Magnetic material of high strength and toughness
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic material of high strength and toughness. 
     In hysteresis motors operating at high The such as are used for example for gas-ultracentrifuges, a material having high strength and toughness in addition to good magnetic properties is needed for the rotor. Thus, it is required that the material have a coercive force greater than 40 oersteds, a yield strength of at least 150 kiloponds/mm 2 , and a toughness such that the rotor does not undergo brittle fracture because of small imperfections in the material. the rotor may, for example, be in the form of an annular disc. 
     It is known that a cobalt-containing alloy named Vicalloy can be used for making the annular discs. However, the required strengths can only be achieved in this material by major cold-working, so that the use of this material is limited solely to relatively thin discs (thicknesses of about 1.8 to 2.5 millimeters), which are produced from cold-worked bands. The toughness of such bands is exceptionally small. 
     SUMMARY OF THE INVENTION 
     An obejct of the invention, therefore, is to provide a material having strength, toughness, and coercive force characteristics which make it suitable for use as rotors in hysteresis motors operating at high r.p.m. 
     This as well as other objects which will become apparent in the discussion that follows are achieved, according to the present invention, by using a steel hardening by precipitation in a martensitic matrix and having an iron-nickel, iron-chromium, or iron-manganese basis. For the purpose of improving the magnetic properties of this steel, it is first austenitized and then subjected to a heat treatment including the step of heating the steel to above 500°C, this step being in addition to the precipitation heat treatment or else in complete replacement of the precipitation heat treatment. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The usual heat treatment of a 18 weight-percent Ni, 8 weight-percent Co, 5 weight-percent Mo steel which hardens by precipitation in a martensitic matrix essentially involves austenitizing the steel for 1 hour at 810°C following rolling. High strength is obtained by a three-hour precipitation hardening at 480°C and a subsequent cooling in air. The following properties are obtained: 
     
         Coercive Force H.sub.c                                                    
                 = 18 to 20 oersteds                                      
Vickers Hardness,                                                         
                 = approx. 560 kiloponds/mm.sup.2                         
 10 kilopond load                                                         
Yield Strength,  = approx. 175 kiloponds/mm.sup.2                         
 0.2% offset                                                              
Tensile Strength = approx. 190 kiloponds/mm.sup.2                         
Elongation       = approx. 9 to 10%                                       
Reduction of Area                                                         
                 = approx. 53%                                            
 
    
       This material is given the necessary strength and toughness by the precipitation hardening heat treatment at 480°C, but the required coercive force is not obtained. However, by subjecting this material to an additional heat treatment at a temperature above 500°C according to the present invention, the coercive force is increased and a material meeting the various requirements for application in a hysteresis motor is achieved. 
     The heat treatment at 500°C according to the present invention gives the precipitation needed for obtaining high strength and it additionally causes a diffusion-controlled decomposition of the martensite. The resulting small particles of austenite in the ferrite matrix yield a coercive force increased over that obtained by only heat treating below 500°C. 
     An optimum alloy composition for the present invention in as follows: Ni from 12 to 20%, Co from 5 to 20%, remainder iron. Nickel can be partially or completely replaced by chromium or manganese. 
     An additional improvement in the magnetic properties can be achieved by including the following elements, alone or in combination, in the steel: titanium--up to 1.5%, niobium--5%, molybdenum--up to 8%, tantalum--5%, tungsten--5%, vanadium--5%, beryllium--1%, and aluminum--1%. The total of all those additions should not exceed 5 atomic percent. 
    
    
     The invention is further illustrated by the following examples: 
     EXAMPLE I 
     Material: Martensitically hardening 18% Ni steel. 
     Analysis: 0.015% C, 4.95% Mo, 17.85% Ni, 8.0% Co, 0.1% Al, 0.36% Ti 
     Initial treatment: Austenitizing by heating for one hour at 810°C, followed by cooling in the air. 
     Additional heat treatment: 30 minutes at 600°C 
     
         Properties:                                                               
         Coercive Force H.sub.c                                           
                         = 50 oersteds                                    
         Vickers Hardness,                                                
                         = 470 kiloponds/mm.sup.2                         
          10 kilopond load                                                
         Yield Strength, = 150 kiloponds/mm.sup.2                         
          0.2% offset                                                     
         Tensile Strength                                                 
                         = 155 kiloponds/mm.sup.2                         
         Elongation      = 13.2%                                          
         Reduction of Area                                                
                         = 38%                                            
 
    
     EXAMPLE II 
     The same material with the same composition as in Example I. 
     The same austenitizing heat treatment as in Example I. 
     Heat treatment for achieving desired properties: 45 minutes at 600°C + 16 hours at 450°C. 
     
         ______________________________________                                    
Properties:                                                               
        Coercive Force H.sub.c                                            
                      = 62 oersteds                                       
        Vickers Hardness,                                                 
                      = 511 kiloponds/mm.sup.2                            
         10 kilopond load                                                 
        Tensile Strength                                                  
                      = approx. 170 kiloponds/mm.sup.2                    
______________________________________                                    
 
    
     EXAMPLE III 
     The same material, composition, and austenitizing heat treatment as in Example I. 
     Heat treatment for achieving desired properties: 2 hours at 650°C + 30 minutes at 810°C + 3 hours at 480°C. 
     
         ______________________________________                                    
Properties:                                                               
         Coercive Force H.sub.c                                           
                        = 43 oersteds                                     
         Vickers Hardness,                                                
                        = .560 kiloponds/mm.sup.2                         
          10 kilopond load                                                
         Yield Strength,                                                  
                        = 188 kiloponds/mm.sup.2                          
          0.2% offset                                                     
         Tensile strength                                                 
                        = 194 kiloponds/mm.sup.2                          
         Elongation     = 14.4%                                           
         Reduction of Area                                                
                        = 48%                                             
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     All percentages given herein for compositions are percentages by weight, unless otherwise noted. 
     Tensile strength is defined as the maximum load in a tensile test divided by the original cross-sectional area. 
     Elongation is defined as the amount of total extension at fracture minus original length expressed as a percentage of the original gage length. ##EQU1## 
     The steels contemplated for the present invention are firstly those which harden by the precipitation of fine particles in a martensitic matrix during an aging heat treatment, following a preliminary heat treatment to provide an austenite microstructure. When reference is made to such a steel as furthermore having an ironnickel basis, it is meant that the steel contains at least 65% iron and between 8 to 25% nickel. For an iron-chromium basis, it is meant Cr between 5 to 20%. For an iron-manganese basis, it is meant Mn 1 to 10%. The further limiting of the steel to one having one of these three bases is important for achieving the combined properties of a coercive force of at least 40 oersteds, a tensile strength of at least 150 kiloponds/mm 2 , and a toughness, as measured by the reduction of area, of at least 15%.  Steels that can be used in the present invention include those having a composition of C less than 0.03%, 4 to 6% Mo, 15 to 18% Ni, 8 to 12% Co and 0.3 to 0.8% Ti. 
     The tensile-test specimens used to obtain the data of the above tables had a gage length of 5 centimeters and a cross sectional area of 0.79 square centimeters over the gage length. They were turned from bars having a 2-centimeter diameter, after heat treatment. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.