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Sintered stainless steel and production process therefor

 

Sintered stainless steel and production process therefor

Still another object of the present invention is to provide a sintered seamless stainless steel pipe, especially a sintered seamless stainless steel pipe which can be advantageously used under highly corrosive conditions as heat exchanging tubesand piping to which sea water or industrial water is fed.


   

The present invention relates to a sintered stainless steel exhibiting markedly improved resistance to stress corrosion cracking and the production thereof, the steel comprising a matrix phase of a substantially ferritic structure and adispersing phase containing an austenitic area. The dispersing phase is selected from the group consisting of a single austenitic structure, an austenitic ferritic structure, an austenitic martensitic structure, and an austenitic ferritic martensiticstructure.


   

As is well known in the art, stainless steel is classified into martensitic, ferritic, austenitic, and duplex types.


   

Ferritic stainless steel is not expensive and it exhibits good resistance to stress corrosion cracking. However, it has poor toughness and its weldability is not good.


   

Austenitic stainless steel exhibits good toughness as well as extremely high resistance to corrosion under usual conditions. However, in general, it is expensive since it contains a relatively large amount of Ni and it does not exhibit goodresistance to stress corrosion cracking ("SCC"hereunder). The incorporation of a relatively large amount of Ni is effective for improving the resistance to SCC to some extent, but the effect derived from the addition of nickel saturates at a certainlevel. Furthermore, the addition of nickel makes the steel expensive, resulting in limited applications therefor.


   

Duplex stainless steel has been proposed so as to eliminate the above-mentioned shortcomings, and it has not only the advantages which the ferritic stainless steels have but also those of the austenitic stainless steels. Duplex steel alsoexhibits the same level of toughness as austenitic stainless steel does and much better SCC resistance.


   

Regarding the SCC resistance of duplex stainless steel, an article by Edeleanu appeared in the Journal Iron Steel Inst., 173, 140 (1953) describing the influence of the amount of ��-ferritic phase in 18Cr-8Ni-Ti steels on SCC resistance. Since then, a number of other articles have been published. It has been reported that alloying elements, heat treatment conditions, and the amount of ferritic phase have an influence on SCC resistance.


   

In another aspect, the present invention resides in a process for producing a sintered stainless steel exhibiting improved resistance to stress corrosion cracking, which comprises a matrix phase and a dispersing phase, the metallurgicalstructures of which are different from each other, comprising the steps of mixing a ferritic stainless steel powder with a powder selected from the group consisting of an austenitic stainless steel powder, an austenitic ferritic duplex stainless steelpowder, an austenitic martensitic duplex stainless steel powder, and an austenitic ferritic martensitic triple phase stainless steel powder, and compacting and sintering the resulting powder mixture.


   

According to still another aspect, the compacting and sintering can be carried out simultaneously using a hot isostatic pressing process. Alternatively, only the compacting may be carried out through a cold isostatic pressing process.


   

In case a sintered seamless stainless steel pipe is produced, the resulting powder mixture is subjected to compacting and sintering, and preferably cold isostatic pressing and then sintering, or hot isostatic pressing. The resulting sinteredstainless steel is formed into a seamless pipe through hot extruding. Cold drawing is applied to the hot extruded product. Cold drawing is applied to the hot extruded seamless pipe of sintered stainless steel to provide a sintered seamless pipe withfinal dimensions