Zhuojin Pipe Fitting Co., LtdManufacture of Surfaced Alloy Sealing Rings for Mechanical Seals
Literature Overview
This 1995 paper by Wang Min and Zeng Xingshao from Zigong Mechanical Seal Factory, published in Fluid Machinery, details the manufacturing process for alloy sealing rings used in mechanical seals. Mechanical seals are critical components in rotating equipment such as pumps, compressors, and mixers, where they prevent fluid leakage between the rotating shaft and the stationary housing. The sealing face must exhibit exceptional hardness, corrosion resistance, and thermal shock resistance, properties that are achieved through alloy surfacing on a more ductile and machinable base material.
Material Selection Strategy
The paper emphasizes the importance of matching the base material and surfacing alloy to the service conditions. The following table summarizes the typical material combinations used in mechanical seal ring manufacturing:
| Service Condition | Base Material | Surfacing Alloy | Hardness Requirement |
|---|---|---|---|
| General water service | 2Cr13 stainless steel | Stellite 6 (Co-Cr-W alloy) | 40–45 HRC |
| Acidic chemical service | Hastelloy C-276 | Alloy 6 (Ni-Cr-Mo) | 35–40 HRC |
| High-temperature service | 17-4PH stainless steel | Tungsten carbide-cobalt | 85–90 HRA |
| Slurry and abrasive service | 316L stainless steel | Carbide-ceramic composite | 70–80 HRA |
The selection of the surfacing alloy depends on the tribological pairing. The two sealing faces in a mechanical seal must be matched in hardness and material compatibility to ensure balanced wear and minimal leakage.
Surfacing Layer Thickness and Hardness Determination
A critical aspect of this paper is the determination of appropriate surfacing layer thickness and hardness. The layer thickness must be sufficient to ensure that the final machined surface is entirely within the alloy layer, free of any base material contamination. The following guidelines apply:
- Minimum surfacing thickness: The overlay thickness must exceed the maximum expected wear depth during the seal's service life. For typical pump seals, this is 0.5–2.0 mm per face.
- Hardness gradient: The hardness at the surface should meet the specification, while the hardness at the interface with the base material should be lower to ensure a strong metallurgical bond. A hardness gradient of 5–10 HRC per 0.1 mm is typical.
- Surface finish: The final machined surface must achieve a surface roughness of Ra 0.05–0.2 μm, which is critical for the hydrodynamic film formation that prevents direct metal-to-metal contact.
Surface Defects and Control Measures
The paper identifies several surface defects that are common in surfacing alloy sealing rings and provides control strategies:
| Defect Type | Root Cause | Detection Method | Control Measure |
|---|---|---|---|
| Cracking | Residual stress; thermal shock during cooling | PT or MT | Reduce heat input; control cooling rate; PWHT |
| Porosity | Gas entrapment; contaminated filler | RT or UT | Clean filler wire; proper shielding gas flow |
| Lack of fusion | Insufficient heat input; poor wetting | Sectioning and microscopy | Increase current; preheat substrate |
| Surface roughness | Unstable arc; electrode wear | Visual inspection; profilometry | Use fresh electrodes; maintain arc stability |
| Inclusion | Contaminated coating; slag entrainment | Metallographic examination | Proper electrode handling; slag removal between passes |
Manufacturing Process Flow
The manufacturing process for a surfaced sealing ring typically follows this sequence:
- Base ring machining: The base ring is machined to near-final dimensions, leaving 0.5–1.5 mm of material for surfacing and final machining.
- Surface preparation: The surface to be surfaced is cleaned by grinding to remove all contaminants.
- Preheating: The base ring is preheated to 150–300°C to reduce thermal gradients and prevent cracking.
- Surfacing welding: TIG or plasma arc surfacing is applied, typically in multiple passes to build up the required thickness.
- Post-weld heat treatment: If required by the alloy specification, the ring is tempered to relieve residual stresses.
- Final machining: The surfaced surface is ground and lapped to the required surface finish and dimensional accuracy.
- Final inspection: Hardness testing, dimensional verification, and non-destructive testing are performed.
Engineering Practice Reflection
The manufacturing of surfaced sealing rings is a process where precision and metallurgical quality must coexist. The tolerance stack-up between the surfacing thickness, the final machining allowance, and the required surface finish leaves very little margin for error. In practice, I have observed that the most common cause of sealing ring failure in service is not the surfacing itself but the subsequent machining process. Aggressive grinding parameters can introduce surface damage, micro-cracks, and residual stresses that compromise the seal face integrity.
The paper's emphasis on surface defect control is well-placed. For critical applications such as nuclear pumps, chemical processing equipment, and high-pressure gas compressors, the sealing ring is a safety-critical component, and any surface defect can lead to seal failure and potential environmental or safety consequences. The application of rigorous quality control protocols, including documented welding procedures, welder qualification, and comprehensive NDT, is not optional but essential.