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| Sintered stainless steel pipe and production process therefor |
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The secondary object of the present invention is to provide a stainless steel exhibiting toughness as high as that of austenitic stainless steel and SCC resistance as good as that of ferritic stainless steel. Another object of the present invention is to provide a method for producing a sintered stainless steel which exhibits not only markedly improved SCC resistance but also a satisfactory level of toughness. 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. As stated above, it has been thought in the art that it is unavoidable to include a few percent of nickel in a ferritic phase of duplex stainless steel pipe. However, the inventors of the present invention, based on the results shown in FIGS. 1 and2, found that if the nickel content of the ferritic phase of duplex stainless steel is reduced to a lower level, the SCC resistance can be improved markedly. The present inventors have carried out a series of experiments to find measures to freelycontrol the nickel content of the ferritic phase of duplex stainless steel. After intensive research, the inventors have found that the above-described prior art problem can be easily overcome by employing a powder metallurgy process in which steelpowders with alloying compositions corresponding to each of the constitutional phases, i.e., a ferritic phase and austenitic phase, are prepared separately through an atomization process. The thus prepared steel powders are mixed at a given ratio toprovide a green compact which is then sintered to give a sintered product in the shape of a pipe, plate, or the like. Therefore, the present invention resides in a sintered stainless steel exhibiting improved resistance to stress corrosion cracking, which comprises a matrix phase and a dispersing phase, the metallurgical structures of which are different fromeach other, the matrix phase comprising a substantially single ferritic structure derived from a ferritic stainless steel powder, and the dispersing phase comprising a structure selected from the group consisting of an austenitic structure, anaustenitic ferritic dual phase structure, an austenitic martensitic dual phase structure, and an austenitic ferritic martensitic triple phase structure, which are derived respectively from an austenitic stainless steel powder, an austenitic ferriticduplex stainless steel powder, an austenitic martensitic duplex stainless steel powder and a triple phase stainless steel powder. 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. |