The Cementite Spheroidization Process in High-Carbon Steels with Different Chromium Contents

T he spheroidization annealing, which results in a microstructure of fine spherical cementite particles in a soft ferritic matrix,[ 1 ] is of significant interest for industrial applications. The main purpose of this process is to reduce the hardness of the material and thus provide a good machinability prior to further processing.[ 2 ] Two types of spheroidization annealing are often used. (1) Subcritical spheroidization below the A 1 temperature, which is mainly applied for hypoeutectoid steels. During subcritical annealing of steels with an initial pearlite structure, the cementite lamellae break up into spheroids driven by the reduction in surface energy.[ 3 , 4 ] (2) Intercritical spheroidization above the A 1 temperature, which is mainly applied for hypereutectoid steels in order to spheroidize and partially dissolve the grain boundary cementite. Therefore, the final microstructure of spheroidized hypereutectoid steels usually contains a bimodal distribution of cementite particles,[ 5 , 6 ] where large particles are located mainly on austenite grain boundaries. Numerical simulations of the intercritical spheroidization process in a high-carbon steel have shown that the rate of dissolution of the cementite lamellae also depends on the surrounding conditions.[ 7 ] This research will focus on the intercritical spheroidization annealing.