CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Water Glass Baumé Degree on Microstructure and Properties of Composite Powder and Solid Wire Cladding Alloys
Literature Overview
This 2024 publication by Gong Jianxun, Liu Shutong, Zhang Minghua, and Ai Xiaowen from the School of Mechanical Engineering and Mechanics at Xiangtan University, supported by the Hunan Provincial Natural Science Foundation (Project No. 2021JJ30669), investigates the influence of water glass flux density — expressed in Baumé degree — on the microstructure and mechanical properties of cladding alloys produced using composite powder and solid wire as filler materials. Water glass (sodium silicate) is widely used as a flux in submerged arc welding and flux-cored arc welding cladding operations, serving as both a shielding agent and a chemical modifier. The Baumé degree directly correlates with the silica content and viscosity of the flux, which in turn affects the arc stability, slag composition, and ultimately the weld metal quality.
Core Technical Findings
The researchers systematically varied the water glass Baumé degree across a range of 25°Bé to 45°Bé and examined its effects on two filler material configurations: composite powder (a mixture of alloy powders and flux particles) and solid wire (conventional cladding wire). The study revealed that the flux density has a profound and non-linear influence on the resulting cladding layer properties.
| Baumé Degree | Silica Content (%) | Viscosity (mPa·s at 1500°C) | Arc Stability | Dilution Rate (%) | Hardness (HV) |
|---|---|---|---|---|---|
| 25°Bé | 40–45 | 180–220 | Moderate | 22–28 | 280–320 |
| 30°Bé | 48–53 | 250–300 | Good | 18–24 | 320–380 |
| 35°Bé | 55–60 | 320–380 | Good | 15–20 | 380–450 |
| 40°Bé | 62–67 | 400–480 | Excellent | 12–18 | 450–520 |
| 45°Bé | 68–73 | 500–600 | Poor (viscous) | 10–15 | 500–580 |
At lower Baumé degrees (25–30°Bé), the flux has lower viscosity and higher fluidity, which facilitates better gas shielding but also promotes greater dilution of the base metal into the weld pool. This results in a cladding layer with lower alloy content, reduced hardness, and potentially compromised corrosion resistance. At higher Baumé degrees (40–45°Bé), the increased flux viscosity creates a more stable slag cover, reducing dilution and preserving the alloy composition of the cladding layer. However, excessively high viscosity impairs slag flowability, leading to incomplete slag removal, slag inclusion defects, and potential undercut formation.
Microstructural Analysis
The microstructural evolution of the cladding alloys was found to be strongly dependent on the flux density. At 30–35°Bé, the cladding microstructure exhibited a balanced combination of ferrite and martensite phases, with carbide precipitates uniformly distributed. The cooling rate in this range was approximately 8–12 °C/s, promoting favorable grain refinement without excessive martensite formation.
At 40–45°Bé, the reduced dilution rate allowed the alloying elements (Cr, Mo, Mn, and C) to remain at higher concentrations in the weld metal, promoting the formation of harder carbide phases such as M7C3 and M23C6. However, the increased cooling rate (12–18 °C/s) due to the more insulating slag layer also promoted martensitic transformation, which, while beneficial for hardness, increased the susceptibility to cold cracking.
The composite powder configuration demonstrated superior performance compared to solid wire across all Baumé degree ranges. The composite powder, with its pre-mixed alloy particles, provided more uniform alloy distribution in the weld pool and reduced the sensitivity to flux density variations. This finding has significant implications for the selection of filler materials in industrial cladding operations.
Process Optimization Recommendations
Based on the study's findings, the following process optimization guidelines are recommended for practical application:
- For solid wire cladding, an optimal Baumé degree range of 32–37°Bé provides the best balance between dilution control, arc stability, and mechanical properties.
- For composite powder cladding, a slightly higher Baumé degree range of 35–40°Bé is recommended to maximize the alloy retention benefits.
- The interpass temperature should be maintained at 150–250°C to prevent excessive cooling rate while avoiding grain coarsening.
- Slag removal between passes must be thorough, particularly at higher Baumé degrees where slag viscosity increases and removal becomes more difficult.
- Post-weld heat treatment at 550–600°C for stress relief is essential for cladding layers with hardness exceeding 500 HV to prevent delayed cracking.
Engineering Practice Integration
In the context of bimetal pressure vessel fabrication, the selection of flux density is a critical process parameter that directly affects the serviceability of the cladding layer. For hydrogenation reactors and other high-pressure vessels where the overlay layer must withstand both mechanical stress and corrosive environments, the dilution rate and resulting alloy composition are of paramount importance. A dilution rate exceeding 25% can significantly reduce the corrosion resistance of stainless steel overlay layers, potentially leading to intergranular corrosion or pitting under service conditions.
The study's findings also have implications for the qualification of welding procedures under standards such as NB/T 47014 and ASME IX. The Baumé degree should be specified as a controlled variable in the welding procedure specification, with acceptable ranges defined based on the specific application requirements. Furthermore, the flux density should be monitored and recorded during production to ensure traceability and quality control.
Study Insights and Implications
This study makes a valuable contribution to the understanding of flux chemistry in cladding operations. The systematic investigation of water glass Baumé degree across a wide range, combined with detailed microstructural and mechanical property analysis, provides a comprehensive framework for flux selection and process optimization. The finding that composite powder filler materials are less sensitive to flux density variations than solid wires is particularly significant for industrial applications where process consistency is challenging to maintain. The research was conducted under the auspices of the Hunan Provincial Natural Science Foundation, reflecting the growing recognition of applied welding research in China's academic community. For practicing engineers, the key takeaway is that flux density is not merely a consumable specification but a critical process variable that must be actively managed to achieve the desired cladding quality.