Method of fabricating a steel part by powder metallurgy, and resulting steel part

Abstract

A method for manufacturing by powder metallurgy a steel part is provided. A pre-alloyed powder is prepared, having the desired composition for said part, except for the O and N contents and optionally C contents, with O and N contents of at most 200 ppm, and with an Mn content from 0.4 to 2% by weight and a Cr content of less than or equal to 3%; the powder is placed in a container for which the walls define a space, the shape of which corresponds to that of the part, a getter being at the periphery of the powder, said getter has the capability at a high temperature of absorbing and reducing CO and absorbing nitrogen, and a vacuum is applied and the container is then sealed; the container and the powder are brought to a temperature causing sintering of the powder and densification of said powder not exceeding 5%, evolvement of nitrogen and CO from the powder and their absorption by the getter; densification of said powder is achieved by hot isostatic compaction in order to obtain said part; said part is separated from the container and from the getter; and peeling, heat treatment and machining of said part are achieved. The thereby produced steel part is also provided.

1 - 20 . (canceled) 
     
     
         21 . A method for manufacturing by powder metallurgy a steel part comprising:
 preparing a pre-alloyed powder having a desired composition for the part, except on the O and N contents and optionally on the C content, with O and N contents of at most 200 ppm, the powder having an Mn content comprised between 0.4 and 2% by weight and a Cr content of less than or equal to 3%;   placing the powder in a container for which walls of the container define a space, a shape of the space corresponding to that of the part to be manufactured, a getter being positioned at least partly at a periphery of the powder, the getter having the capability, at high temperature, of absorbing and reducing CO and of absorbing nitrogen by dissolution, and a vacuum is applied and the container is then sealed;   bringing the container and the powder contained therein to a temperature causing sintering of the powder and densification of the powder not exceeding 5%,   evolving nitrogen and CO from the powder and their absorption by the getter;   densifying the powder by hot isostatic compaction by placing the container and the powder in a pressurized chamber in order to obtain the part;   separating the part is from the container and from the getter; and   peeling, a heat treating and machining of the part for giving the part mechanical properties, a desired surface condition and exact dimensions.   
     
     
         22 . The method as recited in  claim 21  wherein the part is in a steel of a composition, in weight % after densification:
 C≦0.25%; 
 Mn=0.5-1.60%; 
 P≦0.025%; 
 S≦0.025%; 
 Si≦0.4%; 
 Ni=0.4-1.00%; 
 Cr≦0.25%; 
 Mo=0.43-0.6%; 
 V≦0.05%; 
 Nb≦0.01%; 
 Cu≦0.2%; 
 Ca<0.015%; 
 B≦0.003%; 
 Ti≦0.015%; 
 Al≦0.04%; 
 O≦50 ppm; 
 N≦50 ppm; and 
 the remainder being iron and impurities resulting from the manufacturing. 
 
     
     
         23 . The method as recited in  claim 22  wherein O≦20 ppm. 
     
     
         24 . The method as recited in  claim 22  wherein N≦25 ppm. 
     
     
         25 . The method as recited in  claim 22  wherein the part has the composition, in weight %, after densification:
 C≦0.22%; 
 Mn=1.15-1.60%; 
 P≦0.008%; 
 S≦0.008%; 
 Si=0.10-0.30%; 
 Ni=0.50-0.80%; 
 Cr≦0.25%; 
 Mo=0.43-0.57%; 
 V≦0.03%, being aware that for the parts to be coated, this maximum content may be reduced to 0.01%; 
 Cu≦0.20%; 
 Al≦0.04%; 
 O≦50 ppm; 
 N≦50 ppm; and 
 the remainder being iron and impurities resulting from the manufacturing. 
 
     
     
         26 . The method as recited in  claim 25  wherein O≦20 ppm. 
     
     
         27 . The method as recited in  claim 25  wherein N≦25 ppm. 
     
     
         28 . The method as recited in  claim 22  wherein the part has the composition, in weight percent after densification:
 C≦0.25%; 
 Mn=0.5-1.00%; 
 P≦0.025%; 
 S≦0.025%; 
 Si≦0.4%; 
 Ni=0.4-1.00%; 
 Cr≦0.25%; 
 Mo=0.45-0.6%; 
 V≦0.05%; 
 Nb≦0.01%; 
 Cu≦0.2%; 
 Ca≦0.015%; 
 B≦0.003%; 
 Ti≦0.015%; 
 Al≦0.025%; 
 O≦50 ppm, preferably ≦20 ppm; 
 N≦50 ppm, preferably ≦25 ppm; and 
 the remainder being iron and impurities resulting from the manufacturing. 
 
     
     
         29 . The method as recited in  claim 28  wherein O≦20 ppm. 
     
     
         30 . The method as recited in  claim 28  wherein N≦25 ppm. 
     
     
         31 . The method as recited in  claim 22  wherein the getter is in titanium or in a titanium alloy, the temperature of the powder during sintering being between 950 and 1,065° C. 
     
     
         32 . The method as recited in  claim 21  wherein the getter ( 6 ) is in a material selected from among titanium, zirconium, hafnium and alloys thereof, and a stainless steel. 
     
     
         33 . The method as recited in  claim 21  wherein the getter is in titanium or in a titanium alloy, the temperature of the powder during sintering being between 950 and 1,065° C. 
     
     
         34 . The method as recited in  claim 33  wherein the temperature of the powder during sintering is between 1,000 and 1,065° C. 
     
     
         35 . The method as recited in  claim 21  wherein the sintering and densifying by hot isostatic compaction of the powder are successively carried out, without any intermediate cooling of the powder. 
     
     
         36 . The method as recited in  claim 21  wherein after having placed the powder in the space defined by the walls of the container, the powder undergoes cold isostatic compaction at a maximum temperature of 300° C. and under a pressure from 100 to 300 bars. 
     
     
         37 . The method as recited in  claim 36  wherein that the cold isostatic compaction provides a reduction in the volume of a powder by 1 to 3%. 
     
     
         38 . The method as recited in  claim 21  wherein the wall of the container in contact with the powder is made in the material making up the getter. 
     
     
         39 . The method as recited in  claim 21  wherein the getter is a coating of the wall of the container. 
     
     
         40 . The method as recited in  claim 21  wherein the getter forms a separate part placed in the vicinity of the wall of the container in contact with the powder. 
     
     
         41 . A steel part obtained by the method as  claim 21 , the oxygen content being ≦50 ppm, the nitrogen content being ≦50 ppm, and the cumulated oxygen+nitrogen content being ≦80 ppm.