Method for Manufacturing Iron-based Powder Metallurgical Parts

Abstract

A method for manufacturing iron-based metallurgical parts, the method comprising: mixing graphite powder; pressing; presintering; oxidizing the presintered metallurgical part to form an oxide layer having a thickness of 1 μm to 50 μm on its surface to form an oxidized presintered metallurgical part; sintering; machining; carburizing; quenching and tempering. An oxide layer is formed on the surface of a part by oxidization, oxygen in the oxide layer is chemically reacted with the carbon in the surface layer of the product during the sintering, and the resulting product enters a sintering atmosphere in the form of gas to form a decarburized layer having a certain thickness on the surface of the part, so that the decarburization is realized.

1 . A method for manufacturing iron-based metallurgical parts, the method comprising:
 (a) mixing graphite powder, iron powder, alloy element powder, and lubricant powder to obtain a mixed powder;   (b) pressing the mixed powder on a powder metallurgical molding press to obtain a molded green compact;   (c) presintering the molded green compacts in a non-oxidizing atmosphere at 600° C. to 1050° C. for 10 min to 300 min to obtain a presintered metallurgical part;   (d) oxidizing the presintered metallurgical part to form an oxide layer having a thickness of 1 μm to 50 μm on its surface to form an oxidized presintered metallurgical part;   (e) performing a secondary sintering of the oxidized presintered metallurgical part in an non-oxidizing atmosphere at 1050° C. to 1350° C. for 10 min to 200 min to obtain a sintered metallurgical part;   (f) performing a process to increase the density and/or surface compactness of the sintered part to form a compact metallurgical part;   (g) carburizing the compact metallurgical part in an atmosphere having a carbon potential of 0.3% to 2.0% at 700° C. to 1200° C. for 5 min to 400 min to form a carburized metallurgical part, and cooling the carburized metallurgical part to a temperature suitable for quenching; and   (h) quenching and tempering the carburized metallurgical part to achieve the desired mechanical properties, wherein the iron-based metallurgical part comprises iron, 0.2% to 1.5% by mass percentage carbon, the alloy element and the lubricant.   
     
     
         2 . The method of  claim 1 , wherein the oxidizing in the step (d) is steam treatment carried out at 350° C. to 700° C. for 20 min to 300 min. 
     
     
         3 . The method of  claim 1 , wherein the oxidizing in the step (d) is treating the presintered part in an atmosphere containing 0.5 vol % to 100 vol % of oxygen at 200° C. to 600° C. for 20 min to 300 min. 
     
     
         4 . The method of  claim 1 , wherein when the temperature of the part in the cooling stage of the step (c) is 400° C. to 600° C., 10 vol % to 100 vol % of steam is supplied for a steam treatment to carry out the oxidation of step (d). 
     
     
         5 . The method of  claim 1 , wherein 0.5 vol % to 50vol % of air or oxygen is supplied into the non-oxidizing atmosphere after the presintering of step (c) to carry out the oxidation of step (d). 
     
     
         6 . The method of  claim 1 , wherein a continuous sintering furnace is used such that the step (c) is carried out in a presintering region, the step (d) is carried out by maintaining 0.1 vol % to 10 vol % of oxygen or 0.5 vol % to 20 vol % of steam in the non-oxidizing atmosphere, and the oxidized presintered metallurgical part is delivered to a sintering region for carrying out the step (e). 
     
     
         7 . The method of  claim 1 , wherein in the step (c), the presintering is carried out at 750° C. to 1000° C. for 20 min to 120 min. 
     
     
         8 . The method of  claim 1 , the process for increasing the density and/or surface compactness of the sintered part in step (f) is at least one of extrusion molding, reshaping, repressing, surface rolling and cross rolling. 
     
     
         9 . The method of  claim 1 , wherein the alloy element is at least one of Ni, Cu, Mn, Cr and Mo. 
     
     
         10 . The method of  claim 1 , further comprising a shot blasting treatment after step (h). 
     
     
         11 . The method of  claim 1 , wherein the non-oxidizing atmosphere in steps (c) and (e) is a nitrogen-based atmosphere or vacuum. 
     
     
         12 . The method of  claim 1 , wherein the non-oxidizing atmosphere is a nitrogen hydrogen atmosphere containing 5 vol % of hydrogen. 
     
     
         13 . The method of  claim 1 , wherein the iron-based metallurgical part comprises the following components in percentage of weight:
 0.4˜1.5% C, 0˜4% Cu, 0˜5% Ni, 0˜2% Mo, 0˜6% Cr, 0˜5% Mn, and a remainder component being Fe and less than or equal to 2% unavoidable impurities.   
     
     
         14 . The method of  claim 1 , wherein the lubricant is 0.1˜1% (in percentage of weight) powder lubricant, and the lubricant is polyamide wax or stearate or polyacrylamide wax.