Zhuojin Pipe Fitting Co., LtdPlasma Arc Overlay of Iron-Based Alloy with Tungsten Carbide Composite Layers
Literature Overview
This paper by Zhao Kun, Wang Hongying, Cheng Zhiguo, Dong Zuyue, and Wang Chunyi (2002) from the Harbin Welding Research Institute describes a plasma arc overlay process for producing composite overlay layers combining iron-based alloy powder with mechanically crushed tungsten carbide (WC) hard alloy particles. The work was supported by the Heilongjiang Provincial Science and Technology Department project (G97A12-8-01). The key innovation is the use of an externally fed tungsten carbide plasma arc welding gun system, which allows WC particles to be introduced into the plasma arc zone and deposited as a composite layer with an iron-based alloy matrix. The resulting overlay layer contains 40% to 42% WC by mass, with excellent wear resistance and controlled defect levels.
Core Technical Content
Process Configuration
The plasma arc overlay process described in this paper uses a transferred plasma arc with a tungsten electrode. The iron-based alloy powder (F321) and WC particles are fed separately into the arc zone through a specially designed powder/particle delivery system. The plasma arc provides a concentrated, high-temperature heat source that melts the iron-based powder to form the matrix while partially melting the surface of the WC particles to ensure bonding without completely dissolving them.
| Parameter | Value |
|---|---|
| Plasma arc current | 100-200 A |
| Arc voltage | 25-35 V |
| Shielding gas (argon) flow rate | 15-25 L/min |
| Iron-based powder feed rate | 0.5-1.5 kg/h |
| WC particle feed rate | 0.3-0.8 kg/h |
| Travel speed | 50-150 mm/min |
| Powder/particle to arc distance | 5-15 mm |
| Substrate preheat | 150-250°C |
Microstructure Analysis
The composite overlay layer microstructure is characterized by three distinct zones:
- WC particle zone: The WC particles retain their original high hardness (1500-2000 HV) with minimal surface remelting. The remelted surface layer of each particle is thin, typically less than 50 micrometers, which preserves the bulk hardness of the WC particles.
- Eutectic zone: Surrounding each WC particle is a fishbone-shaped eutectic structure consisting of martensite and carbides. This eutectic zone is confined to the immediate vicinity of the particles and does not diffuse into the surrounding iron-based alloy matrix. This confinement is critical because it prevents the embrittlement of the matrix by the hard eutectic phase.
- Matrix zone: The iron-based alloy matrix consists of martensite, retained austenite, and a small amount of eutectic (martensite + carbide). The matrix hardness is typically 50-58 HRC, providing a good combination of toughness and hardness.
Defect Analysis
The authors measured the average pore defect rate of the WC particles and found it to be below 1.5%. This is an important quality metric because pores around WC particles can act as stress concentrators and initiate cracking under impact or cyclic loading. The low defect rate is attributed to:
| Factor | Effect |
|---|---|
| Controlled WC particle size (0.5-2.0 mm) | Uniform melting and bonding |
| Adequate arc energy density | Complete melting of iron powder, partial melting of WC surface |
| Argon shielding | Prevention of oxidation and porosity |
| Moderate travel speed | Adequate heat input for bonding without excessive dilution |
The authors also examined cracking behavior and found that the confined eutectic zone around WC particles prevents crack propagation through the matrix. When a crack initiates at a WC particle, it is arrested at the particle-matrix interface due to the toughness mismatch between the hard WC and the tougher matrix.
Performance Comparison
| Property | Iron-based only | WC composite | Improvement |
|---|---|---|---|
| Hardness (HV) | 800-900 | 1200-1500 | 33-67% |
| Wear resistance (vs. quenched steel) | 3-5x | 8-12x | 60-140% |
| Impact toughness (J/cm2) | 15-25 | 8-15 | -33 to -67% |
| Pore defect rate (%) | <1.0 | <1.5 | Comparable |
Engineering Practice Implications
The plasma arc composite overlay process is particularly valuable for components subject to severe abrasive wear where the combination of hardness and toughness is required. Applications include:
- Pump impellers and diffusers in slurry service
- Valve seats and plugs in high-pressure hydrocarbon service
- Mining equipment wear parts
- Pipeline spools in high-erosion sections
The process requires careful control of the WC particle size distribution. Particles that are too large (>2.0 mm) may not bond properly to the matrix, leading to spalling under impact loading. Particles that are too small (<0.3 mm) may fully melt and dissolve into the matrix, losing their individual particle identity and reducing the composite effect. The optimal particle size range of 0.5 to 2.0 mm provides a good balance between bonding quality and wear resistance contribution.
Study Insights and Reflections
This paper demonstrates the power of composite overlay design in combining the wear resistance of hard particles with the toughness of a ductile matrix. The key metallurgical insight is the confinement of the eutectic zone around WC particles, which prevents the embrittlement of the matrix. This is achieved through careful control of the arc energy density and travel speed, which determines the degree of WC particle surface remelting. For engineers designing composite overlay systems, the lesson is that the interface between hard particles and the matrix is the critical zone for both wear resistance and fracture resistance. The plasma arc process offers the thermal control necessary to achieve this interface engineering, making it a superior choice over lower-energy processes such as TIG or MIG for composite overlay applications.