CLADDING TECHNOLOGY SHANXI CO., LTDCrack Formation Mechanism in Centrifugal SHS Ceramic Composite Steel Pipe
Literature Overview
This seminal 1999 study by Zhang Shuguang, Zhang Baoping, and Wang Kezhi (published in Journal of the Chinese Ceramic Society) investigates the crack formation mechanism in centrifugal silica hollow sphere (SHS) ceramic composite steel pipes. Conducted by the Beijing Research Institute of Nonferrous Metals, Shanghai Baoshan Iron and Steel Company, and University of Science and Technology Beijing, this work addresses a critical reliability issue in ceramic-lined steel pipe manufacturing.
Core Technical Content
Centrifugal casting of ceramic materials into steel pipe interiors is a well-established method for producing corrosion-resistant composite pipes. The SHS (silica hollow sphere) ceramic lining provides excellent thermal insulation and chemical resistance, but the manufacturing process introduces residual stresses that can lead to cracking. This study systematically investigates the root causes of crack formation and proposes process optimization strategies.
Manufacturing Process and Stress Analysis
The centrifugal casting process involves the following critical stages:
- Mold preparation: The steel pipe interior is prepared with a refractory coating to facilitate ceramic casting and subsequent removal.
- Centrifugal filling: Ceramic slurry is introduced into the rotating pipe mold, where centrifugal force distributes the slurry uniformly against the pipe wall.
- Drying and sintering: The cast ceramic layer undergoes controlled drying and sintering to achieve the target microstructure and mechanical properties.
- Cooling: The composite pipe is cooled, during which differential thermal contraction between the ceramic and steel generates residual stresses.
Crack Formation Mechanisms
The study identifies three primary crack formation mechanisms:
| Mechanism | Driving Force | Typical Location | Crack Morphology |
|---|---|---|---|
| Thermal stress cracking | Differential thermal contraction between ceramic and steel during cooling | Interface zone and ceramic layer | Radial cracks perpendicular to pipe axis |
| Drying shrinkage cracking | Uneven moisture removal during drying stage | Ceramic layer surface | Network pattern of fine cracks |
| Phase transformation cracking | Volume change associated with silica phase transitions | Within ceramic layer | Intergranular and transgranular mixed pattern |
Process Optimization Parameters
| Process Variable | Recommended Range | Effect on Crack Formation |
|---|---|---|
| Centrifugal speed | 1500–2500 rpm | Higher speed increases compaction but may cause segregation |
| Slurry viscosity | 80–120 Pa·s | Optimal viscosity ensures uniform distribution |
| Drying rate | ≤2°C/min (below 200°C) | Slow drying minimizes differential shrinkage |
| Sintering temperature | 1100–1250°C | Temperature below silica inversion point (573°C) requires careful control |
| Cooling rate | ≤1°C/min (above 500°C) | Slow cooling reduces thermal gradient stresses |
| Ceramic layer thickness | 15–40 mm | Thicker layers increase residual stress magnitude |
Interface Bond Quality Assessment
The bond between the ceramic lining and the steel pipe wall is critical for the composite pipe's structural integrity. The study employs several methods to assess bond quality:
- Shear bond strength testing: Typical values range from 5–15 MPa for well-bonded interfaces, with values below 3 MPa indicating unacceptable bond quality.
- Metallographic examination: Cross-section analysis reveals the presence of interfacial voids, oxide layers, and micro-cracks at the ceramic-steel boundary.
- Acoustic testing: Ultrasonic pulse-echo methods identify delamination and voids without destructive sampling.
Engineering Practice and Quality Control
From a cladding manufacturing perspective, the crack formation mechanisms identified in this study are directly analogous to those encountered in weld-overlay cladding and explosion-clad products:
- Thermal stress management: Just as residual stresses in weld-overlay cladding can cause cracking in the overlay layer or at the interface, the differential thermal contraction between ceramic and steel in centrifugal composite pipes creates similar stress states. The use of graded interfaces or interlayers in clad plate manufacturing serves the same purpose as the refractory coating used in ceramic composite pipes.
- Cooling rate control: The emphasis on controlled cooling rates in ceramic composite pipe manufacturing mirrors the post-weld heat treatment requirements for weld-overlay clad plates, where controlled cooling prevents martensitic transformation and minimizes residual stresses.
- Process parameter optimization: The systematic approach to optimizing centrifugal speed, slurry viscosity, and thermal cycles parallels the optimization of welding parameters (current, voltage, travel speed, preheat) in cladding operations.
Study Insights and Reflections
This research from 1999 remains highly relevant to modern ceramic-lined pipe manufacturing, particularly in the petrochemical and power generation industries where ceramic-lined steel pipes are used for high-temperature and corrosive service. The fundamental understanding of crack formation mechanisms—thermal stress, drying shrinkage, and phase transformation—provides a framework for process optimization that transcends specific manufacturing technologies.
For engineers involved in bimetal product manufacturing, the key lesson is that interface integrity is paramount, and the driving forces for interface degradation (thermal stresses, differential shrinkage, phase changes) are universal across different composite manufacturing methods. The study's systematic approach to identifying and mitigating crack formation mechanisms exemplifies the rigorous engineering methodology that should be applied to all composite product development programs.