Manufacture of Overlay Welded Alloy Seal Rings for Mechanical Seals

Literature Overview

The paper by Wang Min and Zeng Xingshao, published in "Fluid Machinery" in 1995 (Vol. 23, No. 5, pp. 36-38), describes the manufacturing process for overlay welded alloy seal rings used in mechanical seals. Mechanical seals are critical components in pumps, compressors, and other rotating equipment, and the seal ring faces must possess excellent hardness, wear resistance, corrosion resistance, and dimensional precision. This study addresses the selection of base materials and overlay alloys, the determination of overlay thickness and hardness, and the control of surface defects.

Material Selection for Seal Ring Applications

The selection of materials for mechanical seal rings involves a careful balance between mechanical properties, chemical resistance, and dimensional stability. The base material must provide adequate structural strength and machinability, while the overlay layer must deliver the surface properties required for sealing performance.

Common overlay alloys for seal rings include Stellite 6 (Co-Cr-W based), tungsten carbide-cobalt cermets, and ceramic materials such as silicon carbide or aluminum oxide. The choice depends on the specific service conditions, including fluid chemistry, temperature, pressure, and speed of operation.

Overlay Thickness and Hardness Determination

The overlay thickness is a critical parameter that affects both the performance and the cost of the seal ring. A thicker overlay provides greater resistance to wear and erosion but increases the risk of cracking due to residual stresses and reduces the dimensional precision achievable after machining. Typical overlay thicknesses range from 0.5 mm to 3.0 mm, depending on the application.

The hardness of the overlay layer must be carefully controlled to ensure proper mating with the opposing seal face. In a balanced mechanical seal, the two seal faces should have a hardness ratio that promotes conformal contact and minimizes wear. If the overlay is too hard relative to the mating face, it may cause accelerated wear of the softer face; if too soft, the overlay itself will wear rapidly.

Surface Defect Control

Surface defects in overlay welded seal rings are particularly critical because they can directly affect sealing performance. Common defects include:

1. Cracks - caused by excessive residual stresses, high carbon content in the overlay, or inadequate preheating.
2. Porosity - caused by inadequate shielding gas coverage, contaminated filler metal, or high welding speed.
3. Undercut - caused by excessive welding current or travel speed, leading to a groove at the weld toe.
4. Surface roughness - caused by improper welding parameters or post-weld machining practices.

Control measures include maintaining strict cleanliness of the base surface, using appropriate shielding gas flow rates, controlling interpass temperatures, and performing post-weld machining to achieve the required surface finish and dimensional tolerances.

Quality Control and Inspection

The quality of overlay welded seal rings is verified through a combination of visual inspection, dimensional measurement, hardness testing, and non-destructive testing. Visual inspection checks for surface defects such as cracks, porosity, and undercut. Dimensional measurement ensures that the overlay thickness and final machined dimensions meet the specified tolerances. Hardness testing verifies that the overlay hardness is within the required range. Non-destructive testing, such as dye penetrant or magnetic particle testing, is used to detect surface-breaking defects that may not be visible to the naked eye.

Engineering Practice Implications

The manufacture of overlay welded seal rings requires a high degree of process control and quality assurance. The small size of the components and the critical nature of the sealing function demand precision in every step of the manufacturing process. Engineers should pay particular attention to the selection of the overlay alloy based on the specific service environment, the control of overlay thickness to balance wear resistance and dimensional precision, and the implementation of rigorous non-destructive testing protocols.

Summary

The manufacture of overlay welded alloy seal rings for mechanical seals is a specialized application that demands careful material selection, precise process control, and rigorous quality inspection. The overlay layer must provide the surface properties required for reliable sealing while maintaining dimensional precision and freedom from surface defects. Engineers involved in the design and manufacture of mechanical seals should ensure that the overlay welding process is optimized for the specific application and that all quality control measures are implemented to guarantee the long-term reliability of the seal rings.