Abstract:
A method of treating a wrought IN718 part comprising treating the part to a solution having a temperature of at least 980° C. for a time sufficient to remove precipitant present on the part.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a heat treatment process to improve the stress relaxation resistance of wrought IN718 material.  
           [0003]    2. Description of Related Art  
           [0004]    Stress relaxation is an important property for components that are under constant strain load and high temperature. Stress relaxation is a phenomenon that occurs when an external stress is applied to a piece of metal. The metal reacts by developing an equal and opposite internal stress. If the metal is restrained in the strained position, the internal stress decreases as a function of time. The gradual decrease in internal stress is called stress relaxation and happens because of the transformation of elastic strain in the metal to plastic, or permanent strain. The rate of decrease of internal stress with time is a function of alloy composition, alloy temper, orientation and exposure temperature.  
           [0005]    It is desirable to reduce the rate of decrease, i.e. to increase the resistance to stress relaxation in components used for certain applications. For example, the spoolies used in the steam cooling system of GE Power Systems H machine need good stress relaxation resistance, because spoolies undergo constant compression load during operation and the stress sustained after relaxation must be over a certain value.  
           [0006]    Spoolies are thin tubes that connect various parts used in steam delivery system of GE “H” machines to deliver steam to the buckets without detrimental leakage, which would lead to performance loss and adverse thermal gradients within the rotor structure. The basic concept for power system steam sealing is derived from many years of successful application of spoolies in the GE CF6 and CFM56 aircraft engine families. The function of spoolies requires good stress relaxation resistance.  
           [0007]    INCONEL Alloy 718 (IN718) has the major chemistry of Ni—Fe—Cr—Cb—Mo—Ti—Al and was developed through extensive optimization studies by H. L. Eiselstein at the International Nickel Company (INCO) in the 1950&#39;s. Alloy IN718 is a precipitation hardenable nickel based alloy with high strength and ductility at temperatures up to 704° C., good corrosion resistance, ease of formability and can be welded with good resistance to strain-age cracking. Alloy IN718 was initially developed for the aerospace industry, and it has been used for jet engine and high-speed airframe parts such as wheels, buckets, spacers, and high temperature bolts and fasteners. Most of these applications used wrought IN718. Wrought IN718 is typically formed by forging, rolling, or extruding the superalloy IN718.  
           [0008]    Wrought IN718 material is a candidate for spoolies because of its good manufacturability, generally good mechanical properties, and lower cost. However, wrought IN718 material produced by conventional heat treatment does not have good stress relaxation. Specifically, wrought IN718 material made by conventional heat treatment (solution temperature 960° C. (1760° F.)) has about 54.6% relaxed stress at constant strain of 1.5% and temperature of 640° C. (1200° F.) over a period of 20 hours. This value does not meet the design requirement for spoolies.  
           [0009]    As IN718 is a precipitation hardened superalloy, the heat treatment of IN718 involves a combination of controlled heating and cooling operations in the solid state to develop desired properties and microstructure. The first step in the heat treatment is a solution treatment in which precipitant formed in a work piece during the metal forming cycles (forging, rolling, extruding etc.) are dissolved to form a single-phase solid solution. The solution treatment is performed to control the quantity and the shape of the secondary phase microstructure of superalloys by obtaining either a solid solution free of precipitates or a solid solution with a controlled amount of precipitates. After the solution treatment, the part is quenched or cooled as rapidly as possible to preserve the microstructure established during solution treatment. Then the part is age hardened to obtain the optimum size and distribution of gamma prime, which is the strengthening phase in IN718.  
           [0010]    Typically a wrought IN718 part is prepared by:  
           [0011]    1. Forming a part by forging, rolling, or extruding an IN718 material.  
           [0012]    2. Heat treating the part with solution treatment at a temperature of between about 946° C. (1735° F.) and about 960° C. (1760 ° F.) for at least one hour.  
           [0013]    3. Quenching the part in water or oil.  
           [0014]    4. Aging the part at 718° C. (1325° F.) for 8 hrs followed by another 8 hrs at 621° C. (1150° F.).  
           [0015]    To be able to use wrought IN718 for spoolie applications and other applications where good stress relaxation resistance is needed, the stress relaxation capability of IN718 must be improved.  
           [0016]    In copper and brass alloys, stress relaxation performance can be improved by controlled additions of magnesium, aluminum, iron, or phosphorus as disclosed in, for example, U.S. Pat. No. 6,093,265 and 4,233,069. However, it is difficult to improve the stress relaxation property of IN718 by element addition because IN718 has a very complex chemistry composition. A slight change in element content may change the properties of the wrought IN718 significantly.  
           [0017]    Alternative materials have been sought for components operated at constant strain where good relaxation property is needed. For example, Udimet 720, a superalloy manufactured by Special Metals Corp., has good stress relaxation resistance. However, candidates for spoolies must have good weldability as the making of spoolies involves welding to form hollow thin tubes. Udimet 720 has poor weldability for making spoolies. In addition, Udiment 720 is much more expensive than IN718.  
         BRIEF SUMMARY OF THE INVENTION  
         [0018]    The invention is directed to a method of treating a wrought IN718 part comprising heat treating the part with solution treatment at a temperature of at least 980° C. for a time sufficient to remove precipitant present on the part.  
           [0019]    The solution treatment at temperatures of at least about 980° C. improves the stress relaxation resistance of wrought IN718 material. The material can then be used for conditions where constant strain is encountered.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a comparison of reduced stress of IN718 by different solution temperatures and Udiment 720 with coarse and fine grain sizes.  
         [0021]    [0021]FIG. 2 is a comparison of reduced stress percentage of IN718 by different solution temperatures and Udiment 720 with coarse and fine grain sizes.  
         [0022]    [0022]FIG. 3 is a graph of stress v time at 11200 F of IN718 by different solution temperatures and Udiment 720 with coarse and fine grain sizes. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    It is desirable to use wrought IN718 because of its good manufacturability, good mechanical properties, and lower cost, providing the resistance to stress relaxation can be improved. It was discovered that the resistance to stress relaxation of wrought IN718 material can be improved by solution treatment of the part at temperatures higher than temperatures employed in solution treatments in the past.  
         [0024]    More particularly, it was discovered that the resistance to stress relaxation has a direct relationship with the solution treatment temperature. It was discovered that resistance to stress relaxation is improved by solution treatment of the part at a temperature of at least 980° C., preferably about 980° C. to about 1010 ° C.  
         [0025]    The process comprises:  
         [0026]    1. Forming a part by forging, rolling, or extruding an IN718 material.  
         [0027]    2. Heat treating the part with a solution treatment at a temperature of at least about 980° C., but less than 1010° C. for at least one hour.  
         [0028]    3. Quenching the part in water or oil, or air if the parts are small;  
         [0029]    4. Aging the part.  
         [0030]    After the part is formed in step  1 , the part is usually air-cooled. The surface of the part is then machined prior to the solution treatment to eliminate surface cracks during metal forming process.  
         [0031]    The part may be aged in any suitable manner known by those of skill in the art. A preferred aging process ages the part at about 718° C. (1325° F.) for about 8 hours. Then the part is furnace cooled at 100F/hr to 621° C., soaked at 621° C. for 8 hours, and then air cooled.  
         [0032]    Solution treatment is well-known to those of skill in the art, but at temperatures of 946° C. to 960° C. The present invention uses conventional solution treatment but at temperatures of at least 980° C. The solution treatment at temperatures of at least about 980° C. improves the stress relaxation resistance of wrought IN718 material. The material can then be used for conditions where constant strain is encountered.  
       EXAMPLE 1  
       [0033]    The heat treatment process of wrought IN718 material was simulated for several test specimens. IN718 forging material was used for test pieces. The test pieces were divided and solution treated at 982° (1800° F.), 996° C. (1825° F.) and 1010° C. (1850° F.) for 8 hours, respectively. The parts were quenched in water and then aged at 718° C. (1325° F.) for 8 hrs followed by another 8 hrs at 621° C. (1150° F.).  
         [0034]    Specimens were then taken from each of the pieces and stress relaxation tests were performed. The results were compared with specimens solution treated at conventional temperatures of between 946° C. and 960° C. (1735 to 1760° F.). The stress relaxation tests were performed at two temperatures: 593° C. (1100° C.) and 640° C. (1200° F.). At each temperature, the starting specimens were loaded to achieve 1.0% and 1.5% strain.  
         [0035]    [0035]FIG. 3 shows typical stress versus time curves whereby the initial stress was very high but decreased with time. After about 10 hours, the stress change was minimal. In FIG. 3, “IN718” represents material treated with conventional heat treatment. It is seen that the stress dropped quickly compared with Udimet 720 (both coarse and fine grained materials) at the same testing conditions. The blue line represents IN718 material which had been solution treated at 982° C., which shows great improvement on stress relaxation resistance as stress drops more slowly. The yellow line and red line represent IN718 solution treated at 996° C. and 1010° C. respectively. These materials had more improved stress relaxation resistance and are comparable with Udimet 720.  
         [0036]    [0036]FIG. 1 compares the relaxed stress for each material at various testing conditions, and FIG. 2 compares the percentage of relaxed stress. Baseline IN718 material (conventional heat treatment) was the worst since it had the highest stress reduction. Coarse grain Udiment 720 was very good, but after high temperature solution treatment, the stress relaxation resistance of IN718 is comparable with coarse grain Udiment 720. The wrought IN718 material had better resistance to stress relaxation when solution treated at a higher temperature than was conventionally used.  
         [0037]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.