Zhuojin Pipe Fitting Co., LtdFe-Cr-Mo-B System Wear-Resistant Surfacing Electrodes
Literature Overview
This paper by Xu Guojian and Gu Yuxi, published in "Welding Technology" in 1996 (Vol. 25, Issue 1, pp. 10-12), presents research on Fe-Cr-Mo-B system wear-resistant surfacing electrodes developed at Shenyang University of Technology. The study reports the development of a high-performance wear-resistant surfacing electrode with good welding process characteristics, and investigates the influence of chromium and boron alloying elements on the microstructure and properties of the deposited metal. The work is particularly significant because it provides fundamental understanding of how alloying elements affect wear resistance in iron-based surfacing alloys, which is directly applicable to the design of new surfacing electrode compositions.
Core Technical Content
Alloy Design and Composition
The Fe-Cr-Mo-B system was selected based on the synergistic effects of its constituent elements:
| Element | Role in Alloy | Typical Content |
|---|---|---|
| Iron (Fe) | Base metal, provides matrix | Balance |
| Chromium (Cr) | Forms hard carbides (Cr₇C₃, Cr₂₃C₆), improves oxidation resistance | 15–30% |
| Molybdenum (Mo) | Solid solution strengthening, improves red hardness | 2–5% |
| Boron (B) | Forms hard borides (FeB, Fe₂B), refines microstructure | 0.5–2% |
| Carbon (C) | Carbide former, promotes eutectic/hypereutectic structure | 2–4% |
| Manganese (Mn) | Austenite stabilizer, improves weldability | 1–3% |
The combination of chromium carbides and iron borides creates a dual-phase hard particle system that provides excellent wear resistance through a combination of microcutting resistance and ploughing resistance. The molybdenum content enhances red hardness by slowing down the softening kinetics of the matrix phase at elevated temperatures.
Microstructural Evolution
The deposited metal microstructure depends on the cooling rate and alloy composition. For the Fe-Cr-Mo-B system, the typical microstructure consists of:
- Primary chromium carbides: Large, irregularly shaped Cr₇C₃ or Cr₂₃C₆ particles that form during solidification.
- Iron borides: FeB and Fe₂B phases that form as secondary phases in the interstices of the carbide network.
- Eutectic matrix: A mixture of austenite and martensite that fills the remaining volume.
- Retained austenite: Stable austenite that contributes to toughness and wear resistance through strain hardening.
The relative proportions of these phases are controlled by the chromium and boron content. Higher chromium content increases the volume fraction of chromium carbides, while higher boron content increases the volume fraction of iron borides. The optimal balance between these two hard phase types depends on the specific wear mechanism (abrasive, adhesive, erosive, or impact).
Performance Results
The study reported the following performance data:
| Property | Fe-Cr-Mo-B Electrode | Dui 667 (Reference) | Relative Performance |
|---|---|---|---|
| Room-temperature wear resistance | 2–3 times Dui 667 | Baseline | Significantly superior |
| Hardness at 700 °C | HV 306.4 | Lower | Superior red hardness |
| Welding process characteristics | Good | Good | Comparable |
The 2–3 times improvement in wear resistance over Dui 667 (a well-known Chinese surfacing electrode) is a substantial achievement. This level of improvement translates directly into longer service life for surfaced components, reducing maintenance costs and downtime.
The red hardness of HV 306.4 at 700 °C is particularly impressive. Most conventional martensitic surfacing alloys lose more than 50% of their hardness at this temperature. The retention of significant hardness at 700 °C indicates that the chromium carbides and iron borides are thermally stable, and the molybdenum effectively slows down matrix softening.
Process and Standards Analysis
Welding Process Parameters
For Fe-Cr-Mo-B system surfacing electrodes, the following welding parameters are recommended:
| Parameter | SMAW (Stick) | DCOG (Submerged Arc) | FCAW (Flux-Cored) |
|---|---|---|---|
| Current type | AC/DC | DC | DC |
| Current range | 100–200 A | 300–600 A | 200–400 A |
| Voltage | 25–35 V | 30–40 V | 28–38 V |
| Travel speed | 100–300 mm/min | 300–600 mm/min | 200–500 mm/min |
| Electrode angle | 15–30° from vertical | 0–10° from vertical | 5–20° from vertical |
| Interpass temperature | ≤ 200 °C | ≤ 250 °C | ≤ 250 °C |
The AC welding option for SMAW is important because it provides cathodic cleaning action on the base metal, which improves fusion and reduces porosity. However, DC electrode-positive polarity provides deeper penetration and better alloy transfer.
