2004 年 45 巻 2 号 p. 569-576
This study examines the effect of maximum temperature on cyclic-heating-induced embrittlement. Experimental results indicated a significant deterioration of tensile properties when the maximum heating temperature was 1023 K. Significant recrystallization of ferrite grains occurred when the heating temperature was raised to over 1073 K and this suppressed the embrittlement. Moreover, evidence of partial phase transformation was observed when the maximum heating temperature was raised to 1123 K. When a specimen was heated to 1023 K with a certain number of cycles, a distinct area fraction of intergranular fracture could be recognized from tensile fractography. The initiation site of these thermal cracks was at the prior-solidificational eutectic cell boundary of the ferrite matrix, which is related to magnesium-containing oxide inclusions. These oxide inclusions were analyzed and found to contain mainly magnesium, oxygen, phosphorus and cerium. It has been confirmed that the formation of magnesium-containing inclusions in the solidification process is mainly responsible for intergranular fracture, and consequently plastic deformation is promoted when the number of heating cycles is increased. From observation of the vicinity of crack initiation and propagation paths, significant slip evidence was found and was revealed using etch-pit techniques. In conclusion, the cyclic heating induced severe embrittlement at a specific maximum temperature of 1023 K. It is suggested that this resulted predominantly from periodic strain and stress accumulation of the inclusions which caused plastic deformation in the prior-solidification eutectic cell boundary region.
