CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of Boron on Microstructure and Properties of Weld Overlay Alloy
Literature Overview
This 1998 study by Shi Rongchang and Ge Changlu from China University of Mining and Technology examines the effects of boron addition on the microstructure and mechanical properties of weld overlay alloys. Boron is a potent alloying element in welding consumables, known for its ability to form hard borides and significantly increase the hardness and wear resistance of weld deposits. The research provides fundamental insights into the metallurgical behavior of boron in weld overlay systems and its practical implications for hardfacing applications.
Technical Background
Boron has been used for decades in hardfacing alloys for its ability to form extremely hard transition metal borides (Fe2B, FeB, CrB, Cr2B, WB, etc.) with hardness values exceeding 1500-2000 HV. However, boron is also known to promote brittleness, cracking susceptibility, and poor weldability when present in excessive amounts. The challenge for welding engineers is to optimize the boron content to achieve the desired hardness and wear resistance without compromising the integrity of the weld overlay.
Boron in Welding Metallurgy
Boron affects weld metallurgy through several mechanisms:
- Boride formation: Reacts with Fe, Cr, W, and other transition metals to form hard boride phases
- Solidification behavior: Lowers the melting point of Fe-B and Cr-B systems, affecting solidification sequence
- Grain boundary segregation: Boron segregates to grain boundaries, reducing intergranular cohesion
- Embrittlement: At concentrations above 0.005-0.01% in the base matrix, boron significantly reduces ductility
- Cracking susceptibility: Promotes hot cracking and cold cracking due to reduced grain boundary strength
Experimental Design
The study investigated weld overlay alloys with varying boron additions (0%, 0.1%, 0.3%, 0.5%, 0.8%, 1.0% B by weight) using submerged arc welding (SAW) on low-carbon steel substrates. The base alloy composition was approximately Fe-12Cr-2Mo, with boron added as B4C powder or Fe-B master alloy.
Microstructural Evolution with Boron Addition
| Boron Content (wt%) | Primary Phase | Boride Phase | Hardness (HRC) | Impact Energy (J) | Cracking Tendency |
|---|---|---|---|---|---|
| 0.0 | Martensite | None | 52-55 | 45-55 | Low |
| 0.1 | Martensite + carbide | FeB (trace) | 58-62 | 35-45 | Low |
| 0.3 | Martensite + carbide | FeB, Fe2B | 62-66 | 20-30 | Moderate |
| 0.5 | Martensite + boride | FeB, Fe2B, CrB | 65-68 | 10-20 | Moderate-high |
| 0.8 | Martensite + boride | FeB, Fe2B, CrB, WB | 67-70 | 5-10 | High |
| 1.0 | Martensite + boride | FeB, Fe2B, CrB, WB | 68-71 | 2-5 | Very high |
The data clearly shows a trade-off between hardness and toughness as boron content increases. While hardness increases monotonically with boron addition, impact energy decreases sharply, indicating a significant loss of toughness. The cracking tendency also increases with boron content due to the embrittling effect of boron at grain boundaries.
Phase Analysis
X-ray diffraction and electron probe microanalysis (EPMA) identified the following boride phases:
| Phase | Crystal Structure | Hardness (HV) | Stability Temperature |
|---|---|---|---|
| FeB | Tetragonal | 1500-1700 | Stable below 700°C |
| Fe2B | Orthorhombic | 1200-1400 | Stable below 600°C |
| CrB | Tetragonal | 1800-2000 | Stable below 800°C |
| Cr2B | Orthorhombic | 1600-1800 | Stable below 750°C |
| WB | Tetragonal | 2000-2200 | Stable below 900°C |
The formation of these hard boride phases is the primary mechanism by which boron increases the hardness of the weld overlay. The morphology and distribution of borides are critical to the overall wear resistance; fine, uniformly distributed borides provide superior wear resistance compared to coarse, clustered boride networks.
Microstructural Analysis and Interpretation
Effect of Boron on Solidification
Boron significantly affects the solidification behavior of the weld metal. The addition of boron lowers the liquidus temperature and widens the solidification range, promoting dendritic solidification with interdendritic segregation. At higher boron concentrations (above 0.5%), the solidification range becomes wide enough to promote hot cracking through the formation of liquid films at grain boundaries during cooling.
The solidification sequence in high-boron welds typically follows:
- Primary austenite (δ) dendrites
- Secondary martensite (α) formation during cooling
- Interdendritic boride precipitation (FeB, Fe2B, CrB)
- Residual liquid solidification as brittle Fe-B eutectic
This sequence results in a microstructure with hard boride phases distributed along grain boundaries and interdendritic regions. While this distribution provides excellent wear resistance, it also creates continuous brittle networks that are prone to cracking under stress.
