The aging behavior of types 308 and 308CRE stainless steels and its effect on mechanical properties

Aging of 308 and 308CRE SS was studied at 475 to 850/sup 0/C for aging times up to 10,000 hours. Above 550/sup 0/C, aging of 308 steel resulted in precipitation of carbides and the transformation of ferrite to sigma phase or the formation of sigma phase in initially ferrite-free material. The elevated-temperature aging of 308CRE steel resulted in the precipitation of titanium-rich carbides, nitrides, and sulfides, and the transformation of ferrite to sigma phase. The distribution of precipitates was affected by the initial condition of the materials. The elevated-temperature creep properties, and in particular the improved properties of 308CRE, were related to the precipitate distribution. Below 550/sup 0/C, aging of welded type 308 steel, precipitation of G-phase within the ferrite was observed, as well as the decomposition of ferrite into alpha and alpha prime. With the help of a novel mechanical properties microprobe, which was capable of determining the hardness of the minor constituent ferrite phase, the hardness behavior as a function of aging could be related to the microstructures. These results are interpreted in terms of the potential susceptibility of these alloys to 475/sup 0/C embrittlement

Neural Network Modeling of Weld Pool Shape in Pulsed-Laser Aluminum Welds

A neural network model was developed to predict the weld pool shape for pulsed-laser aluminum welds. Several different network architectures were examined and the optimum architecture was identified. The neural network was then trained and, in spite of the small size of the training data set, the network accurately predicted the weld pool shape profiles. The neural network output was in the form of four weld pool shape parameters (depth, width, half-width, and area) and these were converted into predicted weld pool profiles with the use of the actual experimental poo1 profiles as templates. It was also shown that the neural network model could reliably predict the change from conduction-mode type shapes to keyhole-mode shapes

Advanced Integration in Multi-Scale Mechanics and Welding Process Simulation in Weld Integrity Assessment

In this project, mathematical models that predict the microstructure in pipeline steel welds were to be developed. These models were to be integrated with thermal models that describe the time-temperature history in the weld as a function of location in order to derive the spatial variation of microstructure in the weld. The microstructure predictions were also to be combined with microstructure-hardness relations, based on the additivity principle, to determine the spatial variation of hardness in the weld. EMC2 also developed microstructural models based on empirical relationships. ORNL was to pursue the development of more fundamental, theoretically based models. ORNL applied a previously developed model for inclusion formation to predict the extent and nature of inclusions that form during weld cooling from the liquid. This inclusion model was directly integrated with computational thermodynamics capability. A convenient user interface was developed for both the inclusion model and the thermodynamic phase-stability calculations. The microstructure model was based on the simultaneous transformation theory analysis as applied to the transformation of austenite to various ferrite constituents during weld cooling. The model available on the Materials Algorithm Project web site was used. Extensive modification of this model was required to correct problems with compilation and calculations as a function of the computational platform (Unix, Linux, Windows, etc.) that was used. The user interface for the inclusion model and thermodynamic phase-stability calculations was delivered to EMC2 along with the modified and correct microstructure model. Evaluation of the theoretically based model will be carried out and the predictions will be compared with experimental results as well as predictions based on the empirical models developed by EMC2

Direct Observations of Sigma Phase Formation in Duplex Stainless Steels using In Situ Synchrotron X-Ray Diffraction

The formation and growth of sigma phase in 2205 duplex stainless steel was observed and measured in real time using synchrotron radiation during 10 hr isothermal heat treatments at temperatures between 700 C and 850 C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. Differences between the calculated and measured amounts of sigma, ferrite and austenite suggest that the thermodynamic calculations underpredict the sigma dissolution temperature by approximately 50 C. The data were further analyzed using a modified Johnson-Mehl-Avrami (JMA) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMA exponent, n, at low fractions of sigma was found to be approximately 7.0, however, towards the end of the transformation, n decreased to values of approximately 0.75. The change in the JMA exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMA equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high temperature value to room temperature

Modeling phase transformations in ternary systems: Ferrite dissolution during continuous cooling

The diffusion-controlled phase dissolution (or growth) in a ternary system of finite length has been modeled numerically using an implicit finite-difference method. The analysis has been applied to study the ferrite to austenite transformation in austenitic stainless steel weldments. The iron-chromium-nickel ternary system was taken as representative of this class of materials. The effect of system geometry was evaluated by considering planar, cylindrical, and spherical geometries. The numerical analysis was extended to the case of continuous cooling, for a range of cooling rates from 0.1 to 100 K/s. The results provide information on how quickly the system deviates from equilibrium during cooling, and what the final compositions and phase fractions are as a function of cooling rate. In most cases, the deviation from equilibrium, in terms of residual ferrite content and composition, increased as the cooling rate increased, as expected. However, under some conditions, it was found that the lowest cooling rates actually deviated further from equilibrium than intermediate cooling rates. This curious phenomenon was investigated in detail and was explained in terms of the indirect path toward final. Such indirect equilibration is often found during and typical of diffusion-controlled transformation behavior in multi-component systems

Tensile behavior of three commercial ferritic steels after low-temperature irradiation

The ferritic (martensitic) steels and the austenitic stainless steels are being considered for use as first wall and blanket structural components for fusion reactors. Tensile specimens of normalized-and-tempered 9 Cr-1 MoVNb and 12 Cr-1 MoVW steels, normalized-and-tempered and isothermally annealed 2-1/4 Cr-1 Mo steel, and 20%-cold-worked type 316 stainless steel were irradiated at approximately 50/sup 0/C to damage levels of up to about 9 displacements per atom (dpa) in the High Flux Isotope Reactor (HFIR). The preirradiated microstructures of the 9 Cr-1 MoVNb and 12 Cr-1 MoVW steels were a tempered martensite; the microstructure of the normalized-and-tempered 2-1/4 Cr-1 Mo steel was tempered bainite, and that of the isothermally annealed 2-1/4 Cr-1 Mo steel was primarily polygonal ferrite

Phase stability in austenitic stainless steels -- New approaches, results, and their relation to properties

In recent years, the phase stability of austenitic stainless steels, and its effect on the mechanical properties of stainless steels, have been the subject of much interest. With the availability of new experimental techniques, new theoretical methods, and new computational procedures, significant advances have been made in understanding, and being able to predict, phase stability and mechanical properties of stainless steel welds. This paper reviews some of these developments, with an emphasis on recent work that has been done at Oak Ridge National Laboratory

Modeling and Simulation of Microstructural Development During Weld Solidification

Techniques for numerical calculations of phase transformation kinetics have recently become available. These methods are integrated with computational thermodynamics to allow for the description of diffusion-controlled transformations as a function of time. Such calculations have been applied to the modeling of solidification behavior in the current study. Three examples are considered which relate to the prediction of microstructure development and solute redistribution during conditions corresponding to welding conditions. The examples evaluate dendritic growth, planar growth, and competition between alternate solidification modes. It is shown that these techniques are particularly powerful when dealing with multi-component (>2) alloy systems. For such multi-component alloys, new considerations must be taken into account to describe the solute redistribution and the conditions leading to planar front growth. Finally, when studying global behavior covering a wide range of alloy compositions and thermal conditions, individual computations become impractical. For such calculations, neural network analysis may be beneficial and an example is given where such an analysis is shown to be suitable in describing a complex series of phase transformations