The Effect of Cementite Particles on Ultra-Fine Ferrite Grain Growth Observed in Low-Carbon Steel after Subcritical Ageing

O. Balancin1, A.M. Jorge Junior1, D.D. Coimbrão1, O.V. Silva Neto2 and J. Gallego3*

1 Department of Materials Engineering/UFSCar, São Carlos, Brazil 2 Department of Mechanical Engineering/UNIP, São José do Rio Preto, Brazil 3* Department of Mechanical Engineering/MAPROTEC/UNESP – Campus de Ilha Solteira, Brazil, gallego@dem.feis.unesp.br The Hall-Petch model has established the most important relationships between microstructure and mechanical properties, where was observed that grain refining processes are able to increase both yield strength and toughness. Better commercial steels have been made with an optimized combination between chemical composition and thermomechanical processing, which has got decrease the ferrite grain size to about 5μm. Ultra-fine ferritic grains (UFFG) could be achieved through warm severe plastic deformation of steels by following subcritical heat treatment [1]. This microstructure is thermodynamically stable if the ferritic grain growth could be inhibited by of an amount of small particles (pinning effect). In this work the effect of cementite particles on UFFG growth in commercial steel was investigated using transmission electron microscopy. UFFG microstructure was obtained in commercial low-carbon steel (0.16C; 1.34Mn; 0.46Si; %wt). The details of the thermomechanical treatment were published elsewhere [1]. The UFG microstructure was obtained after ageing at 958K during 0.5h, 1h, 12h and 24h and samples were prepared for TEM. Some 3mm diameter round have been punched into discs of 200μm in thickness, which were mechanically ground between 50 to 75μm. These discs were thinned in a twin-jet electro-polisher (Strüers Tenupol 3) using an acetic and 5% (v/v) perchloric acid electrolyte at 17°C. Thin foil observation was carried out in a Philips CM120 microscope operated at 120kV. Quantitative analyses were performed on digitized images with ImageTool freeware. TEM analysis have determinated that UFFG was obtained after ageing, as show the BF micrographs presented in Fig.1. The averaged ferritic grain size increased from 1.0μm to 1.4μm during longer ageing treatment. In all samples it was possible to identify cementite particles, whose distribution can be classified in two groups: fine globular (up to 10nm) and rod shape (up to 250nm length). Selected area diffraction patterns extracted from thin foils have shown a typical ring shape (Fig.2). These results confirm that both ferritic grains and cementite particles were fine distributed and have shown different crystallographic orientations. During ageing both phases presented similar normal coarsening behavior (Fig.3(a)), which was mainly controlled by Ostwald ripening of cementite. The effect of size (d) and volume fraction (f) of cementite on UFFG growth was compared with several models [2] and the Rios’ grain growth model (Fig.3(b)) presented better agreement with our experimental results [3]. References [1] J. Gallego et al., Materials Science Forum, vols. 558-559, (2007), pp. 505-510. [2] F. J. Humphreys., Acta Materialia, v. 45, (1997), pp. 5031-5039. [3] P. A. Manohar et al., ISIJ International, v. 38, (1998), pp. 913-924. [4] This research was supported by FAPESP and CNPq (Brazil).

FIG. 1. Typical thin foil bright field micrographs showing ultra-fine ferrite grains obtained after aging at 958K after 0.5h (a); 1h (b); 12h (c) and 24h (d). Magnification: 10,000X.

FIG. 2. Ring shape diffraction patterns extracted from same thin foil areas presented at Fig. 1. Aging at 958K for 0.5h (a), 1h (b), 12h (c) and 24h (d). Nominal camera length 16.58Ǻ.mm.

FIG. 3. Variation of ferrite grain and cementite particle size during aging treatment in (a). Pinning effect of cementite particles on ferrite grain size, showing good agreement with literature in (b).