The Microstructure of High-Cr Ferritic Steels

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  • 高クロム-フェライト鋼の金属組織
  • コウクロム-フェライト コウ ノ キンゾク ソシキ

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High-Cr ferritic steels such as AISI Type 446 stainless steel may have a duplex structure of α+γ at elevated temperatures, depending on their chemical composition, which variously influences their high temperature physical and mechanical properties. In this study, the effects of alloying elements on the α/α+γ transformation and the behavior of “veining” structure in ferrite grains of high Cr steels were investigated, the results of which may be summarized as follows: (1) C and N lower slightly the α/α+γ-transformation temperature of Type 446 steel. Ni also lowers that transformation temperature, remarkably while Si raises it. (2) In low-C steels, the γ-phase increases notably and then decreases gradually as the temperature rises over the transformation temperature, while, in the high-C steels, the γ phase is not changed nearly at all by the temperature. (3) The linear relation between the effective Cr percent and the heating temperature as reported by Post and Eberly was not found but and a somewhat new relation was found. This inconsistency may be attributed to the change in diffusion velocity at elevated temperatures. (4) The “veining” structure observed in 25%Cr steels having more than 0.03% of C+N appears by water cooling from high temperatures, say over 1100°C. This structure is more clear as the heating temperature is higher and the heating time is longer but it does not appear after slow cooling. It is not visible near the ferrite boundaries but visible only inside of ferrite grains. It agglomerates by reheating at 550° to 650°C. It is proposed that the origin of the veining structure of high-Cr steels is precipitates of carbide and/or nitride produced by reduction of the solubility of C and/or N during rapid cooling from high temperatures or prestage of the precipitation accompanied by high internal stress.

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