An Alternate Approach to Accelerated Spheroidization in Steel by Cyclic Annealing Atanu Saha, Dipak Kumar Mondal, and Joydeep Maity (Submitted September 30, 2009; in revised form February 6, 2010) In this work an annealed 0.6 wt.% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810 °C (above Ac 3 temperature) followed by cooling in a flowing air medium (flow rate: 6 m 3 /h). After 8 cycles (about 1 h and 20 min), the microstructure mostly contains spheroidized cementite and ferrite along with trace amount (3%) of pearlite. In addition to the diffusion within lamella, the disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac 3 temperature, and the generation of defects (lamellar faults) during non-equilibrium cooling in a flowing air medium are the main reasons of accelerated spheroidization. Keywords accelerated spheroidization, carbon steel, cementite dissolution, cyclic heat treatment, lamellar faults 1. Introduction The conventional spheroidization process of steel consists of a subcritical annealing treatment that takes a long time. In an annealed Fe-0.79 wt.% C alloy, the spheroidization remains incomplete even after 100 h of holding at 700 °C (Ref 1). In 0.72%C steel with fine pearlitic structure, isothermal annealing at 680 °C requires 72 h of holding for nearly 100% spheroidi- zation, whereas with coarse pearlitic structure, 50 h of holding only causes 50% spheroidization (Ref 2). Therefore, it has been a challenge for steel industry to accelerate the spheroidization of pearlitic structure. Since the isothermal annealing slightly below Ac 1 temperature is a long process for spheroidization, several other processes have been developed such as (a) thermal cycling near Ac 1 temperature, (b) isothermal annealing with the aid of prior cold work, (c) hot deformation before, during, or after the transformation of austenite to pearlite, and (d) decomposition of supercooled austenite at a temperature slightly below Ac 1 (Ref 3). The thermal cycling (swinging annealing) of ±5 °C around Ac 1 temperature facilitates the dissolution of cementite lamellae when temperature is raised above Ac 1 . At subsequent cooling below Ac 1 this dissolution process is interrupted and the broken cementite particles coagulate more easily and quickly (Ref 4). Baranova and Sukhomlin (Ref 5) observed that the prior cold working generates defects in the atomic crystal structure of both ferrite and cementite that affects mechanism and kinetics of pearlite spheroidization. Under the influence of previous cold work, during subcritical annealing of steel, the process of cementite separation into pieces is made easier. During subcrit- ical annealing, the cementite in worked steel is found to disintegrate along slip bands, grain, or subgrain boundaries of ferrite. Zhang et al. (Ref 6) applied a uniaxial compression at pearlitic transformation incubation temperature in order to accelerate spheroidization in steel. The microstructural evolu- tion process of the steel during deformation included pearlitic transformation, cementite spheroidization, and ferrite recrystal- lization. Two microprocesses of cementite spheroidization were observed. The first one was the dissolution and subsequent breakdown of cementite lamellae; whereas the other involved precipitation of finer cementite particles in the ferrite matrix during recrystallization of ferrite. In contrast, Zhu and Zheng (Ref 7) studied a direct spheroidization process during hot rolling of hypereutectoid GCr15 steel. The direct spheroidiza- tion is promoted by a combination of low deformation temperature and slow cooling rate that happens to augment the divorced eutectoid transformation reaction. Under such conditions, during phase transformation carbide particles are likely to grow independently on the pre-existing carbide ÔnucleiÕ and result in a poor carbon area to form ferrite. This generates a final microstructure consisting of a ferrite matrix and sphero- idized carbide particles. Another method of accelerated sphero- idization (known as Ôcomplete thermal cycling processÕ) involves austenitizing followed by rapid quenching to a temperature slightly above the martensite-start transformation temperature (M s ), finally up-quenching to a temperature slightly below the Ac 1 temperature for isothermal annealing. The supercooled austenite possesses large dislocation density and during subcritical annealing just below the Ac 1 temperature, these dislocations aid the nucleation of cementite particles. This causes a rapid spheroidization through the abnormal decompo- sition of austenite (Ref 3). Cyclic heat treatments are found to accelerate several solid state metallurgical processes. Sista et al. (Ref 8) applied a cyclic austempering technique to accelerate bainitic transfor- mation in 1080 steel. Sahay et al. (Ref 9) found an accelerated grain growth behavior in a cold rolled AlK-grade steel (0.05%C, 0.05%Al, and 45 ppm N) subjected to cyclic annealing Atanu Saha, NDT and Metallurgy Group, Central Mechanical Engineering Research Institute, Durgapur 713209 West Bengal, India; and Dipak Kumar Mondal and Joydeep Maity, Department of Metallurgical and Materials Engineering, National Institute of Technology Durgapur, Durgapur 713209 West Bengal, India. Contact e-mail: joydeep_maity@yahoo.co.in. JMEPEG (2011) 20:114–119 ÓASM International DOI: 10.1007/s11665-010-9653-x 1059-9495/$19.00 114—Volume 20(1) February 2011 Journal of Materials Engineering and Performance