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in creator/contributor: "David, S.A."

Boron-induced toughness loss in Ti-6A1-2Nb-1Ta-0. 8Mo

Because boron has been added to the weld filler metal to refine the weld grain structure and is a common impurity in Al, the poor weld and HAZ properties of the Al-stabilized alloys could have been caused by this B. This thermal cycling study showed that the precipitated phase was Ti B and the the B raised the BETA-transus. The phase relation of B in Ti6211 permits a plausible explanation for the toughness loss of ..cap alpha.. + BETA annealed alloys caused by welding and weld thermal cycles. The alloys with B contents under this solubility limit of 0.0018% B were unaffected. (DLC)

Date: January 1, 1984

Creator: Inouye, H. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

High-power laser and arc welding of thorium-doped iridium alloys

The arc and laser weldabilities of two Ir-0.3% W alloys containing 60 and 200 wt ppM Th have been investigated. The Ir-.03% W alloy containing 200 wt ppM Th is severely prone to hot cracking during gas tungsten-arc welding. Weld metal cracking results from the combined effects of heat-affected zone liquation cracking and solidification cracking. Scanning electron microscopic analysis of the fractured surface revealed patches of low-melting eutectic. The cracking is influenced to a great extent by the fusion zone microstructure and thorium content. The alloy has been welded with a continuous-wave high-power CO/sub 2/ laser system with beam power ranging from 5 to 10 kW and welding speeds of 8 to 25 mm/s. Successful laser welds without hot cracking have been obtained in this particular alloy. This is attributable to the highly concentrated heat source available in the laser beam and the refinement in fusion zone microstructure obtained during laser welding. Efforts to refine the fusion zone structure during gas tungsten-arc welding of Ir-0.3 % W alloy containing 60 wt ppM Th were partially successful. Here transverse arc oscillation during gas tungsten-arc welding refines the fusion zone structure to a certain extent. However, microstructural analysis of this alloy's laser welds indicates further refinement in the fusion zone microstructure than in that from the gas tungsten-arc process using arc oscillations. The fusion zone structure of the laser weld is a strong function of welding speed.

Date: May 1, 1980

Creator: David, S.A. & Liu, C.T.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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.

Date: January 1, 1987

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Solidification behavior and microstructural analysis of austenitic stainless steel laser welds

Solidification behavior of austenitic stainless steel laser welds has been investigated with a high-power laser system. The welds were made at speeds ranging from 13 to 60 mm/s. The welds sowed a wide variety of microstructural features. The ferrite content in the 13-mm/s weld varied from less than 1% at the root of the weld to about 10% at the crown. The duplex structure at the crown of the weld was much finer than the one observed in conventional weld metal. However, the welds made at 25 and 60 mm/s contained an austenitic structure with less than 1% ferrite throughout the weld. Microstructural analysis of these welds used optical microscopy, transmission electron microscopy, and analytical electron microscopy. The austenitic stainless steel welds were free of any cracking, and the results are explained in terms of the rapid solidification conditions during laser welding.

Date: January 1, 1981

Creator: David, S.A. & Vitek, J.M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Solidification behavior of type 308 stainless steel filler metal

A series of experiments that combined thermal analysis and interrupted solidification was carried out to understand the solidification behavior of type 308 stainless steel weld metal. Results indicate the following sequence of phase separations for the alloy investigated: L ..-->.. L + delta ..-->.. L + delta + ..gamma.. ..-->.. ..gamma.. + delta. Ferrite observed at room temperature was identified as the untransformed primary delta-ferrite that formed during the initial stages of solidification. The mode of freezing and the extent of solid-state diffusion both control the amount and distribution of ferrite. Microstructural and microprobe analyses indicated both extensive solid-state transformation and solute redistribution during solidification and cooling from the nonequilibrium solidus to room temperature.

Date: January 1, 1979

Creator: David, S. A. & Goodwin, G. M.

