ステンレス鋼とIV?VI族金属との異材拡散接合に関する研究

Abstract

本研究はステンレス鋼SUS304LとIV?VI族金属との異種材料拡散接合に関する研究と片状黒鉛鋳鉄の熱処理特性と変態挙動に関する研究とに大別され、後者は芝浦工業大学との共同研究で行ったものである。ステンレス鋼SUS304LとIV?VI族金属との異種材料拡散接合部は各種の金属間化合物を生成する。ここではSUS304LとIV?VI族金属との異種材料拡散接合を行い、接合部に生成する金属間化合物相、接合部における各種元素の拡散挙動、および接合割れとの関係を周期律表との関連で系統的に調べた。いずれの接合部も接合強さが最大を示す接合温度が存在した。これよりも接合温度が低い場合は接合部に未接合部が存在し、接合温度が高い場合は接合部に金属間化合物層が厚く生成して接合強さを低下させた。このような温度で接合した接合部はボイドあるいは割れが特定の金属間化合物相に発生し、これが接合強さの低下を招いたと考えられる。片状黒鉛鋳鉄の材質改善方法として熱処理法が考えられる。しかし、熱処理法を確立するためには片状黒鉛鋳鉄の熱処理データを完備する必要がある。ここではSi量を約0.7?4.5%含む片状黒鉛鋳鉄を用い、熱処理特性に及ぼすSiの影響を調べるとともに、片状黒鉛鋳鉄のフェライト・パーライト変態の機構を明らかにした。Si量の増加によりCCT(連続冷却変態)図のフェライト変態域は高温短時間側へ移動し、パーライト変態域は若干高温側へ移動した。M(sub s)温度はSi量の増加とともに高温側へ移動した。ガンマ化温度の上昇によりフェライトおよびパーライト変態域は約4%Si以上になるとやや低温側へ移動した。片状黒鉛鋳鉄のCCT(連続冷却変態)図中でフェライト変態域がパーライト変態域中に存在する、すなわちフェライト変態開始温度がパーライト変態開始温度よりも低くなるSi量の範囲は約1?3%であり、ガンマ化温度に影響されなかった。この特異な現象は片状黒鉛鋳鉄中に黒鉛が存在することによるものであり、冷却途中でマトリックス中のC量が変化することに起因すると考えられ、その変態の機構を明らかにした。

This study is divided into two works, the one on dissimilar metal diffusion joining between stainless steel SUS304L and IV to VI group metals, and the other on heat treatment characteristics and transformation behavior of flake graphite cast irons which was carried out in a collaboration with Shibaura Institute of Technology. In the dissimilar metal joints of stainless steel SUS304L and IV to VI group metals, various intermetallic compounds are formed. The relationship between intermetallic compound phases formed at joints, the diffusion behaviors of various elements at joints, and the joining cracks were systematically examined in relation to the periodic table of elements. For all of joints there existed a joining temperature at which the joining strength was the maximum. Below the joining temperatures, unjoined parts existed in the joints, and above the joining temperatures thick intermetallic compound phase were formed with the decrease in the joining strength. Voids or cracks were formed in a particular intermetallic compound phase in the joined parts at these temperatures, resulting in the decreases of the joining strength. A heat treatment process is considered as a method for improving material properties of flake graphite cast irons. But it is necessary to complete heat treatment data for flake graphite cast irons in order to establish the process. In this study, using flake graphite cast irons with about 0.7 to 4.5 percent of Si, the effects of Si concentration on the heat treatment characteristics were examined and the mechanism of ferrite-pearlite transformation for flake graphite cast irons were clarified. In CCT (Continuous Cooling Transformation) diagram, the ferrite transformation region was shifted to the high temperature and short time sides and the pearlite transformation region was shifted a little to high temperature side with increase of the Si concentration. The M(sub s) temperature was shifted to the higher temperature side as the Si concentration increases. Due to increases of austenitizing temperatures, the transformation regions of ferrite and pearlite above about 4 percent of Si were shifted a little to low temperature side. In the CCT diagram of flake graphite cast irons, the ferrite transformation region existed in the pearlite transformation region, that is, the range of the Si concentration where the starting temperature of ferrite transformation was lower than that of pearlite transformation by about 1 to 3 percent and was not affected by the austenitizing temperature. It was considered that this peculiar phenomenon was caused by the existence of graphite in flake graphite cast irons and resulted from the change of C concentration in the matrix during cooling. The mechanism of the transformation was clarified.