Effect of Sodium Silicate Baume Degree on Composite Powder Overlay Welding Alloy Microstructure and Wear Resistance

Literature Overview

This study published in Hot Working Technology (2024, Vol. 53, Issue 19) by Gong Jianxun et al. from Xiangtan University investigates a novel self-shielded open-arc welding method that combines composite powder particles with H08A solid wire to produce high-chromium wear-resistant alloys. The core variable examined is the Baumé degree of sodium silicate (water glass) used as a binder for powder agglomeration, with particular attention to how this seemingly simple process parameter influences elemental transfer coefficients, microstructure evolution, and ultimately wear performance.

Core Technical Findings

The research employs XRD, SEM, EDS, and pin-on-disc wear testing to systematically characterize the overlay deposits across different water glass Baumé degrees. The key finding is that water glass serves not merely as a mechanical binder but as a functional process modifier that fundamentally alters the arc stability and metallurgical outcomes.

Key Results Summary

Water Glass Baume Degree	Arc Stability	Spatter Level	Microstructure Type	Primary Phase	Relative Wear Resistance
0 (no binder)	Unstable	High	Sub-eutectic	M7C3 + austenite matrix	Baseline (1.0x)
Intermediate	Moderate	Moderate	Transition	Mixed carbides	3–5x
25	Stable	Low	Hypereutectic	Primary M7C3 significantly increased	7.6x

Elemental Transfer Coefficient Analysis

A critical insight from this work is the progressive increase in alloying element transfer coefficients as the Baume degree rises. This is attributed to the improved arc confinement and reduced spatter loss when the powder particles are properly agglomerated. The composite powder, once bound by sodium silicate, burns more uniformly in the arc zone, allowing more complete vaporization and subsequent condensation of Cr, Mo, and other alloying elements into the molten pool.

Microstructure Evolution Mechanism

The transition from sub-eutectic to hypereutectic structure is the most metallurgically significant finding. In the sub-eutectic regime (low Baume degree), the cooling sequence favors austenite dendrite formation with interdendritic M7C3 carbide precipitation. As the Baume degree increases to 25, the higher Cr content in the weld pool shifts the solidification path, promoting primary M7C3 carbide nucleation ahead of the austenite dendrites.

This structural transformation has direct implications for wear resistance through the following mechanisms:

Hard phase volume fraction — Primary M7C3 carbides provide a much higher hard phase content than interdendritic carbides, creating a more continuous hard-phase network.
Carbide morphology — Primary carbides tend to be more blocky and well-distributed, resisting fracture under abrasive contact.
Matrix-carbide interface integrity — The hypereutectic structure maintains stronger bonding between the hard phase and the tough austenite matrix, reducing interfacial debonding during sliding.

Wear Mechanism Analysis

The wear mechanism transition from combined micro-ploughing and micro-flaking (low Baume degree) to predominantly micro-flaking (high Baume degree) is particularly instructive. Micro-ploughing indicates that soft matrix material is being displaced by hard asperities on the counterface, while micro-flaking involves the fracturing and removal of hard phase particles embedded in a tougher matrix. The shift toward micro-flaking dominance at Baume degree 25 confirms that the hard-phase network is sufficiently robust to resist ploughing, and the matrix is tough enough to retain the carbides under load.

Engineering Practice Implications

Process Window Considerations

For production implementation of this composite powder + solid wire method, several practical considerations emerge:

Baume degree control: The optimal range appears to center around 25°Bé, but the viscosity of sodium silicate at this concentration must be compatible with the powder feeding system. Higher Baume degrees increase viscosity, potentially causing bridging in powder hoppers.
Arc stability: The improved arc stability at 25°Bé is critical for multi-pass overlay welding on thick sections. Unstable arcs at low Baume degrees would lead to uneven dilution and inconsistent hardness across the overlay.
Heat input management: With improved elemental transfer, the effective heat input to the workpiece may be slightly reduced compared to unbonded powder, which could benefit distortion control in overlay welding of large pipe sections.

Application Scenarios

This technology is particularly relevant for:

Overlay repair of rotating equipment components (crushers, grinders, mill liners)
Surface hardening of carbon steel pipe fittings in abrasive service
Field repair welding where self-shielded processes offer flexibility without external gas supply

Critical Reflections

The 7.6x improvement in wear resistance at 25°Bé is remarkable, but it is essential to consider that wear resistance alone does not determine service life. The hypereutectic structure with high primary M7C3 volume fraction may exhibit reduced impact toughness and increased susceptibility to thermal cracking during multi-pass welding. In engineering practice, the trade-off between wear resistance and crack resistance must be evaluated for the specific service conditions.

Furthermore, the sodium silicate binder introduces additional Si and Na into the weld system. While the study focuses on the positive effects of improved arc stability, the potential for Na-induced hot cracking in the heat-affected zone of thick-section steel components warrants further investigation. Sodium silicate fluxes are well-known to contribute to alkaline slag systems, which can influence hydrogen pickup and cold cracking susceptibility in high-carbon equivalent steels.

The study also does not address the long-term stability of the composite powder particles during storage. Water glass binder systems can age, leading to increased particle hardness and potential feeding difficulties over time. For production environments, powder storage conditions and shelf life specifications should be established.

Study Insights and Outlook

This work demonstrates that a simple process parameter — the concentration of a common industrial chemical — can profoundly influence the metallurgical outcome of overlay welding. The systematic approach of varying Baume degree and tracking the cascading effects on arc behavior, elemental transfer, microstructure, and wear performance provides a clear methodology that can be applied to other powder-binder systems.

Future research should explore: (1) the combined effect of Baume degree and powder composition on overlay properties; (2) multi-pass overlay welding with different Baume degree settings per pass to achieve tailored microstructures from surface to root; (3) field trials on actual industrial components to validate laboratory wear test results under real operating conditions. The potential to optimize a self-shielded overlay process for field applications without external shielding gas is particularly attractive for pipeline repair and maintenance operations.