It is known that the cobalt-based family of Multiphase alloys (Multiphase is a registered trademark of SPS Technologies, Inc.) derive their strength, in part, from cold working and that additional strengthening can be obtained by subsequent ageing heat treatments [1][2][3][4][5][6][7][8][9]. In the case of alloy MP35N ( 3 5 C o -3 5 N i -2 0 C r10Mo, in wt %), the strengthening associated with cold working is attributed to a face-centred cubic (fc c) -~ hexagonal close-packed (h c p) transformation and/or formation of mechanical twins depending on the deformation temperature relative to the Md temperature [3,4] (Md is the temperature below which the f c c --* h c p transformation can be stressinduced). The strengthening which results from subsequent ageing heat treatments is correlated with the f c c ~ h c p transformation [4] or the formation of intermetallic compounds [6].Haynes alloy No. 25 (Haynes is a registered trademark of Cabot Corporation) is a cobalt-base superalloy with a nominal chemical composition of C o10Ni-20Cr-15W. Typically, 'the alloy is heat treated at 1205 ° C for 15 min followed by rapid air cooling. In this condition, the matrix is a fc c solid solution with a lattice constant of 0.357 nm. In view of the behaviour of alloy MP35N, it may be expected that alloy 25 is also susceptible to the fc c ~ h c p transformation particularly as it contains less nickel than alloy MP35N. Therefore, the present investigation was undertaken to determine the susceptibility of alloy 25 to the fc c ~ h c p transformation with emphasis on the role of cold working and ageing heat treatments.Sheet samples (1.3 mm thick) were cold reduced by up to 20%. Ageing heat treatments were conducted at temperatures in the range 370 to 760 ° C. The effect of cold working on the mechanical properties was determined from tensile tests. Light optical metallography and thin-foil transmission electron microscopy were used for the microstructural characterizations. Samples for light optical metallography were etched in a solution consisting of 95% HC1 and 5% H202 by volume. Thin foils for transmission electron microscopy and diffraction work were prepared by the jet polishing technique in a solution of 30vol % nitric acid in methanol at about -20 ° C. All foils were examined at 100 kV. Fig. I shows characteristic optical microstructures of alloy 25 in the annealed condition ( Fig. la) and after ageing for 48h (Fig. lb) and 500h (Fig. lc) at 760 ° C. It can be seen that after ageing at 760 ° C, the microstructure contains a phase with a widmanst~iten-type morphology. This phase was identified by electron diffraction as h c p s-phase with lattice constants of a = 0.25nm and c = 0.41 nm. The electron diffraction pattern of Fig. 2 illustrates the coexistence of

Alloy selection and alloy design both require consideration of an array of material attributes, including in-service properties, weldability, and fabricability. Critical properties of modern heat-resistant alloys for gas turbine applications include high-temperature strength, thermal stability, oxidation resistance, and fatigue resistance. In this paper, the properties of 12 solid-solution-strengthened and six age-hardenable heat-resistant alloys are compared. Weldability is an important attribute and can be a major limiting factor in the use of certain alloys. Weldability test methods are discussed, and the resistance of alloys to solidification cracking and strain-age cracking is compared. The use of weldability testing in the development of modern heat-resistant alloys is discussed with several examples cited. Finally, alloy selection for gas turbine components is outlined, taking into account both alloy properties and fabricability.

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A series of studies were undertaken to determine the optimum rejuvenation heat treatment of HAYNES 230 alloy and to examine its weld repairability. To simulate service exposure, samples of plate nominally 4.75 mm (0.187 in.) thick were exposed at temperatures of 760°C (1400°F) and 871°C (1600°F) for a period of 1500 hours. Room temperature tensile properties and stress-rupture properties at 927°C (1700°F)/62 Mpa (9 ksi) were determined and compared to the material in the unexposed condition. Rejuvenation heat treatments of exposed materials were performed at a temperature of 1177°C (2150°F) for times of 30 minutes to 3 hours followed by a rapid air cool. In all cases, the room temperature tensile and the stress-rupture properties were found to be comparable to those for the unexposed condition. Based on these data, a heat treatment time of 30 minutes was judged to be adequate. Plates representing the as-received, exposed, and exposed and rejuvenated conditions were gas tungsten arc welded (GTAW) using 230-W™ filler metal. Transverse tensile samples containing the weldments were tested at room temperature, and face and root weldment samples were bent around an approximate 2T radius. All of the results, including those for the exposed condition, were found to meet the requirements of Section IX of the ASME Boiler and Pressure Vessel Code.

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