Effect of Additives on Microstructure and Properties of Centrifugal SHS Ceramic Composite Steel Pipes

Literature Overview

This paper by Zhang Shuguang and colleagues, published in the Chinese Journal of Nonferrous Metals in 2002 (Volume 12, Issue 4, pages 639–642), investigates the influence of ZrSiO₄ and TiO₂ additives on the microstructure and mechanical properties of centrifugally manufactured Self-Propagating High-temperature Synthesis (SHS) ceramic composite steel pipes. The study focuses on the Al–Fe₂O₃ system, which is a well-established SHS reaction pair for producing alumina-based ceramic coatings on steel substrates for corrosion and wear resistance applications.

Core Technical Findings

The research systematically examines two additive types and their effects on ceramic layer density, phase composition, and bond strength with the steel substrate. The key findings are summarized below:

Technical Analysis of Additive Mechanisms

TiO₂ Additive Mechanism

The TiO₂ additive demonstrates superior performance through multiple mechanisms. During the SHS reaction, TiO₂ participates in the formation of Fe₄(TiO₄)₃, which acts as a liquid-phase sintering agent at elevated temperatures. This liquid phase fills the intergranular voids in the forming Al₂O₃ matrix, resulting in the observed 42% reduction in porosity at 10% TiO₂ addition. The densification effect directly improves the mechanical properties of the ceramic layer and enhances the bond strength with the steel substrate by reducing the defect population at the interface.

From a welding and joining perspective, the improved densification is particularly significant because porous ceramic coatings are prone to delamination under thermal cycling or mechanical impact. In pipeline applications, where the ceramic-lined steel pipe may be subjected to welding during installation, the reduced porosity and improved bond strength translate to better resistance against coating spallation at weld heat-affected zones.

ZrSiO₄ Additive Mechanism

The ZrSiO₄ additive decomposes during the SHS reaction, releasing ZrO₂ which transforms to the monoclinic phase (m-ZrO₂) at lower temperatures. The m-ZrO₂ phase introduces microcracks through stress-induced phase transformation, providing a crack-bridging toughening mechanism. However, the absence of densification means that the overall porosity of the ceramic layer remains high, and the bond strength initially decreases with increasing ZrSiO₄ content. The subsequent slight recovery in bond strength at higher ZrSiO₄ levels may be attributed to the improved fracture toughness of the ceramic layer, which reduces the driving force for interface debonding.

Process Parameters and Manufacturing Considerations

The centrifugal SHS process involves spinning a steel tube at high speed while igniting a packed powder mixture of Al and Fe₂O₃ (with additives) inside the tube. The centrifugal force compacts the reacting mixture against the steel tube wall, creating a dense ceramic layer. Key process parameters include:

1. Rotational speed: Typically 500–1500 rpm, controlling the compaction pressure on the reacting mixture
2. Ignition energy: Sufficient to initiate the exothermic reaction (approximately 1500 kJ/mol for Al + Fe₂O₃)
3. Reaction temperature: Peak temperatures of 2500–3000°C are reached locally
4. Cooling rate: Influences the final phase composition and residual stress state

The centrifugal SHS method offers several advantages over conventional ceramic coating techniques: it produces thick coatings (typically 5–20 mm), requires no external heat source after ignition, and can be applied to pipes of various diameters and lengths. However, the process is sensitive to powder packing density, tube surface cleanliness, and rotational stability.

Defect Analysis and Countermeasures

Engineering Application Implications

For pipeline engineers, the TiO₂-additive approach represents a practical solution to improve the durability of SHS ceramic-lined steel pipes used in aggressive environments such as acid mine drainage, chemical processing, and offshore oil and gas production. The 42% porosity reduction translates to significantly improved corrosion resistance because the primary corrosion pathway through porous ceramic coatings is the penetration of corrosive media through interconnected pores to the steel substrate.

The study's findings also have implications for welding procedures applied to ceramic-lined pipes. When welding the steel substrate of a ceramic-lined pipe, the heat input must be carefully controlled to avoid thermal cracking of the ceramic layer. The TiO₂-improved ceramic layer, with its reduced porosity and enhanced thermal crack resistance, should tolerate moderate welding heat input more effectively than the unmodified coating. However, the thermal expansion coefficient mismatch between the Al₂O₃-based ceramic (approximately 8 × 10⁻⁶/°C) and carbon steel (approximately 12 × 10⁻⁶/°C) remains a fundamental challenge that must be managed through welding procedure qualification.

Study Insights and Outlook

The research clearly demonstrates that TiO₂ is the superior additive for improving the performance of SHS ceramic composite steel pipes in the Al–Fe₂O₃ system. The mechanism of action—liquid-phase sintering through Fe₄(TiO₄)₃ formation—is well understood and can be further optimized by varying the TiO₂ content and particle size. Future work should investigate the combined effect of TiO₂ with other additives such as SiO₂ or CaO to further improve both densification and thermal shock resistance. Additionally, long-term durability testing under actual service conditions, including thermal cycling and mechanical impact, would provide valuable data for engineering design. The centrifugal SHS technology, when properly optimized with appropriate additives, offers a cost-effective solution for producing corrosion-resistant steel pipes with thick ceramic linings suitable for demanding industrial applications.