Viscous coupling plate hardening and flattening method

Abstract

A method of hardening and flattening a viscous coupling plate generally combining a heat treatment process followed by roller leveling is disclosed. The heat treatment process produces an epsilon iron nitride surface layer of the desired hardness on the plate. Subsequent to the heat treatment, the plates are passed through a series of leveling rolls to produce a sufficiently flat plate. The flat rolling process can incorporate a single pass or multiple passes through the leveling rolls. The method of the present invention eliminates any distortion in the plates and brings them within any required flatness specifications, as well as producing viscous coupling plates that exhibit improved resistance to permanent deformation under operating conditions.

What is claimed is: 
     
       1. A method for hardening and flattening viscous coupling plates used in drivetrains of motor vehicles and subjected to frictional contact with one another for torque transmission to provide wear resistance and improved resistance to plastic deformation during use, said method comprising the steps of: (a) placing a plurality of viscous coupling plates spaced apart from one another in a fixture and while in said fixture sequentially: (1) ferritic nitrocarburizing said viscous coupling plates from step (a) above at a temperature of about 1050° F. to about 1200° F. in a nitriding media for a time period of about three to about three and one-half hours and producing an epsilon iron nitride surface layer on said viscous coupling plates that is about 0.0010 inches to about 0.0015 inches thick and has a hardness of about Rockwell C62;   (2) oil quenching said viscous coupling plates from step (a)(1) to room temperature;   (3) tempering said viscous coupling plates from step (a)(2) for at least about two hours at a temperature of at least about 350° F.;   (4) air cooling said viscous coupling plates from step (a)(3) to room temperature;     (b) removing said viscous coupling plates from step (a)(4) from said fixture; and   (c) passing said viscous coupling plates from step (b) through a series of leveling rolls at least once to bring said plates to within about 0.002 inches to about 0.013 inches flatness and to more uniformly redistribute any residual stresses that remain in said plates and to render said plates less susceptible to deformation during use in the drivetrain of a motor vehicle.