Patent Publication Number: US-8986817-B2

Title: Nitrided component surface repair

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for repairing a damaged surface and more particularly to repairing of a hardened surface of a machine component. 
     BACKGROUND 
     Electroless metal coatings on machine components for enhanced performance are known in the art. The electroless coating process is typically used for enriching the surface of metallic components for protection against corrosion, wear, etc. For example, U.S. Published Application No. 2011/206532 relates to an electroless coating process including coating a substrate with an electroless coating and then heating to a specific temperature. 
     Some engines are assembled with surface hardened components. A nitriding process is widely used for surface hardening of the components. The nitriding process is a heat treatment process that diffuses nitrogen into the surface of the metallic component to create a case hardened surface. The component may be made from steel, iron, titanium, and the like. The component that is subjected to the nitriding process may already be in a hardened and tempered condition. This may allow for the nitriding process to take place at a temperature lower than the last tempering temperature. Further surface treatment, such as, for example, coating or cladding, may be conducted at a lower temperature than that of the nitriding temperature, in order to avoid damaging of the nitride hardened layer. 
     During operation, the nitrided surface of the component may be subjected to wear, cavitation, erosion, tear, corrosion, or other damage. The damage may be due to the application of higher force, surface strain, torsion, bending moment, or other environmental conditions. Damages to the nitrided surface may result in unsatisfactory performance, sometimes requiring replacement of the parts with new components. This may lead to an overall increase in costs. 
     Various methods for repairing these damaged components are known in the art. For example, micro arc welding and laser beam cladding can be used for coating a material on the nitrided surface. However, these techniques are thermal based repair processes that may damage a base material strength of the nitrided surface. High temperatures involved in thermal based repair processes can lead to out-gassing of nitrogen from the nitrided surface, causing a reduction in the hardness of the nitrided surface. The out-gassing of nitrogen may also lead to porosity issues in the coated material. 
     Other known techniques include chrome plating and High Velocity Fuel Oxygen (HVOF). However, the chrome plating technique is not environment friendly. In the HVOF method, bonding between the coated material and the nitrided surface is not metallurgical, thus affecting the bond strength. 
     Hence, there is a need of providing an improved method of repair for nitrided components. 
     SUMMARY 
     In one aspect of the disclosure, a plunger for fuel injection includes a nitrided surface. The nitrided surface includes a damaged area and an electroless material coated on the damaged area. 
     In another aspect of the disclosure, a method includes introducing a nitrided surface having a damaged area for a predetermined time period into a bath containing an electroless material. The method also includes coating the damaged area with the electroless material, and heat treating the electroless material at a predetermined temperature for a predefined time. 
     In yet another aspect, a nitrided surface is provided. The nitrided surface includes a damaged area. An electroless material is coated on the damaged area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an exemplary fuel injection assembly having a plunger, according to one embodiment of the present disclosure; 
         FIG. 2  is an isometric view of the plunger shown in  FIG. 1 , a damaged area clearly visible on a nitrided surface of the plunger; 
         FIG. 3  is an sectional view of an exemplary fuel injector assembly having a poppet; 
         FIG. 4 . is an isometric view of the poppet shown in  FIG. 3 , the damaged area clearly visible on the nitrided surface of the poppet; and 
         FIG. 5  is a flowchart of an exemplary method for repairing the damaged area on the nitrided surface. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Generally, corresponding or similar reference numbers will be used, when possible, to refer to the same or corresponding parts. 
     Referring now to  FIG. 1  an exemplary fuel injection assembly  100  of a diesel engine is illustrated. The fuel injection assembly  100  may include a plunger  102  slidably and rotatably placed in a barrel  104 . The barrel  104  may be configured to have an inlet port  106  and a spill port  108 . The plunger  102  may be configured to rotate and slide in the barrel  104 , in order to effectuate substantial delivery of pressurized fuel into a cylinder of the diesel engine at a predetermined time. 
