Low-Cost Heat Treatment Process For Superalloy Disks
Operating conditions imposed on a component often require different, distinct mechanical properties at different locations within the component. Examples include a crankshaft in an internal combustion engine, gears in an automobile transmission and disks for a gas turbine engine.
Gas turbine disks are often made of nickel-base superalloys because they must withstand the temperatures and stresses involved in the gas-turbine cycle. In the bore of the disk where the operating temperature is somewhat lower, the limiting material mechanical properties are often tensile and fatigue strength. In contrast, resistance to both creep and crack growth are often the limiting properties in the rim of the disk, where the operating temperatures are higher than those of the bore due to its proximity to the combustion gases.
Recently introduced advanced nickel-base superalloys should improve engine performance through higher disk temperatures compared with current engines. This is achieved by using alloy compositions with high levels of gamma prime (y') and refractory elements. However, there is a need in the long term for disks with even higher rim temperature capabilities of 1400 F (760 C) or higher. Increased temperature capability would allow higher compressor exit temperatures, thereby increasing the efficiency of the gas turbine engine. Increased temperature capability of the rim can be achieved, in part, by using disks with a coarse grain microstructure in the rim, which provides optimal creep resistance, and a fine grain microstructure in the bore, which provides optimal fatigue resistance .
In most gas-turbine applications, nickel-base superalloy disks are currently heat treated at a uniform solution temperature either below the y'solvus temperature (subsolvus heat treatments), or above the solvus temperature (supersolvus heat treatments). Several new approaches that differ from the traditional subsolvus and supersolvus heat treatments have been established. One approach (U.S. Patent 5,312,497) uses induction heating to preferentially heat the rim of a disk while passing a pressurized gas through the bore of the disk to keep the bore and web cooler. Another approach (U.S. Patent 5,527,020) uses top and bottom thermal caps placed over the bore of the disk to blow pressurized air through the center of a single disk while holding the disk at a constant temperature in a gas-fired furnace. The bore is thus maintained at a sufficiently lower temperature than the rim. This achieves the desired subsolvus solution of the bore with a fine grain microstructure and the desired supersolvus solution of the rim with a coarse grain microstructure.
Both approaches produce the desired dual microstructure (fine grain bore and coarse grain rim) in the disk, but both add to the cost and complexity of the solution heat treatment. Also, they can only be applied to one disk at a time, and, therefore, are very expensive. The latter reduces complexity compared with the former, but still requires specialized air pressure lines going into a furnace, which must remain operable for process viability. Accordingly, there still is a need for a heat treatment technology to produce different microstructures in the bore and rim of nickel-base superalloy disks without the drawbacks of the two approaches mentioned above.
This article demonstrates the viability of a new solution heat-treatment technology that produces nickel-base superalloy disks with a fine grain bore and coarse grain rim using conventional gas-fired production furnaces without auxiliary cooling. An additional goal is to achieve the differential microstructures while still maintaining the option for rapid cooling upon completion of the solution heat treatment with minimal delay in transferring the disk from the furnace to the fan or oil quenching station.
Dual microstructure heat treatment technology
The basic concept of dual microstructure heat treatment (DMHT) technology uses the inherent thermal gradient between the bore and rim of a disk during the initial phase of conventional heat treatment. When a cold disk is placed into a furnace, the disk outer skin becomes hot while the interior remains cooler for a significant period of time. It is possible to design a solution heat treatment that can produce a fine grain bore and coarse grain rim in a typical turbine disk by enhancing/modifying the thermal gradient using heat sinks, solid metal cylinders (thermal blocks) having large thermal mass that chill the central portion of the disk.
In general, the diameter of the thermal blocks is smaller than that of the disk being heat treated. One thermal block is used on the top face and one on the bottom face of the disk, an arrangement that provides direct exposure of the rim to the radiant energy of the furnace while shielding the bore of the disk. To enhance the effectiveness of the thermal blocks, an insulating jacket can be used to slow the temperature rise of the thermal blocks. The insulation is applied to all thermal block surfaces exposed to the radiant energy in the furnace. A clean metal-to-metal contact is desired between the disk and the thermal blocks.
In the heat-treating process in a conventional production furnace, the disk with heat sinks is maintained at a temperature above the solvus of the disk alloy and is removed when the rim of the disk exceeds the solvus but before the bore reaches the solvus temperature. Timing of the DMHT process is critical to its success. While modeling the transient thermal behavior of the disk and heat sinks is desirable, a more practical approach is to monitor the temperature of the thermal block near the bore of the disk by means of an embedded thermocouple. When the thermocouple reaches a predetermined temperature below the solvus temperature, the disk and heat sinks are removed from the furnace. The simplicity of the disk-heat sink assembly also allows the components to be easily separated after removal from the furnace, and the disk can be quenched with minimal delay.
FEA of the DMHT process
Before the actual DMHT trials can be performed, the shape and size of the heat sinks and the heat-treatment parameters must be determined. This is most readily accomplished using any commercially available finite element analysis (FEA) computer package. An ALGOR finite element computer package, which has a transient thermal analysis module, was used for the analysis presented in this article.
