CLADDING TECHNOLOGY SHANXI CO., LTDBoride Hard Phase Wear-Resistant Alloy Overlay Welding Electrode Research
Literature Overview
This 1995 study by Xu Guojian and Liu Li from the Welding Teaching and Research Section at Shenyang University of Technology, published in the Journal of Cemented Carbides, investigates the development of overlay welding electrodes incorporating boride hard phases for enhanced wear resistance. Borides, particularly CrB₂ and CrB, have attracted attention as alternative hard phases due to their high hardness, good thermal stability, and lower cost compared to carbides and nitrides. The research contributes to the broader effort of developing cost-effective, high-performance overlay welding consumables for industrial wear applications.
Boride Phase Characteristics and Selection
Borides occupy a unique position in the hierarchy of hard phases used in overlay welding. Chromium borides (CrB₂ and CrB) offer hardness values comparable to many carbides while being significantly less expensive and more readily available. The selection of boride phases for overlay welding is influenced by several factors:
The following table compares boride phases with other common hard phases:
| Property | CrB₂ | CrB | WC | TiC |
|---|---|---|---|---|
| Hardness (HV) | 1,800-2,200 | 1,200-1,500 | 1,800-2,200 | 2,500-2,900 |
| Thermal Stability | Good | Moderate | Excellent | Excellent |
| Cost | Low | Low | High | High |
| Brittleness | Moderate | Moderate | High | High |
| Oxidation Resistance | Moderate | Poor | Good | Poor |
| Availability | Good | Good | Limited | Limited |
The study selected CrB₂ as the primary hard phase due to its superior hardness and thermal stability, with CrB serving as a secondary phase to improve toughness and reduce brittleness. The composite boride system provides a balance between wear resistance and crack resistance that is difficult to achieve with single-phase boride overlays.
Electrode Development and Microstructure
The electrode development process involved careful optimization of wire composition, flux formulation, and welding parameters to achieve optimal hard phase formation and distribution. The wire composition was designed to promote CrB₂ formation during solidification while maintaining a tough austenitic matrix.
Key composition parameters include:
- Chromium content: 18-25 wt% in the wire to ensure sufficient CrB₂ formation
- Boron content: 1.5-3.0 wt% to control boride phase fraction
- Carbon content: 0.5-1.5 wt% to provide additional hardening through carbide formation
- Nickel content: 8-12 wt% to stabilize austenite and improve toughness
- Molybdenum content: 1-3 wt% to enhance thermal stability and corrosion resistance
The weld metal microstructure consists of a matrix of austenite and martensite with dispersed CrB₂ and CrB particles. The boride particles are typically plate-like or needle-shaped, with sizes ranging from 5 to 50 μm. The distribution and morphology of the boride particles significantly influence the wear resistance and fracture behavior of the overlay.
Wear Performance and Fracture Behavior
Wear testing was conducted using standard abrasion and erosion test methods. The boride overlay demonstrated wear resistance comparable to carbide overlays in many applications, with the advantage of lower cost and improved toughness. The wear mechanism was primarily abrasive, with the boride particles providing the primary wear resistance contribution.
The following table summarizes wear test results for the boride overlay:
| Test Condition | Wear Rate (mg) | Hardness (HV) | Wear Mechanism |
|---|---|---|---|
| Dry sliding | 20-30 | 1,000-1,400 | Abrasive |
| Sand abrasion | 15-25 | 1,000-1,400 | Abrasive |
| Erosion (30°) | 25-40 | 1,000-1,400 | Abrasive-adhesive |
| Impact-abrasion | 35-55 | 1,000-1,400 | Mixed |
Fracture analysis revealed that the boride overlay exhibits better fracture toughness than carbide overlays due to the ductile matrix supporting the hard phases. Crack initiation occurs preferentially at the matrix-boride interface rather than within the boride particles themselves, indicating good interfacial bonding.
Process Parameters and Weld Quality
The welding process parameters significantly influence the boride phase formation and distribution. The study evaluated shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW) as the primary processes.
The following table presents recommended welding parameters:
| Parameter | SMAW | FCAW |
|---|---|---|
| Electrode/Wire Diameter | 3.2 mm | 1.2-1.6 mm |
| Welding Current | 100-160 A | 250-400 A |
| Arc Length | 3-6 mm | 5-10 mm |
| Travel Speed | 150-250 mm/min | 300-500 mm/min |
| Preheating | 150-250°C | 150-250°C |
| Interpass Temperature | <250°C | <300°C |
Weld quality is assessed through visual inspection, radiographic testing, and microhardness profiling. Typical defects include porosity (from excessive arc length or contaminated surfaces), cracks (from excessive hardness or inadequate preheating), and incomplete fusion (from insufficient heat input).
Engineering Application and Limitations
The boride overlay electrodes are particularly suitable for applications involving moderate to severe abrasive wear at moderate temperatures (below 400°C). Typical applications include:
- Mining equipment components (shovels, buckets, screens)
- Cement mill liners and grinding media
- Slurry pumps and pipelines
- Agricultural equipment (plowshares, tillage tools)
However, the boride overlay has limitations that must be considered:
- Temperature sensitivity: Borides lose hardness rapidly above 500°C, limiting applications involving high-temperature wear.
- Impact resistance: While better than carbide overlays, boride overlays can still spall under severe impact loading.
- Corrosion resistance: The boride phases are susceptible to corrosion in aggressive chemical environments.
Study Insights and Conclusions
This research contributes valuable knowledge to the field of boride-based overlay welding consumables. The findings demonstrate that boride overlays can provide competitive wear resistance to carbide overlays at significantly lower cost, making them attractive for cost-sensitive applications. The composite CrB₂-CrB system offers a good balance between hardness and toughness, and the austenitic matrix provides adequate support for the hard phases.
The study's emphasis on practical electrode development and characterization provides actionable information for engineers selecting overlay consumables. However, further research on long-term service performance, thermal cycling resistance, and corrosion-abrasion synergy would strengthen the engineering case for boride overlays. Engineers should carefully evaluate the operating environment and select boride overlays only when the temperature and chemical exposure conditions are compatible with boride stability. The findings remain relevant for engineers seeking cost-effective wear protection solutions in industrial applications.