C. V. Robino

Modeling solute redistribution and microstructural development in fusion welds of Nb-bearing superalloys

Abstract Solute redistribution and microstructural evolution have been modeled for gas tungsten arc fusion welds in experimental Ni base and Fe base Nb-bearing superalloys. The multi-component alloys were modeled as a ternary system by grouping the matrix (Fe, Ni, Cr) elements together as the “solvent” to form the γ component of the γ-Nb–C “ternary system”. The variation in fraction liquid and liquid composition during the primary L→γ and eutectic type L→(γ+NbC) stages of solidification were calculated for conditions of negligible Nb diffusion and infinitely fast C diffusion in the solid phase. The proposed model is based on modifications of solute redistribution equations originally developed by Mehrabian and Flemings. Results of the calculations were superimposed on the pseudo-ternary γ-Nb–C solidification surfaces to predict the solidification reaction sequences along with the total and individual amounts of γ/NbC and γ/Laves eutectic-type constituents which form during solidification. Comparison was made to experimentally measured values, and reasonable agreement was found. The model results permit quantitative interpretations of composition–microstructure relations in these Nb-bearing experimental alloys and should provide useful insight into comparable commercial alloy systems as well.

Heat treatment of investment cast PH 13-8 Mo stainless steel: Part I. Mechanical properties and microstructure

The microstructure of investment cast PH 13-8 Mo stainless steel heat-treated to various conditions was studied using light and electron microscopy, electron probe microanalysis, and Mössbauer spectroscopy. The mechanical properties were investigated by using uniaxial tensile testing, hardness testing, and Charpy impact testing. TheΒ-NiAl strengthening precipitates, though detectable by electron diffraction, were difficult to resolve by transmission electron microscopy (TEM) in specimens aged at low temperatures (566 °C and below). A high dislocation density was observed in the lath martensitic structure. The higher strength and lower ductility observed at low aging temperatures was attributed to both the high dislocation density and the precipitation ofΒ-NiAl. When samples were aged at high temperatures (> 566 °C), a lower dislocation density and a reverted austenite fraction on the order of 15 pct were observed. SphericalΒ-NiAl precipitates were observed in the overaged condition. The decrease in strength and corresponding increase in ductility observed in samples aged at temperatures above 566 °C were attributed to the reverted austenite and recovery. Mechanical properties were improved when the homogenizing temperature and time were increased. Electron probe microanalysis quantified the increased homogeneity realized by increasing homogenizing temperature and time. Elimination of the refrigeration step, which normally follows the solution treatment, did not degrade the mechanical properties. Mössbauer spectroscopy showed only minor decreases in the fraction of retained austenite when refrigeration followed the solution treatment.

The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation

A digital image correlation (DIC) method has been used to characterize the constitutive tensile stress-strain response in 304L austenitic stainless steel weldments produced by both continuous-wave (CW) and pulsed-wave (PW) laser welding. The method provides quantitative two-dimensional (2-D) strain maps of the deformation field across the transverse weld samples throughout the tensile test. Local stress-strain response was extracted from regions within the fusion zone and compared to base metal response. The weldments were found to have a higher yield strength than the base metal. The metallurgical origin for the fusion zone strengthening was largely attributed to Hall-Petch and ferrite content effects. While failures localized in the fusion zone with little appreciable necking, the material within the fusion zone retained considerable local ductility: more than 45 pct strain at failure. Significant weld root porosity found in the PW condition and absent in the CW condition appeared to have no deleterious effect on the mechanical performance under the present test conditions in this very ductile, flaw-tolerant alloy.

High-temperature metallurgy of advanced borated stainless steels

The high-temperature metallurgy of advanced borated stainless steels has been evaluated through differential thermal analysis, aging studies, quantitative metallography, and impact toughness measurements. Differential thermal analysis (DTA) was conducted on alloys containing a range of boron concentrations and was used to determine the temperatures associated with melting/ solidification reactions. Aging studies, conducted at temperatures near the solidus, were used to determine the effects of elevated temperature exposures on impact properties and micro-structure. Differential thermal analysis quantified the solidus and liquidus temperatures as a function of boron concentration. Impact testing of samples aged at temperatures near the solidus indicated only moderate reductions in toughness. Particle shape measurements indicated that the boride particles were initially nearly spherical and remained so during elevated temperature exposure, although some faceting and agglomeration of the borides did occur. Measurements of boride particle size distributions were used to define the time, temperature, and composition dependence of the boride coarsening. Coarsening data were analyzed in terms of current coars-ening models. These analyses indicated that the boride particle coarsening followed the theo-retically predictedt1/3 time dependence and that the coarsening rate increased with increasing volume fraction of the boride phase. Analysis of the particle size data for aging at various temperatures indicated that the boride coarsening was consistent with the activation energy for Cr diffusion in austenite. Scanning electron microscopy of the impact fracture surfaces showed that the failure mode in isothermally heat-treated samples was ductile and essentially identical to the failure mode for as-received material.

Heat treatment of investment cast PH 13-8 Mo stainless steel: Part II. Isothermal aging kinetics

The hardening response of investment cast PH 13-8 Mo stainless steel has been evaluated by hardness measurements following aging in the temperature range normally specified for this alloy (510 °C to 593 °C). A new relationship between fraction transformed and hardness was developed, and analysis of the data in terms of the kinetics of precipitation, in a manner similar to that frequently applied to other precipitation-hardenable martensitic steels, yielded low time exponents and a low value for the apparent activation energy. The values of the time exponents were 0.49, 0.37, 0.56, and 0.53 at 510 °C, 538 °C, 566 °C, and 593 °C, respectively, and that for the apparent activation energy was 139 kJ/mole. As has been proposed for other maraging type steels, these estimates suggest thatΒ-NiAl precipitates along or near dislocations and that growth of the precipitates is dominated by dislocation pipe diffusion. However, these predictions were neither supported nor refuted by transmission electron microscopy (TEM) because of difficulties in imaging theΒ-NiAl precipitates at the aging times and temperatures used. Further, analysis of the data using the formalism of Wert and Zener for the growth of precipitates with interfering diffusion fields indicated that the estimates of fraction transformed from hardness data are not fully appropriate for maraging type steels. Consideration of the nature of the Avrami analysis and the electron microscopy results suggests that other phenomena, including dislocation recovery and reversion of martensite to austenite, occur at rates sufficient to convolute the Avrami analysis. It is further suggested that these results cast doubt on the fundamental implications of previous analyses of precipitation kinetics in age-hardening martensitic steels. Although the Avrami analysis was found not to provide a tenable description of the precipitation kinetics, it does provide a reasonable methodology for portrayal of the hardening response of PH 13-8 Mo stainless steel.

Microstructural evolution and mechanical properties of fusion welds in an iron-copper-based multicomponent steel

NUCu-140 is a copper-precipitation-strengthened steel that exhibits excellent mechanical properties with a relatively simple chemical composition and processing schedule. As a result, NUCu-140 is a candidate material for use in many naval and structural applications. Before NUCu-140 can be implemented as a replacement for currently used materials, the weldability of this material must be determined under a wide range of welding conditions. This research represents an initial step toward understanding the microstructural and mechanical property evolution that occurs during fusion welding of NUCu-140. Microhardness traverses and tensile testing using digital image correlation show local softening in the heat-affected zone (HAZ). Microstructural characterization using light optical microscopy (LOM) revealed very few differences in the softened regions compared with the base metal. Local-electrode atom-probe (LEAP) tomography demonstrates that local softening occurs as a result of dissolution of the Cu-rich precipitates. MatCalc kinetic simulations (Vienna, Austria) were combined with welding heat-flow calculations to model the precipitate evolution within the HAZ. Reasonably good agreement was obtained between the measured and calculated precipitate radii, number density, and volume fraction of the Cu-rich precipitates in the weld. These results were used with a precipitate-strengthening model to understand strength variations within the HAZ.

Development and testing of an advanced neutron-absorbing gadolinium alloy for spent nuclear fuel storage

The U.S. Department of Energy requires nuclear criticality control measures for storage of its highly enriched spent nuclear fuel. A new alloy based on the Ni-Cr-Mo alloy system with a gadolinium addition has been developed. Gadolinium has been chosen as the neutron absorption alloying element because of its high thermal neutron absorption cross section. The metallurgical development, mechanical and physical properties, thermal neutron absorption properties, and accelerated corrosion-testing performance of this Ni-Cr-Mo-Gd alloy is described. A brief comparison is also included of the corrosion performance of this alloy as compared to borated stainless steel, which is commonly used as a neutron-absorbing, structural alloy.

