Applied Surface Science 258 (2012) 7348–7353 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Microstructural study of surface melted and chromium surface alloyed ductile iron M. Heydarzadeh Sohi a,∗ , M. Ebrahimi b , H.M. Ghasemi a , A. Shahripour a a School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran b Department of Mining, Metallurgy and Petroleum Engineering, Amirkabir University of Technology, P.O. Box 1875-4413, Tehran, Iran a r t i c l e i n f o Article history: Received 17 October 2011 Received in revised form 2 April 2012 Accepted 2 April 2012 Available online 25 April 2012 Keywords: TIG Ductile iron Surface melting Chromium surface alloying a b s t r a c t In this study, ductile iron was surface melted and chromium surface alloyed via pre-placing of fer- rochromium powder with different thicknesses and subsequently surface melting by tungsten inert gas (TIG) process. Optical and scanning electron microscopy, as well as micro-hardness testing and X-ray diffraction analysis were used for characterization of the treated samples. Surface melting and chromium surface alloying resulted in formation of ledeburitic structure and high chromium white cast iron in the treated layers, respectively. It was also noticed that hardness of the treated layers was considerably higher than that of the base material. Increasing thickness of ferrochromium layer increased the amount of M 7 C 3 carbides and hardness of the alloyed layer. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ductile iron is widely used in industry because of their good combination of strength and toughness, low cost, good castability, excellent machinability, as well as good mechanical strength and wear resistance. However, under severe conditions the wear resis- tance of the ductile iron decreases [1–4]. Surface hardness is one of the most important factors in wear resistance. Surface melting and surface alloying are relatively new trends in surface treatment of ductile iron hardening [5,6]. In surface melting, a surface layer of the sample melts and, consequently, solidifies in a short time to form a microstructure like white iron that has high hardness [7,8]. Surface alloying is accomplished by adding the alloying elements such as chromium, titanium, molybdenum, vanadium, and tungsten before or simultaneously with surface fusion so that hard carbides of alloy- ing elements could form [9]. Among alloying elements for surface hardening of ductile iron, chromium has attracted more attention than other elements [3,6]. So far, surface melting and alloying of ductile iron have been mainly carried out by using of laser and elec- tron beams. Tungsten inert gas (TIG) heat source also has potential to be used for these treatments [10–12]. This research aims to study surface alloying of ductile iron by pre-placing of ferrochromium powder on ductile iron specimen and subsequently TIG surface melting of the specimen. ∗ Corresponding author. Tel.: +98 21 82084077; fax: +98 21 88006076. E-mail address: mhsohi@ut.ac.ir (M.H. Sohi). 2. Experimental procedure In this research, rectangular samples (75 mm long, 60 mm wide and 10 mm thick) were machined from the pearlitic-ferritic duc- tile iron with 35% ferrite cast via the Y-block method. Chemical composition of the ductile iron is shown in Table 1. Surface alloying was initially carried out by preplacing 0.5–3 mm thick layer of ferrochromium powder containing 65% chromium. The powder was mixed with a small amount of sodium silicate to keep the powder on the surface under the flow of argon during surface melting. The pre-coated specimens were then dried in a furnace at a temperature of 200 ◦ C for 2 h. MERKLE TIG 200 AC/DC unit in direct-current electrode-negative (DCEN) was used for surface treatment. A 2% thorium oxide tungsten electrode of 2.4 mm diameter and a constant distance 3 mm from the surface of the specimens was used to produce a stable arc. Surface melting and alloying were carried out at current intensity of 130 A and 180 A, respectively. The voltage was kept at constant value of 15 V for both treat- ments. Pure argon (with purity of 99.999%) was used as shielding gas. In order to increase the heat sink through the specimens, they were mounted on a brass back up with the dimension of 120 mm × 60 mm × 20 mm. After surface treatment, the microstructures of the treated sur- faces were studied using optical and scanning electron microscopes (SEM) equipped with energy dispersive spectroscopy (EDS) ana- lyzer. Philips Xpert X-ray diffractometer with Cu K ( = 1.5418 ˚ A) was used to analyze the phases formed in the treated surfaces. The surface melted specimen was etched with nital (2%), whereas, the 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.014