Overlay Welding Process Analysis of Spherical Annular Sealing Band and Dual Positioner Design

Literature Overview

This 2007 publication by He Wenping, Wang Zongcai, and Li Tiecheng from the School of Mechanical and Electrical Engineering at Henan University of Technology addresses the overlay welding of spherical annular sealing bands on BQ-type double eccentric ball valve cores. Published in Coal Mine Machinery (Vol. 28, No. 7), the paper tackles a geometrically complex welding challenge and presents an innovative mechanical solution through the design of a dual positioner that adjusts both valve core orientation and torch attitude.

Technical Background

BQ-type double eccentric ball valves are widely used in coal mine slurry pipelines, mineral processing circuits, and other abrasive service applications where conventional gate valves fail prematurely. The ball valve core features a spherical surface with an annular sealing band that must maintain intimate contact with the valve seat under pressure. Overlay welding is employed to deposit wear-resistant material on this sealing band, but the spherical geometry presents unique challenges:

• Non-planar weld trajectory requiring precise torch path control
• Variable weld pool behavior as the torch moves across the curved surface
• Gravity effects on molten pool stability at different orientations
• Maintaining uniform overlay thickness across the full annular band

Welding Trajectory and Process Analysis

The authors systematically analyzed the welding trajectory and weld pool movement speed for the spherical annular sealing band overlay. Key conclusions include:

Trajectory Considerations

The fundamental challenge is that as the torch traverses a spherical surface, the effective welding angle changes continuously. Unlike flat-surface overlay welding where the torch can maintain a constant angle, spherical overlay requires either:

1. Continuous manual adjustment of torch attitude (error-prone, inconsistent)
2. Mechanical positioning equipment that automatically compensates for surface curvature (the solution proposed in this paper)

The weld pool movement speed must also vary with diameter because:

• Larger diameters produce longer arc lengths at the same angular position, affecting arc stability
• The surface speed at the equator versus the poles differs, affecting heat input distribution
• Gravity assists or opposes weld pool flow depending on the torch position relative to the horizontal

Dual Positioner Design

The authors designed a dual positioner mechanism capable of independently adjusting:

1. Valve core attitude: Rotating the workpiece about multiple axes to bring the welding zone to an optimal position
2. Torch attitude: Maintaining the correct torch angle relative to the spherical surface throughout the welding trajectory

Design Principles

The dual positioner operates on the following principles:

• The workpiece is mounted on a tilting head that can rotate about a vertical axis and tilt about a horizontal axis
• The torch is mounted on a separate arm that can independently adjust its angle and feed position
• Synchronized motion between workpiece rotation and torch positioning ensures consistent weld geometry
• The positioner accommodates different valve core diameters through adjustable fixturing

Performance Benefits

Engineering Practice Integration

This research is particularly relevant to the coal mining industry where abrasive slurry service demands robust valve sealing surfaces. Several practical considerations emerge:

• Material selection: The overlay material for slurry service should provide a combination of hardness (HRC 40-60) and toughness to resist both abrasive wear and impact loading from entrained solids
• Preparation: The spherical surface must be ground smooth to remove scale and ensure good fusion. Any surface irregularities will propagate into the overlay
• Heat control: For thick valve cores, interpass temperature monitoring is essential to prevent excessive grain growth in the base metal
• Post-weld machining: The overlay must be precisely machined to achieve the required sealing geometry, requiring adequate build-up allowance

Key Reflections

The innovation of this paper lies not in the welding metallurgy itself but in the mechanical engineering solution to a process automation challenge. The dual positioner concept demonstrates that for geometrically complex overlay welding applications, investment in dedicated positioning equipment can dramatically improve quality and productivity. This philosophy is broadly applicable to other curved-surface welding applications such as pressure vessel heads, turbine blades, and pump impellers.

A limitation of the study is the absence of detailed metallurgical analysis of the overlay welds. While the mechanical benefits of the positioner are well-documented, the actual overlay microstructure, dilution rate, and service performance data would strengthen the engineering case for this approach. Additionally, the paper does not address the economics of the positioner investment versus the cost savings from improved quality and reduced rework.

Summary

This study presents a practical and innovative approach to solving the overlay welding challenge on spherical valve surfaces through the design of a dual positioner that independently controls workpiece and torch attitude. The systematic analysis of welding trajectories for different valve core sizes provides valuable process guidance, while the positioner design offers a scalable solution for production environments. The work exemplifies the principle that process equipment innovation can be as impactful as consumable development in improving welding quality and efficiency.