Preparation and Wear Resistance Analysis of Cobalt-Based Tungsten Carbide Cladding Overlays

Overview and Background

Cobalt-based tungsten carbide (Co-WC) cladding overlays are among the most effective surface engineering solutions for extreme wear environments, particularly in applications involving abrasive, adhesive, and erosive wear at elevated temperatures. These coatings find extensive use in mining equipment, oil and gas drilling components, power generation turbine parts, and chemical processing equipment. The literature reviewed examines the preparation of Co-WC cladding overlays by various welding processes, including plasma transferred arc (PTA) cladding, submerged arc welding (SAW) overlay, and laser cladding, with a focus on the relationship between coating composition, microstructure, and wear resistance.

Coating Composition and Microstructure

The typical composition of Co-WC cladding overlays includes 55–65% Co (balance), 25–35% WC, 5–10% Cr, and trace amounts of Mo, Ni, and Fe. The microstructure consists of a cobalt-chromium solid solution matrix with dispersed tungsten carbide particles. During welding, partial dissolution of WC occurs, leading to the formation of M6C and M23C6 carbides (where M = Co, Cr, W, Mo) in addition to the remaining primary WC particles. The extent of WC dissolution depends on the peak temperature and cooling rate, which are process-dependent parameters.

Process	Peak Temperature (°C)	WC Dissolution (%)	Primary WC Retained (%)	Hardness (HV)
PTA Cladding	1500–1700	30–50	50–70	850–1050
SAW Overlay	1800–2000	40–60	40–60	800–950
Laser Cladding	1200–1500	15–30	70–85	950–1150
GMAW Overlay	1900–2100	50–70	30–50	750–900

The hardness of Co-WC coatings is primarily governed by the volume fraction and size distribution of the retained WC particles. Laser cladding produces the finest microstructure with the highest hardness due to the rapid cooling rates (10³–10⁴ K/s) that minimize WC dissolution and promote fine carbide precipitation.

Wear Resistance Testing and Results

The wear resistance of Co-WC cladding overlays was evaluated using pin-on-disk tribometry, dry sliding wear tests, and three-body abrasive wear tests. The wear rate was measured under conditions simulating industrial applications, including room temperature dry sliding against GCr15 bearing steel balls and high-temperature (500–800°C) sliding against SiC counterfaces.

Test Condition	Co-WC (PTA) Wear Rate (mg)	Co-WC (Laser) Wear Rate (mg)	Base Steel (42CrMo) Wear Rate (mg)	Improvement Factor
Room temp, 20 N, 5 km	2.5–4.0	1.5–2.5	15–25	5–10×
500°C, 20 N, 5 km	4.0–6.5	2.5–4.0	30–45	5–8×
800°C, 20 N, 5 km	6.0–9.0	4.0–6.0	60–80	5–7×
Three-body abrasion	8.0–12.0	5.0–8.0	40–60	4–6×

The wear mechanisms transition from abrasive wear at room temperature to a combination of abrasive and oxidative wear at elevated temperatures. The presence of a continuous Cr2O3 oxide layer at temperatures above 500°C contributes significantly to the wear resistance by providing a protective barrier against further oxidation and reducing the friction coefficient.

Process Optimization and Defect Prevention

The quality of Co-WC cladding overlays is critically dependent on process parameters that control the dilution ratio, cooling rate, and defect formation. A systematic approach using the 5W2H methodology (What, Why, Where, When, Who, How, How much) was applied to optimize the PTA cladding process for maximum wear resistance.

Parameter	Optimal Value	Effect on Quality
Arc Current	200–280 A	Controls penetration and dilution
Travel Speed	150–300 mm/min	Affects heat input and cooling rate
Powder Feed Rate	150–300 g/min	Controls deposition rate and porosity
Arc Voltage	28–35 V	Affects arc stability and penetration
Shielding Gas Flow	15–25 L/min (Ar)	Prevents oxidation and porosity
Interpass Temperature	<150°C	Prevents excessive WC dissolution

Common defects in Co-WC cladding overlays include porosity, cracking, delamination, and excessive dilution. Porosity is primarily caused by insufficient shielding gas coverage or contaminated powder, while cracking is associated with high residual stresses and hydrogen pickup. Delamination occurs at the coating-substrate interface when the bond strength is insufficient, typically due to poor surface preparation or excessive dilution.

Study Insights and Implications

The literature clearly demonstrates that laser cladding produces the highest quality Co-WC overlays with the best wear resistance due to the minimal dilution and rapid solidification that preserve the primary WC particles. However, the high cost and limited build rate of laser cladding make it impractical for large-area applications. For industrial-scale production, PTA cladding offers the best compromise between cost, productivity, and coating quality. The key insight is that the wear resistance of Co-WC coatings is not solely determined by hardness but also by the toughness of the cobalt matrix, which provides crack-arresting capability during abrasive wear. Engineers should consider the balance between hardness and toughness when selecting the Co-WC composition and process parameters for specific applications. Additionally, the post-weld heat treatment of Co-WC overlays (solution treatment at 1050–1150°C followed by aging at 850–950°C) can precipitate fine secondary carbides and further enhance wear resistance without significantly reducing hardness.