Investigation of Austenitization in Low Carbon Microalloyed Steel During Continuous Heating
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enitization is one of the most important processing steps and significantly influences the final mechanical properties of a given steel. The austenitization process in general depends on the composition, initial microstructure, and processing parameters like heating rate, austenitization temperature, soaking time, etc.[1–4] The formation of austenite from a mixture of pearlite and ferrite can broadly be divided into two stages.[2–9] At first, pearlite begins to transform into austenite at Ac1 and it continues up to Ach. This is followed by proeutectoid ferrite to austenite transformation which is completed at Ac3. As discussed later, the second stage, i.e., ferrite to austenite transformation, can further be divided into two sub-stages. The corresponding equilibrium temperatures are known as Ae1, Aeh, and Ae3, respectively. Depending on the heating rate, there can be an overlap between these stages of austenite formation.[3] In the present work, studies using KAVITHA GUNABALAPANDIAN, RAVI RANJAN, and SHIV BRAT SINGH are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India. Contact e-mail: [email protected] SANTIGOPAL SAMANTA is with the Research and Development (R&D) Division, Tata Steel Ltd., Jamshedpur 831 001, India. Manuscript submitted October 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
a dilatometer were conducted to understand the different stages of austenite formation in a low carbon microalloyed steel during heating at different rates. In addition, the experimental dilatation curve at the slowest heating rate studied was compared with the calculated dilatation curve using the equilibrium phase transformation and lattice parameter data. Dilatometry is one of the experimental techniques that is frequently used to understand the phase transformation behavior in steels.[2–12] The dilatometer utilizes the fact that the change in length of a material is influenced by the temperature and phase transformation. An increase in the temperature generally leads to expansion of the material while phase transformation may result in an overall contraction or expansion depending on the phases involved. Any deviation from linear thermal expansion (or contraction during cooling) indicates transformation. The composition of the as-received hot-rolled steel plate used in the current study is given in Table I. Cylindrical samples, with 4 mm diameter and 10 mm length, for dilatometer tests were prepared using a wire EDM machine. All the experiments were performed in vacuum (5 9 104 mbar) or in inert atmosphere (during cooling cycle) in Ba¨hr 805 A/D dilatometer having a resolution of 5 lm/5 C. The temperature change was measured and controlled by a Pt and Pt-10 wt pct Rh thermocouple spot welded on the surface of sample, and argon gas was used to control the cooling rates. The samples were heated to 1173 K (900 C) at different heating rates (0.05, 0.5, and 5 K s1), followed by soaking at that temperature for 300 seconds and cooling to room
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