著者

福井 彰一

出版者

The Iron and Steel Institute of Japan

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.55, no.2, pp.151-161, 1969-02-01 (Released:2010-10-12)

参考文献数

13

被引用文献数

16 10

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The influence of tempering on the delayed fracture characteristics in 0.1N HCl solution, the initiationand propagation behavior of cracks in delayed fracture and the hydrogen embrittlement characteristics in-tensile tests have been studied in metallurgical view point for several low-alloy steels with the difference in, carbon content and tempering behavior.In general, the delayed fracture strength ratio (fracture strength at 30hr in life divided by static strength)-varied depending on the hardness of the steel, and showed almost constant value of 0.85 in all steels with thehardness up to Hv 350 and rapidly decreased the value with increase in hardness above Hv 350.Contradictorily to the general aspects described above, an abnormal increase appeared in the delayed.fracture strength ratio for the steel tempered at about 250°C, independently on the hardness of the steel.This phenomenon shifted to the higher tempering temperature as silicon content in the steel was increased.Considering the stabilizing effect of silicon on epsilon carbides, it is supposed that the phenomenon is attributedto the precipitation of epsilon carbides.The cracks in delayed fracture initiated at the prior austenite grain boundaries, but the propagation pathdiffered according as the tempering condition of the steel. It has been observed that the cracks propagatedat the prior austenite grain boundaries in the steels tempered in the temperature range of primary temper-brittleness (500°F embrittlement), and mainly propagated across the interior of the grains the steelstempered in the temperature out of the range of primary temper-brittleness.From the results of the electron microscopic observation of the precipitation behavior of carbides, ithas been observed that cementites precipitated in a line at the prior austenite grain boundaries in the specimenof which crack in delayed fracture propagated along the grain boundaries. The delayed fracturestrength of these steels were inferior to those cracks propagated across the interior of the grains.The dependence of the degree of hydrogen embrittlement, which was indicated as the decrease of the reductionof area in tensile test for the steel dipped in 0.1N HCl solution, on the tempering temperature was in goodagreement with that of the delayed fracture strength in the same solution. This fact suggests that themicrostructure of the steel contributed in the same way to both of the delayed fracture strength and the hydrogen embrittlement.