CLADDING TECHNOLOGY SHANXI CO., LTDManufacturing Process Analysis and Optimization of Internal Cladding Polished Dished Heads
Literature Overview
This study, published in 2021 in the journal China Chemical Equipment, was conducted by Zhang Wenlong, Ma Tao, Wang Zhenlin, Li Shulong, Du Jintao, and Zhang Tao from Lanzhou Lanchen Heavy Equipment Co., Ltd. and the Gansu Province Key Laboratory of Special Materials Welding for Pressure Vessels. The paper addresses the manufacturing process challenges associated with dished heads (elliptical or torispherical) that require internal weld-overlay cladding and subsequent polishing. This is a critical component type in pressure vessel fabrication, particularly for chemical processing equipment where the internal surface must withstand corrosive media. The study provides a comprehensive analysis of the manufacturing process, identifies quality issues, and proposes optimization measures.
Core Technical Content
The manufacturing of internal cladding polished dished heads involves multiple sequential operations, each presenting unique quality challenges. The process flow typically includes: dished head forming, internal cladding welding, cladding layer inspection, polishing, and final inspection. The complexity arises from the curved geometry, which makes uniform cladding and polishing significantly more challenging than on flat plates.
Process Flow and Critical Control Points
| Operation | Key Challenge | Critical Control Point | Inspection Method |
|---|---|---|---|
| Dished head forming | Residual stress, thinning | Forming temperature, reduction ratio | UT thickness measurement |
| Internal cladding welding | Position access, weld quality | Welder skill, heat input, dilution | MT, UT, hardness test |
| Cladding layer inspection | Defect detection on curved surface | Technique selection, coverage | MT, PT, RT |
| Polishing | Surface finish, thickness control | Tool selection, material removal rate | Surface roughness, thickness |
| Final inspection | Overall quality verification | Hydrostatic test, dimensional check | Pressure test, dimensional |
Internal Cladding Welding Challenges
The internal cladding of dished heads presents several specific challenges that distinguish it from flat plate cladding:
- Welder access and positioning: The curved internal surface limits welder access, making it difficult to maintain consistent welding parameters and bead placement. In all-position welding, gravity effects on the molten pool vary with weld position.
- Thermal distortion: The localized heat input from cladding welding can cause distortion of the dished head geometry, leading to out-of-tolerance dimensions and potential fit-up problems during assembly.
- Residual stress accumulation: The combination of forming stresses and welding stresses can lead to high residual stress levels, potentially causing deformation during subsequent operations or in service.
- Dilution variation: The dilution rate varies with weld position due to changes in the thermal gradient, leading to non-uniform cladding layer composition and properties.
Optimization Measures Proposed
The study proposes several optimization measures based on the analysis of quality issues encountered in production:
1. Welding sequence optimization: A systematic welding sequence was developed to minimize thermal distortion and residual stress. The sequence divides the dished head into symmetric zones, with welding progressing from the crown toward the knuckle and then to the straight flange. This approach ensures that each weld pass is balanced by a diametrically opposite pass, reducing net distortion.
2. Heat input control: The welding parameters were optimized to achieve a heat input of 1.5–2.5 kJ/mm, which balances the need for adequate fusion with the requirement to minimize dilution and thermal distortion. Multi-pass welding with thin layers (each pass 2–3 mm) was recommended to achieve a total cladding thickness of 5–8 mm with controlled dilution.
3. Preheat and interpass temperature management: A preheat temperature of 150–200°C was specified to reduce the thermal gradient and minimize cracking susceptibility. The interpass temperature was maintained below 200°C to prevent excessive grain growth and dilution.
4. Polishing process optimization: The polishing operation was optimized by:
- Using a stepped approach with progressively finer abrasives
- Controlling the material removal rate to maintain a minimum cladding thickness of 3 mm
- Employing CNC-controlled polishing tools for improved consistency
- Implementing in-process thickness monitoring to prevent over-polishing
Quality Control and Inspection Protocol
A comprehensive quality control protocol was established for the manufacturing process:
| Inspection Stage | Method | Acceptance Criteria | Frequency |
|---|---|---|---|
| Cladding welds | Magnetic Particle Testing (MT) | No linear indications > 2 mm | 100% of welds |
| Cladding layer | Hardness test | 200–350 HV (for 304/316L) | Every 500 mm |
| Cladding layer | Dilution test (spectroscopy) | ≤ 30% base metal dilution | Every 500 mm |
| Cladding thickness | UT measurement | ≥ 3 mm after polishing | 100% of surface |
| Surface finish | Roughness measurement | Ra ≤ 0.8 μm (polished) | Every 200 mm |
| Final component | Hydrostatic test | 1.25 × design pressure, 30 min hold | 100% of components |
Engineering Practice and Case Study
The study includes a case study of a dished head manufactured for a chemical processing reactor. The component required a 316L stainless steel internal cladding over a Q345R carbon steel base, with a polished surface finish of Ra ≤ 0.8 μm. The manufacturing process encountered several quality issues during initial production:
- Weld cracking: Cold cracks were observed in the cladding welds due to insufficient preheat and excessive hydrogen pickup. This was resolved by increasing the preheat temperature to 200°C and using low-hydrogen welding consumables.
- Excessive dilution: The dilution rate exceeded 35% in some areas, particularly at the knuckle radius where the welder had difficulty maintaining consistent parameters. This was addressed by implementing a multi-pass welding strategy with a backing layer and by providing additional welder training for curved surface welding.
- Polishing over-removal: In the initial polishing trials, the cladding thickness was reduced below the specified minimum in some areas. This was resolved by implementing in-process thickness monitoring and by adjusting the polishing tool pressure and speed.
After implementing the optimization measures, the final product met all specified requirements with a cladding thickness of 4–6 mm, a surface roughness of Ra 0.4–0.7 μm, and no detectable defects.
Key Reflections and Study Insights
This study provides valuable practical insights into the manufacturing of internal cladding polished dished heads, which are among the most challenging components in pressure vessel fabrication. The key lessons are:
- Process integration is critical: The manufacturing of these components requires a holistic approach that considers the interactions between all process steps. A defect introduced during welding can be propagated or exacerbated during polishing, and a distortion from forming can compromise the weld fit-up.
- Welder skill and training are paramount: The curved geometry and all-position welding requirements demand a high level of welder skill. Investment in welder training and qualification for specific component geometries is essential.
- In-process monitoring saves costs: Implementing in-process thickness monitoring during polishing, and hardness/dilution testing during welding, allows for early detection and correction of issues, avoiding costly rework or scrap.
- Standardization improves consistency: Developing detailed welding procedure specifications (WPS) and polishing procedure specifications (PS) with defined parameters, sequences, and inspection requirements is essential for achieving consistent quality.
The study also highlights the importance of the Gansu Province Key Laboratory of Special Materials Welding for Pressure Vessels as a resource for addressing manufacturing challenges in the Chinese pressure vessel industry. The findings have broad applicability to other curved components requiring internal cladding, including spherical tanks, heat exchanger shells, and reactor vessels. This work represents a practical bridge between academic research and industrial manufacturing, demonstrating how systematic process analysis and optimization can significantly improve manufacturing quality and efficiency.