Zhuojin Pipe Fitting Co., LtdPre-Placed Silicon Iron Effects on High-Chromium Overlay Alloy Microstructure and Wear Resistance
Literature Overview
This study by Gong Jianxun and colleagues from the School of Mechanical Engineering at Xiangtan University, published in 2018 in the Journal of Iron and Steel Research International (Volume 30, Issue 7, pages 570-575), investigates the effect of pre-placed silicon iron powder on the microstructure and wear resistance of high-chromium overlay alloys produced by flux-cored wire self-shielded arc welding. Supported by the Hunan Provincial Natural Science Foundation-Xiangtan Joint Fund (2015JJ5031), this research addresses a significant challenge in high-chromium overlay welding: the formation of brittle eutectic carbides that reduce the toughness of the overlay layer.
Core Technical Innovation
The study introduces a novel approach to overlay welding: pre-placing alloy powder on the weld bead surface before depositing the next pass with a flux-cored wire. This method allows for the introduction of alloying elements—specifically silicon—into the overlay composition without changing the base filler metal. The pre-placed powder is melted by the arc of the subsequent pass, effectively alloying the weld metal.
The key findings regarding silicon iron addition are:
| Silicon Iron Addition | Microstructural Change | Wear Resistance Improvement |
|---|---|---|
| Increasing amounts | Primary M7C3 transforms from lamellar to hexagonal blocky | Progressive improvement |
| Surrounding eutectic | Lamellar eutectic transforms to granular eutectic | Sustained improvement |
| 40-60% pre-placed powder by mass | Optimal carbide morphology | 40% wear resistance increase |
| Thermal input per unit deposit | Reduced by 28-38% | Lower residual stress |
| Deposition efficiency | Increased by 36-55% | Cost and productivity benefit |
Microstructural Analysis
The microstructural evolution with increasing silicon iron addition is particularly significant for understanding the wear resistance improvement. In conventional high-chromium overlay alloys, the primary M7C3 carbides form as elongated lamellar structures, and the eutectic carbides between the dendrites form as continuous lamellar networks. These continuous lamellar carbide networks are inherently brittle and act as crack initiation and propagation paths, limiting the toughness of the overlay.
With increasing silicon iron addition, the primary M7C3 carbides transform from lamellar to hexagonal blocky morphology with dispersed distribution. More importantly, the surrounding eutectic carbides transform from lamellar to granular morphology. This granular eutectic structure is fundamentally different from the conventional lamellar eutectic and provides a significant improvement in fracture resistance. The granular carbides are more isotropic and do not provide continuous crack paths, resulting in improved toughness without sacrificing hardness.
Wear Mechanism Analysis
The study identifies two wear mechanisms in the high-chromium overlay alloy:
- Micro-cutting by abrasive particles: Hard abrasive particles plow through the softer matrix phase, removing material as chips.
- Micro-spalling: Fragments of the material break away from the surface due to subsurface crack initiation and propagation.
The improvement in wear resistance with increasing silicon iron is attributed primarily to the reduction in micro-spalling. As the eutectic carbide morphology improves from lamellar to granular, the susceptibility to micro-spalling decreases continuously. The granular carbides provide better crack deflection and bridging, reducing the volume of material lost through spalling. The micro-cutting component of wear remains relatively unchanged, as it is primarily governed by the hardness contrast between the abrasive particles and the matrix.
Process Efficiency Benefits
The pre-placement of alloy powder provides significant process efficiency benefits beyond microstructural improvement. By adding alloy powder to the surface of the previous weld bead before depositing the next pass, the effective deposition efficiency is increased because the powder is melted and incorporated into the weld metal without requiring additional filler metal consumption. The study reports deposition efficiency improvements of 36-55%, which translates to reduced material costs and increased productivity.
Additionally, the pre-placed powder reduces the thermal input per unit of deposited metal by 28-38%. This is because the powder acts as a heat sink during the welding process, absorbing heat that would otherwise raise the temperature of the previously deposited layers. The reduced thermal input per unit deposit lowers the residual thermal stress in the overlay, reducing the risk of cracking and improving the overall integrity of the multi-pass overlay.
Engineering Application Considerations
The pre-placed powder technique is particularly suitable for applications where high-chromium overlay layers are required on large components, such as mining equipment, cement mill liners, and industrial pumps. The technique can be integrated into existing welding procedures with minimal equipment modification. The key process parameters to control are:
- Powder particle size: Fine powder (below 100 micrometers) melts more readily and provides better alloying
- Powder coverage: Uniform distribution is critical for consistent composition
- Powder mass ratio: 40-60% of the flux-cored wire deposit mass per pass provides optimal results
- Interpass temperature: Must be controlled to prevent premature oxidation of the powder
Summary
This study presents a novel and practical approach to improving the microstructure and wear resistance of high-chromium overlay alloys through the pre-placement of silicon iron powder. The transformation of eutectic carbide morphology from lamellar to granular is the key mechanism behind the improved wear resistance, and the 40% improvement in wear resistance is a significant practical benefit. The additional benefits of increased deposition efficiency and reduced thermal input per unit deposit make this technique economically attractive for industrial applications. The combination of metallurgical insight and process efficiency improvement represents a valuable contribution to the field of overlay welding technology.