3D FINITE ELEMENT SIMULATION OF STEEL QUENCHING IN ORDER TO DETERMINE THE MICROSTRUCTURE AND RESIDUAL STRESSES A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY BY CANER ŞİMŞİR IN PARTIAL FULFILLMENT OF REQUIREMENT THE DEGREE OF DOCTOR OF PHILOSOPHY IN METALLURGICAL AND MATERIALS ENGINEERING FEBRUARY 2008