Manufacturing Process Analysis and Optimization of Inner-Wall Surfaced and Polished Heads for Pressure Vessels

Literature Overview

The paper published in China Chemical Equipment (Vol. 23, No. 1, 2021) by Zhang Wenlong and colleagues from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Pressure Vessel Special Material Welding addresses the manufacturing challenges of heads (end caps) that require inner-wall surfacing and subsequent polishing. This is a highly specialized manufacturing process relevant to chemical and petrochemical pressure vessel fabrication, where the inner surface must meet strict metallurgical and surface finish requirements for corrosion resistance and fluid flow characteristics.

Core Technical Points

Manufacturing Process Flow

The manufacturing of inner-wall surfaced and polished heads involves a complex sequence of operations:

Blank preparation and hot forming (stamping or spinning)
Dimensional inspection and correction of forming deviations
Inner-wall surfacing (typically using submerged arc welding or electroslag welding)
Post-surfacing inspection and surface preparation
Mechanical or chemical polishing of the surfaced layer
Final dimensional and surface quality verification

Each step introduces potential quality issues that must be controlled to ensure the final product meets specifications.

Challenge 1: Forming Deviation

Hot forming of heads from plate blanks introduces geometric deviations that significantly complicate subsequent surfacing operations. The primary concerns include:

Thickness variation: Non-uniform material thinning during forming creates variable base metal thickness across the head surface, affecting surfacing dilution rates and penetration
Shape deviation: Departures from the designed profile create variable standoff distances for surfacing torches, leading to inconsistent bead geometry
Residual stress: Forming introduces significant residual stresses that can cause distortion during subsequent heating operations

The authors emphasize that forming deviation is the root cause of many downstream quality issues. A poorly formed head requires excessive surfacing material to achieve uniform cladding thickness, increasing cost and introducing more residual stress.

Challenge 2: Surfacing Process

Inner-wall surfacing of curved heads presents unique challenges compared to flat plate surfacing:

Challenge	Impact	Mitigation
Variable curvature	Inconsistent torch standoff and bead width	Multi-axis CNC torch positioning
Gravity drainage of molten metal	Bead sagging on vertical/overhead surfaces	Reduced heat input, smaller wire diameter
Heat accumulation on curved surface	Excessive HAZ, dilution, and distortion	Interpass temperature control, cooling
Access limitations	Difficulty in positioning torch and consumables	Specialized fixtures and jigs

The surfacing process must be carefully designed to account for the three-dimensional geometry of the head. For submerged arc welding (SAW) surfacing, which is commonly used for this application due to its high deposition rate, the flux distribution and wire feed consistency become critical on curved surfaces.

Challenge 3: Polishing

The polishing of the surfaced inner wall is the final critical operation that determines the functional performance of the head. The requirements typically include:

Surface roughness (Ra) below a specified value (commonly Ra ≤ 0.8 μm for critical applications)
Uniform surface without scratches, pits, or tool marks
Preservation of the cladding layer thickness without excessive removal
Maintenance of metallurgical properties without introducing cold work

The polishing challenge is compounded by the curved geometry, which requires specialized tooling and operator skill. Over-polishing can remove too much cladding material, potentially exposing the base metal. Under-polishing leaves surface defects that can serve as corrosion initiation sites.

Engineering Practice Integration

PDCA Approach to Process Optimization

Applying the Plan-Do-Check-Act (PDCA) cycle to this manufacturing process:

Plan: Establish target forming tolerances, surfacing parameters, and polishing specifications based on the pressure vessel design requirements. Conduct a design of experiments (DOE) to identify optimal surfacing parameters for the specific head geometry.

Do: Implement the manufacturing process with in-process monitoring of forming quality, surfacing parameters, and interpass temperatures. Document all process parameters and deviations.

Check: Perform comprehensive inspection at each stage: dimensional inspection after forming, metallurgical examination of surfacing welds, hardness and microstructure verification, and surface roughness measurement after polishing.

Act: Feed inspection results back into process parameter optimization. Adjust forming tolerances, surfacing parameters, and polishing procedures based on the observed quality trends.

Common Defects and Countermeasures

Defect	Root Cause	Detection Method	Countermeasure
Undercut at surfacing bead edges	Excessive heat input, improper torch angle	Visual + dye penetrant	Reduce current, adjust torch angle
Cracking in cladding layer	Hydrogen embrittlement, high residual stress	Magnetic particle + ultrasonic	Preheat, post-weld heat treatment
Surface scratches after polishing	Inadequate polishing technique, tool contamination	Visual + roughness measurement	Improve polishing technique, clean tools
Cladding layer thickness variation	Forming deviation, inconsistent surfacing	Ultrasonic thickness measurement	Improve forming, use CNC surfacing
Base metal exposure	Excessive polishing, insufficient cladding	Visual + magnetic testing	Monitor cladding thickness, limit polishing depth

Case Study Insights

The authors' case study from Lanzhou Lanshi Heavy Equipment provides practical validation of the process optimization approach. The key lessons include:

Pre-forming quality inspection and correction should be mandatory before surfacing begins
Surfacing parameters must be adjusted for different curvature zones of the head (crown, knuckle, straight flange)
Polishing should be performed in a controlled sequence from coarse to fine, with intermediate inspection at each stage
The entire process requires tight coordination between forming, welding, and machining departments

Key Questions and Reflections

A critical question that the study does not fully address is the impact of the polishing process on the metallurgical properties of the cladding layer. Mechanical polishing introduces cold work and residual compressive stresses in the surface layer, which can affect the corrosion resistance and fatigue performance of the cladding. For austenitic stainless steel cladding layers, cold work can increase the ferrite content and reduce corrosion resistance. For nickel-based alloy cladding layers, cold work can affect the precipitation behavior and creep resistance.

Another important consideration is the inspection strategy. How should the surfaced and polished head be inspected to ensure quality without damaging the carefully prepared surface? Conventional magnetic particle inspection requires magnetic flux leakage testing, which may not be suitable for non-magnetic cladding layers. Eddy current testing is a viable alternative but requires careful calibration for the specific cladding material and thickness.

Study Insights and Implications

This study provides valuable practical guidance for manufacturers of surfaced pressure vessel heads, identifying the three critical manufacturing challenges and offering optimization strategies. The emphasis on forming quality as the foundation for successful surfacing and polishing is particularly important and often overlooked in practice. For engineers involved in pressure vessel fabrication, the key takeaway is that the quality of inner-wall surfaced and polished heads is determined by the integration of all manufacturing stages, not by any single operation. Process optimization requires a systems approach that considers the interactions between forming, surfacing, and polishing operations. The practical experience shared by the authors from a major Chinese heavy equipment manufacturer provides credible guidance for improving manufacturing yield and quality consistency in this specialized application.