Patent Publication Number: US-2020291497-A1

Title: Method of making stainless steel

Description:
FIELD 
     The subject matter herein generally relates to stainless steel, and more particularly to a method of making stainless steel having reduced magnetic permeability. 
     BACKGROUND 
     A commercial stainless steel substrate generally contains 10-12% manganese, and a nitrogen atmosphere is applied during a sintering process to allow nitrogen to diffuse into the stainless steel substrate to increase the nitrogen content, which promotes formation of austenite to minimize magnetic permeability of the stainless steel. However, the melting point of manganese is 1241-1247° C. Stainless steel is generally sintered at 1300° C., which may burn the manganese. Since the presence of manganese increases diffusion of nitrogen in the stainless steel substrate, when the manganese is burned during sintering, the amount of nitrogen diffused in the stainless steel substrate is reduced, which reduces formation of austenite and increases the magnetic permeability of the stainless steel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present disclosure will now be described, by way of embodiments, with reference to the attached figures. 
         FIG. 1  is a flowchart of an embodiment of a method of making stainless steel according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. Additionally, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the embodiments described herein. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other word that “substantially” modifies, such that the component need not be exact. For example, “substantially cylindrical” means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series and the like. 
       FIG. 1  shows a flowchart of an embodiment of a method for making stainless steel. The method is provided by way of embodiment, as there are a variety of ways to carry out the method. Each block shown in  FIG. 1  represents one or more processes, methods, or subroutines carried out in the example method. Furthermore, the illustrated order of blocks is by example only, and the order of the blocks can be changed. Additional blocks can be added or fewer blocks can be utilized, without departing from this disclosure. 
     At block S 1 , metal material is smelted and then atomized and powdered to obtain a first alloy powder. 
     The metal material may include iron, chromium, and manganese. The smelting temperature is about 1500-1600° C. 
     At block S 2 , a mass fraction of manganese in the first alloy powder is detected. If a mass fraction of manganese in the first alloy powder is less than 12%, block S 3  is implemented. If the mass fraction of manganese in the first alloy powder is 12-15%, block S 5  is implemented. 
     At block S 3 , a predetermined amount of manganese-containing material is added to the first alloy powder and then heated and kneaded together with plastic to obtain a second alloy powder having a mass fraction of manganese reaching about 12-15%. 
     The manganese-containing material may include at least one of pure manganese, manganese iron powder, or other manganese-containing compound. In one embodiment, the plastic is polyoxymethylene. A mass percentage of the plastic relative to the first alloy powder is about 6-10%. 
     In the second alloy powder, a mass fraction of chromium is about 15.5-17.5%, a mass fraction of molybdenum is about 3-4%, and a mass fraction of nickel is not more than about 0.2%. Phosphorus, sulfur, carbon, oxygen, and nitrogen have a total mass fraction not more than about 1%. A remaining mass fraction of the second alloy powder is iron. 
     At block S 4 , the second alloy powder is granulated, injection molded, and then sintered to obtain stainless steel. 
     Particles obtained from granulation are substantially cylindrical and have a diameter of about 2-5 mm and a length of about 3-6 mm, so that a good molding effect can be achieved. In one embodiment, the molding process is injection molding, and sintering can be carried out in a continuous furnace or a nitrogen atmosphere having a temperature of about 1290° C. In other embodiments, sintering may be carried out by vacuum sintering. The stainless steel formed by the above processes has low magnetic permeability. 
     At block S 5 , if the mass fraction of manganese is 12-15%, the first alloy powder is heated and kneaded together with the plastic, such as polyoxymethylene, having a mass percentage of about 6-10% of the first alloy powder. 
     The first allow powder heated and kneaded with the plastic is then granulated, injection molded, and sintered to obtain the stainless steel having low magnetic permeability. 
     It should be noted that the magnetic permeability of stainless steel in the related art can be as high as 1.023-1.045. In contrast, the magnetic permeability of the stainless steel obtained by the current disclosure is about 1.001-1.002 (the magnetic permeability of air is 1.000). Thus, the stainless steel obtained by the current disclosure is more stable and essentially non-magnetic, which is applicable in fields requiring low magnetic permeability in stainless steel. 
     The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, including in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including, the full extent established by the broad general meaning of the terms used in the claims.