Item Type: Report

Partner: UNT Libraries Government Documents Department

Computer modeling of arc welds to predict effects of critical variables on weld penetration

In recent years, there have been several attempts to study the effect of critical variables on welding by computational modeling. It is widely recognized that temperature distributions and weld pool shapes are keys to quality weldments. It would be very useful to obtain relevant information about the thermal cycle experienced by the weld metal, the size and shape of the weld pool, and the local solidification rates, temperature distributions in the heat-affected zone (HAZ), and associated phase transformations. The solution of moving boundary problems, such as weld pool fluid flow and heat transfer, that involve melting and/or solidification is inherently difficult because the location of the solid-liquid interface is not known a priori and must be obtained as a part of the solution. Because of non-linearity of the governing equations, exact analytical solutions can be obtained only for a limited number of idealized cases. Therefore, considerable interest has been directed toward the use of numerical methods to obtain time-dependant solutions for theoretical models that describe the welding process. Numerical methods can be employed to predict the transient development of the weld pool as an integral part of the overall heat transfer conditions. The structure of the model allows each phenomenon to be addressed individually, thereby gaining more insight into their competing interactions. 19 refs., 6 figs., 1 tab.

Date: January 1, 1991

Creator: Zacharia, T. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Solidification behavior of austenitic stainless steel filler metals

Thermal analysis and interrupted solidification experiments on selected austenitic stainless steel filler metals provided an understanding of the solidification behavior of austenitic stainless steel welds. The sequences of phase separations found were for type 308 stainless steel filler metal, L + L + delta + L + delta + ..gamma.. ..-->.. ..gamma.. + delta, and for type 310 stainless steel filler metal, L ..-->.. L + ..gamma.. ..-->.. ..gamma... In type 308 stainless steel filler metal, ferrite at room temperature was identified as either the untransformed primary delta-ferrite formed during the initial stages of solidification or the residual ferrite after Widmanstaetten austenite precipitation. Microprobe and scanning transmission electron microscope microanalyses revealed that solute extensively redistributes during the transformation of primary delta-ferrite to austenite, leading to enrichment and stabilization of ferrite by chromium. The type 310 stainless steel filler metal investigated solidifies by the primary crystallization of austenite, with the transformation going to completion at the solidus temperature. In our samples residual ferrite resulting from solute segregation was absent at the intercellular or interdendritic regions.

Date: February 1, 1980

Creator: David, S.A.; Goodwin, G.M. & Braski, D.N.

Item Type: Report

Partner: UNT Libraries Government Documents Department

Effect of initial composition distribution on the phase transformation behavior in the Fe-Cr-Ni system

A finite-difference implicit numerical model was used to study the diffusion-controlled {alpha}{minus}{gamma} (ferrite-to-austenite) solid-state phase transformation in the Fe-Cr-Ni system. The influence of a nonuniform initial composition distribution was examined in order to assess the impact that nonuniform solute profiles resulting from solidification may have on subsequent transformation behavior in weldments and castings. A significant impact on the transformation kinetics and transformation path was found in some cases. Factors that affect the degree of influence are discussed.

Date: December 31, 1995

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Microstructure modeling in weld metal

Since microstructure development in the weld metal region is controlled by various physical processes, there is a need for integrated predictive models based on fundamental principles to describe and predict the effect of these physical processes. These integrated models should be based on various tools available for modeling microstructure development in a wide variety of alloy systems and welding processes. In this paper, the principles, methodology, and future directions of modeling thermochemical reactions in liquid, solidification, and solid state transformations are discussed with some examples for low-alloy steel, stainless steel, and Ni-base superalloy. Thermochemical deoxidation reactions in liquid low-alloy steel lead to oxide inclusion formation. This inclusion formation has been modeled by combining principles of ladle metallurgy and overall transformation kinetics. The model`s comparison with the experimental data and the ongoing work on coupling this inclusion model with the numerical models of heat transfer and fluid flow are discussed. Also, recent advances in theoretical and physical modeling of the solidification process are reviewed with regard to predicting the solidification modes, grain structure development, segregation effects, and nonequilibrium solidification in welds. The effects of solid state phase transformations on microstructure development and various methods of modeling these transformations are reviewed. Successful models, based on diffusion-controlled growth and plate growth theories, on microstructure development in low-alloy steel and stainless steel weld metals are outlined. This paper also addresses the importance of advanced analytical techniques to understand the solid state transformation mechanisms in welds.

Date: December 1995

Creator: David, S. A. & Babu, S. S.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Modeling microstructure development in weld materials

Microstructure development in the weld metal region is controlled by various physical processes such as thermochemical reactions in liquid, solidification, and solid state transformations. There is a need for fundamental and generalized models that can predict the effect of these physical processes on microstructure development in a wide variety of alloy systems during welding processes. This paper describes certain advances made in the area of modeling the microstructure development in low-alloy steel, stainless steel, and Ni-base superalloy. In addition, this paper describes the importance of advanced analytical techniques for fundamental understanding of phase transformation mechanisms in welds.

