High carbon steel wire with bainitic structure for spring and other cold-formed applications

Abstract

According to the present invention, there is disclosed a heat treated high carbon or alloy steel wire and method of heat-treating the wire. The wire is produced by the steps of initially heating the high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F. for between 10 seconds and 2 minutes; rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F.; holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and cooling the resulting wire to ambient temperature.

1 . A method of making a bainite wire coil spring comprising the steps of: 
 a. initially heating a continuous length of high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F.;    b. rapidly cooling the continuous length of wire to an intermediate point temperature of between 400 to about 1000 degrees F.;    c. holding the continuous length of wire at the intermediate point temperature for about 2 minutes to about 40 minutes;    d. cooling a resulting continuous length of lower bainite heat treated wire to ambient temperature; and    e. winding the continuous length of lower bainite heat treated wire into a coil spring.    
   
   
       2 . The method of making a bainite wire coil spring of  claim 1  wherein the step of initially heating the continuous length of wire is to a temperature of between 1625 degrees F. and 1700 degrees F.  
   
   
       3 . The method of making a bainite wire coil spring of  claim 1  wherein the step of in-process cooling the wire includes cooling the continuous length of wire to an intermediate point temperature of between 550 and 950 degrees F.  
   
   
       4 . The method of making a bainite wire coil spring of  claim 4  wherein the step of in-process cooling the wire includes cooling the continuous length of wire to an intermediate point temperature of about 650 degrees F.  
   
   
       5 . The method of making a bainite wire coil spring of  claim 1  wherein the step of initially heating the continuous length of high carbon or alloy steel wire is for between 10 seconds and 2 minutes.  
   
   
       6 . The method of making a bainite wire coil spring of  claim 1  wherein the step of wire holding the continuous length of wire at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation.  
   
   
       7 . The method of making a bainite wire coil spring of  claim 1  wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a tensile strength of between 160 and 290 ksi.  
   
   
       8 . The method of making a bainite wire coil spring of  claim 7  wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a tensile strength of between 200 and 270 ksi.  
   
   
       9 . The method of making a bainite wire coil spring of  claim 1  wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a yield strength of between 130 and 242 ksi.  
   
   
       10 . The method of making a bainite wire coil spring of  claim 1  wherein the resulting lower bainite wire has a hardness of between 32 and 56 Rockwell C.  
   
   
       11 . The method of making a bainite wire coil spring of  claim 1  wherein the resulting lower bainite wire has a high ductility as measured by reduction in area of about 35% to about 60%.  
   
   
       12 . (canceled)  
   
   
       13 . A high carbon steel, heat treated bainite wire coil spring comprising; 
 a continuous length of high carbon steel, heat treated wire having a lower bainitic microstructure and a tensile strength of between 160 and 290 ksi, a yield strength of between 130 and 242 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.    
   
   
       14 . (canceled)  
   
   
       15 . The bainite wire coil spring of  claim 13  wherein the wire has a tensile strength of between 200 and 270 ksi, a yield strength of between 175 and 230 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.  
   
   
       16 . A heat treated high carbon or alloy steel bainite wire coil spring produced by the steps of: 
 a. initially heating a continuous length of high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F.;    b. rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F.;    c. holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and    d. cooling the resulting lower bainite heat treated wire to ambient temperature; and    e. winding the heating lower bainite wire into a coil spring.    
   
   
       17 . The bainite wire coil spring of  claim 16  including the step of initially heating the continuous length of high carbon or alloy wire to a temperature of between 1625 degrees F. and 1700 degrees F.  
   
   
       18 . The bainite wire coil spring of  claim 16  wherein the step of in-process cooling the wire includes cooling the continuous length of high carbon or alloy wire to an intermediate point temperature of between 550 and 950 degrees F.  
   
   
       19 . The bainite wire coil spring of  claim 16  wherein the step of initially heating high carbon or alloy steel is for between 10 seconds and 2 minutes.  
   
   
       20 . The bainite wire coil spring of  claim 16  wherein the step of holding the wire at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation.