Martensite Transformation During Continuous Cooling: Analysis of Dilatation Data
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RTENSITE transforms by a displacive or shear mechanism of transformation; the kinetics of the transformation are extremely rapid in a large number of alloys, including steels. The transformation in such cases is characterized as ‘‘athermal,’’ where the amount of martensite that forms is a function of the degree of supercooling below the martensite start (Ms) temperature only and does not depend on the time of holding at that temperature. Several empirical equations were proposed to describe the amount of martensite below the Ms temperature. Some of these used in this work for comparison are Koistinen and Marburger,[1] van Bohemen and Sietsma,[2] van Bohemen,[3] and Lee and Tyne[4]; these equations are identified here as KM, BS, B, and LT, respectively. Though these equations were formulated for a restricted range of composition and other variables, such as grain size and cooling rate,[5] they are very useful and are used extensively to understand the evolution of
RAVI RANJAN and SHIV BRAT SINGH are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, 721302 Kharagpur, India. Contact e-mail: [email protected] Manuscript submitted February 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure and to optimize the processing parameters in advanced high-strength steels, such as quenching and partitioning steels and transformation-induced plasticity (TRIP)-aided steels,[6–26] where retaining an optimum amount of austenite in the final microstructure is of prime importance to exploit the TRIP effect. In the current work, the length change or dilatation associated with martensite transformation was used to estimate its amount as a function of the temperature using a novel method developed recently,[27,28] and the results were compared with the empirical equations that describe the athermal martensite transformation referred to previously.[1–4] The lattice parameters of a given phase are usually expressed as a linear function of the concentration of alloying elements, where the contribution of each solute atom is quantified by a coefficient.[29–39] The lattice parameter coefficient is not known for aluminum (Al) in martensite. The present approach[27,28] was extended to determine this value, and it is compared with the value obtained through the Bain model of martensitic transformation.
II.
EXPERIMENTAL
Two steel compositions, namely, Si-steel and Al-steel, were selected for the current work.[6,7] The composition
of the two steels is given in Table I. The as-cast materials of these steels were first cut to appropriate sizes (100 9 20 9 20 mm3) and soaked at 1473 K (1200 C) for 2 hours in a muffle furnace.[27] After soaking, these materials were hot forged in the temperature range of 1373 K to 1473 K (1100 C to 1200 C), giving 50 pct reduction in thickness and then air cooling to room temperature.[27] Solid cylindrical samples with diameter of 4 mm and length of 10 mm were prepared from the as-forged material for experiments in a Quenching and Deformati
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