二相ステンレス鋳鋼の耐孔食性に及ぼす金属組織の影響

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The microscopic structural investigation was made to explain about the pitting corrosion resistance of a ferrite-austenite stainless cast steel DCS1.Temperature for solution treatment was from 900 up to 1150°C and sensitization was made between 500 to 1050°C after solution treatment at 1080°C.The results are as follows:(1) The corrosion rate of DCS1 decreases when solution treatment temperature increases. But it becomes less than 2(mg/cm2) in case the temperature is over 1080°C.(2) When cooling rate is slow, chromium carbide Cr23C6 and sigma phase recipitate at the ferrite-austenite grain boundaries and in the ferrite grains. These precipitations cause the selective corrosion and in this case the corrosion rate increases largely.(3) The time-temperature-sensitization curve of DCS1 consists of two C curves with nose temperature at 850°C and 700°C. The high temperature nose results from the precipitation of sigma phase at the ferrite-austenite grain boundary and in the ferrite grain. The lower nose is the sensitization caused by the precipitation of chromium carbide at the ferrite-austenite grain boundaries.(4) With the cooling rate of water quenchd specimen from 1080°C, solution treatment can be done without sensitization up to the thickness of 100mm.
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