Influence of solution treatment on the microstructure of a 9wt.%Cr-1wt.%Mo-0.07wt.%C steel

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Abstract

This paper discusses the influence of solution treatment on the morphology and chemistry of microstructural constituents in a 9wt.%Cr-1wt.%Mo-0.07wt.%C steel, solutionized in the temperature range 1273–1573 K and cooled at two different rates, namely 100 and 0.1 K s−1. The techniques employed are conventional light and electron microscopy and electron probe microanalysis. The temperature range of stability of the single-phase austenite field and the duplex-phase field consisting of austenite and δ-ferrite for the steel has been determined. The change in solution temperatures has a profound influence on the prior austenite grain size, the packet size of the product martensite and the amount of ferrite. Variations in the hardness levels with heat treatment condition relate to the synergistic effects of the change in the packet size of the martensite and that of the amount of ferrite. These features have an opposing influence on the hardness. Solute repartitioning takes place Between the two coexisting equilibrium phases above 1423 K, i.e. austenite and δ-ferrite. The magnitude of the repartitioning depends on the stabilizing effect of the individual solutes. In order to understand this feature of a new concept of the “microscopic chromium equivalent” of the constituent phases has been introduced. This concept also serves as an index to determine the extent of completion of equilibration.

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