Figure 1 shows the disk used in the analysis, which is representative of a typical disk shape having a thick bore with a central hole and a thinner rim. Thermal blocks of varying dimensions were analyzed to determine the desired thermal gradient for this disk in a reasonable time. It was determined that blocks 6 in. in diameter by 2 in. thick (152 by 50 mm) were satisfactory for this analysis.
Figures 2 and 3 show the results of the analyses obtained after a given time at 2150F (1180C) for thermal blocks without and with insulation, respectively. Values for density, conductivity and heat capacity for the disk and thermal blocks in these analyses were 0.3 lb/in.3, 1.0 Btu/h-in.-F and 0.2 Btu/lb-F, respectively. The effective heat transfer coefficient of the metallic surfaces was assumed to be 0.5 Btu/h-F-in.2, while the insulated surfaces were assumed to block all heat transfer. A comparison of Figs. 2 and 3 shows the advantage of insulating the thermal blocks. These two cases serve as upper and lower bounds for the experimental DMHT trials run in this program. The temporal evolution of temperature at the ID and OD of the disk are presented in Fig. 4 for the insulated heat sinks. The shaded region in Fig. 4 represents an estimated time interval for creation of an acceptable dual grain macrostructure for a disk alloy having a 2100 F (1150 C) solvus temperature.
While modeling the DMHT process can provide valuable insight and guidance, estimates of the thermal gradients are only as reliable as the material properties and process boundary conditions that go into these analyses. Therefore, a DMHT trial was performed on a disk with insulated heat sinks incorporating thermocouples to validate modeling and establish the exact configuration and parameters for the DMHT process.
The disk and heat sink configuration for the first DMHT trial is shown in Fig. 5. The disk shape is identical to that shown in Fig. 1, and 6 in. (152 mm) diameter plain carbon steel thermal blocks were used. The thermal blocks have small alignment pins that fit in the bore hole of the disk, which ensures that the disk and thermal blocks remain concentric throughout the heat treatment. The thermal blocks were insulated using Kaowool(tm) (Thermal Ceramics)-filled insulating jackets. The outer shells of the insulating jackets were fabricated from sections of 8-in. (~200 mm) diameter steel pipe. The upper shell rests on the disk, while the lower shell is cut to a length that leaves a tiny gap between the disk and the shell. This ensures that the lower thermal block and the disk maintain maximum thermal contact. Both of the thermal shells are positioned with 8-in. diameter alignment plates, which are bolted to the thermal blocks. This maintains concentricity of the insulating jackets, thermal blocks and disk. Thermocouples were also embedded in the disk and heat sink for this trial as shown in Fig. 6.
The disk and heat sinks were placed in a gas-fired furnace at 2000 F (1095 C). The furnace temperature was intentionally set below the solvus temperature of the disk alloy to allow several DMHT trials to be run if needed without producing any significant grain growth in the disk. Figure 7 shows the time-temperature response of the thermocouples. These data show that a significant temperature gradient can be maintained between the bore and rim of the disk, and also suggest that there is sufficient lag time to allow coarsening of the grain size in the rim while maintaining a fine grain size in the bore. Further, the temperature in the heat sink is almost equivalent to the bore temperature of the disk near the end of the run. This provides a simple and reliable method to determine the time at which the disk and heat sinks should be removed from the furnace.
A DMHT conversion at a furnace temperature above the y'solvus of the disk alloy was attempted on machined forgings with fine grain microstructures based on the data from the first DMHT trial and finite element analyses. The first DMHT conversion was tried at a temperature of 2140 F (1170 C) with the heat sink assembly shown in Fig. 5. A single thermocouple was embedded in the upper thermal block to monitor temperature response near the bore of the disk. The disk and heat sink were removed from the furnace when the thermocouple showed a temperature value of 2070 F (1130 C), and cooled in still air. The resulting macrostructure revealed a narrow band on the outer periphery of the disk, which was converted to a coarse grain microstructure while the remainder of the disk retained a fine grain microstructure.
A second DMHT conversion was tried at a slightly higher temperature of 2170 F (1190 C), and the disk and heat sink assembly were removed from the furnace when the thermocouple in the upper thermal block reached 2100 F (1150 C). The disk was quenched in oil with a total transfer time from furnace to quench tank of less than one minute, which included removal of the heat sinks. The resulting macrostructure and microstructures are shown in Figs. 8 and 9, respectively. A band over 2 in. (50 mm) wide on the outer periphery of the disk was converted to a coarse grain microstructure, while the interior of the disk retained a fine grain microstructure. Further, no quench cracks were observed in the disk.
Based on the results achieved with the generic disk, the DMHT process was tried on a Rolls Royce turbine disk from the AE2100 engine using finite element analysis to set the size of the heat sinks. A dual grain structure was successfully produced as shown in the lead photo of this article.
A solution heat treatment technology called dual microstructure heat treatment (DMHT) was developed to produce nickel-base superalloy disks with a fine grain bore and coarse grain rim. The process requires nothing more than two relatively simple heat sinks on the top and bottom of the disk to be heat treated, and can be performed using conventional gas-fired production furnaces without auxiliary cooling. The heat sinks are easy to remove, which allows rapid quenching of the disk with minimal delay.
Continued development and evaluation of the DMHT technology is planned. Areas to be investigated include refinement of models for the DMHT process, assessment of mechanical properties of DMHT disks, and continued development of the DMHT technology from a production and cost standpoint. IH