Examining Effect of Speheroidization Heat Treatments on Impact Toughness of High Carbon Sintered Steel

Spheroidization heat treatments by two different routes were performed on the high carbon powder metallurgy steels. For this purpose, 0.9 % in wt. natural graphite was admixed to NC100.24 pure iron powder and compacted in 5x10x55 mm(3) size under 700 MPa pressure at room temperature in a die congruent with ASTM E-23 standard and then sintered at 1180 degrees C under protective gas atmosphere. Firstly, spheroidization proces of lamellae cementite phases was practiced at 705 degrees C for 5 hours below A(c1) eutectiod temperature in the sintered parts. In the second sphreoidization route, the sintered specimens were firstly austenisated at 850 degrees C for 6 min and then immediately quenched in water medium at room temperature to obtain fully martenzitic structure in the microstructure. Then, to produce fine spherical cementite dispersion in the specimens having martenzitic structure, the specimens were annealed at 505 degrees C and 705 degrees C temperatures for 5 hours separately. 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. In terms of hardness and impact toughness, properties of specimens spheroidizated from martenzitic structure were higher in comparison to classical spheroidization route.