熱サイクルによる原子炉用金属材料の変形について

Abstract

In order to clarify the basic characteristics on the thermal fatigue strength of metals, the authors have investigated on the changes in dimension, deformation, hardness, specific gravity, microstructure, temperature distribution and yield strength at elevated temperatures during simple thermal cycling of the various metals and alloys for reactor use, and the following results were obtained:(1) Armco iron and 304L stainless steel elongated in longitudinal direciton, but carbon steel having various carbon contents, 21/4Cr-1 Mo steel and four kinds of Fe-Al-Cr alloy contracted after the thermal cycling.(2) The grain growth due to thermal cycling of both 304L stainless steel and Fe-7.5 Al-6 Cr alloy containing 0.5% Ti and 1% Nb which suppressed the contraction was faily small. But in both alloys, surface crack occurred in the early stage of thermal cycling and propagated to the interior of samples. In the other Fe-Al-Cr alloys which displayed contraction by thermal cycling remarkable grain growth occurred and, in these cases, crack initiated only on the surface layer of sample at later stages than in 304L stainless steel, where that did not propagated. Therefore the grain growth itself could not be seemed as the essential cause of dimensional changes, because thermal stresses due to thermal cycling are relieved during the grain growth and not to promote but to retard the dimensional changes.(3) The temperature and yield strength distributions in sample during thermal cycling of 304L stainless steel and two kinds of Fe-Al-Cr alloy (8% Al and 12% Al) were measured. For latter alloys the intense decrease of the yield strength was observed at higher temperature than 500-550°C but for former alloy such marked change of yield strength was not observed through a temperature range up to 750°C. From these results, the authors concluded that the essential cause of the dimensional changes in such metals and alloys having simple phase as armco iron, 304L stainless steel and Fe-Al-Cr alloys depends generally upon the amount of thermal strain due to temperature gradient and the change in yield strength.Besides, the authors suggested that whether elongation or contraction was dependent apparently upon the existence of the marked decrease of yield strength within the cycling temperature range. However, the critical values could not be decided quantitatively because of lack in data. The authors will study further quantitatively to clarify the mechanism of the above mentioned phenomena.