1 0 0 0 OA 1.8GPa級超高強度フェールセーフボルトの耐遅れ破壊特性
- 著者
- 木村 勇次 井上 忠信 中田 隆之 福田 又一
- 出版者
- 一般社団法人 日本機械学会
- 雑誌
- 日本機械学会論文集 (ISSN:21879761)
- 巻号頁・発行日
- vol.84, no.860, pp.17-00493, 2018 (Released:2018-04-25)
- 参考文献数
- 24
- 被引用文献数
- 1 7
Delayed fracture susceptibility of 1.8 GPa-class ultra-high strength bolts was evaluated by an outdoor exposure test. Low-alloy steel bars with a chemical composition of Fe-0.4%C-2%Si-1%Cr-1%Mo (mass%) were quenched and tempered at 500 °C for 1 h, caliber-rolled at 500 °C with a rolling reduction of 78%, in order to create fail-safe steels (FS steels) with an ultrafine elongated grain structure. Subsequently, FS steel bars were formed into JIS M12 hexagon head bolts with a nominal length of 60 mm (FS bolts). Bolt heading was performed at 700~730 °C, and screw part was formed by thread-rolling at 500 °C to maintain the ultrafine grain structure with a strong <110>//rolling direction fiber texture. Conventionally quenched and tempered bolts (QT bolts) with a tempered martensitic structure were prepared for comparison. A steel plate was fastened by the bolts on 23 April 2013, and the outdoor exposure test was started on 21 May 2013 in Miyako Island test site of Japan Weathering Test Center. The bolt fastening force was 122 kN (=0.85 σ0.2 at thread part) for FS bolts and 89 kN (=0.70 σ0.2 at thread part) for QT bolts. The outdoor exposure test demonstrated an excellent delayed fracture resistance of FS bolts at the ultra-high tensile strength of 1.8 GPa; FS bolts have not been broken for 53 months, in contrast to QT bolts exhibiting delayed fracture. Intergranular cracking along the prior-austenite grain boundaries was commonly observed in the broken QT bolts. The accelerated laboratory test using tensile tests of pre-hydrogen-charged specimens and immersion test (30 °C, pH2) showed that the FS bolts had high resistance to hydrogen embrittlement, and it was in good agreement with the result of outdoor exposure test of the FS bolts.
1 0 0 0 OA 超強加工による鋼の結晶粒超微細化とセメンタイトの分解許容量
- 著者
- 飛鷹 秀幸 木村 勇次 高木 節雄
- 出版者
- 社団法人日本鉄鋼協会
- 雑誌
- 鐵と鋼 : 日本鐡鋼協會々誌 (ISSN:00211575)
- 巻号頁・発行日
- vol.85, no.1, pp.52-58, 1999-01-01
- 被引用文献数
- 6
Mechanical milling using high energy planetary ball mill was applied to Fe-C alloy powders with (ferrite+cementite) two-phase structures to give an ultimate large strain into the powders. Dissolution behavior of cementite during mechanical milling was investigated in relation to ultra grain refining of ferrite matrix, and dissolution capacity of cementite was discussed in terms of carbon content in the powders. Ultra grain refining of ferrite matrix to about 10nm results in full dissolution of cementite in the powders with carbon up to 2 mass% C. Most of carbon, which has been rejected from decomposed cementite, is suggested to segregate at grain boundary to form amorphous layer. Thus, it was proposed that the dissolution limit of cementite depends on both volume fraction of the grain boundary amorphous layer and carbon concentration therein. For example, as the maximum carbon content of the grain boundary amorphous layer was to be about 4.2 mass% C, the dissolution limit of cementite was estimated at 30 vol% from the mass valance for carbon content in the case ferrite grains were refined to around 10nm. This volume fraction of cementite is just correspondent to that in Fe-2mass%C alloy.