Date: September 1, 1996

Creator: David, S.A. & Babu, S.S.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Recent developments in the study of phase stability of austenitic stainless steels and its relation to properties

Much work has been done over the years in alloy development of stainless steels and in the characterization of stainless steel microstructures and properties. However, in recent years there have been significant new advances made, and insights gained, into the physical metallurgy of these materials. In particular, advanced techniques have led to new information on the phase stability of stainless steels and the influence of the phase stability on mechanical properties. This paper will highlight some of these new advances, with an emphasis on work that has been done at ORNL on these alloys. For stainless steel alloys, the phase stability can be influenced by several factors. They include solidification behavior, the ferrite/austenite solid-state transformation, other high temperature phase transformations, and low temperature phase transformations. Recent advances in theoretical and experimental methods have led to new developments in understanding and characterizing these factors. Advanced solidification theory has been applied to understand the influence of rapid solidification on phase formation during solidification. New thermodynamic evaluation methods have shown great potential in providing details on the overall phase stability, including details on the influence of composition on phase stability. finite-difference techniques have been applied to the stainless steel alloy system to gain much insight into the ferrite/austenite transformation behavior. Finally, advanced techniques such as analytical electron microscopy, atom probe field ion microscopy, nano-indentation techniques, and specimen miniaturization techniques have provided valuable information on the response of stainless steel microstructures and properties to thermal treatment. All of these new methods and approaches are described in detail in this presentation.

Date: December 31, 1995

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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.

Date: December 31, 1995

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Bibliography of the technical literature of the Materials Joining Group, Metals and Ceramics Division, 1951 through June 1987

This document contains a listing of the written scientific information originating in the Materials Joining Group (formerly the Welding and Brazing Group), Metals and Ceramics Division, Oak Ridge National Laboratory during 1951 through June 1987. It is a registry of about 400 documents as nearly as possible in the order in which they were issued.

Date: August 1, 1987

Creator: David, S.A.; Goodwin, G.M. & Gardner, K. (comps.)

Item Type: Report

Partner: UNT Libraries Government Documents Department

Prediction of non-equilibrium solidification modes in austenitic stainless steel laser welds

A primary austenitic, non-equilibrium mode of solidification can be induced in many 300 series austenitic stainless steels by laser welding and other rapid solidification techniques. The change in solidification mode can often result in a fully austenitic microstructure compared to the two phase ferrite plus austenite microstructure that is commonly found after primary ferrite solidification. A predictive capability for the solidification mode under rapid solidification conditions such as laser welding is needed since the microstructure and properties are closely related to solidification behavior. Conventional constitutional diagrams do not predict with any reliability the change to non-equilibrium austenitic solidification mode. Several means that have been attempted to predict the solidification behavior under extreme conditions are reviewed. The Cr{sub EQ}/Ni{sub EQ} ratio is shown to be unreliable, at least with the equivalence factors commonly used. Theoretical calculations of growth rate competition between ferrite and austenite solidification show promise, but some ambiguity results from inaccuracy in thermophysical material parameters. Use of calculated thermodynamic parameters such as equilibrium and non-equilibrium liquidus and solidus temperatures also shows some promise. However, this approach is not completely satisfactory and reliable either. It is concluded that a reliable and accurate method for predicting the tendency of austenitic stainless steels to solidify in the non-equilibrium mode is not yet available.

Date: August 1, 1993

Creator: Vitek, J. M. & David, S. A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Electron-beam welding of thorium-doped iridium alloy sheets. [Ir--0. 3% W 200 ppM Th-50 ppM Al]

Modified iridium alloys containing 100 ppM Th were found to be very susceptible to hot-cracking during gas tungsten-arc and electron-beam welding. However, the electron-beam welding process showed greater promise of success in welding these alloys, in particular Ir--0.3% W doped with 200 ppM Th and 50 ppM Al. The weldability of this particular alloy was extremely sensitive to the welding parameters, such as beam focus condition and welding speed, and the resulting fusion zone structure. At low speed successful electron-beam welds were made over a narrow range of beam focus conditions. However, at high speeds successful welds can be made over an extended range of focus conditions. The fusion zone grain structure is a strong function of welding speed and focus condition, as well. In the welds that showed hot-cracking, a region of positive segregation of thorium was identified at the fusion boundary. This highly thorium-segregated region seems to act as a potential source for the nucleation of a liquation crack, which later grows as a centerline crack.

