Investigating the effect of spheroidization heat treatments on the microstructure and impact toughness properties of high carbon powder metal steels

High carbon steels produced by powder metallurgy route were exposed to spheroidization heat treatments by different routes. It was aimed to produce fine dispersed sphere cementite in the microstructure of high carbon powder metal steels through these heat treatments. After spheroidization studies, microstructure, hardness and impact properties of high carbon steels were examined. For these studies, 0,9 % and 1,4 % natural graphite powder and 0,5 % zinc stearate as a lubricant were added separately to Höganäs NC 100.24 grade pure iron powder and they were directly mixed. Mixed powders were cold shaped in the single act die under 700 MPa pressing pressure at room temperature and were sintered at 1180 0C for 20 min under Argon gas atmosphere. Spheroidization operations by three different heat treatment cycles were applied to two different high carbon steels. To the first group specimens, spheroidization heat treatment at 705 0C under Ac1 temperature was applied. In the second spheroidization route, the specimens were held at 705 0C for different periods of time and then they were heated up to 735 0C and held at this temperature for different periods of time again and the same process was repeated between these temperatures to accelerate the spheroidization. In the first two heat treatments, spheroidization time was between 5 to 72 hours.In the last procedure, the martensitic transformations were obtained by quenching the specimens with 0,9 % and 1,4 % graphite austenitized at 850 0C and 950 0C for 6 min. respectively. Then, it was aimed to produce fine spherical cementite dispersion in the ferritic matrix by annealing the specimens at 505 0C, 605 0C, and 705ºC temperatures for 0,5 to 5 hours. Spheroidization time was longer for the specimens produced through the first two methods in comparison to the ones produced by the third way. According to XRD analysis results, transition carbides except cementite phase weren?t detected in all the specimens. At the end of these spheroidization treatments, it was understood that there wasn?t a direct relation between hardness and impact toughness; however, the toughness was more related to morphology and dispersion of cementite in the microstructure. Among the specimens, the highest hardness (?340 HV) was obtained in the specimen with 1,4 % graphite content produced through the third way at 505 0C temperature. The highest impact toughness (?34 j.cm-2) was provided in the specimens with 0,9 % graphite produced through the first two ways.