International Journal of Engineering and Technology Volume 4 No. 5, May, 2014 ISSN: 2049-3444 © 2014 – IJET Publications UK. All rights reserved. 325 Mechanical Properties Enhancement of Conventional Mild Steel For Fastener Application O. I. Sekunowo 1 , S. O. Adeosun 2 and O. E. Ojo 3 1, 2 Department of Metallurgical and Materials Engineering, University of Lagos 3 Federal Institute of Industrial Research Oshodi (FIIRO), Lagos ABSTRACT Due to the rampant brittle fracture of most mild steel fasteners, an innovative hardening process of microstructure modificaton is employed on conventional mild steel to enhance its mechanical properties. The steel specimens were subjected to inter-critical austenitising heat treatment in the temperature range of 800 0 -950 0 C and soaked for 30 minutes, quenched in water at ambient temperature and tempered at varying temperatures of 250 0 , 350 0 , 450 0 and 550 0 C. Results of mechanical characterisation of the test specimens demonstrate improvement in ultimate tensile strength and impact energy by 58.3 and 53.8 percent respectively coupled with a modest increase in hardness from 25.7HRB to 37.5HRB. Contribution to improved mechanical properties stem from the inducement of fine needle-like lower bainite particles within the dislocation-rich ferrite matrix at 450 0 C and 550 0 C tempering temperature regime. Keywords: Mild steel, hardening, austenitising, mechanical properties 1. INTRODUCTION Mechanical joining of engineering components is commonly carried out using various types of materials. Besides welding and laser processes, fastening is another veritable joining technique depending on the service environment. Mild steel due to its availability coupled with a relatively low cost is often employed as metallic fasteners. However, the rampant brittle fracture of mild steel fasteners such as nails, screws, studs, etc on application of sudden load has become a growing concern (Teresa and Manuela, 2008). Various reasons have been adduced for the abysmal failure of mild steel fasteners in service. Hence, the imperative of devising a means by which the impact property and other relevant characteristics of conventional mild steel can be improved. Major impact toughness enhancement techniques that have been employed include alloying, heat treatment and most recently, impulse electric current treatment. Varied amounts of copper, titanium and nickel have been added to steel by quite a number of researchers to achieve improved impact toughness. In the work of Mone and Scott (2011), significant improvement in impact property was obtained as the copper content was increased from 0.2-1.5wt%. Enhanced impact toughness was noticed from 0.25% copper and peaked at 1.5wt%. Philip and Richard (1970) achieved improved toughness as the silicon content was increased. Zirconium, titanium and nickel have also been employed by other researchers. Although the alloying technique of toughness enhancement has been found to be remarkably effective, the rather prohibitive cost implication has rendered the method unpopular except in situations where the service benefits by far outweigh the cost burden. High impulse electric current technique (IECT) was employed by to increase the impact toughness of low carbon steel (Stepanov, et al., 2007). The high-density current passed through the specimen seemed to affect only the grain size and not the morphology. Therefore contribution to improved toughness was reported to have been due to the homogeneous dispersion of grains within the matrix occasioned by the passage of impulse current. However, it has been observed that IECT toughness enhancement has limited applications especially in structural components. The foregoing, properties optimisation techniques notwithstanding the heat treatment option of impact toughness improvement may be most versatile considering the volume of works carried out in that area. Heat treatment involves a cycle of heating and cooling that alters the material microstructures resulting in a modified mechanical properties. Of the various heat treatment methods, hardening through quenching followed by tempering has been established as the most effective method of enhancing the strength characteristics of metal and their alloy. According to Offor and Ezekoye (2011), improved notch impact toughness was exhibited by 0.14wt% carbon steel after being subjected to varied inter-critical (810 0 C-890 0 C) normalizing. Similarly, the hardening of HT 480 and HT 600 having 0.27wt%C (HT represents high tensile) and subsequent quenching in oil followed by tempering between 480 o C and 600 o C resulted in impact energy of 195J and 225J respectively (Muhammed; et al., 2011). One of the major processing parameters in hardening heat treatment is the tempering temperature. Rugly and Strohaecker (2004) worked on toughness improvement of low carbon steel independent of the austenitising temperature. The result established that a relatively high tempering temperature between 500 0 C and 650 0 C favours improved impact toughness. This was also corroborated by Danilo and Jaime, (2010) and further strengthened by the findings of Dosseth and Boyer, 2006 in which it was observed that tempering in the range of 250 0 C- 370 0 C should be avoided to stem the phenomenon of temper embritlement. Considering all of the above, the issues germane in the heat treatment of low carbon steel for mechanical properties enhancement bore down to the interplay of three parameters namely the austenitising temperature, cooling rate