Jeffrey W. Sowards

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.

Weld Solidification Cracking in Solid-Solution Strengthened Ni-Base Filler Metals

The weld solidification cracking susceptibility of several solid-solution strengthened Ni-base filler metals was evaluated using the transverse Varestraint test. The alloys tested included Inconel 617, Inconel 625, Hastelloy X, Hastelloy W, and Haynes 230W.* Susceptibility was quantified by determining the solidification cracking temperature range (SCTR) which is a direct measurement of the range over which cracking occurs. This temperature range was then compared to the equilibrium solidification temperature range derived from Calphad-based ThermoCalc™ calculations, Scheil-Gulliver solidification simulations, and in-situ measurements using the single sensor differential thermal analysis (SS-DTA) technique.

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

SE(T) Testing of Pipeline Welds

Single edge-notch tension (referred to as SE(T) or SENT) tests are increasingly being used in the pipeline community, as they are a laboratory-scale fracture toughness test, capable of being performed on linepipe steels and welds. The constraint and loading conditions of the SE(T) specimens more closely correspond with actual field flaws than those of the conventional three-point-bend CTOD (crack tip opening displacement) specimens. The test matrix covered in this paper consists of two nominally X65 pipes and one X80 pipe. Two welding procedures were applied to one of the X65 pipes, resulting in two different welds. Consequently four girth welds were in the test matrix. Notches were cut with electrical discharge machining (EDM) from the outer-diameter (OD) surface of the pipe with the target locations in the base metal, weld centerline, and heat-affected zone (HAZ). The EDM notches were grown by fatigue precracking in a three-point bend fixture to generate sharp flaws. The specimens were loaded in tension and periodically unloaded to generate J-integral resistance curves. The specimens with the weld and HAZ flaws were tested at room temperature and three to four lower temperatures. This paper covers the specimen preparation and the comparison of test results among specimens with different flaw locations at a wide range of temperatures. The specimen preparation and fatigue crack front straightness presented significant challenges. In general, at a given temperature, cracks propagated at lower energies in the weld material than in the HAZ or base material. Comparison of the J-integral curves for the even-matched and over-matched welds showed greater toughness in the over-matched weld at lower temperatures (but still on the upper-shelf of the curves of the ductile-to-brittle transition temperature (DBTT)). Testing at low temperatures appears to affect the HAZ differently than the weld material, as significant increases in toughness were observed between room temperature and −80°C in the HAZ.Copyright

Solidification Behavior and Weldability of Dissimilar Welds Between a Cr-Free, Ni-Cu Welding Consumable and Type 304L Austenitic Stainless Steel

The solidification behavior of a Cr-free welding consumable based on the Ni-Cu system was evaluated in conjunction with Type 304L stainless steel. The weld metal microstructure evolution was evaluated with optical and secondary electron microscopy, energy dispersive spectroscopy, X-ray diffraction, button melting, and thermodynamic (CALPHAD-based) modeling. Solidification partitioning patterns showed that higher dilutions of the filler metal by Type 304L increased segregation of Ti, Cu, and Si to interdendritic regions. Button melting experiments showed a widening of the solidification temperature range with increasing dilution because of the expansion of the austenite solidification range and formation of Ti(C,N) via a eutectic reaction. The model predictions showed good correlation with button melting experiments and were used to evaluate the nature of the Ti(C,N) precipitation reaction. Solidification cracking susceptibility of the weld metal was shown to increase with dilution of 304L stainless steel based on testing conducted with the cast pin tear test. The increase in cracking susceptibility is associated with expansion of the solidification temperature range and the presence of eutectic liquid at the end of solidification that wets solidification grain boundaries.

Henry Granjon Prize Competition 2006 Winner, Category D “Human related subjects” Method for Sampling and Characterizing Arc Welding Fume Particles

Arc welding fume may pose a serious risk to the health and safety of welders and operators in the welding industry. Many methods have been employed to collect and characterize welding fume in the past but previous studies have often not used a combination of techniques that give the full picture on the nature of welding fume. This study was employed to collect fume generated by a variety of arc welding processes, including shielded metal arc welding (SMAW: E6010, E7018, E308–16) and gas metal arc welding (GMAW: ER70S-6 with 100% CO2 and 75% Ar-25% CO2 shielding gases), and to characterize the fume by size distribution, chemical composition, structure, and morphology with state-of-the-art techniques. This requires the use of multiple imaging and analysis techniques since the size variation of welding fume particles is quite large. Collection of welding fume generated by a variety of common electrodes was performed with an electrical low-pressure cascade impactor (ELPI) to size particles by their aerodynamic diameters and develop particle size distributions. A fume collection hood was also used to collect bulk fume samples and measure fume generation rates. Fume particles on the impactor stages were imaged using high resolution scanning electron microscopy (HR-SEM) and high resolution transmission electron microscopy (HR-TEM), revealing the presence of three fume particle morphologies including spherical, agglomerated, and irregular. TEM revealed the presence of a core-shell particle structure. Chemical analysis and phase identification was also performed for individual particles and bulk stages with energy dispersive X-ray spectroscopy (SEM-XEDS and TEM-XEDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Bulk fume and individual particles analyzed were largely metal-oxides with a Fe3O4-type crystal structure. Using these advanced characterization techniques in conjunction with one another provides an overall picture of fume that has been previously unattainable.

