Mechanism of Coarsening of Ferrite Grains in the Welding Deposit of Low C-2 1/4 Cr-1Mo Steel on Annealing

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  • 低炭素2 1/4 Cr-1Mo鋼溶着金属の焼鈍による炭化物変化とフェライトの粗大化の機構について
  • テイタンソ 21 4Cr-1Mo コウ ヨウチャク キンゾク ノ ショウドン ニ ヨル タンカブツ ヘンカ ト フェライト ノ ソダイカ ノ キコウ ニ ツイテ

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Abstract

Coarse ferrite develops in low-carbon 2 1/4 Cr-1Mo multilayer weld deposit during the annealing process below the Ac1 transformation point. In this report, changes of the deposit in structure, hardness, and X-ray diffraction patterns after heating tests at 760°C for one to ten hours are studied. The results obtained were as follows: (1) In the heat affected zone of the weld deposit, fine ferrite and bainite are mixed. Under this condition, carbides are remarkably changed during annealing, and in about five hours such unstable carbides as M3C, Mo2C etc. are dissolved and very fine M23C6 precipitates in matrix as stable phase. (2) With dissolving of M3C, Mo2C etc. around the grain boundary the growth of ferrite is facilitated. The precipitation of the very fine M23C6 causes a large lattice strain and recrystallization starts in these regions, then ferrite grains grow quickly. The growth starts in five hours, and in the condition just before it, ferrite is much hardened. The X-ray diffraction patterns in this condition are much broader. (3) For the growth of ferrite, it is a necessary condition that the primary austenite remains undamaged (not heated to the Ac3 point or above). In this condition, fine ferrite has a uniform crystal array, which is convenient for the growth of ferrite.

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