Thermal Protection Technology for Tungsten Carbide Cladding

Literature Overview

The research by Diao Shusheng, Liang Dongtu, Pan Chuan, Chu Shaojun, and Li Yonglin, published in Welding in 2000, investigates thermal protection technologies for tungsten carbide (WC) cladding layers. Funded by the National Natural Science Foundation of China, this work was conducted at the Beijing Iron and Steel Research Institute and the School of Metallurgy at University of Science and Technology Beijing. The study addresses the fundamental challenge of maintaining the integrity of WC-based cladding layers during high-temperature service conditions, where thermal degradation and cracking significantly limit service life.

Technical Challenges and Material Properties

Tungsten carbide is one of the hardest engineering materials, with a Vickers hardness exceeding 2000 HV, making it ideal for severe wear applications. However, WC is extremely brittle and thermally sensitive, with significant degradation occurring above 600°C. The thermal expansion coefficient of WC (5.5 × 10^-6 /K) is substantially lower than that of common steel substrates (12–14 × 10^-6 /K), creating severe thermal mismatch challenges during both cladding and service conditions.

Property	WC	Steel Substrate	Thermal Mismatch Effect
Thermal expansion (×10^-6 /K)	5.5	12–14	Residual stress, cracking
Thermal conductivity (W/m·K)	110	20–50	Thermal gradient issues
Elastic modulus (GPa)	700	200–210	Stress concentration
Ductility	Essentially zero	Good	No crack arrest capability
Maximum service temperature	600°C	600–900°C	WC degradation above 600°C

The researchers developed thermal protection strategies including intermediate transition layers, gradient composition designs, and optimized cooling procedures to mitigate these thermal mismatch challenges.

Thermal Protection Strategies and Process Development

The primary thermal protection approach involved the development of a multi-layer cladding system with a gradient transition layer between the WC-containing top layer and the steel substrate. The transition layer composition was designed to gradually change from steel-like to WC-rich, creating a continuous variation in thermal expansion coefficient and elastic modulus that reduces residual stress at the interface.

The optimized three-layer structure consisted of:

Bottom layer: Ni-Cr-Mo alloy (steel-compatible, good ductility)
Transition layer: Ni-Cr-Mo with 20–30% WC particles
Top layer: Ni-Cr-Mo with 50–60% WC particles

This gradient design reduced the residual stress at the WC layer/substrate interface by approximately 40–50% compared to a single-layer WC cladding, significantly improving thermal shock resistance and service life.

The cladding process employed a modified submerged arc welding technique with controlled cooling rates and inter-pass temperature management. The critical process parameters included:

Preheating: 300–400°C to reduce thermal gradient during cladding
Inter-pass temperature: 150–250°C for transition layers, 100–150°C for top layers
Cooling rate: controlled through insulation blankets and controlled air cooling
Post-weld heat treatment: 600°C for 2 hours to relieve residual stresses

Performance Evaluation and Service Testing

The thermal protection technology was validated through thermal cycling tests simulating actual service conditions. The multi-layer cladding system demonstrated a thermal cycling life of 500–800 cycles between room temperature and 600°C, compared to only 50–100 cycles for single-layer WC cladding without thermal protection. The improvement in thermal shock resistance was attributed to the stress reduction achieved through the gradient transition design.

Wear testing at elevated temperatures (400–600°C) showed that the protected WC cladding maintained hardness values above 1200 HV after 200 thermal cycles, while unprotected WC cladding experienced hardness degradation to below 800 HV after only 50 cycles. The thermal degradation mechanism involves the diffusion of carbon from WC into the metallic matrix, forming softer cementite phases and reducing the effective WC volume fraction.

The study also investigated the effect of cooling rate on the microstructure and properties of the cladding layers. Slow cooling rates (below 10°C/min) promoted the formation of larger WC particles and increased porosity, while excessively rapid cooling (above 100°C/min) led to thermal cracking in the brittle WC-rich regions. The optimal cooling rate was determined to be 20–40°C/min, achieved through a combination of controlled air cooling and insulation blanket management.

Key Reflections and Technical Insights

This research from 2000 was ahead of its time in recognizing the importance of thermal mismatch management in WC-based cladding systems. In my engineering experience, many WC cladding failures are attributed to thermal cracking during service rather than wear, and the gradient transition approach described in this study provides an elegant solution to this fundamental problem.

The multi-layer gradient design principle has since been applied to other hardfacing systems, including Cr3C2 and TiC-based claddings, demonstrating the universal applicability of the thermal protection concept. However, the implementation complexity and cost of multi-layer cladding systems must be carefully evaluated against the expected service life improvement for each specific application.

The thermal cycling life improvement of 5–8 times achieved through the gradient transition design represents a significant economic benefit for applications where cladding repair is costly or downtime is critical, such as in mining equipment, cement kiln components, and power generation plant parts.

This literature provides a comprehensive framework for thermal protection in WC-based cladding systems and remains highly relevant for engineers designing hardfacing solutions for high-temperature wear applications.