Effect of spheroidizing heat treatment on the microstructure, hardness and toughness of high carbon powder metallurgy steel

Microstructure and toughness properties of high carbon powder metal steels, to which spheroidizing heat treatments were applied by using three different methods, were investigated. For this purpose, 1.4 wt.% natural graphite was added to NC100.24 pure iron powders. They were compacted under 700 MPa pressure in the dimensions of 5 x 10 x 55 mm(3) at room temperature in a mold in accordance with ASTM E-23 standard, and then they were sintered at 1180 degrees C under shielding argon gas atmosphere. In sintered samples, primary cementite + lamellar pearlitic structures were produced. After sintering process, spheroidizing process was applied by using three different methods. First one of these methods was to expose samples to conventional spheroidizing heat treatment for 5-40 h at 705 degrees C below A(c1) eutectoid temperature. By using the second method, the samples were subjected to spheroidizing process repetitively for 5 and 40 h in the temperature range of 705 and 735 degrees C below and above the A(c1) eutectoid temperature. Thirdly, the samples quenched at 950 degrees C were over-tempered for 0.5-5 h at 505 and 705 degrees C. It was understood that at the end of the spheroidizing process, there was no direct correlation between toughness and hardness but toughness was more correlated with the cementite distribution and morphology in the microstructure. After quenching, the spheroidized samples showed more superior toughness and hardness properties by using the over-tempering method.