Microstructure and Tribological Properties of Spherical WC-Reinforced Iron-based Composite Plasma Cladding Layer

Research Context and Industrial Relevance

Wear-resistant cladding layers are essential for components subjected to severe sliding and abrasive wear conditions, including mining equipment, cement mill liners, crane wheels, and hydraulic cylinder rods. This study focuses on a plasma transferred arc (PTA) cladding layer composed of an iron-based matrix reinforced with spherical tungsten carbide (WC) particles. The spherical morphology of the WC particles, achieved through gas atomization or plasma spheroidization, is a critical design feature that distinguishes this composite from conventional angular WC-reinforced overlays. The spherical shape reduces stress concentration at particle-matrix interfaces, improves particle dispersion uniformity, and enhances the toughness of the composite cladding layer.

Microstructural Characterization

The PTA cladding layer exhibits a complex microstructure consisting of a martensitic iron-based matrix (BCC α-Fe with dissolved Cr, Mo, and V) reinforced by uniformly distributed spherical WC particles. The matrix hardness is approximately 550–650 HV, providing a hard yet somewhat ductile background. The spherical WC particles (average diameter 15–25 μm) retain their integrity after the PTA process, with minimal dissolution due to the relatively short residence time in the molten pool. However, at the particle-matrix interface, a thin WC dissolution zone (1–3 μm) forms, where tungsten diffuses into the matrix and carbon is released to form secondary carbides (Fe3C, M7C3). This interfacial zone is critical for bonding strength and crack resistance.

The dilution rate with the low-carbon steel substrate is controlled at 12–18 percent by optimizing the PTA parameters: current 160–200 A, travel speed 50–70 mm/min, wire feed speed 30–40 cm/min, and preheating to 150–200°C. The multi-pass cladding strategy builds up the overlay to a total thickness of 3–5 mm with each pass achieving 0.8–1.2 mm deposition.

Tribological Performance

Test Condition	Friction Coefficient	Wear Rate (mm³/N·m)	Hardness (HV)	Surface Roughness Ra (μm)
Base steel (45#)	0.42–0.48	2.8×10⁻⁶	180–220	3.2
Ni-based PTA overlay	0.38–0.42	1.5×10⁻⁶	320–360	1.8
Iron-based + 10wt% WC overlay	0.30–0.35	0.8×10⁻⁶	680–750	1.2
Iron-based + 20wt% WC overlay	0.28–0.32	0.4×10⁻⁶	780–850	0.9
Iron-based + 30wt% WC overlay	0.32–0.38	0.9×10⁻⁶	820–880	1.5

The pin-on-disk wear test against a GCr15 bearing steel counterpart demonstrates that the 20wt% WC-reinforced overlay achieves the optimal balance of wear resistance and toughness. The friction coefficient is lowest (0.28–0.32) and the wear rate is reduced by approximately 86 percent compared to the base steel. The wear mechanism transitions from adhesive-abrasive wear on the base steel to predominantly abrasive wear on the WC-reinforced overlay, where the hard WC particles plough through the counterface material. At 30wt% WC, the wear rate increases slightly due to particle pull-out and matrix cracking, indicating that there is an optimal reinforcement fraction beyond which the composite becomes brittle.

Defect Analysis and Process Control

The primary defects observed in WC-reinforced iron-based PTA overlays include particle agglomeration, unmelted particles, and microcracking at the particle-matrix interface. Particle agglomeration occurs when the powder blend is not adequately homogenized before feeding, leading to localized high WC concentration zones that create stress risers. Unmelted particles appear when the PTA heat input is insufficient to fully wet the WC particles, creating voids that reduce the effective load-bearing area. Microcracking initiates at the WC-matrix interface during cooling when the thermal contraction mismatch (WC: 4.5×10⁻⁶/°C, iron matrix: 12×10⁻⁶/°C) generates tensile stresses exceeding the interface strength.

The countermeasures include: pre-mixing the powder blend in a V-mixer for a minimum of 30 minutes to ensure uniform dispersion; optimizing the PTA heat input to ensure complete wetting of WC particles (current above 160 A); and applying a post-weld tempering treatment at 550°C for 2 hours to relieve residual stresses and transform retained austenite in the matrix.

Engineering Application and Process Integration

This composite overlay technology has been successfully applied to mining shovel buckets, cement mill grinding rings, and hydraulic cylinder rods in industrial service. In a field trial on a hydraulic cylinder rod with 3 mm iron-based + 20wt% WC PTA overlay, the service life increased from 8 months to over 30 months under identical operating conditions, representing a 3.75-fold improvement. The overlay maintained its hardness and surface integrity throughout the extended service period, with minimal measurable wear after 20,000 operating cycles. For pressure vessel applications where wear-resistant overlays are required on rotating shafts or sliding surfaces, this technology offers a practical solution that combines high hardness with acceptable toughness, avoiding the brittleness issues associated with pure carbide overlays. Engineers should note that the WC-reinforced overlay requires careful consideration of the underlying substrate compatibility, as the high hardness of the overlay may cause accelerated wear of the mating surface if not properly matched. A comprehensive tribological system analysis, considering both the overlay and the counterface material, is essential for optimizing the overall component performance.