Development of Gd-Enriched Alloys for Spent Nuclear Fuel Applications—Part 1: Preliminary Characterization of Small Scale Gd-Enriched Stainless Steels

The influence of Gd additions on the microstructure and hardness of type 316L stainless steel was investigated by various microstructural characterization techniques. This work was conducted as a first step toward the development of Gd-enriched alloys for spent nuclear fuel applications. Small (∼10 g) gas tungsten arc melt buttons were prepared to produce 316L stainless steel with Gd levels from 0.1–10 wt.% Gd. Electron microprobe measurements showed that Gd is essentially insoluble in the austenite/ferrite matrix. All of the alloys formed an interdendritic (Fe, Ni, Cr)3Gd intermetallic, and the amount of the (Fe, Ni, Cr)3Gd phase increased with increasing Gd concentration. Depending on the P and O levels, various amounts of Gd phosphides and oxides were also observed. The relatively high Ni concentration (∼28 wt.% Ni) and low Cr concentration (∼3 wt.% Cr) of the (Fe, Ni, Cr)3Gd phase led to Ni depletion and Cr enrichment in the matrix which, in turn, affected the matrix stability. Alloys with 0.1−6 wt.% Gd exhibited a two-phase ferrite/austenite matrix. Alloys containing 8 and 10 wt.% Gd exhibited a fully ferritic matrix due to extensive Ni depletion/Cr enrichment and concomitant stabilization of ferrite. Hardness increased with increasing Gd concentration due to the formation of the (Fe, Ni, Cr)3Gd intermetallic and the change in matrix structure from austenite to ferrite. A mass balance procedure is presented that accounts for changes to the matrix composition with Gd concentration. This procedure can be used to determine the nominal alloy composition required to produce a 316L-type matrix composition for any Gd level.

A solidification diagram for Ni-Cr-Mo-Gd alloys estimated by quantitative microstructural characterization and thermal analysis

A γ-Gd solidification diagram is proposed as an aid to understanding solidification behavior of Ni-Cr-Mo-Gd alloys. In this system, the Ni-Cr-Mo solid solution γ primary austenite phase is treated as the “solvent” and Gd is treated as the solute. The proposed diagram, which has features characteristic of a binary “eutectic” system, was constructed by combining differential thermal analysis and quantitative microstructural analysis data. As a result of the partially divorced solidification microstructure in the ingots studied, determination of the fraction eutectic, and hence the eutectic composition, requires the use of advanced image analysis techniques. The diagram displays a number of features that are very similar to the Ni-Gd binary system and can be used to assess the influence of the Gd concentration on solidification behavior.

Surface alloy depletion and martensite formation during glass to metal joining of austenitic stainless steels

Abstract Preoxidised and glass to metal (GtM) sealed austenitic stainless steels displayed a ferritic (bcc) layer near the metal/oxide interface, as determined by electron backscatter diffraction and X-ray diffraction. Through electron probe microanalysis, it was determined that this layer was depleted of alloying elements due to the oxidation and sealing processes. Characterisation of the layer morphology suggested that it formed through the martensite transformation mechanism. Thermochemical modelling with ThermoCalc also supported a martensitic transformation as opposed to diffusional ferrite formation. The composition gradient through the layer was correlated to the Eichelman and Hull empirical relationship for martensite start (Ms) temperatures. Because of Cr, Mn and Si depletion during preoxidation and glass sealing, Ms temperatures near ambient are possible in this surface region. The martensite layer was non-uniform, however, with laths extending deeper into the alloy due to stabilised growth in the material above its Ms temperature. This behaviour was characterised by image analysis techniques and discussed in terms of martensite stability and microstructural effects. Possible negative aspects of bcc phase formation on GtM seal properties are discussed, and analyses of alternative alloys 21-6-9 (tradename Nitronic 40; Armco Holding Corp., West Chester, OH, USA) and 22-13-5 (Nitronic 50) showed reduction or elimination of martensite after GtM joining.