Findings and Recommendations from the NIST Workshop on Alternative Fuels and Materials: Biocorrosion.

In 2013, the Applied Chemicals and Materials Division of the National Institute of Standards and Technology (NIST) hosted a workshop to identify and prioritize research needs in the area of biocorrosion. Materials used to store and distribute alternative fuels have experienced an increase in corrosion due to the unique conditions caused by the presence of microbes and the chemistry of biofuels and biofuel precursors. Participants in this workshop, including experts from the microbiological, fuel, and materials communities, delved into the unique materials and chemical challenges that occur with production, transport, and storage of alternative fuels. Discussions focused on specific problems including: a) the changing composition of “drop-in” fuels and the impact of that composition on materials; b) the influence of microbial populations on corrosion and fuel quality; and c) state-of-the-art measurement technologies for monitoring material degradation and biofilm formation.

Measuring laser power as a force: A new paradigm to accurately monitor optical power during laser-based machining operations

In laser manufacturing operations, accurate measurement of laser power is important for product quality, operational repeatability, and process validation. Accurate real-time measurement of high-power lasers, however, is difficult. Typical thermal power meters must absorb all the laser power in order to measure it. This constrains power meters to be large, slow and exclusive (that is, the laser cannot be used for its intended purpose during the measurement). To address these limitations, we have developed a different paradigm in laser power measurement where the power is not measured according to its thermal equivalent but rather by measuring the laser beam’s momentum (radiation pressure). Very simply, light reflecting from a mirror imparts a small force perpendicular to the mirror which is proportional to the optical power. By mounting a high-reflectivity mirror on a high-sensitivity force transducer (scale), we are able to measure laser power in the range of tens of watts up to ~ 100 kW. The critical parameters for such a device are mirror reflectivity, angle of incidence, and scale sensitivity and accuracy. We will describe our experimental characterization of a radiation-pressure-based optical power meter. We have tested it for modulated and CW laser powers up to 92 kW in the laboratory and up to 20 kW in an experimental laser welding booth. We will describe present accuracy, temporal response, sources of measurement uncertainty, and hurdles which must be overcome to have an accurate power meter capable of routine operation as a turning mirror within a laser delivery head.

Measurements of Fatigue Crack Growth Rates of the Heat-Affected Zones of Welds of Pipeline Steels

Pipelines are widely accepted to be the most economical method for transporting large volumes of hydrogen, needed to fuel hydrogen-powered vehicles. Some work has been previously conducted on the fatigue crack growth rates of base metals of pipeline materials currently in use for hydrogen transport and on pipeline materials that may be used in the future. However, welds and their heat-affected zones are oftentimes the source and pathway for crack initiation and growth. The heat-affected zones of welds can exhibit low resistance to crack propagation relative to the base metal or the weld itself. Microstructural irregularities such as chemical segregation or grain-size coarsening can lead to this low resistance. Therefore, in order to have adequate information for pipeline design, the microstructures of the heat-affected zones must be characterized, and their mechanical properties must be measured in a hydrogen environment. With that in mind, data on the fatigue crack growth rate is a critical need. We present data on the fatigue crack growth rate of the heat-affected zones for two girth welds and one seam weld from two API 5L X52 pipes. The materials were tested in hydrogen gas pressurized to 5.5 MPa and 34 MPa at a cyclic loading rate of 1 Hz, and an R ratio of 0.5.Copyright

Understanding the High-Temperature Mechanical Properties of A710 (HSLA-80) Steel With Use of Complementary Atom Probe Tomography and Electron Microscopy

Alloy ASTM A710 (HSLA-80) is a co-precipitation hardened steel with a desirable combination of mechanical properties and weldability, making it an excellent candidate for use in structural applications such as bridges, shipbuilding, and pipelines. There have been extensive studies of the strengthening mechanisms, effect of heat treatment and continuous cooling, and kinetics of the precipitation process in this and similar materials, but little work has been performed to understand the high-temperature mechanical properties [1-6]. Understanding the effect of high temperatures on the mechanical properties of these and similar alloys is very important in terms of engineering for structural integrity and well-understood failure mechanisms in a fire. In this study we use local-electrode atom probe tomography, transmission electron microscopy, and optical metallography as complementary techniques that lend fundamental insight into the high-temperature mechanical behavior of A710 on multiple length scales.