The cementite spheroidization process is investigated in hypereutectoid steels with different chromium (Cr) contents. A spheroidized structure in high-carbon steel is usually obtained by a divorced eutectoid transformation (DET) reaction, which occurs during slow cooling of austenite with fine cementite particles. A bimodal distribution of spheroidized cementite particles is experimentally observed. It is shown that Cr addition allows one to obtain the spheroidized structure after austenitization at a higher temperature and a longer annealing time as compared with low-Cr steel. It is found that the DET reaction takes place at low undercoolings compared with the pearlitic reaction. The occurrence of the DET reaction also depends strongly on the spacing between cementite particles during the austenitization process, i.e., on austenitization temperature and time.

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

The spheroidizing behavior of cementite in Fe–0.8C mass% steel during cyclic heat treatment was investigated. The effect of cyclic numbers on the microstructure and mechanical properties of the experimental steel was discussed. In the present work, each cyclic heat treatment consisted of a short-duration (5 min) held at 1043 K (above A1 temperature 1003 K) and a short-duration (3 min) held at 953 K. The spheroidizing ratio of cementite and the average size of particles grow with the increase of cyclic numbers. After five cyclic heat treatments, the cementite spheroidizes completely, the average size of cementite particles is about 0.49 μm and the larger particles ratio is below 4%. After cyclic heat treatment, the plasticity of the experimental steel clearly increases. The elongation of the as-transformed specimen is about 8%, the elongation of the 1-cycle specimen is 25.4%, and the elongation of the 5-cycle specimen reaches a peak value of 33.9%. The ultimate tensile strength (UTS) of as-transformed specimens is about 680 MPa, and the UTS of the 1-cycle specimen decreases to about 620 MPa; however, the UTS of the 3-cycle specimen increases to about 650 MPa. The ratio of yield strength to tensile strength is about 0.70 after cyclic heat treatments.

Effect of cyclic heat treatments on spheroidizing behavior of cementite in high carbon steel

The effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels has been investigated on the basis of microstructural analysis and thermodynamic calculations. The results showed that an increase of silicon content in high carbon chromium bearing steels retards the spheoridization of cementite. The thermodynamic calculations revealed that the shrinkage of the austenite phase field in bearing steels with increasing silicon content gave rise to an increase of volume fraction of cementite at an annealing temperature, possibly resulting in incomplete spheroidization. Furthermore, due to the low solubility of silicon in cementite, an increase of silicon content can raise the activity or chemical potential of carbon atoms in austenite at the austenite/cementite interfaces. Consequently, the difference in chemical potential of carbon atoms at the interfaces would be reduced with increasing silicon content, causing a decrease of the driving force for their diffusion from cementite to austenite.

Effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels

Warm deformations have been applied to a low-alloy medium carbon steel (AISI 5140) to promote faster spheroidization during soft annealing treatments. The application of warm deformation leads to the fragmentation of cementite lamellae and the formation of defects on both cementite and the matrix. This induces faster lamellae break-up according to a boundary splitting mechanism, which is responsible for the improved spheroidization after annealing. The substructure developed in the matrix enhances pipe diffusion through the sub-boundaries, which helps the lamellae terminations to coarsen and causes lamellae fast splitting and finally yields a coarse cementite particle distribution. When deforming up to e = 0.3, almost fully spheroidized microstructures are obtained after annealing at 993 K (720 °C), independently of the initial pearlite features. By means of the EBSD technique, it has been observed that the applied warm deformation, in addition to enhancing the degree of spheroidization, allows a much finer microstructure to be formed after annealing. Grain refinement takes place as a consequence of a continuous recrystallization process, which is directly related to cementite spheroidization in the long term.

Influence of Prior Warm Deformation on Cementite Spheroidization Process in a Low-Alloy Medium Carbon Steel

The fatigue characteristics of bearing steel in ultrasonic fatigue tests are investigated with regard to the role of spheroidized carbides. The results show that, despite a similar hardness level, the bearing steel with spheroidized carbide has a much longer fatigue life than steel without, and that the failure mechanism differs in each case. All the samples of SAQT-processed steel examined using ultrasonic fatigue tests were characterized by a conventional fish-eye structure, which implies they went through subsurface-originated or interior inclusion-induced failure. On the contrary, it was shown that surface-induced failure prevailed in the QT-processed steel without spheroidized carbide. The results suggest spheroidized carbide may strengthen the tempered martensitic matrix of the bearing steel and concentrate the stress around an inclusion rather than at the surface, allowing for prolonged fatigue life over the steel without spheroidized carbide.

Role of spheroidized carbides on the fatigue life of bearing steel

The spheroidization of cementite at subcritical and intercritical temperatures was studied quantitatively by evaluation of a shape factor for the cementite particles. It was found that the degree of spheroidization, as determined by the values of the shape factor, was markedly accelerated by treatments consisting in either (1) subcritical annealing after cold deformation or (2) intercritical annealing followed by subcritical annealing. Nevertheless, some differences were found among the microstructures resulting from the above treatments, which were related to the mechanisms of cementite formation in each of them. Also, the kinetics of austenitization during intercritical annealing was found to be accelerated by previous deformation, and the analysis of the formation of austenite at intercritical temperatures in terms of an Avrami equation was consistent with an "effective" saturation of ferrite-pearlite boundaries with austenite nuclei, followed by a planar mode of growth into the pearlite nodules.

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D. Hernández-Silva

Papers

The Spheroidization of Cementite in a Medium Carbon Steel by Means of Subcritical and Intercritical Annealing