Complex Precipitates of TiN-MCx in GCr15 Bearing Steel

metals Article Complex Precipitates of TiN-MCx in GCr15 Bearing Steel Qianren Tian 1 , Guocheng Wang 1,2,3, *, Xinghu Yuan 1 , Qi Wang 1,2 and Seetharaman Sridhar 3 1 2 3 * School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China; (Q.T.); (X.Y.); (Q.W.) Key Laboratory of Chemical Metallurgy Engineering Liaoning Province, University of Science and Technology Liaoning, Anshan 114051, China Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA; Correspondence: Received: 7 May 2019; Accepted: 31 May 2019; Published: 3 June 2019



 Abstract: Nitride and carbide are the second phases which play an important role in the performance of bearing steel, and their precipitation behavior is complicated. In this study, TiN-MCx precipitations in GCr15 bearing steels were obtained by non-aqueous electrolysis, and their precipitation mechanisms were studied. TiN is the effective heterogeneous nucleation site for Fe7 C3 and Fe3 C; therefore, MCx can precipitate on the surface of TiN easily. The chemistry component of MCx consists of M3 C and M7 C3 (M = Fe, Cr, Mn) and Cr3 C2 . TiN-MCx with high TiN volume fraction, TiN forms in early stage of solidification, and MCx precipitates on TiN surface after TiN engulfed by the solidification advancing front. TiN-MCx with low TiN volume fraction, TiN and MCx form in late stage of solidification, TiN can not grow sufficiently and is covered by numerous precipitated MCx particles. Keywords: non-aqueous electrolysis; TiN-MCx ; precipitation; bearings; high carbon chromium bearing steel 1. Introduction Controlling microstructures and second phase in steel plays a vital role in the quality of steel. Carbide (M3 C, M3 C2 , M7 C3 , M = Fe, Cr, Mn) [1,2] and TiN inclusion [3,4] are common second phases in high carbon chromium steel. As a result of good wear resistance and solid solubility with alloy elements (Cr, Mn) [5,6], carbide can retain good mechanical properties of bearing steel during heat treatment [7,8]. Recently, utilization of inclusions has become attractive to improve steel performance. TiN is more harmful to bearing steel than Al2 O3 in the same size [9]. Many studies have investigated TiN and Al2 O3 , MgAl2 O4 and MnS, and NbC complex precipitation with inclusion [10–13]. Our previous study [14] found that TiN inclusion was covered by carbide in the etched GCr15 bearing steel metallographic specimens. It is necessary to observe their three-dimensional (3-D) morphologies in steel because the two-dimensional (2D) nature of the particles cannot reflect their real morphologies. The non-aqueous electrolysis extraction of second phase from steel is an effective method to study its 3D morphologies and composite interfaces. Fang and Ni [15] studied the behaviors of rare earth dissolved in α-Fe, Fe-Ce intermetallic compounds and rare earth inclusions via non-aqueous electrolysis. Bi et al. [16] analyzed 3D morphology, quantity, and chemistry of inclusion in ferroalloys by the electrolysis method. Wang et al. [17] observed Al2 O3 -MnO-SiO2 (-MnS) inclusion in steel by non-aqueous electrolysis. Zhang et al. [18,19] analyzed Ca-Mg spinel in cord steel and MnS in heavy rail steel by the electrolysis. Zhang et al. [20] studied the suitable electrolytic conditions for 16MnCrS5 steel. In this study, 3D morphologies of the carbide (MCx , M = Fe, Cr, Mn) and TiN-MCx precipitates extracted from GCr15 bearing steel specimens by the non-aqueous electrolysis were observed by Metals 2019, 9, 641; doi:10.3390/met9060641 www.mdpi.com/journal/metals Metals 2019, 9, 641 2 of 12 field emission scanning electron microscope-energy dispersive spectrometer (FESEM-EDS). The MCx chemistry component was confirmed by X-ray diffraction (XRD) and FactSageTM phase diagram calculation. The precipitation mechanism of TiN-MCx with different volume fraction in GCr15 bearing steels was elucidated. 2. Experiment 2.1. Chemical Components Analysis The chemical compositions of GCr15 bearing steel produced by the basic oxygen furnace (BOF)-landle furnace (LF)-vacuum degas (VD)-continuous casting (CC) process in a foundry were determined by direct-reading spectrometer (Model: ARL-3460 Optical Emission Spectrometer, Thermo Fisher Corporation, Waltham, MA, USA). The total oxygen and total nitrogen contents were analyzed using a nitrogen-oxygen analyzer (Model: TC-600, LECO Corporation, St. Joseph, MI, USA). The chemical compositions of the GCr15 bearing steel are shown in Table 1. Table 1. Chemical Compositions of GCr15 Bearing Steel (in mass percent). Composition C Si Mn P S Ti Cr V N Alt Ca Concentration 1.01 0.25 0.36 0.012 0.0014 0.0078 1.46 0.0099 0.0049 0.012 <0.005 0.0009 O (T) 2.2. Non-Aqueous Electrolysis and XRD Detection The non-aqueous electrolysis method was used to extract TiN-MCx particles from the GCr15 bearing steel. Samples with diameter of 10 mm and height of 100 mm were used as anode and copper as cathode. The electrolyte consists of 1% tetramethylammonium chloride, 5% triethanolamine, 5% glycerol, and 89% anhydrous methanol (in volume percentage). The constant voltage direct current (DC) power supply (model: DH1720A-1) was used to keep the current density between 40–60 mA/cm2 . The temperature of the electrolyte was kept at 268–278 K (−5–5 ◦ C). Argon gas was used to stir organic electrolyte. After electrolysis, steel samples were placed in a beaker containing ethanol and vibrated with ultrasonic wave to separate all particles from the samples surface. MCx and inclusions in ethanol were further separated by the magnetism. The inclusion particles were transferred directly to the double-sided carbon bands attached to the conductive material and then were observed by FESEM-EDS. After magnetic separation, MCx was analyzed by XRD (Model: X’Pert Powder, Malvern PANalytic Ltd., Malvine, UK, the detection parameters are that Cu Kαλ = 0.154178 nm, tube current 40 mA and tube voltage 40 kV, scanning scope 30–85 ◦ C, step length 0.013 s, residence time 5 s). 3. Result 3.1. Observation for Particles 2D morphologies of TiN-MCx in the metallographic specimens etched by 4% nitric acid alcohol were observed by FESEM-EDS and are shown in Figure 1. The EDS points are the black crosses, and the analysis for elements can both be seen in Figure 1. The dark grey particles are TiN inclusions, and the light grey particles are MCx . Figure 1a shows a long strip and large size TiN with a small amount of MCx around it. Figure 1b–d show TiN with less pronounced aspect ratios and it is covered by a larger number of MCx , which, in some cases, form a continuous layer rather than discrete particles. Metals 2019, 9, 641 Metals 2019, 9, x FOR PEER REVIEW Metals 2019, 9, x FOR PEER REVIEW 3 of 12 3 of 12 3 of 12 Figure 1. TiN-MC energy spectrometer (EDS) point analysis part in TiN-MCxxx particles analysis for for TiN Figure 1. TiN-MC particles a (...truncated)