Spherical Annular Sealing Band Overlay Welding Process and Dual-Positioner Design

Literature Overview

This 2007 paper by He Wenping, Wang Zongcai, and Li Tiesheng from the School of Mechanical and Electrical Engineering at Henan University of Technology presents a systematic analysis of overlay welding processes for spherical annular sealing bands on BQ-type double eccentric ball valve cores. The work was published in Coal Mine Machinery (Vol. 28, No. 7, pp. 23–25) and addresses a practical manufacturing challenge in the coal mining and bulk material handling industry, where ball valves must provide reliable sealing under abrasive and high-pressure conditions.

Core Technical Content

The BQ-type double eccentric ball valve employs a spherical ball core with an annular sealing band that contacts the valve seat to achieve shutoff. The sealing band is typically produced by overlay welding a wear-resistant alloy onto the spherical surface of the ball core. The key technical challenges identified in this study include:

• Maintaining consistent weld bead geometry across the curved spherical surface
• Controlling weld pool flow and solidification behavior on inclined and horizontal surface orientations
• Achieving uniform deposition thickness around the full circumference of the ball
• Adapting welding parameters to different ball sizes while maintaining weld quality

Process Parameter Analysis

The authors analyzed the relationship between welding trajectory, weld pool movement speed, and deposition quality. Their key findings can be summarized as follows:

The analysis of weld pool dynamics on spherical surfaces reveals that the gravitational component of the weld pool force varies continuously as the torch traverses the sphere, from a maximum at the bottom (6 o'clock position) to zero at the sides (3 and 9 o'clock positions) and a maximum in the opposite direction at the top (12 o'clock position). This variation causes the weld pool to sag more at the bottom and flow less at the top, resulting in non-uniform bead profiles if the welding speed is held constant.

Dual-Positioner Design

To address the challenges of welding on spherical surfaces, the authors designed a dual-positioner mechanism capable of independently adjusting the orientation of both the ball core workpiece and the welding torch. This dual-axis control enables:

1. Workpiece rotation control: The ball core can be rotated at a controlled angular velocity to maintain a consistent weld pool orientation relative to gravity, effectively simulating flat-position welding at any point on the sphere.
2. Torch attitude control: The torch can be tilted at a controlled angle relative to the workpiece surface normal, ensuring optimal arc force direction and weld pool confinement regardless of the welding position on the sphere.
3. Trajectory adaptation: The positioner can be programmed to follow the optimal welding trajectory for each ball size, compensating for the geometric curvature and maintaining consistent bead overlap.

Engineering Practice Integration

The dual-positioner design concept has direct applicability to other spherical or complex-curved overlay welding applications, including:

• Ball valve manufacturing for oil and gas pipelines
• Pressure vessel head reinforcement
• Turbine rotor blade root repair
• Bearing raceway overlay welding

In practice, the implementation of such a positioner requires careful integration with the welding power source and wire feeder control systems. The positioner's angular velocity must be synchronized with the torch travel speed to maintain a constant linear welding speed along the desired trajectory. For a ball of radius R, the linear speed v at a latitude angle θ is v = ωR cos θ, where ω is the angular velocity of the ball. This means that to maintain constant linear speed, the angular velocity must be adjusted as the torch moves to different latitudes on the sphere.

Key Technical Reflections

The paper's emphasis on trajectory and speed adaptation for different workpiece sizes represents a fundamental principle in automated overlay welding: the welding process parameters must be scaled to the geometry being welded. This is analogous to the principle in pipe welding where longer weld lengths require different heat input management than shorter welds.

However, the study could have benefited from more detailed metallographic analysis of the deposited layers. The microstructure of overlay welds on spherical surfaces is influenced by the cooling rate, which varies with the local thickness of the deposited material and the thermal mass of the underlying substrate. Thicker deposits at the bottom of the sphere (due to gravitational sagging) would cool more slowly than thinner deposits at the top, potentially leading to coarser grain structures and lower hardness in certain regions.

Study Insights and Implications

This research demonstrates that successful overlay welding on complex geometries requires not only appropriate filler metal selection and welding parameter optimization but also sophisticated mechanical positioning systems that can adapt the welding process to the geometric constraints. The dual-positioner design is a practical engineering solution that bridges the gap between the idealized flat-surface welding conditions assumed in laboratory studies and the complex geometries encountered in industrial applications.

For engineers working on ball valve manufacturing or similar applications, this paper underscores the importance of considering the entire welding system—power source, wire feeder, torch, positioner, and control system—as an integrated unit rather than optimizing individual components in isolation. The quality of the overlay weld is ultimately determined by the synergy between all these elements, and any weakness in the system will manifest as defects in the final product.