Date: April 1, 1979

Creator: David, S.A.; Liu, C.T. & Hudson, J.D.

Item Type: Report

Partner: UNT Libraries Government Documents Department

Bibliography of the technical literature of the Materials Joining Group, 1951--1991

This document contains a listing of the written scientific information originating in the Materials Joining (formerly the Welding and Brazing Group), Metals and Ceramics Division, Oak Ridge National Laboratory during 1951 through June 1991. This registry of documents is as much as possible, in the order of issue date.

Date: December 1, 1991

Creator: David, S.A.; Goodwin, G.M. & Gardner, K. (comps.)

Item Type: Report

Partner: UNT Libraries Government Documents Department

Effect of oxide inclusions on the solid state transformation in low-alloy steel fusion welds

Non-metallic inclusions are known to influence the properties of low alloy steel weld metal by altering the microstructure development. Isothermal transformation kinetics of austenite to acicular ferrite and allotriomorphic ferrite were measured in reheated low alloy steel weld deposits with similar weld compositions and austenite grain size but different inclusion characteristics. Accelerated kinetics of the transformation to acicular ferrite were observed in the weld metal containing coarser titanium-rich inclusions. The results are also discussed in relation to the predictions of inclusion model. The kinetics of the transformation to allotriomorphic ferrite were not influenced by a change in the inclusion characteristics, but, rather, by a change in austenite grain size. A theoretical analysis of austenite grain development during weld cooling is considered in this work. The austenite grain size was found to depend on the driving force for transformation from 6 ferrite to austenite ({Delta}G{sup {sigma}->{gamma}}) calculated from ThermoCalc{trademark} software.

Date: December 31, 1995

Creator: Babu, S.S.; David, S.A. & Vitek, J.M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Modeling phase transformation behavior during thermal cycling in the heat-affected zone of stainless steel welds

An implicit finite-difference analysis was used to model the diffusion-controlled transformation behavior in a ternary system. The present analysis extends earlier work by examining the transformation behavior under the influence of multiple thermal cycles. The analysis was applied to the Fe-Cr-Ni ternary system to simulate the microstructural development in austenitic stainless steel welds. The ferrite-to-austenite transformation was studied in an effort to model the response of the heat-affected zone to multiple thermal cycles experienced during multipass welding. Results show that under some conditions, a transformation ``inertia`` exists that delays the system`s response when changing from cooling to heating. Conditions under which this ``inertia`` is most influential were examined. It was also found that under some conditions, the transformation behavior does not follow the equilibrium behavior as a function of temperature. Results also provide some insight into effect of composition distribution on transformation behavior.

Date: December 31, 1995

Creator: Vitek, J.M.; Iskander, Y.S. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Modeling the ferrite-to-austenite transformation in the heat-affected zone of stainless steel multi-pass welds

The ferrite/austenite transformation in austenitic stainless steel welds was modeled by considering the transformation as a diffusion-controlled process. A finite-difference method was used to solve the diffusion equations. The transformation behavior was evaluated for both isothermal aging and linear cooling over a range of cooling rates. The analysis provides information on the change in ferrite content as a function of heat treatment as well as the rate of the formation (or dissolution) of ferrite. The compositions of the ferrite and austenite are also obtained in the analysis. the results show that the approach to equilibrium can often be indirect and sometimes counter to intuition. The analysis is useful in providing information on the microstructural stability in austenitic stainless steel welds that cannot be obtained experimentally.

Date: December 31, 1995

Creator: Vitek, J.M.; Vitek, S.A. & David, S.A.

Item Type: Report

Partner: UNT Libraries Government Documents Department

Weldability and microstructure development in nickel-base superalloys

The integrity of nickel-base superalloy single-crystal welds depends on the weld cracking tendency, weld metal dendrite selection process, stray crystal formation, and macro- and microstructure development. These phenomena have been investigated in commercial nickel-base superalloy single crystal welds. During electron beam and laser beam welding, transverse and longitudinal weld cracking occurred. However, the weld cracking tendency was reduced with preheating. Most of the dendritic growth pattern development in these welds can be explained by a geometric model. However, the welds also contained misoriented stray crystals, which were frequently associated with weld cracks. The formation of stray crystals was related to thermal and constitutional supercooling effects. Fine-scale elemental partitioning between {gamma} and {gamma}{prime} phase was measured with atom-probe field-ion microscopy. Marked differences in partitioning characteristics in two welds were observed and are related to differences in cooling rates. In this paper, the modeling tools available to describe the above are reviewed.

