Thesis Type: Doctorate
Institution Of The Thesis: Gazi University, Turkey
Approval Date: 2017
Thesis Language: Turkish
Student: Mücahit Kaplan
Supervisor: ADNAN AKKURTAbstract:
In this study, bainite + martensite dublex structures in AISI 52100 bearing steel were produced by heat treatment, its effects on mechanical properties, cutting forces and surface roughness were examined. In order to obtain the hardness value (~62 HRC) necessary for bearing applications, test specimens having 25 mm diameter and spheroidized starting microstructure were prepared from AISI 52100 steel. Heat treatment cycles were applied at different austenitizing and austempering temperature and time. In this study, after austenitizing at 950 ºC for 30 min., austempering at 275 ºC for 30 min. and quenching in water, aimed hardness value was achieved. Specimens achieved by this heat treatment give a 2253 MPa tensile strength and 19 J impact toughness values. Also from the same starting specimens different bainite volume fraction (BVF) (10%, 30, 40 and 50 BVF) has been achieved by austenitizing at 950 ºC for 30 min., then austempering at 275 ºC for different time periods (15 min. 24 hours) and water quenching. In order to examine the effect of processing parameters on cutting forces and surface roughness, hard turning was applied to specimens having four different bainite volume fraction, with fixed cutting depth (0,25 mm), 4 different cutting speeds of 60, 80, 100, 120 m/min and 3 different feed rates of 0.075, 0.1, 0.125 mm/rev. The value of feed force, radial force and main cutting force were decreased as there was an increase in BVF. The specimen having 10% BVF exhibited the highest radial component of cutting forces due to the very high ratio of fresh martensite and unstable retained austenite in the microstructure effect on strain hardening. In the specimens BVF 30, BVF 40 and BVF 50, the main cutting force was observed to be the highest and a decrease was observed in all the cutting forces depending on a decrease in hardness. The value of cutting forces was determined to be correlated with the phase volume fractions and morphologies rather than the hardness value of the specimens. No linear correlation was determined between BVF and surface roughness. The minimum value of average surface roughness (Ra) was 0.38 μm for feedrate of 0.075 mm/rev, BVF of 50% and cutting speed of 60 m/min. The maximum Ra was 1.98 μm for feedrate of 0.125 mm/rev, BVF of 10% and cutting speed of 120 m/min.