Patent Publication Number: US-5293026-A

Title: Hardsurfacing material for engine components and method for depositing same

Description:
The present invention relates to hardsurfaced engine components and more particularly to a hardsurface material for heavy duty aircraft engine components, and a method for depositing such material. 
     Current heavy duty aircraft tappet bodies are made by placing a medium carbon steel body in a cooled copper chill fixture, placing a cast iron disc on the exposed face of the tappet body, and then melting the cast iron disc by means of a carbon arc. The resulting welded structure is then finished to provide a carbon steel tappet body with a cast iron wear face. While this method has been used successfully, there are certain inherent problems associated with it, such as difficulty in controlling the carbon arc, the need for a high degree of manual skill to perform the method, the tendency for graphite to form at the wear surface, and difficulty in controlling porosity. 
     The present invention provides an improved material and process for forming such wear face wherein the material of the deposit and the deposition conditions are so chosen as to obtain a unique microstructure consisting of wear resistant carbides in a tough martensitic matrix. 
     The product made by this process has a more consistent microstructure, a more uniform face hardness and a more uniform thickness of the carbide wear surface than that obtained with previous methods. 
     The deposit material of the invention can be melted onto the wear surface by any one of several types of heat sources such as a carbon arc, tig arc or plasma transferred arc process. The material which is deposited can be in the form of a rod or cast disc for the tig process and powder for the plasma arc process. 
     In a preferred process, a powder of desired chemistry is deposited by a plasma transferred arc process. Controlled solidification of the melt is accomplished by setting up a thermal gradient toward the body using a copper chill block and by water cooling. Deposits produced in this manner have been built up to 8 mm with no significant porosity and graphite formation. Melt control is achieved by controlling (a) current (b) cooling (c) powder flow rate (d) rotation speed and (e) torch scanning speed. 
    
    
     Other objectives and advantages of the invention will be apparent from the following description when considered in connection with the drawings, wherein: 
     FIG. 1 is a schematic view of apparatus for applying a hardsurfacing material in accordance with the invention; 
     FIG. 2 is a sectional view of a tappet body to which a hardened wear surface has been applied; and 
     FIG. 3 is a photomicrograph showing the microstructure of the hardsurfacing material of the invention. 
    
    
     Referring to FIG. 1, there is illustrated a preferred apparatus suitable for performing the method of the present invention. The apparatus, designated generally by the numeral 10, comprises a solid copper chill block 12 mounted on a rotary stage 14 which is geared or otherwise connected to the output of a drive motor 16, a plasma transferred arc (PTA) torch 18 positioned to deposit molten material onto a member supported by the chill block, and means 19 for applying a coolant directly onto the member. In operation, the chill block is rotated about one revolution per minute. 
     Plasma transferred arc welding is well known in the art and will not be described herein in detail. A particular torch configuration suitable for the present application is described in U.S. Pat. No. 4,104,505 to Rayment et al., which is incorporated herein by reference. In accordance with the PTA process, a powder is fed into the torch by means of an inert gas such as argon. Simultaneously an inert gas such as argon is formed into an ionized plasma by passing the inert gas through an electric arc after which the powdered material and the plasma are combined. The resulting plasma carries and melts the powder material which exits from the torch and is directed toward the article to be hardsurfaced. In the preferred embodiment illustrated, the plasma gas enters the torch through line 20, a shielding gas enters through line 22 and the powder enters the torch through line 24. 
     Referring to FIG. 2, for purposes of illustration, a tappet body 26 of so-called &#34;nailhead&#34; configuration is shown having a tubular body portion 28, and an increased diameter head portion 30. In a fairly typical application wherein the surface 32 of the head portion defines the cam contact surface of a tappet for a reciprocating aircraft engine, the body diameter is about 1.0 in. and the head diameter is about 1.5 in. To receive the hardsurface material the unfinished head is counterbored to a minimum depth of about 0.020 in. 
     To apply the hard surface layer 33, the body 26 is placed in the chill block 12, which is configured such that the outer surfaces of the body are in contact with the chill block. The bottom of the tappet body engages a seal 35 in the bottom of the cavity. Cooling water flows from a convenient source through an inlet line 38 which opens into the block cavity, the water flowing freely into the bore 40 of the tappet body and onto the underside of head portion 30 and then cascading down into a stationary collector 41 to a return line 42. In a preferred system, the cooling water is maintained at a temperature between 150° F. and 180° F. The chill block can also be water cooled. The cooling system thus provided establishes a sharp temperature gradient in the head portion of the tappet body thus ensuring that solidification of the hardsurfacing material will progress upward from the bottom of the counterbored area. Such controlled solidification is instrumental in producing the desired wear resistant carbides with minimal formation of graphite. 
     The torch 18 is mounted for linear movement parallel to the surface of the tappet body; for example, on a lead screw 44 received through a nut 45 fixed to the torch and rotated by a stationary drive motor 46, as shown schematically in FIG. 1. In operation, the torch traverses a linear distance substantially equal to the radius of the top face of the tappet. In the exemplary process, the torch moves at a rate of one stroke per second during processing. 
     Once the tappet body is mounted in the chill block, the torch traverse and rotation of the chill block is initiated and the torch is energized, ramping up to its full power level of 175 to 200 amps in 10 to 12 seconds. About 5 seconds after the torch is energized, the hardfacing powder is applied at a rate of 30 to 50 grams per minute. The powder is applied at full power for 35 seconds, ensuring that the entire puddle thus formed is the molten state to minimize graphite formation, followed by a 10 second ramp down of torch power and then stoppage of the powder flow. When deposition is complete, a layer 33(a) of the hardsurface material will be built up on the tappet face, as shown in broken line in FIG. 2, to a thickness of 0.050 in. to 0.25 in. measured from the bottom of the counterbore. 
     After cooling, but within eight hours of deposition, the tappet body is subjected to deepfreezing in solid carbon dioxide or liquid nitrogen for a period of about 30 minutes, the deepfreeze process serving to transform the retained austenite in the weld area to martensite. 
     After deepfreezing, the tappet body is tempered at 600° F. for two hours to temper the stem and the heat affected zone, resulting in a hardness of 30-40 Rc in the stem area of the tappet body and 58-63 Rc at the wear surface. The part is then finish ground to produce the hardfaced tappet body as shown in FIG. 2. 
     In accordance with the invention, the preferred hardfacing material is an iron based powder having the following composition: 
     
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Wt % of           Min    Max                                              
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Carbon            3.2    4.2                                              
Manganese         0.5    1.5                                              
Phosphorous               0.10                                            
Sulphur                   0.10                                            
Silicon           1.0    2.5                                              
Molybdenum        0.5    2.0                                              
Nickel            1.5    3.0                                              
Chromium          0.5    2.0                                              
Iron              Bal.   Bal.                                             
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     FIG. 3 is a photomicrograph of the wear surface of an aircraft tappet showing a microstructure consisting of wear resistant carbides of iron,, chromium and molybdenum in the white areas identified by reference numeral 48, in a martensitic matrix denoted by the black areas identified by reference numeral 50. The composition of the material shown in FIG. 3 is as follows: 
     
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              %                                                           
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       Carbon   3.86                                                      
       Manganese                                                          
                0.90                                                      
       Phosphorous                                                        
                 0.015                                                    
       Sulphur   0.017                                                    
       Silicon  1.71                                                      
       Molybdenum                                                         
                1.38                                                      
       Nickel   2.11                                                      
       Chromium 1.11                                                      
       Iron     Bal.                                                      
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