Effect of Heat Treatment on Magnetic Properties of EN8D Steel Used in Automotive Applications

Abstract

The present study focuses on achieving the optimum combination of mechanical and magnetic properties for EN8D steel. SAE 1010 and EN8D steels are the candidate materials to manufacture the fixed nucleus of the disk-type electro-mechanical horn. Both materials have either good magnetic or mechanical properties. The main function of fixed nucleus is to get magnetized and pull the mobile nucleus towards it. However, as magnetization and demagnetization take place in the range of 300-500 Hz, fixed and mobile nucleus continuously strikes each other at high frequency. Due to such high frequency working condition, fixed nucleus is prone to failure under wear and tear or under fatigue. For this kind of application, a combination of magnetic and mechanical properties is required in the material used to manufacture fixed nucleus. Thus, in current paper, the effects of chemical composition, microstructure, hardness, strength and toughness of EN8D steel before and after austenizing and tempering heat treatment are evaluated and compared with as-rolled EN8D and SAE 1010 steels, which are mostly used for manufacturing fixed nucleus. Magnetic permeability was evaluated using JMatPro software and compared. Austenizing treatment is performed at 900 °C for 1 hour, followed by water quenching. Tempering treatment of the same was performed at 550 °C for 2.5 hours. The Ultimate Tensile Strength (UTS) and hardness of EN8D after heat treatment decreased from 917 to 781 MPa and 269 to 233 HV0.1 respectively. Charpy V-Notch (CVN) toughness of EN8D steel after heat treatment increased from 12 to 45 joules. It indicates that the decrease in strength and hardness resulted into increase in toughness. Microstructure changes from ferrite and pearlite to combination of tempered martensite, ferrite and bainite. Magnetic permeability evaluated by JMatPro software shows increment from 450 to 564 Gauss/Oersted in EN8D steel after heat treatment.
 Keywords: Electro-mechanical horn, fixed nucleus, magnetic permeability, JMatPro, SAE1010 steel, EN8D steel