Pre-placed Ferrosilicon Effects on High-Chromium Surfacing Alloy Microstructure and Wear Resistance

Literature Overview

This paper by Gong Jianxun, Zhang Licheng, and Yao Huiwen (2018), published in Journal of Iron and Steel Research, investigates the effects of pre-placed ferrosilicon powder on the microstructure and wear resistance of high-chromium surfacing alloys produced by self-shielded flux-cored wire open-arc welding. Supported by the Hunan Provincial Natural Science Foundation (2015JJ5031), this study from Xiangtan University addresses a significant challenge in high-chromium alloy surfacing: the brittleness associated with eutectic carbide morphology.

Core Technical Approach

High-chromium surfacing alloys (typically 20-30% Cr) are widely used for severe abrasive wear protection due to their excellent wear resistance. However, the formation of coarse lamellar eutectic carbides (M₇C₃ type) during rapid solidification creates significant brittleness, limiting the alloy's practical application in applications subject to impact loading or thermal cycling.

Pre-placed Powder Concept

The innovative approach in this study involves pre-placing alloy powder on the weld surface before applying the flux-cored wire. This technique effectively increases the alloy content of the deposited metal without proportionally increasing the welding heat input, because the pre-placed powder melts and mixes with the weld pool without requiring additional arc energy.

Effect of Ferrosilicon Content

The study systematically varied the ferrosilicon content in the pre-placed powder and examined its effects on:

Ferrosilicon Content	Primary M₇C₃ Morphology	Eutectic Carbide Morphology	Wear Loss Reduction
0% (baseline)	Lamellar/plate-like	Lamellar eutectic	Baseline
20%	Coarse plate-like	Coarse lamellar	~15%
40%	Hexagonal block-like	Granular eutectic	~25%
60%	Fine hexagonal block-like	Fine granular	~35-40%

The transformation from lamellar to granular eutectic carbide morphology represents a fundamental improvement in the alloy's fracture toughness and fatigue resistance. The hexagonal block-like primary M₇C₃ particles provide superior resistance to crack propagation compared to elongated lamellar structures.

Thermal Input and Efficiency Benefits

A remarkable finding was the significant reduction in effective thermal input per unit of deposited metal:

Parameter	Conventional Method	Pre-placed Powder Method	Improvement
Thermal input per kg deposited metal	Baseline	Reduced by 28-38%	Significant
Deposition efficiency	Baseline	Increased by 36-55%	Substantial
Residual thermal stress	High	Reduced	Improved

This dual benefit of reduced thermal input and increased deposition efficiency is achieved because the pre-placed powder contributes alloy content without requiring additional arc energy. The flux-cored wire provides the arc energy and some alloy, while the powder supplements the alloy content at no additional energy cost.

Wear Mechanism Analysis

The wear behavior was characterized through pin-on-disk testing, revealing two primary wear mechanisms:

Micro-cutting: Hard carbide particles act as cutting edges that abrade the counterface material
Micro-flaking: Progressive removal of carbide particles from the matrix due to fatigue

The improvement in eutectic carbide morphology (from lamellar to granular) specifically reduces the micro-flaking component, as granular carbides are more uniformly supported by the matrix and less susceptible to progressive detachment.

Engineering Practice Integration

The pre-placed powder technique has significant potential for industrial application in several scenarios:

Pipe and fitting hardfacing: For large-diameter pipe components requiring thick hardfacing overlays, the pre-placed powder method can significantly reduce production time and thermal distortion.
Repair applications: In field repair of worn pipe internals, pump casings, or valve components, the technique allows for rapid buildup of alloy content without excessive heat input that could damage the base material.
Multi-layer surfacing: For thick overlays requiring multiple passes, the pre-placed powder method can be applied selectively to intermediate layers to optimize the gradient of alloy composition and carbide morphology.
Cost optimization: By increasing deposition efficiency by 36-55%, the technique reduces both consumable costs and labor costs per unit of deposited metal.

The optimal ferrosilicon content range of 40-60% of the deposited metal mass provides a practical target for process optimization. In practice, this translates to specific powder application rates that must be calibrated based on wire feed rate, travel speed, and powder distribution uniformity.

Study Insights and Reflections

This study demonstrates a sophisticated understanding of the relationship between solidification microstructure and mechanical performance in high-chromium alloys. The ability to control eutectic carbide morphology through alloy composition adjustment, combined with the practical benefits of reduced thermal input and increased efficiency, makes this technique highly attractive for industrial adoption.

The concept of pre-placed powder represents a paradigm shift in surfacing technology: rather than relying solely on the welding consumable to deliver alloy content, the technique leverages the base surface as a delivery platform for additional alloy. This approach opens possibilities for creating functionally graded overlays with tailored composition profiles through selective powder placement.

One practical challenge not fully addressed is the uniformity of powder distribution. In automated welding systems, powder distribution can be precisely controlled, but in manual or semi-automated applications, achieving consistent powder coverage is more challenging. Future developments should focus on automated powder application systems integrated with the welding process.