Method of magnetically processing an iron-carbon alloy

Abstract

A magnetic field assisted processing method entails heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure; cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature. The field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix in order to produce a magnetically-processed alloy having improved ductility and strength.

1 . A method of magnetically processing an iron-carbon alloy, the method comprising:
 heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure;   cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and   applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature, the field being applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix to produce a magnetically-processed alloy having improved ductility and strength.   
     
     
         2 . The method of  claim 1 , wherein the intermediate temperature is below a Curie temperature of the one or more iron carbide phases in the ferrite matrix. 
     
     
         3 . The method of  claim 2 , wherein the intermediate temperature is above a Curie temperature of one or more iron carbide phases not present in the ferrite matrix. 
     
     
         4 . The method of  claim 1 , wherein the one or more iron carbide phases in the ferrite matrix are selected from the group consisting of Fe 3 C, Fe 7 C 3 , Fe 2 C, Fe 23 C 6 , Fe 4 C, and Fe 5 C 2 . 
     
     
         5 . The method of  claim 1 , wherein at least one of the iron carbide phases further comprises a metal M selected from the group consisting of Mo, V, Cr, Nb, and Ti. 
     
     
         6 . The method of  claim 1 , further comprising at least one additional metal carbide phase in the ferrite matrix, wherein the metal carbide phase includes a metal M selected from the group consisting of Mo, V, Cr, Nb and Ti. 
     
     
         7 . The method of  claim 1 , wherein the intermediate temperature lies between about 150° C. and 550° C. 
     
     
         8 . The method of  claim 1 , further comprising holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field. 
     
     
         9 . The method of  claim 1 , further comprising cooling the iron-carbon alloy from the intermediate temperature to a lower temperature during the application of the high field strength magnetic field. 
     
     
         10 . The method of  claim 9 , wherein the lower temperature is ambient temperature. 
     
     
         11 . The method of  claim 1 , wherein the time duration of the application of the magnetic field after reaching the intermediate temperature is between about 1 minute and 30 minutes. 
     
     
         12 . The method of  claim 1 , wherein the rate of cooling of the iron-carbon alloy to the intermediate temperature is between about 1° C./s and 400° C./s. 
     
     
         13 . The method of  claim 1 , further comprising applying the high field strength magnetic field to the iron-carbon alloy during the cooling of the alloy to the intermediate temperature. 
     
     
         14 . The method of  claim 1 , wherein the high field strength magnetic field is between about 0.2 Tesla and about 40 Tesla. 
     
     
         15 . The method of  claim 14 , wherein the high field strength magnetic field is in the range of from about 0.2 Tesla and to less than 1 Tesla. 
     
     
         16 . The method of  claim 1 , wherein the heating at the austenitization temperature occurs in a furnace outside a high field strength magnet, and further comprising moving the iron-carbon alloy from the furnace into a bore of the magnet during the cooling to the intermediate temperature. 
     
     
         17 . The method of  claim 1 , further comprising applying the high field strength magnetic field to the iron-carbon alloy during the heating at the austenitizing temperature. 
     
     
         18 . The method of  claim 1 , wherein the high field strength magnetic field is applied to the iron-carbon alloy only after the alloy has reached the intermediate temperature. 
     
     
         19 . The method of  claim 1 , wherein the austenitizing temperature is at least about 25° C. higher than an austenite finish temperature of the iron-carbon alloy. 
     
     
         20 . The method of  claim 1 , wherein the iron-carbon alloy includes between about 0.1% carbon and about 6.7% carbon.