PURPOSE: To produce a chromized steel pipe which is less degraded in the strength and creep rupture strength by subjecting the steel pipe to a chromizing treatment then to a specific cooling treatment thereby suppressing the decarburization to the prescribed value or below of the thickness of the decarburized layer.
CONSTITUTION: The steel pipe is embedded into chromium powder compounded with Al2O3, NH4Cl, etc., and while gaseous H2 or Ar is passed therein, the steel pipe is heated to a high temp. of about 900W1,200°C to diffuse and penetrate the chromium. The steel pipe is subjected to air cooling, etc., after the above-mentioned chromizing treatment, by which the steel pipe is cooled at ≥30°C/hr cooling rate in the temp. region down to at least 600°C. The formation and splashing of CO by reaction of the C in the steel pipe with the O2 in the treating atmosphere and the consequent formation of the decarburized layers over a wide range are thereby prevented. As a result, the thickness of the decarburized layer is decreased to ≤800μm and the chromized steel pipe with which the degradation in the strength and creep rupture strength by the chromizing treatment is obtd.
ARAI TETSUZO
UEDA TOMONORI
YOSHIKAWA MASAYASU
NIPPON KARORAIZU KOGYO KK
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