Method for treating a cast iron workpiece and workpiece formed thereby

Abstract

A method for treating a cast iron workpiece to increase a useful life thereof includes machining the workpiece to provide a finish surface thereon and deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool ( 80,80′ ), thereby forming a nanocrystallized surface layer ( 70 ). The workpiece is nitrocarburized, the nanocrystallized surface layer accelerating diffusion of nitrogen atoms and carbon atoms therethrough. The nitrocarburizing taking place: i) if the workpiece is stress relived prior to machining, for about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or ii) if the workpiece is not stress relieved prior to machining, for about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C. The nitrocarburizing renders the nanocrystallized surface layer into i) a friction surface, or ii) a corrosion-resistant surface.

1 . A method for treating a cast iron workpiece to increase a useful life thereof, the method comprising:
 either i) stress relieving the workpiece, or ii) refraining from stress relieving the workpiece;   machining the workpiece to provide a finish surface thereon;   deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool, thereby forming a nanocrystallized surface layer at the finish surface; and   nitrocarburizing the workpiece, the nanocrystallized surface layer accelerating diffusion of nitrogen atoms and carbon atoms therethrough, the nitrocarburizing taking place:
 i) if the workpiece is stress relieved, for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or 
 ii) if the workpiece is not stress relieved, for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., 
   
       thereby rendering the nanocrystallized surface layer into i) a friction surface, or ii) a corrosion-resistant surface by the nitrocarburizing. 
     
     
         2 . The method as defined in  claim 1  wherein the workpiece is a rotational member of a vehicle brake. 
     
     
         3 . The method as defined in  claim 1  wherein the workpiece is a shaft or an engine block cylinder liner. 
     
     
         4 . The method as defined in  claim 1  wherein machining is accomplished by a process selected from turning, milling, sand blasting, grit blasting, grinding, and combinations thereof. 
     
     
         5 . The method as defined in  claim 1  wherein nitrocarburizing includes a gas nitrocarburizing process, a plasma nitrocarburizing process, or a salt bath nitrocarburizing process. 
     
     
         6 . The method as defined in  claim 1  wherein the nitrocarburizing comprises:
 immersing at least the nanocrystallized friction surface of the workpiece into a nitrocarburizing salt bath; and then 
 immersing the at least the nanocrystallized friction surface into an oxidizing salt bath. 
 
     
     
         7 . The method as defined in  claim 1  wherein rubbing the finish surface against the blunt tool is accomplished by rotating the finish surface against the blunt tool. 
     
     
         8 . The method as defined in  claim 7  wherein four passes are made over the finish surface with the blunt tool. 
     
     
         9 . The method as defined in  claim 7  wherein deforming further comprises advancing the blunt tool into the rotating finish surface of the workpiece by about 0.03 mm beyond first contact between the rotating workpiece and the blunt tool. 
     
     
         10 . The method as defined in  claim 1  wherein the blunt tool includes a blunt pellet operatively associated therewith, the pellet to rubbingly contact the finish surface. 
     
     
         11 . The method as defined in  claim 10  wherein the pellet is formed from a material chosen from iron-tungsten alloys, silicon carbide, boron nitride, titanium nitride, diamond, and hardened tool steel. 
     
     
         12 . The method as defined in  claim 10  wherein the pellet has a shape chosen from a sphere shape, a spherical cap shape, a roller shape, and a parabolic shape. 
     
     
         13 . The method as defined in  claim 1  wherein a thickness of the nanocrystallized surface layer ranges from about 3 μm to about 15 μm. 
     
     
         14 . A rotational member formed by the method of  claim 1  wherein the rotational member comprises a brake rotor, a brake drum, or a combination thereof. 
     
     
         15 . The rotational member as defined in  claim 14  wherein the rendered surface is a friction surface, and wherein the friction surface exhibits hardness of between about 56 HRC and about 64 HRC.