The classic work from this period is that of Greninger7) who determined M* in pure Fe-C alloys. The lowest carbon studied in this work was 0.20/0 C and it should be noted that extrapolation of Greninger's M, values to pure iron gives a value of 550~l O'C, Fig. 16(a). Fig. 9,40) was termed massive ferrite.

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An intercritical annealing step was introduced in the treatment of a PH13-8 stainless steel to improve the toughness of the alloy in aged condition. Four different treatment cycles, i.e. austenitisation (Q-treatment) and intercritical anneal (L-treatment), LQ, 2B (QLQL) and 2K (LQLQ) were carried out before aging treatment at 510°C for 4 h (the commercial H950 treatment). Optical and scanning electron microscopies, and Xray diffraction analysis were employed to study the microstructures of the alloys after different heat treatments. Hardness and Charpy impact strength of the samples were measured.Results show that significant grain refinement was observed after 2K and 2B treatment, but not after QL and LQ treatments. Such refinement of prior austenite grain did not lead to significant increase of hardness either before or after ageing. The Charpy impact strength of the alloy in aged condition was improved after the four pre-ageing treatments. The formation of a 'dual-phase' martensitic structure through intercritical annealing is thought to make the main contribution to the better toughness obtained, with beneficial effects also from grain refinement. All the four treatments offer better combined properties than the commercial treatment, whereas QL and LQ treatments may be cost-competitive. Relationships among heat treatment, microstructure and mechanical properties are discussed.KEY WORDS: stainless steel; toughness; grain refinement; intercritical annealing; heat treatment.Such grain refinement increased the strength of the alloy at room temperature, and a significant increase in ultimate tensile strength was observed at elevated temperatures. Recently Guo et al. reported that introducing an intercritical annealing step before the solution treatment of AerMet 100 (Fe-13.4Co-11.1Ni-3.1Cr-1.2Mo-0.23C in wt%), a high strength lath martensitic steel, may result in fine effective grain size.11) It should be noted that when maraging steels, i.e., martensitic precipitation hardening steels were studied, [7][8][9][10] attempts on grain refinement was through rapid austenitisation procedures, whereas studies on other alloys employed intercritical annealing in their thermal treatments. [2][3][4][5][6]11) In the latter cases, even when tempering treatments were included, 3,4) the purpose was to introduce thermally stable austenite but not to achieve precipitation hardening. The grain size was retained through the tempering step.3,4) It is not yet clear how the introduction of intercritical annealing will affect the precipitation kinetics and mechanical properties of precipitation hardening steels.The aim of the current research was to refine the grain size of PH13-8 stainless steel through the introduction of intercritical annealing treatment. The influence of thermal treatment on grain size, mechanical properties of the alloy before and after ageing, and precipitation kinetics was studied. The relationships among heat treatment, microstructure, and mechanical properties are also discussed. Experimental Procedures Alloy ...