Gaylord D. Smith

An Investigation of Structure Stability and Its Improvement on New Developed Ni-Cr-Co-Mo-Nb-Ti-Al Superalloy

A new nickel-base superalloy Ni-25Cr-20Co-0.5Mo-2Nb-1.7Ti-0.8Al has just developed by Special Metals Corporation in the application to ultra-supercritical boilers with steam temperatures up to 700°C. The structure stability of the alloy was studied in detail in combination of SEM, TEM, XRD and micro-chemical phase analyses. Experimental results show that the most important structure instability is as follows: 1) g¢ coarsening; 2) g¢ to h transformation; 3) G phase formation. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level were designed and melted for experimental study. These 2 modified new alloys exhibit more stable structure at 760oC long time aging. A bright perspective has been shown for the development of this new alloy in the near future.

Materials Selection for High Temperature Metal Recuperators

One method of increasing the efficiency of gas turbines is the use of a heat exchanger to capture energy from the exhaust gas of the system. In particular, prime-surface and plate-fin recuperators are used to simply transfer heat from the turbine exhaust to the air leaving the compressor discharge and entering the combustor. In so doing, less fuel is required to heat the inlet air to the final combustion temperature. Desirable material requirements include a low coefficient of thermal expansion, high thermal conductivity, high temperature strength, adequate environmental resistance and good fabrication characteristics. For most industrial applications the 300 and 400 series stainless steels are currently used for the material of construction. This paper discusses the properties of higher Ni-containing alloys and their possible use as recuperator materials for advanced microturbines currently being developed.Copyright

Modification of Ni-Cr-Co-Mo-Nb-Ti-Al Superalloy for USC Power Plant Application at Temperature above 750°C

A recent developed Ni-Cr-Co-Mo-Nb-Ti-Al type nickel-base superalloy, INCONEL 740, has been selected for the application of USC boilers at the temperature above 750°C. This paper focuses on the structure stability improvement of this alloy. Phase computation by Thermo-Calc has been adopted to study main influencing factors on precipitating phases of the alloy and the results show that the ratio of Al/Ti plays an important role. Four new modified alloys in adjustment of Al and Ti contents and in control of Si level were designed and melted for experimental study. The results indicate that the modified alloys exhibit more stable structure stability at 750, 800 and 850°C long time exposure. The newly developed alloy can be adopted for engineering production and application for USC power plants at temperature above 750°C.

Capitalizing on computational tools in industrial alloy development

Computational tools based on the CALPHAD (calculation of phase diagrams) approach are increasingly being used by industry to expedite alloy development, thus cutting cost and cycle times of the traditional experimental trials. One concrete example (i.e., the development of a new alloy for superheater tubing in advanced coal-firedpower plants), is presented illustrating the capabilities of the current modeling software. Data for the validity of the predictions are provided. Moreover, some general estimates for actual time and cost savings are given.

Microstructure and Microanalysis of Crept Alloy 740

Alloy 740 is a new Ni-Cr-Co superalloy developed by Special Metals Corp. for ultra-supercritical steam boiler tubing applications [1]. To evaluate the potential for using alloy 740 above 700°C, this work examines the stability or aging effects on the initial precipitation-hardened microstructure after creep-rupture testing at 816°C. Here, material was solution annealed 0.5h at 1200°C, followed by 16h at 800°C, then crept 2500h at 816°C/20ksi. Both uncrept and crept samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

The Effect of Cold Rolling on the Grain Boundary Character and Creep Rupture Properties of INCONEL Alloy 718

In order to evaluate the effects of sheet processing on the grain boundary character distribution (GBCD) of INCONEL alloy 718 (IN 718), electron backscattered diffraction (EBSD) mapping was performed on samples cold rolled between 0-40%. Increased cold rolling increased the fraction of low-angle boundaries at the expense of the coincident site lattice boundaries. The tensile-creep rupture life (Tr) and elongation-to-failure (ef) were evaluated at 649°C and 758MPa, and the data indicated that increased cold rolling significantly increased both the Tr and ef values. In addition the GBCD and room-temperature (RT) tensile properties were evaluated for superplastically formed INCONEL alloy 718 (IN 718SPF). The tensile results indicated the exceptional strength of the fine-grained IN 718SPF material, however the GBCD parameters were intermediate to those of the 10% and 20% cold rolled IN 718 materials.