Method of fabricating spheroidal graphite cast iron parts of high precision, geometrically and dimensionally, and having improved mechanical characteristics

Abstract

Disclosed is a method of fabricating spheroidal graphite cast iron parts including a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities; b) casting this liquid mixture at a temperature in the range 1350° C. to 1550° C. into a mold to obtain a blank; c) extracting the blank from the mold at a temperature between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of the composition, or at a temperature less than AR3; d) shaping the blank at a temperature in the range 1050° C. to AR3; e) cooling to a temperature lying in the range 260° C. to 420° C. situated in the bainitic range, and maintaining this temperature for 60 min to 180 min, and cooling to ambient temperature.

1 . A method of fabricating spheroidal graphite cast iron parts of improved mechanical characteristics and high precision, geometrically and dimensionally, the method comprising the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%;    b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part;    c) extracting said blank from the mold at a temperature Ts lying between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of said composition;    d) shaping the blank at a temperature Tf lying in the range between 1050° C. and AR3, by hot plastic deformation, directly in the heat of casting or after being maintained at temperature Tm=Tf+20° C. to 50° C. for a duration lying in the range between 10 min and 60 min, in order to obtain the part in its final shape and dimensions;    e) quenching said part directly in the heat of forming at a temperature Tb lying in the range between 260° C. and 420° C. and situated in the bainitic range, and maintaining the part at said temperature Tb for a duration tb lying in the range between 60 min and 180 min; and    f) cooling said part to ambient temperature;    the method being characterized in that the blank obtained by molding possesses a volume substantially identical to that of the part, and the shaping operation by hot plastic deformation is a calibration operation in closed containers or dies enabling the calibrated cast part to be obtained without any lateral flash.    
     
     
         2 . A method of fabricating spheroidal graphite cast iron parts having improved mechanical characteristics and high precision, geometrically and dimensionally, which method is characterized in that it comprises the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%;    b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part and has a volume substantially identical to the volume of the part;    c) extracting said blank from the mold at a temperature Ts lying between the solidus and AR3, where the solidus and AR3 represent the limit temperatures for the austenitic range of said composition;    d) calibrating said blank of the part in containers or dies at a temperature Tf lying in the range between 1050° C. and AR3, by hot plastic deformation, directly in the heat of casting or after being maintained at temperature Tm=Tf+20° C. to 50° C. for a duration lying in the range 10 min to 60 min, in order to obtain the part in its final shape and dimensions; and    e) cooling said calibrated casting to ambient temperature in free manner, under control, or by quenching, in order to confer the desired mechanical characteristics on the part.    
     
     
         3 . A method of fabricating spheroidal graphite cast iron parts having improved mechanical characteristics and high precision, geometrically and dimensionally, the method being characterized in that it comprises the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%;    b) casting this mixture in the liquid state at a temperature lying in the range between 1350° C. and 1550° C. into a mold to obtain a blank of the part that is to be obtained, which blank is of a shape close to the shape of the part and has a volume substantially identical to the volume of the part;    c) extracting said blank of the part from the mold after cooling to below AR3 and finishing at ambient temperature;    d) heating and maintaining said blank of the part at a temperature lying in the range between AC3 and 1050° C. for a duration lying in the range 10 min to 90 min to ensure highly uniform temperature and chemical composition inside said blank;    e) calibrating said blank of the part in closed containers or matrices by hot plastic deformation at a temperature lying in the range between AC3 and 1050° C. in order to obtain a calibrated casting without lateral flash; and    f) cooling said calibrated casting to ambient temperature in free manner, or in controlled manner, or by quenching, depending on the desired mechanical characteristics.    
     
     
         4 . A method according to any one of  claims 1  to  3 , characterized in that the shape given to the cast blank of the part is determined from the final shape for the part so as to enable a deformation ratio to be obtained in all of the portions of the blank during the calibration operation that lies in the range 1% to 20% maximum, and to direct the flow of metal during this operation in a direction that is beneficial to the mechanical characteristics, and in particular to the fatigue stresses on the part in service.  
     
     
         5 . A method according to any one of  claims 1  to  3 , characterized in that the operation of calibration in closed containers or dies is implemented by pressing or striking the blank between at least two tools or by using moving elements and/or punches sliding inside the dies or the tools, in particular in order to obtain multiaxial plastic deformation of the blank.  
     
     
         6 . A method according to  claim 1  or  claim 2 , characterized in that, between the blank leaving the mold c) and the operation of hot plastic deformation d), or between the blank leaving the mold c) and the additional operation of maintaining the blank at the temperature Tm, an intermediate operation is added of separating casting heads (feeders, chutes, . . . ) from the blank by cutting or some other method, in order to obtain the blank alone at a volume that is substantially identical to the volume of the calibrated casting that is to be obtained.  
     
     
         7 . A method according to any one of  claims 1  to  3 , characterized in that the volume of the cast blank is equal to the volume of the calibrated casting, with maximum tolerance lying in the range 0 to +6%.  
     
     
         8 . A method according to any one of  claims 1  to  3 , characterized in that the volume of excess material on the blank corresponding to allowable tolerance on the volume of the blank is directed, during the calibration operation, into an excess metal housing placed either in one or more blind holes or through holes in the part that are to be machined, or in one or more cavities provided for this purpose in the part in zones that do not affect the geometrical precision of the calibrated casting.  
     
     
         9 . A method according to any one of  claims 1  to  3 , characterized in that the mold used for casting the blank is preferably a permanent mold constituted by at least two metal half-portions coated in a release agent, but could also be a non-permanent mold of sand or other material.  
     
     
         10 . A method according to  claim 3 , characterized in that the operation of finishing the blank after it has left the mold includes, where necessary, an operation of milling or cutting in order to obtain a volume for said blank that is substantially identical to the volume of the part and/or that is designed to perfect the shape of said blank.  
     
     
         11 . A method according to  claim 2  or  claim 3 , characterized in that the cooling of the calibrated casting in free manner, controlled manner, or by quenching, is performed in free air, in blown air, or in a confined medium or atmosphere.  
     
     
         12 . A method according to  claim 2  or  claim 3 , characterized in that it further comprises annealing heat treatment after the operation of cooling the calibrated casting, for the purpose of adjusting the structure and/or the mechanical characteristics of said calibrated casting.  
     
     
         13 . A spheroidal graphite cast iron having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%, prepared and shaped in accordance with  claim 1 , and characterized in that it has a structure that is essentially bainitic.  
     
     
         14 . A spheroidal graphite cast iron having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo; with a residual Mg content adapted to the thickness of the parts and lying in the range 0.025% to 0.080%; the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015% and P at a content of less than 0.10%, prepared and shaped in accordance with any one of claims  2 ,  3 ,  11 , or  12 , and characterized in that it has a structure that is either essentially ferritic, or essentially perlitic, or ferrito-perlitic.  
     
     
         15 . A cast iron part, characterized in that it is constituted by a spheroidal graphite cast iron according to  claim 13  or  claim 14.