Effect of titanium on the boronizing behaviour of pure iron Yucel Gencer a, ⁎, Mehmet Tarakci a , Adnan Calik b a Gebze Institute of Technology, Department of Materials Science and Engineering, Gebze/Kocaeli, 41400, Turkey b Suleyman Demirel University, Technical Education Faculty, Isparta, Turkey abstract article info Article history: Received 28 February 2008 Accepted in revised form 16 July 2008 Available online 22 July 2008 Keywords: Boronizing Titanium boride Fe–Ti alloys Iron boride Microhardness The effect of titanium on the boronizing behavior of pure iron with additions of 2, 5 and 10 wt.% titanium is reported. Pack boronizing of pure iron and the Fe–Ti alloys was carried out at 1100 °C for 3 h and the microstructure and the types of borides formed on the surface of pure iron and the Fe–Ti alloys substrate were studied by optical microscopy, SEM, EDS and XRD. Microhardness measurements of the substrates and boride layer formed on the substrates were also carried out. The microhardness of the boride layers were approximately 1774 HV and 1900–2220 HV for pure iron and the Fe–Ti alloys, respectively. A single boride layer of Fe 2 B with saw-tooth morphology was obtained on the pure iron while a double boride layer of FeB, Fe 2 B with TiB 2 as precipitates and a transition zone with TiB 2 were found on the Fe–Ti alloys. The saw tooth- like morphology changed to a compact morphology of the boride layer on Fe–Ti alloys with increasing amount of Ti. TiB 2 phase developed in the form of precipitates with different geometrical shapes. The volume and size of TiB 2 precipitates increased with increasing Ti content. The TiB 2 precipitates were distributed finely in the boride layers, but were coarser in size and with a relatively higher volume fraction in the transition zone. The thickness of the boride layer decreased exponentially with addition of Ti. The average thickness ranged between 68 μm and 320 μm for Fe–10 wt.% Ti and for pure iron respectively. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Boronizing is an important surface modification method used to enhance the surface hardness, wear resistance and corrosion resistance of a variety of metallic materials. It is widely applied to ferrous materials and the boronizing behaviour of various steels has been studied extensively. Generally single Fe 2 B phase coatings or coatings also containing FeB phases are formed on the surface of borided steels [1–10]. Experimental studies have been performed on the boronizing of pure iron [11,12] and although the effects of alloying elements on the boronizing of steel are recognized, the individual influences of alloying additions on the boronizing mechanism of steel are not clear. It is reported that the morphology, growth, phase composition, and consequently microhardness and thickness of the resultant boride layer were affected by the alloying elements in the steels [6,7,9,13–19]. The mechanical properties of the borided materials are strongly depending on the chemical composition and the structure of the boride layers [13]. The effect of alloying elements on boronizing behavior of substrate material is a complex issue and they can effect the properties of boride layers formed by various ways when there are more than one alloying element present. In steels, the alloying elements may enter the boride coatings, thus modifying their properties by substituting for iron borides. They may also form distinct particles within the iron boride layers or they can produce a separate continuous boride layer [16]. The effects of carbon, chromium, nickel, silicon on boronizing behaviour of steels have been reported [7,9,13– 19]. Chromium either enters iron borides or accumulates at the interface between the boride coating and steel and also forms a distinct CrB boride layer. Nickel concentrates underneath the boride coating and enters the Fe 2 B phase [16]. Chromium and nickel reduce the boride layer thickness and flattens out the saw-tooth configura- tion which is generally observed in low carbon steels [16–19]. Carbon is relatively insoluble in the boride layer and diffuses away to the matrix to form a boroncementite zone between coating and matrix [13,16]. The behavior of silicon is similar to carbon and it forms ironsilicoborides beneath the boride coating [7,13]. Determination of the individual effects of alloying elements on the boronizing behaviour of pure iron is a convenient way of assessing the likely behaviour of ferrous alloys. The effect of the nickel and chromium on the boronizing behavior of pure iron has been reported [20]. It was shown that the boride layer thickness was decreased and the hardness was increased by both nickel and chromium, with chromium being the most effective. Titanium is an important alloying addition to steels, especially because of its grain refining effect [21] and its ability to form carbides and nitrides [22]. In hot formed or continuously-cast steels, a small amount of titanium is effective as a grain-refiner because the grain growth of recrystallized austenite is retarded by the formation of titanium nitride [22]. Furthermore, when titanium is used in conjunction with boron, it increases the effectiveness of the boron on the hardenability of steel [22]. It is also well known that a titanium– boron compound like titanium boride has very high hardness, high Surface & Coatings Technology 203 (2008) 9–14 ⁎ Corresponding author. Tel.: +90 262 60517 80; fax: +90 262 653 84 90. E-mail address: gencer@gyte.edu.tr (Y. Gencer). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.07.009 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat