Manufacturing Process Analysis and Optimization of Overlay Welded and Polished Dished Heads

Literature Overview and Technical Challenges

Dished heads, particularly torispherical and elliptical heads, are fundamental components in pressure vessel fabrication, serving as end closures that must withstand internal pressure while maintaining structural integrity. When the service environment demands corrosion resistance, overlay welding on the internal surface followed by precision polishing becomes a critical manufacturing process. The research under review addresses the unique challenges of applying weld overlay to the complex geometry of dished heads, where curvature variations, thin wall sections, and the transition from cylindrical shell to head crown create non-uniform heat input conditions. The study provides a systematic analysis of process optimization covering welding sequence, parameter selection, distortion control, and post-weld polishing to achieve a surface finish suitable for sanitary or chemical applications.

Geometric Challenges and Welding Sequence Design

The curvature of a dished head creates a fundamental challenge: the welding torch angle, travel speed, and heat input distribution vary significantly from the head crown to the knuckle region. At the crown, the curvature is most pronounced, requiring precise torch orientation to maintain a consistent weld bead profile. The knuckle region presents a different challenge due to its steep curvature and proximity to the straight flange section, where thermal expansion is constrained.

The welding sequence is designed to minimize angular distortion and warping. A spiral pattern starting from the head crown and progressing outward is recommended for GMAW overlay, as it allows heat to be distributed radially and reduces the tendency for the head to flatten. For SAW overlay, a segmented approach with alternating weld passes in diametrically opposite sectors is more effective, as the higher heat input of SAW requires more aggressive distortion compensation. The study emphasizes that the first pass must be laid with particular care to ensure full penetration and a sound bond with the base metal, as subsequent passes build upon this foundation and any defect in the first pass propagates through the overlay.

Welding Parameter Optimization

Parameter selection for overlay welding on dished heads requires balancing deposition rate against distortion control and microstructural quality. The study presents a systematic optimization using orthogonal experimental design, varying current, voltage, travel speed, and interpass temperature across multiple trials.

A critical finding is that the optimal travel speed must be adjusted continuously as the welder progresses from the crown to the knuckle. At the crown, where the surface is most concave relative to the torch, the effective heat input is reduced, necessitating a 10–15% reduction in travel speed to maintain consistent penetration. Conversely, at the knuckle, the convex curvature relative to the torch increases heat concentration, requiring a slight increase in travel speed to prevent excessive dilution and undercut. The study recommends the use of a programmable wire feeder that can adjust travel speed in real time based on the head geometry, or alternatively, pre-marked speed zones on the head surface for manual welding.

Post-Weld Polishing and Surface Finish Requirements

After overlay welding, the internal surface must be polished to achieve the specified surface roughness, typically Ra 0.4–0.8 μm for sanitary applications and Ra 1.6–3.2 μm for chemical service. The polishing process must remove the weld bead profile, eliminate surface porosity, and produce a uniform surface without exposing the base metal. The study identifies three polishing stages: rough grinding to remove weld bead irregularities, intermediate grinding to achieve a uniform surface, and fine polishing to achieve the target roughness.

The study emphasizes that the overlay layer thickness must be designed with sufficient margin to allow for polishing removal. A minimum overlay thickness of 2.5–3.0 mm is recommended to accommodate 1.0–1.5 mm of material removal during polishing while maintaining a minimum finished overlay thickness of 1.0–1.5 mm. The polishing direction should follow the curvature of the head to avoid creating transverse scratches that can serve as stress concentration sites. Visual inspection, magnetic particle inspection, and roughness measurement are performed after each polishing stage to ensure quality.

Distortion Control and Engineering Insights

Distortion during overlay welding of dished heads is a persistent challenge, particularly for thin-walled heads (wall thickness below 8 mm) where the heat input to wall thickness ratio is high. The study recommends a combination of preheating to 150–200 °C, low interpass temperature control, and post-weld stress relief at 600–650 °C to manage residual stresses. For heads with wall thickness above 12 mm, a lower preheat of 100–150 °C is sufficient, and stress relief may be omitted if the residual stress is confirmed to be within acceptable limits by UT measurement. The overall manufacturing philosophy advocated by the study is one of process integration: welding parameters, sequence, and polishing are not independent operations but must be designed as a cohesive process chain where each stage is optimized with awareness of its impact on the next. This systems-level thinking is essential for producing high-quality overlay welded dished heads in a repeatable and cost-effective manner.