Mechanism of Burn Loss in Tungsten Carbide Particle Cladding

Literature Overview

This study note examines the fundamental mechanisms governing tungsten carbide (WC) particle burn loss during weld overlay cladding processes. The literature investigates why WC particles, despite their exceptional hardness (2200–2600 HV) and wear resistance, suffer significant degradation when subjected to the thermal cycles of arc welding processes. The research spans multiple cladding methods including submerged arc welding (SAW), gas metal arc welding (GMAW), plasma transferred arc (PTA), and laser cladding, providing a comparative framework for understanding burn loss severity across different thermal input regimes.

Fundamental Mechanisms of WC Burn Loss

The burn loss of WC particles during cladding is governed by three primary mechanisms: thermal decomposition, oxidation, and mechanical fragmentation. When WC is heated above approximately 700 °C in an oxygen-containing atmosphere, it begins to decompose according to the reaction WC + O2 → WO3 + C, producing brittle tungsten oxides and free carbon. The decomposition is further accelerated at temperatures above 1000 °C where the reaction rate becomes significant. In an inert or reducing atmosphere, WC remains stable up to approximately 1350 °C, at which point it begins to dissociate into W and C.

The following table presents the thermal stability and burn loss characteristics of WC under different cladding conditions:

Microstructural Evolution and Phase Transformation

Metallographic analysis of WC-containing overlay welds reveals a complex phase transformation sequence. In the as-welded condition, partially decomposed WC particles are surrounded by a matrix of martensite, retained austenite, and carbide phases (M6C, M23C6). The decomposition products include WC → W2C → WO3 transitions, with the extent depending on the local thermal history. The literature demonstrates that the thermal gradient from the fusion line to the top surface creates a gradient in burn loss severity, with the bottom layers experiencing the most severe decomposition due to cumulative thermal cycling from subsequent passes.

A critical observation from the literature is that WC particle size distribution significantly influences burn loss behavior. Coarse WC particles (>75 μm) exhibit more severe decomposition at their edges due to thermal stress concentration at particle-matrix interfaces, while fine particles (<25 μm) tend to dissolve more readily into the matrix. The optimal particle size for minimizing burn loss while maintaining wear resistance is generally in the 25–75 μm range, which provides a balance between thermal stability and mechanical interlocking with the matrix.

Process Optimization Strategies

The literature identifies several effective strategies for minimizing WC burn loss in engineering practice. Reducing the thermal input per pass through lower current, higher travel speed, or multi-pass strategies with thinner individual layers significantly limits the time WC particles spend above their decomposition temperature. Laser cladding, with its inherently low heat input and rapid cooling rates, achieves the lowest burn loss among thermal processes. The use of pre-mixed feedstock compositions containing controlled amounts of chromium, nickel, and cobalt as binder phases also helps stabilize WC particles by creating a more compatible matrix that reduces thermal stress at particle interfaces.

Post-weld heat treatment presents a double-edged sword. While controlled tempering can relieve residual stresses and improve toughness, excessive heating can accelerate WC decomposition. The literature recommends tempering temperatures not exceeding 700 °C for WC-containing overlays, with a dwell time limited to 1–2 hours to minimize further degradation.

Defect Analysis and Countermeasures

Common defects associated with WC burn loss include surface microcracking, porosity from gas evolution during decomposition, and loss of hardness in the top layer. Cracking typically initiates at decomposed WC particle boundaries where brittle WO3 phases concentrate, propagating through the matrix under residual stress. The countermeasures include: using low-stress welding sequences, applying back-rolling or hot-rolling of the overlay surface, and incorporating ductile binder phases (Co, Ni) that accommodate thermal strain.

Study Insights and Engineering Implications

The study of WC burn loss mechanisms underscores the fundamental challenge in hardfacing technology: preserving the beneficial properties of the hard phase while achieving sound metallurgical bonding with the matrix. Engineers must recognize that some degree of WC decomposition is inevitable in thermal cladding processes, and the design objective should be to minimize rather than eliminate burn loss. The selection of cladding process should be driven by the specific service requirements—laser cladding for critical wear applications requiring maximum hardness retention, and conventional arc welding for less demanding applications where cost efficiency takes priority. A thorough understanding of the burn loss mechanism enables engineers to make informed trade-offs between overlay performance, process cost, and production throughput.