Method for producing sintered compact by powder metallurgy

Abstract

A high-strength sintered compact is produced at low cost using an alloy steel powder for powder metallurgy containing no nickel or copper by compacting the alloy steel powder for powder metallurgy or a mixed powder containing the alloy steel powder at a pressure of 700 MPa or more and sintering the compact at a temperature of 1,150° C. to 1,300° C. The alloy steel powder used contains 0.3% to 0.7% by mass of chromium, 0.1% to 0.5% by mass of manganese, 0.1% to 0.5% by mass of molybdenum, and 0.25% to 0.5% by mass of oxygen, the balance being iron and incidental impurities.

1 - 4 . (canceled) 
     
     
         5 . A method for producing a sintered compact by powder metallurgy comprising:
 compacting an alloy steel powder for powder metallurgy comprising 0.3% to 0.7% by mass of chromium, 0.1% to 0.5% by mass of manganese, 0.1% to 0.5% by mass of molybdenum, and 0.25% to 0.5% by mass of oxygen, the balance being iron and incidental impurities, or a mixed powder containing the alloy steel powder at a pressure of 700 MPa or more; and   sintering the compact at a temperature of 1,150° C. to 1,300° C.   
     
     
         6 . The method according to  claim 5 , wherein the mixed powder further comprises a graphite powder. 
     
     
         7 . The method according to  claim 5 , wherein the mixed powder further comprises a graphite powder in an amount equivalent to a sum of an amount of carbon allowed to remain in the sintered compact and an amount of carbon that reacts with oxygen in the alloy steel powder in the sintering. 
     
     
         8 . The method according to  claim 6 , wherein the graphite powder is present in an amount equivalent to a sum of an amount of carbon allowed to remain in the sintered compact and an amount of carbon that reacts with oxygen in the alloy steel powder in the sintering. 
     
     
         9 . The method according to  claim 6 , wherein the graphite powder is added to the alloy steel powder in an amount [% Gr] (% by mass based on the mass of the alloy steel powder) satisfying the condition of equation (1):
   [% Gr]=t×(α×0.46×[% Cr]+β×0.29×[% Mn])+γ=[% C]  (1)
   
       wherein
 [% Cr] is the chromium content of the alloy steel powder (% by mass); 
 [% Mn] is the manganese content of the alloy steel powder (% by mass); 
 [% C] is the amount of carbon, in the graphite powder, allowed to remain in the sintered compact (% by mass based on the mass of the alloy steel powder; 
 α is the oxidation rate of chromium in the alloy steel powder; 
 β is the oxidation rate of manganese in the alloy steel powder; 
 γ is the amount of carbon, in the graphite powder, consumed by reaction with oxygen adsorbed on the alloy steel powder and oxygen contained in a sintering atmosphere (% by mass based on the mass of the alloy steel powder for powder metallurgy), wherein γ≦0.2% by mass; and 
 t is 0.25 to 0.75.