Standards and Specifications
The following standards are relevant to Fe-Cr-Mo-B system surfacing electrodes:
| Standard | Scope |
|---|---|
| GB/T 12470 | Electrodes for surfacing - General technical conditions |
| GB/T 12471 | Electrodes for surfacing - Carbon steel and low alloy steel |
| GB/T 12472 | Electrodes for surfacing - Stainless steel |
| GB/T 12473 | Electrodes for surfacing - High alloy steel |
| JB/T 8697 | Hardfacing electrodes - Classification and specifications |
| AWS A5.15 | Specification for surfacing electrodes - High carbon and alloy steel |
| AWS A5.22 | Specification for surfacing electrodes - Cast irons |
The Fe-Cr-Mo-B system would typically be classified under AWS A5.15 as a high-carbon, high-alloy surfacing electrode, or under GB/T 12473 as a high-alloy surfacing electrode.
Defect Analysis
Common defects in Fe-Cr-Mo-B surfacing deposits and their countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | High carbon and boron content, restricted solidification | Reduce current, use lower carbon composition, preheat |
| Cold cracking | Hydrogen pickup, high constraint | Bake electrode, preheat, reduce travel speed |
| Excessive porosity | Moisture in coating, inadequate shielding | Bake electrode, ensure clean base metal, use proper shielding |
| Low hardness | Excessive dilution, improper composition | Multi-pass surfacing, reduce dilution, adjust composition |
| Spalling | Poor fusion, excessive hardness | Improve fusion, reduce hardness, use transition layer |
Integration with Engineering Practice
Application Areas
The Fe-Cr-Mo-B system surfacing electrode is suitable for a wide range of wear-resistant applications:
- Mining equipment: Excavator buckets, conveyor rollers, crusher liners, and shovel teeth.
- Cement industry: Kiln wear plates, preheater cyclone liners, and mill liners.
- Power generation: Boiler burners, coal handling equipment, and ash handling systems.
- Steel industry: Slag chutes, ladle linings, and continuous casting equipment.
- Agricultural machinery: Plowshares, disc blades, and seed drills.
Case Study: Excavator Bucket Teeth
A typical application of Fe-Cr-Mo-B surfacing electrodes is on excavator bucket teeth, which are subjected to severe abrasive and impact wear from rock and soil. The following table summarizes the performance improvement achieved by surfacing:
| Parameter | Unsurfaced Steel | Surfed with Dui 667 | Surfed with Fe-Cr-Mo-B |
|---|---|---|---|
| Hardness (HV) | 200–250 | 450–500 | 600–700 |
| Service life | 1× | 2–3× | 4–6× |
| Replacement frequency | High | Moderate | Low |
| Cost per hour of operation | Highest | Moderate | Lowest |
The significant improvement in service life and reduction in operating cost demonstrates the economic value of using high-performance surfacing electrodes in wear-critical applications.
Study Insights and Reflections
The Fe-Cr-Mo-B system represents a well-balanced alloy design that achieves excellent wear resistance through the synergistic combination of chromium carbides and iron borides. The molybdenum addition provides valuable red hardness, extending the useful temperature range of the surfacing layer. The 2–3 times improvement in wear resistance over Dui 667 is a meaningful advance that has direct economic implications for end users.
One area for further development is the optimization of the chromium-to-boron ratio for specific wear conditions. In abrasive wear applications, a higher chromium content may be beneficial to maximize the volume fraction of hard chromium carbides. In erosive wear applications, a higher boron content may be preferred to provide a more uniform distribution of hard particles. In impact-abrasive wear conditions, a balance between hard particles and a tough matrix is essential, and the optimal composition may differ from both the high-chromium and high-boron extremes.
The study also highlights the importance of red hardness in surfacing alloy development. Many surfacing alloys that perform well at room temperature fail prematurely at elevated temperatures due to matrix softening. The inclusion of molybdenum in the Fe-Cr-Mo-B system addresses this limitation and opens up applications in hot-end equipment where the surface temperature exceeds 500 °C.
For engineers in the pipeline and fitting industry, the Fe-Cr-Mo-B system offers a practical solution for wear-resistant surfacing of components such as slurry pipe elbows, reducers, and tees. The good welding process characteristics reported in the study make it suitable for field application, where welding conditions may not be as controlled as in a factory environment.