The precipitation process of Fe-12Cr-7Ni-4Mo-0.7Ti steel has been studied to make clear the behavior of the embrittlement produced by aging on maraging stainless steels.
It was identified from the measuring the changes in electrical resistivity that the two types of metastable precipitations P
1 and P
2, tentatively named, were present at low aging temperatures.
From the results of derivatives of resistivity changes obtained by low temperature aging after high temperature aging, P
1 seems to be precipitate contained predominantly the solute atoms other than Mo, and P
2 to be Mo rich precipitate.
The embrittlement occurred after some incubation time when aged at low aging temperatures, regardless of the heat treatment condition before the low temperature aging.
The activation energy for precipitation reaction determined from resistivity measurements was about 180 kJ/mol for the P
1 precipitation and 250 kJ/mol for the P
2. The activation energies conformed to the values determined from the embrittlement behavior.
It is inferred from the activation energies that the embrittlement is rate controlled by P
2 precipitation, although P
1 precipitation takes place in an earlier stage than P
2 precipitation when the low temperature aging is performed immediately after solution treatment. However, the embrittlement is rate controlled by P
1 precipitation when the low temperature aging is performed after the high temperature aging.
It has become clear that the precipitation of P
1 is essential for the embrittlement, while the notch toughness depends strongly on hardness and Mo content in matrix before or after P
1 precipitation. This may be the reason that the rate controlling process of the embrittlement differs with heat treatment before low temperature aging.
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