Zhuojin Pipe Fitting Co., LtdBoride Hard Phase Wear-Resistant Alloy Cladding Electrode Research
Literature Overview
This paper by Xu Guojian and Liu Li from the Welding Teaching and Research Section of Shenyang University of Technology, published in Cemented Carbide (Vol. 12, No. 4, 1995, pp. 223–227), investigates a Cr-Mn-B alloy system for wear-resistant cladding electrodes. The research focuses on the formation and performance of boride hard phases in the cladding deposit, demonstrating significant wear resistance improvements and practical application success. Classified under TG422.1, this work explores an alternative hard phase strategy to carbide-based hardfacing systems.
Alloy System Design
The Cr-Mn-B alloy system was selected based on several metallurgical considerations:
- Manganese: Provides austenitic matrix stability and excellent strain hardening capability. Manganese also promotes the formation of manganese borides (MnB, Mn2B) which contribute to wear resistance.
- Chromium: Enhances oxidation resistance, promotes carbide and boride formation, and contributes to solid solution strengthening. Chromium borides (CrB2, CrB) are extremely hard phases with Vickers hardness exceeding HV2000.
- Boron: Forms a variety of boride phases including CrB2 (HV2500–HV3000), Mn2B (HV1500–HV1800), and complex ternary borides. Boron is also an effective hardening element in austenitic matrices.
The combination of these elements creates a microstructure consisting of an austenitic matrix with dispersed boride hard phases in eutectic configurations. This microstructural architecture provides a favorable combination of toughness (from the austenitic matrix) and abrasion resistance (from the boride hard phases).
Microstructural Characteristics
The cladding deposit microstructure exhibits the following features:
| Microstructural Feature | Description | Function |
|---|---|---|
| Austenitic matrix | Face-centered cubic (FCC) iron | Ductility, strain hardening, toughness |
| CrB2 particles | Monoclinic boride | Primary hard phase, wear skeleton |
| Mn2B particles | Orthorhombic boride | Secondary hard phase |
| Eutectic boride-matrix | Interconnected network | Wear skeleton, load-bearing framework |
| Grain boundaries | Austenite grain boundaries | Potential crack initiation sites |
The eutectic boride-matrix configuration is particularly important because it creates a continuous wear-resistant skeleton throughout the deposit. During abrasive wear, the hard boride phases resist penetration by abrasive particles while the austenitic matrix deforms plastically, accommodating local stress concentrations and preventing catastrophic spalling.
Performance Characterization
The wear resistance performance of this Cr-Mn-B cladding electrode was evaluated through both laboratory testing and field application:
| Performance Metric | Cr-Mn-B Cladding | 13% Mn Steel (Quenched) | 45 Steel (Quenched) |
|---|---|---|---|
| Room temperature wear resistance | ~3× Mn steel | Baseline (1×) | Lower than Mn steel |
| Field service life improvement | 5× vs. 45 steel quenched | Reference | Baseline |
| Strain hardening effect | Excellent | Good | Moderate |
| Machinability | Good | Poor (austenitic) | Good |
| Hardness (typical) | HV400–HV600 | HV400–HV500 | HV300–HV400 |
The 5-fold improvement in service life observed in the brick-making machine double-shaft mixer blade application is particularly significant. This field validation demonstrates that the laboratory wear resistance advantage translates directly into practical service benefits.
Strain Hardening Mechanism
The excellent strain hardening effect of the Cr-Mn-B cladding alloy is attributed to the austenitic matrix. Austenite (FCC) has a lower stacking fault energy than ferrite (BCC), which promotes extensive deformation twinning and martensitic transformation during plastic deformation. This transformation-induced plasticity (TRIP) effect results in:
- Work hardening: Progressive increase in hardness during wear, maintaining surface integrity.
- Transformation toughening: Stress-induced austenite-to-martensite transformation absorbs energy and delays crack propagation.
- Self-hardening: The worn surface becomes progressively harder, creating a favorable condition for sustained abrasion resistance.
Engineering Application Assessment
The practical application on brick-making machine mixer blades provides valuable insights into the real-world performance of boride-based hardfacing:
- Service environment: Abrasive wear from clay and additive materials in the brick-making process.
- Loading conditions: Combined impact and sliding wear with moderate impact energy.
- Temperature: Room temperature to slightly elevated temperatures.
- Previous solution: 45 steel with quenching treatment, providing limited service life.
- New solution: Cr-Mn-B cladding electrode, providing 5× extended service life.
The good machinability of the Cr-Mn-B cladding deposit is an additional practical advantage. Unlike high-chromium white cast irons or cobalt-based alloys, this cladding can be machined to precise dimensions after welding, enabling post-weld finishing of critical surfaces.
Key Questions and Reflections
Several technical aspects warrant further investigation:
- Boron segregation: Boron is known to segregate to grain boundaries, potentially creating brittle intergranular films susceptible to intergranular cracking. The grain boundary boride distribution should be carefully controlled through composition optimization and solidification rate management.
- Thermal stability: The stability of boride phases at elevated temperatures requires evaluation, particularly for applications involving thermal cycling.
- Multi-pass effects: In thick cladding builds, the thermal cycling from subsequent passes may alter the boride morphology and distribution, potentially affecting wear resistance.
- Cracking susceptibility: The combination of austenitic matrix and hard boride phases creates thermal expansion mismatches that may promote cracking during cooling. Preheating and interpass temperature control are essential.
The good machinability reported in this study is somewhat unusual for hardfacing deposits and merits careful verification. While the austenitic matrix provides ductility, the presence of hard boride particles can still cause rapid tool wear during machining operations.
Summary
This study demonstrates that the Cr-Mn-B alloy system offers an effective approach to developing wear-resistant cladding electrodes with boride hard phases providing a wear skeleton within a tough austenitic matrix. The 5-fold improvement in field service life and the excellent strain hardening behavior make this system particularly suitable for abrasive wear applications involving combined impact and sliding wear. The good machinability of the deposit provides an additional practical advantage for post-weld finishing. Engineers should consider this alloy system as a viable alternative to traditional carbide-based hardfacing, particularly when strain hardening and machinability are important requirements.