Manufacturing Process Analysis and Optimization of Internal Surfaced Polished Heads

Literature Overview

This paper, authored by Zhang Wenlong, Ma Tao, Wang Zhenlin, Li Shulong, Du Jintao, and Zhang Tao, published in China Chemical Equipment (2021, Vol. 23, No. 1, pp. 41-44), presents a detailed analysis of the manufacturing challenges associated with internally surfaced and polished heads (dished ends) for pressure vessels. The research was conducted at Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Pressure Vessel Special Material Welding. The study addresses three critical manufacturing difficulties: forming deviation, surfacing, and polishing, and provides practical solutions for similar production scenarios.

Background and Application Context

Internal surfaced and polished heads are specialized pressure vessel components used in chemical processing, pharmaceutical manufacturing, and food processing industries where the internal surface must meet stringent requirements for corrosion resistance, smoothness, and cleanliness. The internal surfacing layer, typically a stainless steel or alloy overlay, provides corrosion protection while the polishing ensures a smooth surface that meets sanitary or process requirements. The combination of these requirements creates unique manufacturing challenges that require careful process planning and execution.

Three Critical Manufacturing Challenges

Challenge 1: Forming Deviation

The forming of dished heads from plate or forged blanks involves significant plastic deformation, which can introduce geometric deviations that affect subsequent surfacing and polishing operations. The forming process, whether by spinning, pressing, or hydroforming, must produce a head with uniform thickness and accurate geometry to ensure consistent surfacing coverage and achievable surface finish.

Forming Parameter	Typical Range	Impact on Surfacing
Forming temperature	800-950°C (hot forming)	Affects grain size and residual stress
Reduction ratio	20-30%	Higher reduction increases residual stress
Final thickness tolerance	±0.5-1.0 mm	Affects surfacing layer uniformity
Geometric accuracy	±2-3 mm	Critical for surfacing path planning

The authors emphasize that forming deviations must be minimized through careful control of the forming process parameters and thorough inspection of the formed blank before surfacing begins. Any significant deviation in thickness or geometry will result in non-uniform surfacing coverage and potentially inadequate corrosion protection.

Challenge 2: Surfacing

The internal surfacing of a dished head presents unique challenges compared to flat plate surfacing. The curved geometry of the head interior creates varying angles of approach for the welding torch, which affects arc stability, heat input distribution, and bead profile. Additionally, gravity effects cause the molten weld pool to sag on the lower portions of the head, leading to uneven bead profiles and potential defects.

The surfacing process typically involves multiple passes to build up the required overlay thickness. Each pass must be carefully planned to ensure adequate overlap and uniform coverage. The choice of welding process (typically GTAW or FCAW for stainless steel overlays) and consumable selection must be made with consideration for the following factors:

Compatibility with the base material (typically carbon steel or low-alloy steel).
Dilution rate and its effect on the corrosion resistance of the overlay.
Residual stress generation and its impact on subsequent polishing.
Surface roughness of the deposited beads, which affects the polishing effort required.

Challenge 3: Polishing

The final polishing of the internally surfaced head is a critical operation that determines the surface quality and, consequently, the service performance of the component. Polishing must be performed to achieve the specified surface roughness (typically Ra ≤ 0.4-0.8 μm for sanitary applications) while maintaining the integrity of the surfacing layer.

The polishing process introduces its own set of challenges:

The curved geometry of the head interior limits access for polishing tools.
Excessive polishing pressure or speed may remove the surfacing layer, exposing the base metal.
Heat generation during polishing may affect the microstructure of the surfacing layer.
Uniform polishing across the entire internal surface is difficult to achieve manually.

Process Optimization Strategy

The authors propose a systematic approach to addressing these challenges:

Pre-forming inspection: Implement rigorous dimensional inspection of the formed head blank to identify and correct any deviations before surfacing begins.
Surfacing path planning: Develop a systematic surfacing sequence that accounts for the head geometry, ensuring uniform coverage and minimizing residual stress accumulation.
Welding parameter optimization: Conduct trial welds to determine the optimal combination of current, voltage, travel speed, and interpass temperature for the specific geometry and consumable combination.
Polishing strategy: Use a combination of coarse and fine polishing passes, with careful monitoring of the surfacing layer thickness to prevent over-polishing.
Quality control: Implement in-process inspections at each stage (forming, surfacing, polishing) to detect and correct issues before proceeding to the next operation.

Quality Control and Inspection

Inspection Stage	Method	Acceptance Criteria
Formed blank	Visual, dimensional, UT	Geometry within tolerance, no cracks or folds
Surfaced head	PT, MT, visual	No cracks, pores, or lack of fusion
Polished surface	Ra measurement, visual	Surface roughness within specification
Final component	Hydrostatic test, dimensional	Leak-tight, dimensions within tolerance

Study Insights and Conclusions

This paper provides valuable practical insights into the manufacturing of internally surfaced and polished heads, a component type that is increasingly demanded by the chemical, pharmaceutical, and food processing industries. The authors' systematic approach to addressing the three critical challenges (forming deviation, surfacing, and polishing) offers a framework that can be adapted to similar production scenarios.

The key takeaway is that successful manufacturing of these components requires a holistic approach that integrates forming, welding, and finishing processes, with careful attention to the interactions between each stage. The forming quality directly affects surfacing uniformity, the surfacing quality determines the polishing effort and final surface quality, and the polishing process must be carefully controlled to preserve the integrity of the surfacing layer.

For engineers involved in the design and manufacturing of specialized pressure vessel components, this study underscores the importance of process planning, parameter optimization, and in-process quality control in achieving the required product quality. The practical nature of the study makes it a valuable reference for production engineers and quality assurance personnel working in the pressure vessel manufacturing industry.