CNC Tracking System for Dished Head Overlay Cladding

Technical Background and Process Challenges

Dished heads (ellipsoidal, hemispherical, or torispherical) are critical pressure-containing components in pressure vessels, reactors, and storage tanks. When corrosion or erosion resistance is required on the internal surface, weld overlay cladding must be applied to the curved geometry of the head. Unlike flat plate cladding, dished head overlay presents significant challenges:

Variable curvature: The radius changes continuously from the crown to the knuckle to the straight flange, requiring constant adjustment of the welding head orientation.
Access limitations: Internal cladding of large heads requires the welding system to navigate confined spaces with limited maneuverability.
Seam continuity: The overlay must form a continuous, defect-free layer across the entire internal surface, including the geometric transition zones.
Dimensional accuracy: Post-overlay machining to achieve specified internal dimensions and surface finish requires precise overlay thickness control.

System Architecture and Technical Specifications

The CNC tracking system described in the study integrates several subsystems to achieve automated, high-quality overlay cladding on dished heads:

Subsystem	Function	Key Specifications
6-axis robotic manipulator	Position welding head on curved surface	Payload ≥ 10 kg, repeatability ±0.05 mm
Vision tracking system	Real-time position feedback	Resolution 0.1 mm, frame rate 30 Hz
Surface scanning system	Pre-scan head geometry	Laser triangulation, accuracy ±0.02 mm
Welding power source	SAW or GMAW overlay	Current 300–600 A, voltage 25–35 V
Wire feed system	Constant wire delivery	Speed control ±1%, diameter tolerance ±0.02 mm
Flux delivery system	SAW flux supply	Gravity-fed with level monitoring
Coolant system	Interpass cooling	Water or air, controlled temperature
CNC controller	Path planning and execution	Interpolation resolution 0.01 mm

The core innovation of this system is the integration of a surface scanning subsystem with real-time adaptive path planning. Before welding begins, a laser triangulation scanner maps the entire internal surface of the dished head, generating a three-dimensional point cloud model. The CNC controller then computes optimal welding paths that account for the actual as-built geometry, which may deviate from nominal due to forming tolerances.

Process Parameters and Quality Control

The overlay process parameters optimized for dished head applications include:

Parameter	SAW Overlay	GMAW Overlay	Notes
Current	400–550 A	200–350 A	Adjusted for curvature
Voltage	28–32 V	22–28 V	Arc length control
Travel speed	200–300 mm/min	150–250 mm/min	Slower at high curvature
Wire diameter	1.6 mm	1.2–1.6 mm	Consumable specification
Preheat temperature	100–150 °C	100–200 °C	Uniform across head
Interpass temperature	≤ 150 °C	≤ 200 °C	Monitored by IR camera
Overlay thickness per pass	2–4 mm	1.5–3 mm	Controlled by parameters
Total overlay thickness	8–15 mm	6–12 mm	3–5 passes typical

The CNC tracking system maintains weld quality through several real-time monitoring functions:

Arc voltage monitoring: Detects arc instability and triggers parameter adjustment or pause.
Wire stick-out measurement: Ensures consistent arc length for uniform bead geometry.
Travel speed feedback: Compensates for speed variations caused by surface irregularities.
Temperature monitoring: Infrared cameras track interpass temperature and enforce cooling requirements.

Welding Sequence Strategy

The welding sequence for dished head overlay follows a systematic approach to minimize residual stress and distortion:

Crown area (top center): Initial pass establishes the reference and allows thermal expansion to occur freely.
Knuckle region (intermediate curvature): Welded in spiral pattern from crown outward, maintaining constant bead overlap of 50%.
Straight flange (low curvature): Final pass along the straight section, providing a smooth transition to the shell-to-head weld.
Seam weld area: The overlay is extended 20–30 mm beyond the head-to-shell weld joint to ensure complete coverage.

The spiral welding pattern, rather than a lathe-turning pattern, is preferred because it distributes heat input more uniformly and reduces the risk of circumferential cracking. The CNC system calculates the exact spiral path based on the scanned geometry, adjusting the robot joint angles continuously.

Common Challenges and Solutions

Challenge	Root Cause	Solution
Bead width variation	Curvature-induced arc asymmetry	Real-time torch angle adjustment
Poor fusion at knuckle	Insufficient heat input at high curvature	Increased current by 10–15% in knuckle zone
Distortion of head	Asymmetric thermal input	Symmetric welding pattern, fixture clamping
Porosity in transition zone	Contamination from forming residue	Pre-weld cleaning protocol, flux coverage
Hardness gradient	Dilution variation with geometry	Multi-pass strategy with consistent parameters

Study Insights and Reflections

This study demonstrates that automation of dished head overlay cladding is technically feasible and economically justified for production volumes exceeding 50 heads annually. The key advantage of the CNC tracking system is not merely speed improvement (which is approximately 20–30% faster than manual welding) but rather the consistency and repeatability of weld quality. Manual overlay of dished heads is highly dependent on welder skill, and quality variation between welders can be significant. The CNC system eliminates this human factor, producing overlay layers with hardness variation of less than ±30 HV30 across the entire surface, compared to ±60–80 HV30 variation in manual work.

However, the study also acknowledges limitations: the system requires significant upfront capital investment, and the setup time for programming new head geometries can be 4–8 hours. For one-off or low-volume production, manual welding with experienced welders may remain more economical. The optimal strategy is a hybrid approach: CNC overlay for high-volume production runs and critical applications (nuclear, offshore), with manual welding for prototype and low-volume work. The technology represents a significant advancement in pressure vessel manufacturing capability, enabling tighter quality control and reduced rework rates in clad pressure vessel fabrication.