Abstract
Various kinds of Cr-Mo steel tubes which are from 1% to 9% in Cr contents are widely used for superheater, reheater, main steam tubes and others. Cr is an effective element for increasing oxidation resistance, creep strength and corrosion resistance, and accordingly the Cr-Mo steel tubes containing Cr above 5% have been used mainly for petroleum or other industries which require both heat resisting and corrosion resisting properties. In this connection, especially, 1 Cr-1/2 Mo, or 21/4 Cr-1 Mo steel has creep strength superior to those of Cr-Mo steels such as 5 Cr-1/2 Mo, 7 Cr-1/2 Mo and 9 Cr-1 Mo at a temperature between 500°and 800°C in non-corrosive atmosphere. The creep strength of all Cr-Mo steels mentioned above are remarkably dependent on their heat treatment, and therefore creep tests have been carried out for test specimens heat-treated in several ways.Five criterions are specified in ASME Boiler and Pressure Vessel Code as the basis for selecting design stress. From the viewpoint of design, among these criterions, the stress of 60% of the average or 80% of the minimum stress to produce rupture in 100000hrs have been considered as the most reasonable value for some tube materials such as Cr-Mo steels having good ductilities at higher temperatures.As the results of our experiment, it has been found that, when 1 Cr-1/2 Mo and 21/4 Cr-1 Mo steel with C contents around the upper limit of their specifications are heat-treated by air cooling at a suitable temperature related to the C-C-T curve and are tempered at a lower side within a given temper temperature range, their creep strength is superior to those of specimens heat-treated by other ways. While their creep strength becomes remarkably lower by the tempering at higher temperatures in the cases mentioned in the above. In addition, the most suitable conditions for heat treatment are decided for other Cr-Mo steels such as 5 Cr-1/2 Mo, 7 Cr-1/2 Mo and 9 Cr-1 Mo.
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