High ductility, high hot tensile strength tungsten wire and method of manufacture

Abstract

A method for manufacturing a high ductility and high hot tensile strength tungsten wire for incandescent lamp filaments is disclosed. The method comprises the steps of preparing a tungsten alloy, swaging a tungsten rod from the alloy, and drawing the swaged rod to wire size in multiple drawing passes. In the method, the wire is annealed between predetermined draws. It is proposed that an annealing is performed before the final drawing pass, by annealing the wire at a temperature between 1100-1300° C. There is also provided a tungsten wire for incandescent lamp filament, which has high ductility and high hot tensile strength. The tungsten wire of the invention has a cold tensile strength-hot tensile strength ratio not exceeding 3.5.

1 . A method for manufacturing a high ductility and high hot tensile strength tungsten wire for incandescent lamp filaments, comprising the steps of
 preparing a tungsten alloy,   swaging a tungsten rod from the alloy,   drawing the swaged rod to wire size in multiple drawing passes,   annealing the wire between predetermined draws,   in which an annealing is performed before the final drawing pass, by annealing the wire at a temperature between 1100-1300° C.   
     
     
         2 . The method of  claim 1 , in which the final drawing pass after said annealing is done at a different drawing speed than the previous drawing passes. 
     
     
         3 . The method of  claim 2 , in which the final drawing pass after said annealing is done at a slower drawing speed than the previous drawing passes. 
     
     
         4 . The method of  claim 3 , in which the final drawing pass after said annealing is done at a drawing speed substantially 0.65 times the drawing speed of the previous drawing pass. 
     
     
         5 . The method of  claim 1 , in which the wire is drawn from the swaged rod to final size in twenty to forty drawing passes. 
     
     
         6 . The method of  claim 1 , in which the wire is pre-heated during the drawing passes. 
     
     
         7 . The method of  claim 6 , in which the wire is pre-heated to 500-900° C. during the drawing passes. 
     
     
         8 . The method of  claim 1 , in which the drawing tools are pre-heated during the drawing passes. 
     
     
         9 . The method of  claim 8 , in which the drawing tools are pre-heated to 300-400° C. during the drawing passes. 
     
     
         10 . The method of  claim 1 , in which the wire is further annealed between drawing passes preceding the final drawing pass. 
     
     
         11 . A tungsten wire for incandescent lamp filament, having high ductility and high hot tensile strength, having a cold tensile strength-hot tensile strength ratio not exceeding 3.5. 
     
     
         12 . The wire of  claim 11 , having a hot tensile strength between 0.16-0.24 N/mg/200 mm, measured at 1620° C. 
     
     
         13 . The wire of  claim 11 , having a cold tensile strength between 0.50-0.75 N/mg/200 mm, measured at room temperature. 
     
     
         14 . The wire of  claim 11 , being formed as a coil, and having a mandrel ratio not exceeding 2. 
     
     
         15 . The wire of  claim 11 , comprising additives selected from the group of Al, K, Si. 
     
     
         16 . The wire of  claim 11 , comprising additives selected from the group of Th, ThO, YO, LaO, CeO, Re.