Investigation of Surface Hardness of Steels in Cyanide Salt Bath Heat Treatment Process S.P Ayodeji, T.E Abioye and S.O Olanrewaju Abstract - Carbonitriding of four types of steels, namely, low carbon steel (LCS), medium carbon steel (MCS), low alloy steel (LAS) and high alloy steel (HAS) was achieved using cyanide salt bath heat treatment process. The steels, after preheating, were heated in a diesel fired salt bath heat treatment furnace and then quickly quenched in different media including air, oil and brine. The heat treatment processes were performed at different temperatures ranging from 790 o C to 920 o C. The heat treatment time was varied from 30minutes to 120 minutes at an interval of 30 minutes so as to investigate its effect on the surface hardness of LCS. The surface hardness of all the heat treated steels samples were measured using an Indentec Rockwell hardness tester. The result revealed that the surface hardness of the heat treated steels is related to the heat treatment temperature and the quenching medium used. Also, the heat treatment time of LCS in salt bath furnace has a significant effect on the surface hardness of the steel. Keywords: Carbonitriding, Heat treatment temperature, Heat treatment time, Surface hardness. I. INTRODUCTION Steels owe their dominance of the field of engineering materials to their ability to provide appropriate properties at economic cost on a production scale. However, the low wear resistance and hardness properties of steels have restricted their use in the engineering field [1]. Existing established methods of improving the surface hardness of steels includes carburizing, nitriding, boronizing and carbonitriding or cyaniding [2], [3], and [4]. These methods change the chemical composition of steels, carburizing by the addition of carbon, nitriding by the addition of nitrogen, boronizing by the addition of borides and cyaniding by the addition of both carbon and nitrogen [2],[3]. Manuscript received January 07, 2010. S.P. Ayodeji is a senior lecturer in the department of Mechanical Engineering, The Federal University of Technology, Akure, PMB 704, Akure, Ondo-State, Nigeria. (phone: 002348036709782; e-mail: ayodejisesan@yahoo.com ). T.E Abioye is a lecturer in the department of Mechanical Engineering, The Federal University of Technology, Akure, PMB 704, Akure, Ondo- State, Nigeria. Currently, he is on study-leave for his PhD degree program in Manufacturing Engineering in the University of Nottingham, UK. (phone: +447596405052; e-mail: tennyjoy2003@yahoo.com ). S.O. Olanrewaju is a senior technologist in the department of Materials and Metallurgical Engineering, The Federal University of Technology, Akure, PMB 704, Akure, Ondo-State, Nigeria. Compared with carburizing and nitriding, Selcuk, Ipek and Karami [3] revealed that carbonitriding treatment is an effective surface hardening method for low carbon and low alloy cold working steels. Though boronizing is the most effective thermochemical treatment for all ferrous materials, the boronized layers are shallow and brittle [3]. Also, the presence of alloy element reduces the diffusivity of boron in the steel and consequently decreases the thickness of the borided layer [3]. Recently, cyanide salt bath heat treatment is preferred over other conventional methods of improving the hardness and wear resistance of steels because of the circulation of molten salt which provides uniform temperature throughout the charge giving accurate temperature control and uniform results, low surface oxidation and decarburisation, and most times parts treated in the salt bath have a clear bright-scale free finish [4]. More so, the inherent cracks and distortions of conventional heat treatment method are eliminated through controlled cooling in molten salt while rapid rate of heat transfer by conduction and convection modes enables a high output to be obtained from a relatively low investment and costs [4]. Specifically, cyanide salt baths offers case hardening and a quick turnaround of the workpiece due to carbon penetration being faster using this process [4]. The hard surface is developed by heating the steel above the austenitizing temperature in a suitable bath of cyanide salt while the carbon and nitrogen diffuse into the steel surface. Thereafter, the heat treated steel is quenched in mineral oil, water or brine. Extensive works had been done by many researchers on enhancing the surface hardness of different steels grades using diverse processes including carburizing, plasma nitriding, boronizing, cyaniding, etc [1], [2], [3], [4] and [5]. Nevertheless, this work aimed at investigating the surface hardness of carbonitrided low carbon steel (LCS), medium carbon steel (MCS), low alloy steel (LAS) and high alloy steel (HAS) in different quenching media. The effect of heat treatment temperature and time on the surface hardness of the steels was also determined. Proceedings of the International MultiConference of Engineers and Computer Scientists 2011 Vol II, IMECS 2011, March 16 - 18, 2011, Hong Kong ISBN: 978-988-19251-2-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) IMECS 2011