Tungsten Carbide Alloy Wear-Resistant Surfacing of Single-Tooth Rollers

Literature Overview

This paper by Ban Jinming, Liu Hongbo, and Xiao Jingjin, published in Welding Technology (2007, Vol. 36, No. 5), addresses the surfacing welding of single-tooth rollers using tungsten carbide alloy materials. The single-tooth roller is a critical component in biomass or waste-to-energy boilers, where it serves to break up and feed solid fuel into the combustion chamber. The extreme wear conditions imposed by abrasive fuel materials necessitate a highly wear-resistant surface, which is achieved through the application of a tungsten carbide-containing overlay layer via surfacing welding.

Core Technical Points

Fundamentals of Surfacing Welding

The paper begins with a concise definition of surfacing welding: a welding process in which a layer of material with specific properties is deposited onto the surface of a component, not to join parts but to impart special surface characteristics such as wear resistance, heat resistance, or corrosion resistance, or to restore and increase component dimensions. This definition underscores that surfacing is a distinct discipline within welding engineering, with its own set of metallurgical challenges, process parameters, and quality criteria.

Tungsten Carbide Alloy Selection

Tungsten carbide (WC) is one of the hardest engineering materials available, with a Vickers hardness exceeding 2000 HV. When incorporated into a surfacing alloy, it provides exceptional resistance to abrasive wear. However, WC is inherently brittle and has a very low fracture toughness, which means that the surfacing alloy must be carefully designed to balance hardness with adequate toughness to resist cracking under impact loading.

Common tungsten carbide surfacing alloys used in industrial applications include:

Alloy Type	Typical Composition	Hardness (HV)	Application
WC-Co (60-80% WC)	60-80% WC, balance Co	1500-1800	Severe abrasive wear
WC-Co-Ni (40-50% WC)	40-50% WC, balance Co-Ni	1200-1500	Moderate abrasive wear with impact
WC-Co-Cr (30-40% WC)	30-40% WC, balance Co-Cr	1000-1300	Abrasive wear with corrosion resistance
WC-TiC-Co (50% WC, 10% TiC)	50% WC, 10% TiC, balance Co	1600-1900	High wear resistance with improved toughness

The choice of alloy for the single-tooth roller depends on the specific operating conditions. If the fuel material is highly abrasive (such as coal with high silica content) and the impact loading is moderate, a high-WC alloy with cobalt binder is appropriate. If the fuel contains corrosive elements (such as sulfur or chlorides) in addition to abrasive particles, a WC-Co-Cr alloy may be preferred to provide simultaneous wear and corrosion resistance.

Welding Process Challenges

The surfacing of tungsten carbide alloys presents several unique metallurgical challenges:

Thermal cracking: The high coefficient of thermal expansion mismatch between the WC particles and the metallic binder, combined with the inherent brittleness of WC, makes the surfacing layer susceptible to thermal cracking during cooling. This is exacerbated if the welding heat input is too high, causing excessive grain growth in the binder phase.
Porosity: Tungsten carbide has a very high melting point (2870 °C), which means that in a typical arc welding process, the WC particles do not fully melt. The unmelted WC particles can create local composition variations and, if gas is trapped, porosity.
Dilution: The base metal of the single-tooth roller is typically a low-alloy steel (such as Q345 or 42CrMo). During surfacing, a significant portion of the base metal melts and mixes with the surfacing alloy, diluting the WC content and reducing the hardness of the final layer. This is the most critical challenge, as the wear resistance of the overlay is directly proportional to the WC content.
Bond strength: The interface between the base metal and the surfacing layer must have adequate bond strength to resist delamination under service loading. A brittle interface with poor metallurgical bonding will fail prematurely, regardless of the surface hardness.

Process Parameters and Welding Sequence

The paper describes a systematic approach to the surfacing welding process, which can be summarized as follows:

Process Parameter	Recommended Range	Purpose
Preheating temperature	200-300 °C	Reduces HAZ hardness, prevents cold cracking
Welding process	SMAW (SMAW preferred for single-tooth rollers)	Good controllability, suitable for complex geometries
Electrode type	WC-Co surfacing electrode (e.g., CHW-01 or equivalent)	Provides appropriate dilution and bond strength
Current range	180-280 A	Balances penetration with dilution control
Arc voltage	22-30 V	Controls arc length and heat input
Travel speed	150-250 mm/min	Higher speed reduces dilution
Number of passes	2-3 passes	First pass for transition, subsequent passes for build-up
Interpass temperature	≤ 150 °C	Prevents excessive heat accumulation
Post-weld cooling	Controlled cooling in furnace or air cooling	Minimizes thermal cracking risk

The welding sequence for the single-tooth roller surface must account for the cylindrical geometry and the tooth profile. The surfacing is typically applied in a helical pattern, starting from one end of the roller and progressing to the other, with each pass overlapping the previous one by 30-50% of the bead width to ensure uniform coverage. The skip-welding technique may also be applied within each helical pass to control distortion.

