EFFECTS OF CRYOGENIC TREATMENT ON MICROSTRUCTURE AND WEAR RESISTANCE OF Fe-0.35C-6.3Cr MARTENSITIC STEEL

The study is conducted to determine the effect of quenching and tempering processes on microstructural evolutions and abrasive properties of medium carbon-high chromium steel. For this purpose, Austenitizing was performed at the temperatures of 1000 degrees C for 15 min followed by oil quenching. To determine the optimum tempering temperature, tempering temperatures were selected in the temperature ranges of 350-550 degrees C. The samples cryogenically treated immediately after quenching in liquid nitrogen (-196 degrees C) for 24 hrs. Dry sand/rubber wheel test was used to evaluate the wear resistance properties. Microstructural observation, fractography and retain austenite was evaluated by optical and scanning electron microscopy and X-ray diffraction analysis. The results show that the best wear resistance can be obtained at the tempering temperature of 500 degrees C, due to the reduction of a tendency to micro-cracking, decrease in internal stresses and improvement of impact energy. Observation of the worn surfaces revealed that the wear mechanisms after tempering at 450 degrees C are a combination of abrasive, adhesive and fatigue wear. However, abrasive wear is the only active wear mechanism for specimen tempered at 500 degrees C. In addition, wear resistance of deep cryogenically treated sample was significantly increased (about 25%) by the removal of retained austenite and formation of uniform fine carbides distribution in the matrix.