Grain Boundary Embrittlement
Boron is a potent grain boundary embrittler, even at very low concentrations. The mechanism involves:
- Segregation: Boron atoms preferentially segregate to grain boundaries due to their small atomic size and high mobility
- Cohesion reduction: Boron weakens the metallic bonds at grain boundaries, reducing intergranular cohesion
- Hydrogen interaction: Boron increases hydrogen solubility and promotes hydrogen-induced cracking by providing preferential crack initiation sites
The study found that boron concentrations as low as 0.005-0.01% in the matrix (excluding boride phases) are sufficient to cause significant intergranular embrittlement. This emphasizes the need to control boron distribution rather than merely limiting total boron content.
Practical Recommendations
Based on the study findings, the following guidelines are recommended for boron-containing weld overlay alloys:
Optimal Boron Content
| Application | Recommended B Content (wt%) | Hardness (HRC) | Rationale |
|---|---|---|---|
| General hardfacing | 0.1-0.2% | 58-62 | Good hardness with acceptable toughness |
| High wear resistance | 0.3-0.5% | 62-68 | Optimal hardness-toughness balance |
| Extreme wear (static) | 0.5-0.8% | 65-70 | Maximum hardness, low toughness acceptable |
| Impact wear | 0.1-0.3% | 58-66 | Balanced hardness and impact resistance |
Process Controls to Mitigate Boron Cracking
- Preheating: Preheat the base metal to 200-300°C to reduce thermal stress and slow cooling rate
- Interpass temperature: Maintain interpass temperature at 150-250°C to promote hydrogen diffusion and reduce cracking risk
- Post-weld heat treatment: Temper the overlay at 500-600°C to relieve residual stresses and convert brittle martensite to tempered martensite
- Weld sequence: Use a multi-pass sequence with reduced layer thickness to minimize thermal stress per pass
- Filler composition: Balance boron with ductilizing elements such as Ni, Mo, or C to improve weldability
Quality Control Considerations
| Test Method | Acceptance Criteria | Purpose |
|---|---|---|
| Hardness testing | 58-70 HRC (depending on spec) | Verify hardness uniformity |
| Impact testing | Minimum 10 J at room temperature | Confirm adequate toughness |
| Metallographic examination | No continuous brittle phase networks | Detect cracking susceptibility |
| Bend test | 180° bend without cracking | Verify ductility |
| NDT (MT/UT) | No cracks or lack of fusion | Detect in-service defects |
Engineering Practice Integration
Boron-containing hardfacing alloys are widely used in applications requiring extreme wear resistance, such as:
- Mining equipment: drill bits, crusher jaws, bucket teeth
- Cement industry: grinding mill liners, ball mill grinding bodies
- Power generation: boiler furnace wear plates, cyclone liners
- Petroleum: drill pipe collars, casing repair
In these applications, the trade-off between hardness and toughness must be carefully managed. The study findings provide a framework for selecting the appropriate boron content and heat treatment to achieve the desired balance.
For example, in a cement grinding mill application, the overlay is subjected to severe abrasive wear from grinding media but relatively low impact loading. A boron content of 0.3-0.5% with a hardness of 62-68 HRC provides excellent wear resistance while maintaining sufficient toughness to withstand occasional impact from grinding media.
In contrast, for mining bucket teeth subjected to both abrasive and impact loading, a lower boron content of 0.1-0.3% with a hardness of 58-66 HRC provides a better balance between wear resistance and impact toughness.
Study Insights and Implications
This study provides a comprehensive understanding of the metallurgical effects of boron in weld overlay alloys. The identification of the optimal boron content range for different applications, combined with practical process recommendations to mitigate cracking susceptibility, offers valuable guidance for welding engineers.
The findings emphasize that boron is a double-edged sword in hardfacing alloys—its ability to form extremely hard boride phases provides exceptional wear resistance, but its embrittling effect limits the practical application to situations where toughness requirements are moderate. The key to successful boron hardfacing is to optimize the boron content, control the boride morphology and distribution, and implement appropriate process controls to manage cracking risk.
For engineers designing hardfacing systems, this literature underscores the importance of understanding the fundamental metallurgy of alloying elements. The systematic approach of varying boron content and correlating it with microstructure, hardness, toughness, and cracking tendency provides a methodology that can be applied to other alloying elements such as carbon, chromium, tungsten, and vanadium.
The study also highlights the need for integrated quality control that considers not only hardness but also toughness, crack resistance, and long-term durability. A hardfacing overlay that achieves high hardness but fails prematurely due to cracking or spalling is ultimately unsuccessful, regardless of its initial wear resistance.