J.M. Vitek

Analysis of solidification microstructures in Fe - Ni - Cr single crystal welds

A geometric analysis technique for the evaluation of the microstructures in autogenous single-crystal electron beam welds has been previously developed. In the present work, these analytical methods are further extended, and a general procedure for predicting the solidification microstructure of single-crystal welds with any arbitrary orientation is established. Examples of this general analysis are given for several welding orientations. It is shown that a nonsymmetric cell structure is expected in transverse micrographs for most welding geometries. The development of steady-state conditions in the weld pool is also examined in terms of the weld pool size, its shape (as revealed by the dendritic growth pattern), and the size of the dendritic cells. It is found that steady state is established within a few millimeters of the beginning of the weld. Furthermore, steady state is achieved faster in welds made at higher welding speeds. A general analysis of the three-dimensional (3-D) weld pool shape based on the dendritic structure as revealed in the two-dimensional (2-D) transverse micrographs is also developed. It is shown that in combination with information on the preferred growth direction as a function of the solidification front orientation, the entire dendritic growth pattern in single-crystal welds can be predicted. A comparison with the actual weld micrographs shows a reasonable agreement between the theory and experiment. Finally, the theoretical analysis of the dendrite tip radius is extended from binary systems to include the case of ternary systems. The theoretical dendrite trunk spacing in a ternary Fe-Ni-Cr alloy is calculated from the dendrite tip radius and is compared with the experimental values for several weld conditions. Good agreement between experiment and theory is found.

Microstructural modification of austenitic stainless steels by rapid solidification

The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.

Effect of evaporation and temperature-dependent material properties on weld pool development

This paper evaluates the effect of weld pool evaporation and thermophysical properties on the development of the weld pool. An existing computational model was modified to include vaporization and temperature-dependent thermophysical properties. Transient, convective heat transfer during gas tungsten arc (GTA) welding with and without vaporization effects and variable properties was studied. The present analysis differs from earlier studies that assumed no vaporization and constant values for all of the physical properties throughout the range of temperature of interest. The results indicate that consideration of weld pool vaporization effects and variable physical properties produce significantly different weld model predictions. The calculated results are consistent with previously published experimental findings.

Time-resolved X-ray diffraction investigation of primary weld solidification in Fe-C-Al-Mn steel welds

In situ time-resolved X-ray diffraction (TRXRD) using synchrotron radiation was used to monitor the phase evolution during welding of Fe-C-Al-Mn steel with 0.05 s resolutions. The primary solidification phase under rapid- and slow-cooling rate conditions was examined. The results showed nonequilibrium austenite solidification during rapid cooling; in contrast to the equilibrium δ-ferrite solidification that occurs under slow cooling conditions. These experimental results were analyzed using computational thermodynamics and interface-response function models. Results of the interface response function calculations considering the effect of carbon alone, predicted a change from primary δ-ferrite to primary austenite solidification with an increase in solid-liquid interface velocity.

Low temperature aging behavior of type 308 stainless steel weld metal

Abstract The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions

The solidification characteristics of the γ phase from the liquid and the subsequent decomposition of the γ phase control the evolution of the microstructure in nickel–base superalloy welds. The precipitation of the γ′ phase from the γ phase during continuous cooling conditions (0.17–75 K s−1) from the solutionizing temperature was characterized in a directionally solidified CM247DS alloy with thermomechanical simulator, and by transmission electron microscopy, atom probe field ion microscopy and atom probe tomography. The number density increased; size decreased and morphology of the γ′ precipitates changed with an increase in cooling rate. Under rapid water-quenched conditions, complex partitioning of the alloying elements between γ and γ′ phases was observed. Atom probe tomography on samples subjected to slower cooling rates showed different partitioning behavior compared to that of water-quenched samples and the presence of secondary γ′ precipitates in the samples subjected to a cooling rate of 1 K s−1.

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.

Helium effects on void formation in 9Cr-1MoVNb and 12Cr-1MoVW irradiated in HFIR☆

Up to 2 wt% Ni was added to 9Cr-1MoVNb and 12Cr-lMoVW ferritic steels to increase helium production by transmutation during HFIR irradiation. The various steels were irradiated to ∼39 dpa. Voids were found in all the undoped and nickel-doped steels irradiated at 400°C, most of them at 500°C, but not in any of them at 300 or 600°C. Bubble formation, however, was increased at all temperatures in the nickel-doped steels. Maximum void formation was found at 400°C, but swelling remained less than 0.5% even with up to 440 appm He. Irradiation at 300 to 500°C caused dissolution of as-tempered M23C6 precipitates and coarsening of the lath/subgrain structure in the 9-Cr steels, whereas the microstructure generally remained stable in the 12-Cr steels. Irradiation in this temperature range also caused compositional changes in the as-tempered MC phase in all the steels, and produced combinations of fine M6C, G-phase, and M2X precipitates in various steels. The subgrain boundaries appear to be strong sinks that enhance resistance to void formation. Higher helium production during irradiation appears to shorten the incubation period for void formation. The effects of helium on steady state void swelling behavior, however, remain unknown.

Modeling of fundamental phenomena in welds

Recent advances in the mathematical modeling of fundamental phenomena in welds are summarized. State of the art mathematical models, advances in computational techniques, emerging high-performance computers, and experimental validation techniques have provided significant insight into the fundamental factors that control the development of the weldment. The current status and scientific issues in the areas of heat and fluid flow in welds, heat source-metal interaction, solidification microstructure, and phase transformations are assessed. Future research areas of major importance for understanding the fundamental phenomena in weld behaviour are identified.

Precipitation Reactions during the Heat Treatment of Ferritic Steels

The precipitation reactions in two ferritic steels, 9Cr-1Mo-V-Nb and 12Cr-1Mo-V-W, were studied. Analytical electron microscopy, optical microscopy, electrolytic extractions, and hardness measurements were used to determine the types, amounts, and effects of precipitates formed as a function of the heat treatment. The effect of variations in the austenitizing treatment was ascertained. In addition to variations in the austenitizing time and temperature, different cooling rates after austenitization were also used. Air cooling after austenitization (normalization) resulted in little precipitation in both alloys. Precipitation in the 12Cr-1Mo-V-W alloy after furnace cooling was found in all cases examined. Under certain conditions precipitation was also found after furnace cooling the 9Cr-1Mo-V-Nb alloy. However, when compared to the amount of precipitate in the fully tempered state, the 9Cr-1Mo-V-Nb showed a much greater variation in the degree of precipitation following furnace cooling. In addition, the matrix microstructure of the 9Cr-1Mo-V-Nb alloy was very sensitive to cooling rate. The precipitation reactions during tempering after a normalizing treatment were followed as a function of tempering treatment. Tempering temperatures were varied from 400 to 780 °C. The carbide precipitation was essentially complete after one hour at 650 °C for both alloys. Analytical microscopy was used to identify the precipitates. In the 9Cr-1Mo-V-Nb alloy, a combination of chromium-rich M23C6 and vanadium-niobium-rich MC carbides was found. The carbides in the 12Cr-1Mo-V-W alloy were identified as chromium-rich M23C6 and vanadium-rich MC. The results give an indication of the sensitivity of these alloys to heat treatment variations.