Bainitic steel materials and methods of making such materials

Abstract

Methods of making bainitic steels may involve austenitizing a quantity of steel by exposing the quantity of steel to a first temperature. A composition of the quantity of steel may be configured to impede formation of non-bainite ferrite, pearlite, and Widmanstätten ferrite. The quantity of steel may be heat-treated to form bainite by exposing the quantity of steel to a second, lower temperature. The second, lower temperature may be stabilized by exposing the quantity of steel to the second, lower temperature in the presence of a thermal ballast.

What is claimed is: 
     
         1 . A bainitic steel material, comprising:
 between 0.62% and 0.78% by weight carbon;   between 1.5% and 2.5% by weight silicon;   between 1.75% and 2.5% by weight manganese;   between 0.22% and 0.5% by weight molybdenum;   between 1.0% and 1.8% by weight chromium;   between 0.0% and 0.3% by weight cobalt;   between 0.25% and 1.0% by weight aluminum; and   the balance consisting essentially of iron.   
     
     
         2 . The material of  claim 1 , wherein:
 the carbon occupies 0.7% by weight;   the silicon occupies 1.9% by weight;   the manganese occupies 2% by weight;   the molybdenum occupies 0.25% by weight;   the chromium occupies 1.4% by weight;   the cobalt occupies 0.15% by weight;   the aluminum occupies 0.75% by weight; and   the balance consists essentially of iron.   
     
     
         3 . The material of  claim 1 , wherein the bainitic steel is free of nickel. 
     
     
         4 . The material of  claim 1 , wherein a microstructure of the bainitic steel is between about 10% and about 20% by volume austenite, the austenite being interspersed throughout a bainite phase. 
     
     
         5 . The material of  claim 1 , wherein a hardness of the bainitic steel is at least substantially uniform throughout the bainitic steel. 
     
     
         6 . The material of  claim 5 , wherein the hardness of the bainitic steel is about 50 HRC or greater. 
     
     
         7 . The material of  claim 6 , wherein the hardness of the bainitic steel is about 56 HRC or greater. 
     
     
         8 . The material of  claim 1 , wherein a yield strength of the bainitic steel in tension is greater than about 1.5 GPa. 
     
     
         9 . The material of  claim 1 , wherein an ultimate tensile strength of the bainitic steel is greater than about 2 GPa. 
     
     
         10 . The material of  claim 1 , wherein a maximum strain at failure of the bainitic steel is about 4% or greater. 
     
     
         11 . A method of making a bainitic steel material, comprising:
 austenitizing a steel material by exposing the steel material to a first temperature in a furnace, a composition of the steel material formulated to retard formation of non-bainite ferrite, pearlite, and Widmanstätten ferrite;   permitting the steel material to air-cool to a second, lower temperature;   after permitting the steel material to cool, heat-treating the steel material to form bainite by:
 exposing the steel material to a third, still lower temperature within an enclosed container distinct from the furnace; and 
 stabilizing a temperature within the enclosed container proximate the third, still lower temperature by surrounding the steel material in a thermal ballast consisting of solid, stationary particles located within the enclosed container; and 
   after heat-treating the steel material, permitting the steel material to cool from the third, still lower temperature to room temperature.   
     
     
         12 . The method of  claim 11 , further comprising homogenizing the steel material by exposing the steel material to a temperature greater than about 1,000° C. before austenitizing the steel material. 
     
     
         13 . The method of  claim 11 , wherein austenitizing the steel material by exposing the steel material to the first temperature comprises exposing the steel material to a temperature of about 1,000° C. 
     
     
         14 . The method of  claim 11 , wherein austenitizing the steel material, the composition of the steel material configured to retard formation of non-bainite ferrite, pearlite, and Widmanstätten ferrite, comprises austenitizing the steel material, the composition of the steel material being between 0.62% and 0.78% by weight carbon, between 1.5% and 2.5% by weight silicon, between 1.75% and 2.5% by weight manganese, between 0.22% and 0.5% by weight molybdenum, between 1.0% and 1.8% by weight chromium, between 0.0% and 0.3% by weight cobalt, between 0.25% and 1.0% by weight aluminum, and the balance consisting essentially of iron. 
     
     
         15 . The method of  claim 14 , wherein austenitizing the steel material comprises austenitizing the steel material, the steel material being free of nickel. 
     
     
         16 . The method of  claim 11 , wherein heat-treating the steel material comprises maintaining the steel material exposed to the third, still lower temperature for 2 days or more. 
     
     
         17 . The method of  claim 16 , further comprising maintaining the steel material exposed to the third, still lower temperature for at least 4 days. 
     
     
         18 . The method of  claim 11 , wherein exposing the steel material to the third, still lower temperature comprises exposing the steel material to a temperature between about 200° C. and about 300° C. 
     
     
         19 . The method of  claim 11 , wherein surrounding the steel material in the thermal ballast consisting of the solid, stationary particles located within the enclosed container comprises exposing the steel material to the third, still lower temperature while the steel material is buried within pre-heated alumina. 
     
     
         20 . The method of  claim 19 , wherein permitting the steel material to cool from the third, still lower temperature to room temperature after heat-treating the steel material comprises air-cooling the steel material.