Aging Behavior and Role of Mo Rich Zone on the Notch Toughness in 18%Ni Maraging Steels

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  • 18%Niマルエージング鋼の析出挙動と切欠靭性に及ぼすMo rich zoneの影響
  • 18パーセント Ni マルエージング コウ ノ セキシュツ キョドウ ト キリ

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Abstract

To investigate the effect of molybdenum rich zone on the notch toughness which was the most significant precipitates in age-hardening of 18%Ni maraging steels, the precipitation behavior during aging of these steels was initially studied by electrical resistivity change, and then the notch tensile strength as a measure of notch toughness was examined on specimens aged under appropriate conditions.<BR>It was found that the molybdenum rich zone precipitated only in the presence of cobalt on aging at lower temperatures. But the precipitation of this zone overlapped that of Ni3Mo which was predominantly formed by aging at higher temperatures.<BR>In the 18%Ni maraging steels containing titanium, the precipitation of the molybdenum rich zone and that of Ni3Mo scarcely overlapped each other on isothermal aging even at higher temperatures. But precipitates based on titanium were produced together with the molybdenum rich zone.<BR>When re-aging was conducted at lower temperatures after aging at higher temperatures, only the molybdenum rich zone was found to precipitate without overlapping other precipitates, although a small amount of nickel rich zone was formed at the begining of the re-aging.<BR>It was found from the tensile testing of notch specimens re-aged after aging at higher temperature that the formation of the molybdenum rich zone resulted in the degradation of the notch toughness in the 18%Ni maraging steel. The reason for it is that even the rich zone would be fine precipitates coherent with the matrix which was able to significantly affect the deformation process.<BR>However, one of the reasons why the 18%Ni maraging steels possess good toughness in spite of their much higher strength on the conventional aging would be that the molybdenum rich zone produced by the combination of cobalt and molybdenum does not consume nickel in the matrix, in contrast to precipitates based on other hardening elements consuming nickel in the matrix.

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