CLADDING TECHNOLOGY SHANXI CO., LTDBoride-Strengthened Iron-Based Cladding Alloys Microstructure and Wear Resistance Study Note
Literature Overview
This 2012 publication by Zong Lin and Ning Jianrong from the School of Mechanical Engineering at Shenyang University of Chemical Technology, appearing in Welding Technology, investigates iron-based cladding alloys strengthened by boride phases for enhanced wear resistance. The study addresses a persistent challenge in surface engineering: achieving high hardness and wear resistance in economically viable iron-based systems without relying exclusively on expensive cobalt- or nickel-based alloys.
Iron-based overlay alloys with boride strengthening represent an important class of wear-resistant materials, particularly for applications involving sliding wear against abrasive particles in mineral processing, cement grinding, and power generation equipment. The incorporation of boron into the alloy system promotes the formation of hard boride phases (FeB, Fe2B, and transition metal borides) that significantly enhance surface hardness while maintaining reasonable toughness.
Core Technical Viewpoints
Boride Phase Formation and Characterization
The microstructure of boride-strengthened iron-based overlays typically consists of a hardened matrix (martensite or austenite, depending on composition) with a network of boride phases distributed along grain boundaries and as discrete particles within the matrix. The boride morphology and distribution are strongly influenced by welding process parameters and cooling conditions.
| Boride Phase | Hardness (HV) | Crystal Structure | Stability Temperature |
|---|---|---|---|
| Fe2B | 1200–1400 | Orthorhombic | Stable below 600°C |
| FeB | 1500–1800 | Orthorhombic | Decomposes above 700°C |
| (Fe,Cr)2B | 1400–1700 | Orthorhombic | Stable below 650°C |
| CrB | 1600–1900 | Orthorhombic | Stable below 700°C |
The authors emphasize that the boride network morphology — whether it forms a continuous intergranular network or discrete islands — is the critical factor governing the wear resistance versus toughness balance. A continuous boride network provides maximum hardness but severely compromises toughness, leading to spalling failure under impact loading. Discrete boride islands dispersed in a tough matrix offer a better overall performance.
Wear Mechanism Analysis
The study identifies three primary wear mechanisms operating in boride-strengthened overlays:
- Abrasive wear — governed by the hardness of the boride phase and its volume fraction. The critical parameter is the ratio of boride hardness to abrasive particle hardness.
- Adhesive wear — controlled by the matrix composition and the presence of surface oxides. Chromium addition improves oxide film stability.
- Fatigue wear — related to subsurface crack initiation and propagation, influenced by the boride network connectivity and matrix toughness.
The wear rate is empirically correlated with the overlay hardness, boride volume fraction, and boride connectivity index. A wear resistance index can be expressed as:
WRI = H × Vb × (1 − Cb)
where H is the overlay hardness (HV), Vb is the boride volume fraction, and Cb is the boride connectivity index (0 for isolated particles, 1 for fully connected network).
Process Parameters and Microstructural Control
Effect of Welding Parameters on Boride Morphology
The welding process parameters directly influence the cooling rate and thermal cycle, which in turn govern the boride precipitation kinetics and morphology.
| Parameter | Effect on Boride Morphology | Recommended Range |
|---|---|---|
| Heat input (kJ/mm) | Lower heat input → finer borides, more discrete | 2.0–4.0 |
| Travel speed (mm/s) | Higher speed → faster cooling, finer borides | 5–15 |
| Wire feed speed (m/min) | Higher feed → higher deposition rate, more dilution | 3.0–6.0 |
| Arc voltage (V) | Higher voltage → wider bead, more dilution | 25–35 |
| Preheat temperature (°C) | Higher preheat → slower cooling, coarser borides | 100–200 |
Multi-Pass Welding Considerations
For thick overlay applications (total thickness > 8 mm), multi-pass welding introduces complex reheat cycles that modify the boride distribution in previously deposited layers. The first pass (root pass) experiences the highest cooling rate and produces the finest boride structure. Subsequent passes impose reheat temperatures on earlier layers, potentially dissolving fine borides and promoting coarsening. The optimal welding sequence involves:
- Root pass with high heat input to minimize dilution and maximize boride content
- Fill passes with moderate heat input to maintain uniform boride distribution
- Cap pass with controlled heat input to achieve a smooth surface finish
The interpass temperature must be maintained between 100–250°C to prevent excessive boride coarsening while avoiding cold cracking.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Intergranular cracking | Continuous boride network | MT/PT | Reduce B content, add B-modifying elements |
| Spalling | High boride hardness, low matrix toughness | Visual/UT | Increase matrix toughness with Ni or Mn |
| Surface porosity | Gas entrapment in slag | RT/UT | Improve flux composition, increase shielding |
| Non-uniform boride distribution | Inconsistent heat input | Metallography | Stabilize welding parameters, use automated welding |
| Excessive dilution | High heat input, thin root pass | Spectroscopy | Use transition layer, reduce heat input |
Engineering Practice Integration
In industrial applications, boride-strengthened iron-based overlays are commonly applied to:
- Cement mill grinding rollers and liners
- Coal mill grinding tables and separators
- Mineral processing chutes and hoppers
- Wind turbine gearbox components
- Hydraulic cylinder barrels
The typical service life improvement over uncoated carbon steel is 3–10 times, depending on the severity of the wear environment. For applications requiring additional corrosion resistance, chromium content can be increased to 20–25% to form a protective oxide layer while maintaining boride hardness.
Study Insights and Implications
The most valuable contribution of this research is the systematic correlation between boride morphology and wear performance. Engineers should recognize that simply increasing boron content is not a panacea — excessive boron leads to continuous intergranular boride networks that promote brittle fracture. The optimal boron content for iron-based overlays typically falls in the range of 1.5–3.5 wt%, with higher values requiring careful process control to maintain discrete boride morphology.
The concept of the boride connectivity index introduced in this study provides a practical tool for quality assessment. In production environments, this can be correlated with macroscopic hardness measurements and microstructural examination of representative coupons. A recommended quality control protocol involves:
- Hardness testing at 5 locations across the overlay surface (target: 800–1200 HV)
- Metallographic examination of a cross-section to assess boride morphology
- Wear testing using standardized dry sliding or abrasion tests
- Impact testing (Charpy V-notch) to verify toughness (target: ≥10 J at 20°C)
The research also underscores the importance of post-weld treatment. A low-temperature temper at 200–300°C can relieve residual stresses without significantly altering the boride structure, while a higher-temperature treatment at 500–600°C may be used to refine the boride morphology by partial dissolution and re-precipitation. The selection of PWHT parameters must be based on the specific alloy composition and the required service conditions.