CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of High-Hardness High-Wear-Resistant Overlay Welding Electrodes
Literature Overview
This research, published in Welding Technology in 2014 by Li Ming, Wu Jingran, Jiang De, and Tang Lisong from Chengde Petroleum College, focuses on the development of shielded metal arc welding (SMAW) electrodes specifically designed for high-hardness, high-wear-resistant overlay applications. The work addresses the practical need for affordable, field-applicable hardfacing electrodes that can be used in repair and maintenance scenarios where specialized equipment is unavailable.
Electrode Design Philosophy
The development of high-hardness overlay electrodes follows a systematic approach based on the 5W2H framework:
- What: Electrode composition design to achieve target hardness (HRC 55–65) and wear resistance
- Why: Extend service life of wear-critical components in mining, construction, and petroleum industries
- Who: Field welders with limited access to specialized equipment
- Where: On-site repair of pumps, valves, excavator buckets, and conveyor components
- When: During planned maintenance or emergency repair
- How: Through optimization of flux composition, wire core alloy, and welding parameters
- How much: Target hardness HRC 55–65 with acceptable toughness
Electrode Composition Design
The electrode is composed of two main parts: the wire core (alloy) and the flux coating.
Wire core composition (typical):
| Element | Content (wt%) | Role |
|---|---|---|
| C | 2.5–4.0 | Forms carbides with Cr, Mo, V, W |
| Cr | 8–12 | Forms Cr7C3 and Cr23C6 carbides |
| Mo | 3–6 | Forms Mo2C, MoC carbides |
| V | 1–3 | Forms VC, V4C3 carbides |
| Mn | 1.0–2.0 | Deoxidizer, austenite stabilizer |
| Si | 0.5–1.5 | Deoxidizer |
| Balance | Fe | Matrix |
Flux coating composition (typical):
| Component | Content (wt%) | Function |
|---|---|---|
| Rutile (TiO2) | 15–25 | Arc stability, slag fluidity |
| Felspar | 10–15 | Slag viscosity control |
| Iron oxide (Fe2O3) | 5–10 | Alloying addition, slag oxidizer |
| Manganese dioxide (MnO2) | 3–5 | Deoxidizer, arc stabilizer |
| Calcium carbonate (CaCO3) | 5–8 | Gas shielding, slag basicity |
| Sodium silicate binder | 5–8 | Coating adhesion |
| Hard alloy powder (WC, Cr3C2) | 10–20 | Direct hard phase addition |
The flux coating serves multiple functions: providing gas shielding, adding alloying elements to the weld metal, controlling solidification rate, and delivering hard alloy particles directly into the weld pool.
Microstructure and Hardness
The hardness of the overlay deposit is primarily determined by the type, size, volume fraction, and distribution of hard carbide phases. The key carbide phases formed include:
- Cr7C3: Orthorhombic, hardness ~1800 HV, forms preferentially at lower carbon activity
- Cr23C6: Monoclinic, hardness ~1400 HV, forms at higher Cr/C ratios
- Mo2C: Hexagonal, hardness ~1800 HV, very hard and wear-resistant
- VC: Cubic, hardness ~2800 HV, extremely hard but forms only at high V concentrations
- Fe3C (cementite): Orthorhombic, hardness ~1200 HV, moderate wear resistance
The matrix microstructure is typically a mixture of martensite and retained austenite. The retained austenite content (controlled by Mn, Ni, C content) is critical for toughness. A retained austenite fraction of 15–30% provides an optimal balance between hardness and crack resistance.
Hardness Optimization Strategy
Based on the study's findings, the following strategies maximize hardness:
- Increase carbon content to 3.0–4.0 wt% to maximize carbide volume fraction
- Add multiple carbide-forming elements (Cr + Mo + V) to create a multi-carbide system with complementary properties
- Incorporate hard alloy powder (WC, Cr3C2) in the flux coating for direct hard phase reinforcement
- Control cooling rate through flux thickness and welding parameters to refine carbide morphology
Welding Performance and Defect Analysis
SMAW overlay welding introduces several potential defects that must be controlled:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | High carbon, low toughness | Add Ni (2–5%) to increase retained austenite |
| Porosity | Gas absorption from flux | Ensure proper flux storage and drying (250 °C × 2 h) |
| Incomplete fusion | Insufficient heat input | Use appropriate current (120–200 A for 3.2 mm electrode) |
| Excessive dilution | High travel speed, large bead size | Use weaved bead pattern, control heat input |
| Hardness variation | Uneven carbide distribution | Maintain consistent travel speed and arc length |
The dilution rate for SMAW overlay is typically 20–40%, which is lower than SAW but higher than GTAW. Multi-pass welding is often required to achieve the target hardness, with the first pass serving as a transition layer and subsequent passes building up the hardfacing layer.
Engineering Application and FMEA
From a failure mode and effects analysis (FMEA) perspective, the critical failure modes for high-hardness overlay electrodes in service are:
- Abrasive wear: The primary wear mechanism. Controlled by carbide hardness and matrix support. Target: >1000 HV for severe abrasion.
- Impact loading: High-hardness coatings are inherently brittle. The retained austenite fraction and carbide morphology determine impact resistance. For impact-critical applications, a dual-layer approach (tough first layer, hard second layer) is recommended.
- Corrosion fatigue: In corrosive environments, the overlay may suffer from selective leaching of the matrix, exposing the carbides. Adding Cr (8–12%) provides adequate corrosion resistance for most industrial environments.
- Thermal cycling: Repeated heating and cooling can cause thermal fatigue cracking. The ductile matrix (retained austenite) absorbs thermal stresses and prevents crack propagation.
Study Insights and Practical Recommendations
The research demonstrates that SMAW electrodes can achieve hardness levels comparable to more specialized processes (PTA, laser cladding) while offering superior field applicability. The key to success lies in the synergistic design of the wire core alloy and flux coating composition.
For industrial implementation, the following recommendations are made:
- Develop a qualified WPS per AWS D10.9 or ISO 14555 for each electrode type
- Perform hardness profiling across the overlay cross-section to verify uniformity
- Conduct wear testing (ASTM G99 for dry sliding, ASTM G65 for pin-on-disc) under representative service conditions
- Establish a maintenance protocol for periodic overlay renewal based on wear rate measurements
- Train field welders on proper technique, including electrode drying, arc length control, and travel speed consistency
The economic advantage of SMAW overlay electrodes is significant: the equipment cost is minimal (a standard SMAW machine), the consumable cost is low, and the process is portable. This makes it ideal for large-scale repair operations in mining, construction, and petroleum industries where component downtime is costly.