Date: November 1, 1997

Creator: David, S.A.; Babu, S.S. & Vitek, J.M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Inclusion formation in low-alloy steel welds

This paper summarizes four models for describing inclusion formation in steel welds. These methods include simple fixed oxidation sequence model, thermodynamic model, thermodynamic-kinetic model, and thermodynamic-kinetic-fluid flow model. Complexities of these models increase with a need to describe details of the inclusion formation. The applicability of the models was illustrated with two examples. In one of the examples, thermodynamic calculations of phase stability between liquid steel, AlN and Al{sub 2}O{sub 3} explained the inclusion formation in self-shielded flux cored arc weld. In the second example, thermodynamic-kinetic calculations illustrated the competing effects of weld metal composition and cooling rate effects on the inclusion formation in electron beam and laser beam welds. Limitations of the current inclusion models and recommendations for future work on the inclusion formation were highlighted.

Date: November 1, 1998

Creator: Babu, S.S.; David, S.A. & DebRoy, T.

Item Type: Report

Partner: UNT Libraries Government Documents Department

Using Neural Networks to Describe Complex Phase Transformation Behavior

Final microstructures can often be the end result of a complex sequence of phase transformations. Fundamental analyses may be used to model various stages of the overall behavior but they are often impractical or cumbersome when considering multicomponent systems covering a wide range of compositions. Neural network analysis may be a useful alternative method of identifying and describing phase transformation beavior. A neural network model for ferrite prediction in stainless steel welds is described. It is shown that the neural network analysis provides valuable information that accounts for alloying element interactions. It is suggested that neural network analysis may be extremely useful for analysis when more fundamental approaches are unavailable or overly burdensome.

Date: May 24, 1999

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Modeling the Ferrite-Austenite Transformation in the Heat-Affected Zone of Stainless Steel Welds

The diffusion-controlled ferrite-austenite transformation in stainless steel welds was modeled. An implicit finite-difference analysis that considers multi-component diffusion was used. The model was applied to the Fe-Cr-Ni system to investigate the ferrite- austenite transformation in the heat-affected zone of stainless steel weld metal. The transformation was followed as a function of time as the heat-affected zone was subjected to thermal cycles comparable to those experienced during gas-tungsten arc welding. The results showed that the transformation behavior and the final microstructural state are very sensitive to the maximum temperature that is experienced by the heat-affected zone. For high maximum exposure temperatures ({approximately} 1300{degree} C), the ferrite formation that occurs at the highest temperatures is not completely offset by the reverse ferrite dissolution at lower temperatures. As a result, for high temperature exposures there is a net increase in the amount of ferrite in the microstructure. It was also found that if compositional gradients are present in the initial ferrite and austenite phases, the extent of the transformation is impacted.

Date: December 1, 1997

Creator: Vitek, J.M. & David, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

Embrittlement of austenitic stainless steel welds

The microstructure of type-308 austenitic stainless steel weld metal containing {gamma} and {delta} and ferrite is shown. Typical composition of the weld metal is Cr-20.2, Ni-9.4, Mn-1.7, Si-0.5, C-0.05, N-0.06 and balance Fe (in wt %). Exposure of austenitic stainless steel welds to elevated temperatures can lead to extensive changes in the microstructural features of the weld metal. On exposure to elevated temperatures over a long period of time, a continuous network of M{sub 23}C{sub 6} carbide forms at the austenite/ferrite interface. Upon aging at temperatures between 550--850 C, ferrite in the weld has been found to be unstable and transforms to sigma phase. These changes have been found to influence mechanical behavior of the weld metal, in particular the creep-rupture properties. For aging temperatures below 550 C the ferrite decomposes spinodally into {alpha} and {alpha}{prime} phases. In addition, precipitation of G-phase occurs within the decomposed ferrite. These transformations at temperatures below 550 C lead to embrittlement of the weld metal as revealed by the Charpy impact properties.

Date: December 31, 1997

Creator: David, S.A. & Vitek, J.M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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