     During operation of fuel injection assembly  100 , the barrel  104  may receive a substantial quantity of fuel through the inlet port  106 . Thereafter, the plunger  102  may compress the fuel in the barrel  104  in order to increase the pressure of the fuel. The high pressure fuel from the barrel  104  may then be introduced into a connector portion  114  through a delivery valve  110 . The delivery valve  110  is located within an outlet portion  112  of the fuel injection assembly  100 . Further, the connector portion  114  may deliver the high pressure fuel to an injector (not shown) in order to inject the fuel into the cylinder. The spill port  108  may be configured to provide the additional fuel in the barrel back to a fuel tank. 
       FIG. 2  illustrates the plunger  102  utilized in the fuel injection assembly  100 , according to various embodiments of the present disclosure. The plunger  102  has an outer surface  202  including an upper portion  204 , a middle portion  206  and a lower portion  208 . As illustrated in  FIG. 1 , the upper portion  204  may be in fluid communication with the fuel in the barrel  104  of the fuel injection assembly  100  for discharging a specific amount of fuel at a predetermined time. The middle portion  206  of the plunger  102  may be in contact with an inner wall of the barrel  104  of the fuel injection assembly  100 . The lower portion  208  of the plunger  102  may include an integrated gear segment  210 . 
     Another exemplary machine component is depicted in  FIGS. 3 and 4 .  FIG. 3  illustrates a fuel injector assembly  300  utilized in connection with the diesel engine. The fuel injector assembly  300  may include a first portion  302 , an intermediate portion  304  and a nozzle portion  306 . The first portion  302  may include a poppet  308  and a plurality of passages  310 . The intermediate portion  304  may include a passage  312 . The intermediate portion  304  may be connected to the first portion  302  and the nozzle portion  306 . 
     During operation, the first portion  302  may receive the pressurized fuel from the fuel injection assembly  100 . The first portion  302  of the fuel injector assembly  300  may then deliver the pressurized fuel to the nozzle portion  306  via the passages  312  located within the intermediate portion  304  and the passages  310  of the first portion  302 . Thereafter, the nozzle portion  306  may inject the pressurized fuel in the cylinder of the diesel engine. 
       FIG. 4  illustrates the poppet  308  utilized in the fuel injector assembly  300 . The poppet  308  has the outer surface  202  including an upper portion  402 , a tapered middle portion  404 , and a lower portion  406 . As shown in  FIG. 3 , the upper portion  402  of the poppet  308  may be in contact with the passage  310  of the first portion  302 . The tapered middle portion  404  at one end is contact with an internal wall of the first portion  302  and at other end is in contact with a spring  314  located within the first portion  302 . Further, the lower portion  406  of the poppet  308  may be connected to an external cap  316  of the fuel injector assembly  300 . 
     The plunger  102  and/or the poppet  308  may be made of a metal. In some embodiments the metal may include an alloy of steel, iron, or titanium. Other embodiments may include any metal known in the art. The outer surface  202  of the plunger  102  and/or the poppet  308  may be hardened by a surface treatment process, such as, but not limited to, a nitriding treatment process. The nitriding treatment process involves diffusing nitrogen into a base material at high temperatures. The high temperatures may include a range of 550-590° C. to provide the required surface hardness. 
     In one embodiment, during the operation of the fuel injection assembly  100 , the plunger  102  may be damaged due to contact with other components in the fuel injection assembly  100 . This damage on the nitrided surface  202  may be caused by a variety of factors including cavitation, erosion, corrosion, wear and tear, and/or higher forces. For example, the middle portion  206  of the plunger  102  may be damaged due to contact with the inner wall of the barrel  104  of the fuel injection assembly  100 . In another example, increased fuel injection pressure within the barrel  104  may cause cavitation damage on the plunger  102 . Usually, the cavitation damage is similar to scars, shown as the on the damaged area  212  in  FIG. 2 . The scar may have a depth of up to 150 micrometer. 
