Application of Tungsten Carbide Particle Surfacing Materials

Literature Overview

The paper by Zhou Haiyun and Huang Tao, published in Mining Machinery (Vol. 18, No. 12, 1990, pp. 56-57), investigates the metallurgical behavior of tungsten carbide (WC) particles during the surfacing process and its impact on the wear resistance of rock-cutting tools. The authors from Sunan Coal Mine Machinery Factory address a critical problem: despite widespread use of WC particle surfacing on rock-cutting tools, the wear performance often falls short of expectations. The root cause, as identified by the authors, is the melting and burnout of WC particles during welding, which reduces their effective size and hardness.

Technical Background

WC particle surfacing is a well-established technique for hardening rock-cutting tools such as:

Wedge teeth on vertical boring machine rock bits
Disc cutters on full-face tunnel boring machines (TBM)
Rock drill bits and chisel bits

The wear mechanism in these applications is primarily abrasive wear caused by hard rock minerals (quartz, corundum, etc.) grinding against the tool surface. WC particles, with hardness exceeding 1500 HV, provide excellent resistance to this type of wear. However, the welding process itself can degrade the WC particles if not properly controlled.

Metallurgical Behavior of WC During Surfacing

The Problem of WC Degradation

During the surfacing process, WC particles are subjected to high temperatures that can cause several detrimental reactions:

Melting and dissolution: WC has a melting point of approximately 2870°C, but in the liquid metal pool during surfacing, it can dissolve in the molten iron-nickel matrix at temperatures well below this value.
Decomposition: WC can decompose into W and C, with the carbon dissolving in the matrix and the tungsten forming lower-hardness carbides (such as Fe3W3C).
Oxidation: At high temperatures, WC can react with oxygen to form WO3, which has low hardness and poor wear resistance.

The net result is that the original WC particles, which provide the primary wear resistance, are partially or completely consumed during the welding process, leading to a surfacing layer with much lower hardness and wear resistance than expected.

Research Findings

The authors conducted systematic studies on:

Study Parameter	Findings
WC particle size	Larger particles (0.5-1.0 mm) resist melting better than fine particles (<0.2 mm)
Flame temperature	Lower flame temperatures reduce WC degradation but may compromise bond strength
Quenching temperature	Optimal quenching temperature maximizes WC hardness retention
Chemical composition	Slight alloying additions (Co, Cr) improve WC stability during welding

Optimal Process Conditions

Based on the research, the authors recommend:

WC particle size: 0.3-0.8 mm for optimal balance of hardness retention and dispersion
Surfacing method: Oxy-acetylene flame surfacing with controlled flame temperature (avoiding excessive preheating of particles)
Particle application: Apply WC particles to the hot substrate surface in layers, allowing partial melting for bonding while preserving particle integrity
Post-weld heat treatment: Quench and temper to optimize the matrix hardness while preserving WC particle hardness

Comparison of Surfacing Methods for WC Particle Application

Method	Heat Input	WC Degradation	Bond Strength	Wear Resistance	Applicability
Oxy-acetylene flame	High	Moderate-High	Good	Moderate	Large surfaces
Plasma arc	Moderate	Moderate	Good	Good	Complex geometries
Submerged arc	High	High	Excellent	Low-Moderate	Flat surfaces
HVOF spraying	Low	Very Low	Moderate	High	Large areas
Laser cladding	Very Low	Very Low	Excellent	Very High	Precision applications

The table illustrates the fundamental trade-off in WC particle surfacing: methods with lower heat input preserve WC integrity better but may have lower bond strength, while methods with higher heat input provide better bonding but degrade the WC particles.

Engineering Practice Cases

Application 1: Vertical Boring Machine Wedge Teeth

For wedge teeth on vertical boring machine rock bits, the authors developed a two-step surfacing process:

Bond layer: Deposit a nickel-based or cobalt-based alloy layer using submerged arc surfacing to ensure strong metallurgical bonding with the steel substrate.
WC layer: Apply WC particles to the hot bond layer surface using oxy-acetylene flame, controlling the flame temperature to minimize particle degradation.

The resulting surfacing layer achieves surface hardness of 80-90 HRC with WC particles retained at 1400-1500 HV, providing excellent abrasion resistance for hard rock boring operations.

Application 2: TBM Disc Cutters

For disc cutters on tunnel boring machines, the challenge is more complex because the cutters experience both abrasive wear (from rock) and impact loading (from rock fragments). The surfacing design must provide both hardness and toughness. The authors recommend:

Using a cobalt-based matrix alloy (with 6-8% Co) for improved toughness
Applying WC particles in a controlled manner to avoid excessive brittleness
Incorporating a graded transition layer between the steel substrate and the hard surfacing layer

Study Reflections

This 1990 paper addresses a fundamental problem in hardfacing technology that remains relevant today: the degradation of hard particles during the welding process. While modern techniques such as HVOF and laser cladding have largely solved this problem by minimizing heat input, the fundamental understanding of WC metallurgy during surfacing remains essential for engineers working with conventional welding processes.

The paper's methodology is particularly instructive: rather than simply reporting a process, the authors investigated the fundamental metallurgical mechanisms governing WC behavior during surfacing. This approach allows the findings to be applied to a wide range of applications, not just the specific tools studied.

For modern engineers, the key lessons from this paper are:

Understand the metallurgy: Always investigate what happens to the hard phase during the welding process, not just the final properties of the surfacing layer.
Control heat input: Minimize heat input to preserve hard particle integrity, even if this requires using less common surfacing methods.
Design graded layers: Use multiple layers with different compositions to balance bond strength and surface properties.
Optimize particle size: Select WC particle size based on the specific application requirements and the surfacing method used.

The paper also highlights an important economic consideration: the cost of WC particles is significant, and their degradation during welding represents a direct economic loss. By optimizing the surfacing process to minimize degradation, the overall cost of the surface engineering solution is reduced.

In summary, this paper demonstrates that effective hardfacing requires deep understanding of the metallurgical processes involved, not just empirical process development. Engineers who invest time in understanding the fundamental mechanisms will be better equipped to develop robust surfacing processes for challenging applications.