Application of Medium-Temperature Phosphating Technology in Plastic Processing of Steel Pipes

Literature Overview

This paper by Chu Xu and colleagues from Shenyang Paka Seimitsu Co., Ltd., published in Materials Protection (Vol. 54, No. 2, 2021), investigates the application of medium-temperature phosphating technology in the plastic forming of steel pipes. The authors systematically characterized the corrosion resistance of phosphated steel pipes using scanning electron microscopy (SEM), X-ray diffraction (XRD), damp heat testing, copper sulfate drip testing, and membrane stripping methods. The study ultimately established optimal process parameters for medium-temperature phosphating treatment of steel pipes used in plastic forming operations.

Core Technical Parameters and Process Optimization

The paper identifies a well-defined process window for medium-temperature phosphating, which is critical for engineers working on pipe forming and welding applications. The optimal parameters are summarized below.

The phosphating membrane composition was identified as Zn₂Fe(PO₄)₂·4H₂O through XRD analysis. The coating weight of 7.42 g/m² and thickness of 3.0 μm indicate a moderately thick phosphating layer, which is important for the subsequent forming and welding operations.

Characterization Methods and Results Interpretation

The authors employed a multi-method characterization approach, which is commendable from a quality assurance standpoint. The SEM examination revealed that the phosphating membrane at the optimal parameters exhibited dense crystallization with uniform grain size distribution. This is significant because non-uniform grain structures often lead to localized corrosion initiation sites, which would compromise the service life of the pipe during plastic deformation.

The copper sulfate drip test demonstrated a corrosion resistance time exceeding 300 seconds, which represents a substantial improvement over untreated steel pipe surfaces. The damp heat test results further confirmed enhanced corrosion resistance. From a practical perspective, these results indicate that medium-temperature phosphating provides a robust pretreatment layer that can withstand the mechanical and thermal stresses encountered during plastic forming operations such as bending, flanging, and welding.

Engineering Practice Implications

From a metallurgical and welding engineering perspective, the application of phosphating in pipe plastic processing deserves careful consideration. In the production of pipe fittings such as elbows, tees, and reducers, the steel pipe undergoes significant plastic deformation. The phosphating layer serves as a lubricant and anti-corrosion barrier during these operations. However, several engineering concerns arise:

• The phosphating layer may crack or spall during severe plastic deformation, exposing the base metal to subsequent corrosion.
• During welding, the phosphating layer can decompose, releasing phosphoric acid vapors that may affect weld pool chemistry and weld quality.
• The presence of the Zn₂Fe(PO₄)₂·4H₂O layer between the base metal and the weld metal could influence the weld metal composition and mechanical properties.

The medium temperature of 70 °C is advantageous compared to high-temperature phosphating (typically 80–95 °C) because it reduces energy consumption and minimizes thermal distortion of the pipe blank. However, the relatively thin coating (3.0 μm) may not provide sufficient protection during aggressive forming operations.

Key Questions and Reflections

The paper does not address the behavior of the phosphating layer during the actual welding process. In engineering practice, when phosphated steel pipes are welded to form fittings, the heat-affected zone (HAZ) experiences temperatures that can exceed the decomposition point of the phosphating layer. This raises questions about whether the phosphating treatment is compatible with subsequent welding operations, or whether the layer must be removed prior to welding. Additionally, the study lacks discussion on how the phosphating treatment affects the formability of the steel pipe during bending and expansion operations, which is a critical parameter for pipe fitting manufacturers.

Study Insights and Reference Value

This paper provides valuable baseline data for engineers selecting surface pretreatment processes for steel pipes destined for plastic forming and welding. The systematic characterization approach and well-defined process parameters offer a reliable reference for process development in pipe fitting manufacturing. However, the study would benefit from additional investigation into the interaction between the phosphating layer and welding processes, as well as the durability of the coating during severe plastic deformation. Engineers should consider this work as a starting point for developing integrated surface treatment and forming-welding process chains.