Microstructure and hardness studies of electron beam melted surface of mild steel M. Ahmad a, *, M.A. Haq a , Ejaz Ahmed a , G. Ali a , J.I. Akhter a , M. Iqbal b a Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan b National Institute of Lasers and Optronics, Nilore, Islamabad, Pakistan 1. Introduction Electron beam (EB) melting and welding is gaining lot of interest in nuclear, chemical and aerospace industries due to its ability of localized melting and high cooling rate during solidification [1]. This technique has been successfully used for surface modification of the materials by adding foreign solute elements or ceramic dispersion reinforcement. These types of surface modifications can lead to the formation of alloyed surface layer (solid solution or bi-phased structure) or surface composite. The composite microstructure helps to improve the hardness and wear resistance of the surface layer [2] depending upon the volume fraction, size, shape and crystal structure of the incorporated phases/reinforcement [3]. SiC is an excellent ceramic material which is widely used in high temperature structural applications and as reinforcement for improvement of mechanical properties of materials [4]. Ahmad et al. [5,6] successfully enhanced the mechanical properties of Zircaloy-4 and Hastelloy C-276 with the addition of SiC and EB melting. Ahmad et al. [7] also noticed an increase in the surface hardness and change in lattice parameter by EB treatment of Hastelloy C-276. Mild steel (MS) is widely used all over the world as construction material and also as minor components to sophisticated industrial devices because it is very cheap and malleable. Corrosion and mechanical properties of the surface layers are generally enhanced by surface coating with corrosion resistant as well as harder materials. However the adhesion of the coating materials with substrate is not strong and can be peeled off very easily. These properties of MS can be enhanced by adding a hard reinforcement in the matrix or alloying the surface with suitable elements under EB and Laser beam (LB) melting. The surface composite of MS was formed by addition of TiC under laser melting and an increase in hardness was reported [8]. Alloying of surface layers with suitable elements in required content can help in increasing the surface properties like wear, corrosion resistance etc. Addition of Ni can play very important role in the modification of the microstructure due to its characteristics of stabilizing the FCC (g) structure [9]. It also enhances the corrosion resistance against the harsh environment [10] by forming NiO 2 that acts as a barrier layer for further oxidation. Ni is also famous for the refinement of grain structure and helps to reduce the formation of cementite [11] in steel. The introduction of the martensitic phase in the matrix of steel has been reported to enhance the hardness [12]. It is well established fact that supercooling is required to produce martensitic structure in carbon steels which introduces large tensile stresses and can cause cracking. When the steel is alloyed with g stabilizer such as Ni, martensitic phase can be introduced at lower undercooling temperature. There is no report available in the literature on the surface modification of MS by the formation of surface composite with the addition of SiC particles as well as surface alloying with Ni under EB melting. The aim of the present study is to modify the surface of MS by adding SiC and Ni under EB melting to produce composite as well as to introduce the martensitic phase to enhance the surface properties. Applied Surface Science 255 (2009) 6721–6723 ARTICLE INFO Article history: Received 20 January 2009 Received in revised form 20 February 2009 Accepted 22 February 2009 Available online 4 March 2009 Keywords: Electron microscopy X-ray techniques Hardness Composite materials ABSTRACT Electron beam surface melting of mild steel with the addition of Ni and SiC is carried out to improve its surface properties. Microstructure of the electron beam molten surface is characterized by scanning electron microscope. Phases are determined using energy dispersive spectroscopy and X-ray diffraction techniques. During electron beam melting SiC dissociated partially, interacted with liquid alloy and resulted in martensitic phases after solidification. Maximum hardness is achieved in electron beam molten zone. It is concluded that the formation of martensitic phase and the presence of Si and Ni in the solid solution are responsible for increase in hardness in the molten zone. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +92 51 2207227; fax: +92 51 9290275. E-mail address: maqomer@yahoo.com (M. Ahmad). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.077