Spheroidal cast iron particularly for piston rings and method for obtaining a spheroidal cast iron

Abstract

A spheroidal cast iron, particularly for piston rings for pistons of engines, with a particular alloyed chemical composition and subjected to a heat treatment known as austempering, which comprises an austenitization treatment followed by an isothermal hardening treatment, has mechanical characteristics that are comparable with those of the steel currently used to produce piston rings, while maintening the best tribological and self-lubrification characteristics of spheroidal cast irons, and a process suitable to obtain said spheroidal cast iron.

What is claimed is:  
     
         1 . Spheroidal cast iron comprising the following elements: Fe, C, Si, Mo and characterized in that it furthermore comprises Ni, RE, and at least one element among Ti, V, Nb.  
     
     
         2 . The spheroidal cast iron according to  claim 1 , wherein the elements of the group formed by Ti, V and Nb are present in percentages by weight that range from 0.1% to 2.00% of the total weight of the cast iron.  
     
     
         3 . The spheroidal cast iron according to  claim 1 , wherein it comprises said elements in the following percentages by weight: 3.20-4.20% C, 2.00-4.00% Si, up to 0.10% P, up to 0.10% S, up to 0.20% Mn, up to 1.30% Cu, up to 0.50% Cr, 1.7% to 5.00% Ni+RE, 0.10-2.00% Mo, 0.1% to 2.0% of the sum of the percentages of the elements of the group constituted by Ti, V and Nb, and up to 0.20% Co.  
     
     
         4 . The spheroidal cast iron according to  claim 3 , wherein the remaining percentage by weight in order to reach the value of 100% of the weight is constituted by Fe.  
     
     
         5 . The spheroidal cast iron according to  claim 1 , wherein it is subjected to a heat treatment, known as austempering, which comprises an austenitization treatment followed by an isothermal hardening treatment.  
     
     
         6 . A process suitable to obtain a spheroidal cast iron comprising the steps that consist in: 
 providing a conventional basic cast iron    melting the base cast iron    checking the molten metal during the preparation and completion of the melting of the basic cast iron by taking samples, which undergo chemical analyses in order to check their composition    adding chemical elements chosen from the class that comprises C, Si, P, S, Mn, Cu, Mo, Cr, Ni, Ti, V, Nb    inoculating spheroidizing agents in the molten mass, said agents being constituted by a silicon base which comprises RE with the addition of at least one of the elements chosen from the class that comprises Mg, Ca, Ce, Ta, Sr, Al, RE.    
     
     
         7 . The process for obtaining a spheroidal cast iron according to  claim 6 , wherein it comprises an austenitization treatment with a holding time of up to 120 minutes at a temperature of 840-1000° C. and an isothermal hardening (austempering) treatment with holding of the cast iron for 5 to 100 minutes at a temperature of 250-450° C.  
     
     
         8 . The process for obtaining a spheroidal cast iron according to  claim 7 , wherein said austenitization treatment also comprises a thermal strain relieving treatment at a temperature that is higher than the temperature of said isothermal hardening treatment.  
     
     
         9 . The spheroidal cast iron according to  claim 1 , wherein it has a matrix structure of the bainitic-austenitic type with an austenite percentage substantially comprised between 20 and 40%.  
     
     
         10 . A piston ring, particularly for pistons of internal-combustion engines wherein it is made of an austempered spheroidal cast iron according to  claim 1.