In earlier N.P.L. work on the strength of carbon steels at high temperatures, steels showing ‘normal’ and ‘abnormal’ creep properties were assessed from tests extending to 1000 hours. Further results on the same steels indicate that the order of merit of these particular steels is not altered in tests extending to 100 000 hours. The rupture properties of carbon steels are reviewed in the light of the N.P.L. results and other published data. A wide range of strength properties is shown to exist. The pattern of results, however, appears reasonably consistent. At the lower end of the strength scale are almost pure irons and at the upper end are steels of moderate carbon content and not low in manganese and silicon. Heavy deoxidation with aluminium of conventional carbon steels results in lower creep and rupture properties than are observed in similar steels deoxidized with silicon; the part played by nitrogen in this effect is considered. The rupture ductility of carbon steels is discussed and in the types covered by the N.P.L. researches, low ductility in long-time service appears unlikely. Further work required in this field is considered and the need for long-time, low-strain creep properties of mild steels for use in nuclear power plants is emphasized.
INTRODUCTIONUNTIL A FEW YEARS AGO the most advanced steam conditions in common use in power stations in Great Britain were 900 lb. per sq. in. and 900 deg. F. (482 deg. C.), and the components operating at high temperatures were normally of either carbon steel or 3 per cent molybdenum steel (with small chromium additions in some instances). The latest stations operated by the Central Electricity Authority (C.E.A.) use steam at 1,500 lb. per sq. in. and 1,050 deg. F. (565 deg. C.), and these conditions necessitated the use of steels of higher alloy content for the components in the sections of the plant operating at the highest temperatures. These steels and the design stresses were chosen, at the design stage, in the light of the best information then available, but it was clear that more comprehensive data were necessary before full advantage could be taken of the properties of which the types of steel in question were capable. In particular, tests of longer duration were required. Furthermore, some of the data then available referred to material in bar form rather than in the form of actual turbine components.With this background in mind, Sub-Committee J/E ('Steels for High Temperatures') of the Electrical Research Association-the Committee in Great Britain primarily concerned with steels for steam power plant-decided to sponsor a series of investigations to provide long-time information on the creep and metallurgical properties of chosen steels. This work was undertaken at the N.P.L., and follows similar investigations on the high-temperature properties of pipes and tubes of carbon steels and + per cent molybdenum steels and of 3 per cent molybdenum4 per The MS. of this paper was$rst received at the Institution
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