Fe–W–C thermodynamics and in situ preparation of tungsten carbide-reinforced iron-based surface composites by solid-phase diffusion

https://doi.org/10.1016/j.ijrmhm.2016.02.001Get rights and content

Highlights

  • In situ synthesis dense WC with solid-phase diffusion

  • Heat treatment at 1423 K for 4 h promotes graphite, γ-Fe, WC and M6C.

  • The size of WC reinforcement was 0.3–12 μm with volume fraction of up to > 80%.

  • The maximum wear resistance was 230 times higher than that of the gray cast iron.

  • High wear resistance was due to in situ formation of dense and hard WCs.

Abstract

Tungsten carbide (WC)-reinforced Fe-based surface composites were prepared by in situ solid-phase diffusion at 1423 K for 4, 6, and 8 h. The thermodynamics, phase composition, microstructure, microhardness, and wear-resistance of the Fe–W–C ternary system of the samples were examined by X-ray diffraction, scanning electron microscopy, Vickers hardness test, and wear test, respectively. Thermodynamic calculations showed that the thermodynamically favored products of the Fe–W–C system were W2C, WC, and Fe3C. W also exhibited a stronger carbide-forming tendency than Fe. The Gibbs free-energies of W2C and WC, which were stable carbides, significantly decreased with increased temperature. The main phases of the composite were WC, γ-Fe, Fe3C, graphite, and η-carbide (M6C) with fishbone-like morphology. The longitudinal section of the composite could be easily divided into three reaction zones, namely, WC layer, “no graphite area,” and M6C-reinforced area. WC particles in the WC layer were irregularly shaped with 0.3–12 μm particle size, with volume fraction of up to > 80%. The average microhardness value of the dense ceramic layer was 2152 HV0.1. The maximum relative wear-resistance, which was 230.4 times higher than that of gray cast iron, was obtained at 20 N. The high wear-resistance of the composite was due to the in situ formation of dense and hard WC particulates that acted as a reinforcement phase.

Introduction

Metal-matrix surface composites reinforced by carbide, nitride, and oxide ceramic, have been intensively investigated and developed because of their high specific modulus and strength, thermal stability, and excellent wear-resistance [1], [2], [3]. The development of iron (Fe)-based surface composites reinforced by carbide ceramic is attracting considerable attention because of the low cost and good mechanical properties of these composites [4], [5]. Similarly, tungsten carbide (WC) is an attractive reinforcing material in composites because of its high melting point (3410 °C), high hardness (17.8 GPa), low coefficient of thermal expansion, and high resistance to oxidation and wear. Zhou et al. [6] prepared crack-free Fe-based 20 wt.% WC coating with a large area by multi-track overlapping laser induction hybrid rapid cladding. The cast WC particles dissolved almost completely and had worse wettability than Fe-based metal matrix. Precipitated carbides such as M12C and M23C6 (M = Fe, W, Cr) formed an intergranular network around the primary Fe-based phase enriched with W. Additionally, several other techniques such as vacuum infiltration casting technique [7], [8], casting method [9], plasma melt injection [10], and plasma transferred arc method [11] have been used to prepare WC-reinforced Fe-based composites. These techniques are based on the addition of cast WC particles to matrix materials. In such cases, the reinforcing phase WC scale is limited by starting powder size, interfacial reactions between reinforcements and matrix, and poor wettability between reinforcements and matrix caused by surface contamination of the reinforcements. The bonding strength of WC particles with the matrix is also usually poor. To date, in situ reaction is considered as one of the most promising methods for producing WC-reinforced Fe-based surface composites. This reaction is advantageous because of the thermodynamic stability of ceramic particulates in the matrix coupled with strong interfacial bonding between reinforcement and matrix. Niu et al. [12] produced an Fe-based surface composite reinforced by in situ-formed WC particles on gray cast iron substrate by centrifugal casting. Experimental results show that primary and secondary WC particles and pearlite with negligible amount of graphite (G) flakes form as the reinforcing phase and matrix, respectively. The composite with a thickness of approximately 3 mm is dense and metallurgically bonded onto the gray cast iron substrate. Meanwhile, Chen et al. [13] prepared WC cermet coating on C steel by in situ laser clad combined with combustion synthesis. However, to the authors' knowledge, these methods produce WC particles with low volume fraction in the matrix and uneven distribution on the substrate.

The present work explains a method of producing WC reinforcement on the surface of an iron matrix by an in situ technique with solid-phase diffusion. This technique has been used elsewhere to prepare TiC/Fe, VCp/Fe, and TaC/Fe surface composite [14], [15], [16], [17]. The route involves a chemical reaction that results in the formation of WC within a gray cast iron matrix. The reinforcement formed in situ is thermodynamically stable. Moreover, the reinforcement–matrix interfaces are clean, resulting in strong interfacial bonding. The method is also low cost, the volume fraction of WC particles can exceed 80%, and the thickness of the reinforcement layer is easily controllable. In this study, Fe–W–C thermodynamics, microstructure, hardness distribution, and wear-resistance of the composite are systematically investigated. The formation mechanism of tungsten carbide-reinforced Fe-based surface composites is also discussed.

Section snippets

Materials

Starting materials were gray cast iron and W plate (99.7 wt.%) with 0.3 mm thickness. These materials were used as C and W sources for the in situ synthesis of WCs on the surface of iron matrix. The chemical composition (wt.%) of gray cast iron was Fe–3.45C–0.56Si–0.268Mn–0.224P–0.024S.

Experimental procedure

A cuboid aluminum-oxide mold with dimensions of 24 mm × 24 mm × 12 mm was prepared. A tungsten plate was cut into the size of 20 mm × 20 mm × 0.3 mm by using a numerically controlled wirecut EDM machine (Suzhou Nutac Electro

Fe–W–C thermodynamics

The Fe–W–C system may be the fundamental system for interpreting the structure of many commercial alloys, such as high-speed and tool steels. The system has been extensively investigated. All stable phases known in the Fe–W–C system are austenite (γ-Fe), G, WC, cementite (Fe3C), and η-carbide (M6C). The ternary eutectic temperatures reportedly to range between 1323 K to 1416 K, and the ternary eutectic point L (liquid)  γ-Fe + WC + G is considered to represent the stable state [20], [21]. Gibbs

Conclusions

Tungsten carbide-reinforced Fe-based surface composites were prepared by in situ solid-phase diffusion near the ternary eutectic temperature (1423 K) for 4, 6, and 8 h in argon atmosphere. The Fe–W–C thermodynamics, phase composition, microstructure, microhardness, and wear-resistance of the samples were examined. The following conclusions can be drawn: (1) The thermodynamically favored products were W2C, WC, and Fe3C according to Fe–W–C thermodynamics calculation. W had a stronger

Acknowledgments

This project was supported by the International S & T Cooperation Program of China (no. 2014DFR50630) and the National Natural Science Foundation of China (no. 51501148). The authors also acknowledge the financial support of the Project of the Shaanxi Key Laboratory of Nano Materials and Technology (15JS054, 14JS048 and 13JS053).

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