. This study has also shown that the CPTT can be used as an alternative, and reliable, tool for ranking the solidification cracking susceptibility of high-Cr, Ni-base filler metals proposed for use in nuclear power plants and other applications.

Effect of magnetic stirring on grain structure refinement: Part 1 – Autogenous nickel alloy welds

Abstract The objectives of this work were to characterise and understand the effects of circular magnetic arc deflection (arc stirring) on grain structure refinement of gas tungsten arc weld beads made in Inconel 690 substrates. Welds were made at various arc stirring frequencies (1·5–50 Hz), and microstructures were analysed using optical and electron backscattered diffraction microscopy. Optimum refinement of grain size occurred at a stirring frequency of ∼7 Hz. Analysis of computational fluid flow, heat transfer and solidification model results suggested that grain detachment was the primary mechanism for grain refinement.

Hot Cracking Study of High Chromium Nickel-Base Weld Filler Metal 52MSS (ERNiCrFe-13) for Nuclear Applications

High chromium nickel-base weld filler metals 52 (ERNiCrFe-7) and 52M (ERNiCrFe-7A) have in recent years replaced filler metal 82 (ERNiCr-3) for new fabrication and for repair applications in commercial nuclear power plants. Filler metals 52 and 52M are selected because they have excellent resistance to primary water stress corrosion cracking (PWSCC). Unfortunately, filler metals 52 and 52M exhibit a higher susceptibility to ductility-dip cracking (DDC) compared to filler metal 82. Filler metal 52MSS (ERNiCrFe-13) is a new high chromium nickel-base alloy with Nb and Mo added to improve resistance to ductility-dip cracking. Increasing Nb has in previous research been shown to widen the solidification temperature range in nickel-base alloys. A wider solidification temperature range can potentially increase susceptibility to hot cracking. This study investigated the solidification behavior and hot cracking susceptibility of three heats of 52MSS and compared the results to a heat of filler metal 52M and a heat of filler metal 52i. The solidification behavior and hot cracking susceptibility were investigated by an optimized Transvarestraint test and by a next generation Cast Pin Tear Test (CPTT). The solidification temperature range and eutectic transformations were measured by a patented Single Sensor Differential Thermal Analysis (SS-DTA) technique. This study showed that filler metal 52MSS was slightly more susceptible to hot cracking than 52M and 52i. This study also demonstrated that the next generation CPTT and SS-DTA technique are effective methods for evaluating the solidification behavior and hot cracking susceptibility of high chromium nickel-base weld filler metals.Copyright

Reducing hot cracking tendency of dissimilar weld overlay by magnetic arc oscillation

Abstract Nickel filler metals are used for joining and repair of dissimilar metal welds in nuclear power plants. However, with some compositions of austenitic stainless steel base metals, weld cracking is observed. In the present work, the solidification cracking behaviour of 52M overlay on stainless steel with and without magnetic stirring was studied. Single, double and triple bead on plate experiments with 52M filler wire were performed on a type 303 stainless steel plate cladding with a single layer of ER308LSi stainless steel. Weldings were then performed with and without magnetic stirring. Although cracking tendency was observed in all experiments, it was seen that magnetic stirring significantly reduced the cracking tendency. Confirmatory electron backscattered diffraction analyses confirmed the grain refinement in 52M beads with magnetic stirring.

Behavior and Hot Cracking Susceptibility of Filler Metal 52 M (ERNiCrFe-7A) Overlays on Cast Austenitic Stainless Steel Base Materials

Operating experience in the nuclear power industry has shown that dissimilar metal welds joined with Inconel 82/182 (ERNiCr-3/ENiCrFe-3) filler metal in the primary loop of pressurized water reactor (PWR) plants are susceptible to primary water stress corrosion cracking (PWSCC). Repair of these dissimilar metal weld joints by weld overlay (WOL) using the PWSCC resistant 52 M (ERNiCrFe-7A) filler metal has been successfully applied in numerous PWRs. The typical dissimilar metal joint consists of a low alloy steel vessel nozzle welded to an austenitic stainless steel safe end. The WOL extends from the low alloy steel nozzle over the safe end and most often onto the adjoining wrought or cast stainless steel pipe. Recent experience shows that 52 M is susceptible to hot cracking when welded over specific heats of centrifugally cast stainless steel pipe. The cracking was attributed to unexpectedly high dilution caused by a synergistic influence of Silicon (Si) and Sulfur (S) on weld pool behavior and bead shape. This paper presents the cause and mechanism of 52 M hot cracking when welding over cast stainless steel, and discusses the synergistic influence of Si, S and other trace elements on the weld bead shape and dilution. In addition, the influence of gas tungsten arc (GTA) welding parameters is discussed in relation to the successful application of 52 M WOL on cast austenitic stainless steel base metals.

Weldability Evaluation of High Chromium, Ni-Base Filler Metals Using the Cast Pin Tear Test

High chromium, nickel-base filler metals have been commonly used throughout the nuclear power industry for the weld overlay repair of dissimilar metal welds. These alloys provide optimum resistance to primary water stress corrosion cracking in nuclear power plant cooling systems. However some of these nickel alloys present weldability challenges including susceptibility to solidification cracking and ductility dip cracking. ERNiCrFe-7A (52M) and ERNiCrFe-13 (52MSS) filler metals, including two heats of 52M and one heat of both 52MSS and 690Nb, have been evaluated in this study. The susceptibility to solidification cracking was evaluated using the cast pin tear test (CPTT). The CPTT was also used to evaluate the effect of dilution between two heats of ERNiCr-3 (FM82) on the solidification cracking behavior. Metallurgical characterization using light optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) in the SEM has been performed in order to identify solidification cracking mechanisms, and to study the effect of liquid film formation and backfilling on cracking susceptibility.

Evaluation of Filler Metal 52M (ERNiCrFe-7A) Hot Cracking When Welding on Cast Austenitic Stainless Steel Base Materials

Dissimilar metal welds of filler metal 182 (ENiCrFe-3) in the primary loop of pressurized water reactor (PWR) nuclear plants are susceptible to primary water stress corrosion cracking (PWSCC) after decades of service. Repair or mitigation has been routinely accomplished by installing a structural weld overlay (SWOL) on the filler metal 182 weld joint with the more PWSCC resistant filler metal 52M (ERNiCrFe-7A). The typical dissimilar metal joint consists of a low alloy steel vessel nozzle welded to an austenitic stainless steel safe end. The SWOL extends from the low alloy steel nozzle over the safe end and most often onto the adjoining wrought or cast stainless steel pipe. Field experience shows that filler metal 52M is susceptible to hot cracking when welding on certain heats of centrifugally cast stainless steel piping. This report evaluates 52M hot cracking when welding on CASS piping and provides the likely cause and mechanism for the cracking. The synergistic influence of silicon (Si) and sulfur (S) elements on the weld bead shape and dilution that leads to hot cracking is investigated. In addition, studies on the influence and use of the gas tungsten arc welding (GTAW) power ratio parameter for 52M overlays are presented.Copyright

Prediction of Ductility-Dip Cracking in Narrow Groove Welds Using Computer Simulation of Strain Accumulation

Ductility-dip cracking (DDC) in high chromium nickel-base weld metals has been an issue during fabrication and repair of nuclear power plant components for many years. DDC is a solid-state cracking phenomenon, and several theories [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] have been proposed for the mechanism. Research conducted to develop these theories has primarily been performed using test methods involving small-scale specimens that may not replicate actual welding conditions (e.g., strain-to-fracture, hot-ductility, and varestraint). Due to the complexities of welding, there are potentially significant differences in the strain, strain-rates, stresses, and thermal cycles that can occur between these small-scale test methods and actual welding conditions. To eliminate this uncertainty, a high-restraint, narrow groove weld mockup was developed to assess DDC in this work. Filler metals 52 and 52M (AWS specifications ERNiCrFe-7 and ERNiCrFe-7A, respectively), compositions considered susceptible to DDC, are deposited with cold wire GTAW in a narrow groove with precise heat input and bead placement controls to isolate the occurrence of DDC to a known region of the weld deposit. Computer modeling using SysWeld™ with validated weld parameter inputs was also performed to simulate the narrow groove weld. Comparison of test specimens to computer simulations shows that the highest occurrence of DDC is in weld regions with multiple reheat cycles and high strain accumulation. This and future work is intended to develop a method to predict DDC susceptibility in multi-pass welds and to develop procedures and techniques that minimize the occurrence of DDC.

Development of New Weld Heat Input and Dilution Equations for Gas Tungsten Arc Welding: Part 1

Predicting weld dilution for machine gas tungsten arc welding (GTAW) is a challenge due to the number of variables associated with the welding process. Proper heat input and power ratio controls are critical in many welding applications to control weld dilution, such as for dissimilar metal welds where low weld dilution is necessary to prevent solidification cracking or for cladding where weld dilution is minimized to maintain corrosion resistance of the clad material. This paper discusses the preliminary development and validation of improved weld dilution, heat input, and power ratio equations for the GTAW process. The new equations incorporate power added for the hot wire GTAW process, filler metal material properties, and the heat used to melt the filler metal when added to the GTAW process. The weld dilution equation was validated by comparing calculated dilution values to measured values from bead-on-plate weld trials performed on a variety of filler metals and substrates. Results of the testing and validation along with limitations of the new equations are discussed.Copyright