Engineering Practice Integration

Application Context

Single-tooth rollers are used in biomass boilers, waste-to-energy facilities, and some industrial furnaces where solid fuel is fed through a rotating mechanism. The tooth profile engages with the fuel material, breaking it into smaller pieces and advancing it into the combustion zone. The wear rate of the tooth surface is typically the limiting factor in roller life, and without a wear-resistant overlay, the roller may require replacement every few hundred operating hours. With a properly applied tungsten carbide surfacing layer, the service life can be extended by a factor of 3 to 10, depending on the fuel abrasivity and operating conditions.

Quality Verification

The quality of the tungsten carbide surfacing layer must be verified through several tests:

Hardness testing: Vickers hardness measurement at multiple locations across the surfacing layer, with a target of ≥ 1200 HV for moderate wear conditions and ≥ 1500 HV for severe wear conditions.
Metallographic examination: Cross-sectional analysis to assess the dilution rate, microstructure of the binder phase, distribution of WC particles, and presence of cracks or porosity.
Bond strength test: Shear or peel test to verify the integrity of the base metal-overlay interface.
Visual and dimensional inspection: Confirmation that the surfacing layer covers the entire tooth profile uniformly without gaps or thin spots.

Common Defects and Countermeasures

Defect	Root Cause	Countermeasure
Cracking in overlay	Excessive heat input, high cooling rate	Reduce current, increase preheat, control cooling rate
High porosity	Gas entrapment, insufficient arc stability	Ensure proper arc length, use clean electrode, preheat
Low hardness	Excessive dilution	Use multiple passes, increase travel speed, use low-penetration arc
Delamination	Poor base metal preparation, contamination	Thoroughly clean base surface, ensure proper preheat
Non-uniform coverage	Inconsistent travel speed or arc length	Use welding positioner or fixture to control roller rotation

Study Insights and Reflections

This paper, though brief, touches upon a critical intersection of materials science and welding engineering: the design of overlay systems for extreme wear environments. The key insight is that the performance of a tungsten carbide surfacing layer is not determined by the alloy composition alone but by the entire process chain, from base metal preparation through welding parameter selection to post-weld cooling. Any deviation in this chain can compromise the final performance, even if the alloy itself is theoretically suitable.

The concept of dilution control deserves particular emphasis. In practice, the dilution rate can vary significantly depending on the welding process, electrode geometry, and base metal thermal properties. For a single-tooth roller with a relatively thin tooth profile, the base metal melts more readily than in a thick plate, leading to higher dilution and lower overlay hardness. This means that the same electrode that performs well on a thick plate may produce an inadequate overlay on the thin tooth surface. Engineers must therefore tailor the process parameters to the specific geometry, not just to the material specification.

The paper also implicitly raises the question of cost-effectiveness. Tungsten carbide surfacing electrodes are significantly more expensive than standard surfacing electrodes, and the welding process requires careful parameter control to achieve acceptable results. A life-cycle cost analysis should compare the cost of the surfacing repair against the cost of roller replacement, factoring in downtime, fuel inefficiency due to worn rollers, and the frequency of repair cycles. In most biomass boiler applications, the surfacing approach is economically justified, but the analysis should be performed on a case-by-case basis.

Summary

The tungsten carbide alloy surfacing of single-tooth rollers is a technically demanding operation that requires a deep understanding of overlay metallurgy, welding process control, and the specific wear mechanisms in biomass and waste-to-energy applications. The paper provides a clear framework for selecting the appropriate alloy, setting process parameters, and verifying quality. The most critical factor is the control of dilution, which directly determines the hardness and wear resistance of the final overlay. Engineers should adopt a systematic approach that integrates materials selection, process optimization, and quality verification, and should always perform a life-cycle cost analysis to confirm the economic viability of the surfacing repair.