     During the operation of the fuel injector assembly  300 , the poppet  308  may be exposed to damage due to contact with other components of the fuel injector assembly  300 . For example, a surface of the tapered middle portion  404  of the poppet  308  may be damaged due to contact with the inner wall of the first portion  302 . In another example, increased friction between the surface of the tapered middle portion  404  and inner wall of first portion  302  as well as abrupt changes in fuel pressure may cause cavitation and/or erosion damage on the poppet  308 . The exemplary damaged area  212  on the poppet  308  is shown in the  FIG. 4 . As described earlier, the damaged area  212  on the nitrided surface  202  of the plunger  102  and/or the poppet  308  may include, for example, scratch marks, scars, deformations, abrasion marks, and erosion marks. A person of ordinary skill in the art will appreciate that the location, type and extent of damage described herein is exemplary. 
       FIG. 5  depicts a method  500  for repairing the damaged area  212  on the nitrided surface  202  of the plunger  102  and/or poppet  308 . The damaged area  212  on the nitrided surface  202  is coated with an electroless material. The electroless material may include nickel (Ni), phosphorus (P), boron (B), or any combination thereof. For example, the electroless material may be a solution of nickel (Ni) with high phosphorus (P) content. One of ordinary skill in the art will appreciate that the electroless solution of nickel (Ni) with high phosphorus (P) content may result in a coated surface with a relatively higher surface hardness. 
     The coating of the electroless material on the nitrided surface  202  is configured to provide a surface hardness in a range of approximately 60-70 HRC. Known electroless Ni coating has plated hardness values in a range a range of approximately 49-62 HRC. These hardness values can be increased to approximately 64-70 HRC on heat treatment. It should be understood that surface hardness in the approximate range of approximately 60-70 HRC is desirable on the machine components, in order to provide protection from wear and tear that these components may undergo during operation. 
     A method  500  for repairing the nitrided surface  202  will be further described in detail in connection with  FIG. 5 . 
     INDUSTRIAL APPLICABILITY 
     The fuel injection assembly plunger  102  and/or the poppet  308  of the fuel injector assembly  300  may be subjected to damage during operation. The damage may result in unsatisfactory overall performance of the plunger  102  and/or the poppet  308 . One solution includes replacing the plunger  102  and/or the poppet  308  by a new one. However this solution poses cost considerations. Other approaches include repairing the damaged area  212  on the nitrided surface  202  of the plunger  102  and/or the poppet  308 . Many surface repairing methods such as thermal processes, plating processes and other deposition methods are known in the art. 
     Thermal repairing processes like micro arc welding, laser beam cladding, etc. may have an effect on surface properties of the repaired surface. During the nitriding process, nitrogen gas is diffused into the surface of the base material, to form a coated base material. The base materials used in the manufacture of the machine components include steel, iron, titanium, and the like. The thermal processes are carried at relatively higher temperatures, causing the out-gassing of nitrogen from the coated base material. 
     For example, in laser or welding based cladding processes, due to high heat input from the cladding processes the temperature attained at the coated base material is higher than the temperature at which nitriding process is performed. As a consequence, the diffused nitrogen is released from the coated base material, leading to the formation of pores. This may reduce the surface hardness of the base material nitrided surface and also cause porosity in the coated layer. 
     Further, the known plating process, such as chrome plating, may provide a surface with relatively lesser hardness. This may be undesirable due to the harsh working environment conditions that the machine components are generally subjected to. The lesser hardness may impact the tendency of these components of being susceptible to damage. Other deposition processes like HVOF provide negligible metallurgical bonding between the coating and the base material. 
     The present disclosure relates to the method  500  of repairing the damaged area  212  on the nitrided surface  202  of the plunger  102  and/or the poppet  308  by using electroless process. The method  500  may be carried out at relatively lower temperatures, preventing the out-gassing of nitrogen from the nitrided surface  202 . Additionally, the method  500  does not require electric current, resulting in uniform and continuous coating on the nitrided surface  202 . The electroless coating may provide a relatively high bond strength of about 40,000-60,000 psi on steel. Further, the method  500  provides for the resultant surface hardness in a range of approximately 60-70 HRC. This may be a desired hardness required for the machine component to be less susceptible to damage. 
       FIG. 5  shows the method  500  for repairing the nitrided surface  202 . Initially at step  502 , the nitrided surface  202  of the plunger  102  and/or the poppet  308  having the damaged area  212  may be subjected to a pre-treatment process. During pre-treatment, the nitrided surface  202  may be cleaned by any standard cleaning process known in the art. This cleaning process may be washing, polishing, precision grinding, wiping, rinsing, chemical treatment or any combination thereof. For example, the damaged area  212  may be precision ground with a tool like a precision grinding wheel in order to remove any irregularities and level-out the nitrided surface  202 . 
     In one embodiment, remaining areas of the nitrided surface  202  of the plunger  102  and/or the poppet  308 , i.e. damage-free areas or areas which do not require coating, may be masked or covered with a polymer mask or any other suitable material mask. The masking may involve tying of a cloth or a polymeric sheet on exposed areas of the nitrided surface  202  by mechanical fasteners like bands, on a temporary basis, until the method  500  is completed. The masking or covering may prevent deposition of electroless material on the damage-free area. 
     At step  504 , the nitrided surface  202  may be introduced into a bath of an electroless solution containing ionic Nickel and ionic Phosphorus (1% to 13% Phosphorus) for a predetermined period of time. The bath may include an open vessel containing the electroless solution. In other embodiments other electroless solutions may be used as would be known in the art. At step  506 , the electroless material may be coated on the damaged area  212 . It should be noted that the predetermined period of time for which the nitrided surface  202  is held in the electroless solution may depend on a thickness of the coating required on the damaged area  212 . 
     During the electroless Nickel coating at step  506 , the coating process auto-catalytically deposits Nickel alloyed with Phosphorus (12% by weight) on the nitrided surface  202 . For example, electroless nickel coating using sodium hypophosphite as the reducing agent proceeds via the following overall reduction/oxidation reaction:
 
Ni 2+ +4H 2 PO 2   − +H 2 O→Ni 0 ↓+3H 2 PO 3   − +H + +P↓+3/2H 2 ↑  Equation 1
 
     At step  508 , the nitrided surface  202  may be subjected to heat treatment. The heat treatment may increase the hardness imparted by the electroless material on the nitrided surface  202 , resulting in hardness of approximately 60 to 70 HRC. In an exemplary case, heat treatment of the nitrided surface  202  of the plunger  102  and/or the poppet  308  may involve baking, wherein there is rise in temperature at the start of the heat treatment and relatively slow decrease in temperature at the end of the heat treatment. This may avoid any micro-cracking in the electroless material coated on the damaged area  212 . 
     The nitrided surface  202  may be heated at a predetermined temperature for a predefined time period. The predetermined temperature at which the nitrided surface  202  is heated is in a range of approximately 200° C. to 400° C. Also, the predefined time for which the nitrided surface  202  is heated lies may be less than 24 hours. One of ordinary skill in the art will appreciate that the predetermined temperature and the predefined time should be selected in such a way that the properties of the nitrided surface  202  is subjected to minimal or no change. More importantly, the predetermined temperature should be low enough in order to prevent the out-gassing of nitrogen from the nitrided surface  202 . 
     At step  510 , the nitrided surface  202  may be subjected to machining, wherein after the heat treatment, one or more dimensions of the resultant surface are processed to achieve desired engineering standards. The engineering standards may be engineering dimensions, surface finish, or any other standard which is necessary for the plunger  102  and/or the poppet  308  being repaired to function in the respective working environment. The machining process could be any standard machining process known in the art. The machining process may include polishing, precision grinding, turning, or any other method known in the art. 
     The present disclosure has been explained with reference to the damaged area  212  present on the plunger  102  and/or the poppet  308 . However, the present disclosure does not restrict itself to the nitrided fuel injection plunger  102  and/or the nitrided poppet  308 . The nitrided component can include any nitrided element that may be used in standard automobile engine applications. 
     While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof.