CLADDING TECHNOLOGY SHANXI CO., LTDProcess Research on Cladding of Bellows Globe Valve Body Sealing Surface
Literature Overview and Technical Background
Bellows globe valves are widely used in chemical, pharmaceutical, and nuclear industries where zero-leakage sealing is a critical requirement. The sealing surface of the valve body, which mates with the bellows seal, is subject to severe wear, corrosion, and thermal cycling during operation. The study addresses the cladding process for depositing a corrosion-resistant and wear-resistant alloy layer on the sealing surface of bellows globe valve bodies, typically made of carbon steel or low-alloy steel, to extend service life and ensure reliable sealing performance.
The sealing surface geometry presents unique challenges for cladding. The surface is typically a tapered or conical seat with a diameter ranging from 25 mm to 200 mm, and the cladding thickness requirement is usually 1.5 to 3.0 mm with a surface finish of Ra 1.6 micrometers or better. The tight geometric tolerances and the need for uniform overlay thickness around the circumference make this a demanding application for weld overlay technology.
Cladding Process Design and Parameters
The study evaluates several cladding methods including submerged arc welding, gas tungsten arc welding, and plasma transferred arc welding, and ultimately recommends a combination of GTAW for打底 and PTA for buildup passes. The rationale for this selection is based on the need for precise heat input control and the ability to achieve fine-grained microstructures with low dilution.
| Parameter | GTAW Base Pass | PTA Buildup Passes |
|---|---|---|
| Heat source | Non-consumable tungsten electrode | Plasma torch with water-cooled nozzle |
| Consumable | ER308L or ER316L wire | Nickel-based powder (Inconel 625 or similar) |
| Current | 120 to 180 A | 250 to 400 A |
| Voltage | 12 to 18 V | 28 to 36 V |
| Travel speed | 150 to 300 mm/min | 200 to 500 mm/min |
| Shielding gas | Argon 99.99% purity | Argon 99.99% purity |
| Powder feed rate | N/A (wire) | 15 to 40 g/min |
| Preheat | 100 to 150 °C | Interpass 150 to 200 °C |
| Overlay thickness | 0.5 to 1.0 mm (base pass) | 0.5 to 1.5 mm per pass |
| Total overlay thickness | 1.5 to 3.0 mm | Including base pass |
The overlay material selection is critical for this application. The study evaluates three material systems: austenitic stainless steel (316L), nickel-based superalloy (Inconel 625), and a proprietary nickel-chromium-iron alloy. The nickel-based alloy is recommended for the most severe service conditions due to its superior resistance to pitting corrosion, crevice corrosion, and stress corrosion cracking in chloride-containing environments.
Surface Preparation and Geometric Control
The quality of the cladding process is heavily dependent on the surface preparation of the valve body sealing surface. The study specifies a rigorous preparation protocol including:
- Mechanical machining of the sealing surface to the required geometry with a tolerance of plus or minus 0.05 mm.
- Grinding with 120-grit followed by 240-grit abrasive paper to remove machining marks and create a uniform surface profile.
- Solvent cleaning with acetone or isopropyl alcohol to remove all contaminants, oils, and dust.
- Visual inspection under magnification to confirm surface cleanliness and absence of defects.
For the cladding process itself, the study proposes the use of a specialized welding fixture that allows precise positioning of the torch relative to the sealing surface. The fixture incorporates a rotational axis that enables the valve body to be rotated during welding, ensuring uniform coverage around the circumference. The torch height is maintained at a constant 2 to 3 mm from the surface using a non-contact arc sensing system.
Defect Evaluation and Quality Control
The study conducts comprehensive quality control on the clad valve bodies, including:
| Inspection Method | Acceptance Criteria | Frequency |
|---|---|---|
| Visual inspection (VT) | No visible cracks, porosity, or undercut | 100% of welds |
| Magnetic particle testing (MT) | No linear indications greater than 3 mm | 100% of welds |
| Ultrasonic testing (UT) | No indications above background | 100% of welds |
| Hardness test | 250 to 350 HV for Ni-based overlay | Per batch |
| Penetrant testing (PT) | No surface-breaking defects | 100% of welds |
| Chemical analysis | Conform to specified composition | Per lot |
| Corrosion test | No pitting after 500 h in 3.5% NaCl | Per design cycle |
The most common defects identified during the study include:
- Crater cracks at the end of each weld pass, caused by insufficient overlap or rapid cooling at the termination point. The countermeasure is to use a craters filler or to overlap each pass by at least 50 percent.
- Tungsten inclusions in the GTAW base pass, caused by excessive arc length or contamination of the tungsten electrode. The countermeasure is to maintain arc length below 3 mm and to use freshly ground electrodes.
- Uneven overlay thickness around the circumference, caused by inconsistent rotational speed or torch positioning. The countermeasure is to use a CNC-controlled welding fixture with constant speed rotation.
Engineering Application and Performance Evaluation
The study documents the application of the cladding process on bellows globe valves used in a nuclear power plant cooling water system. The valves, originally made of ASTM A216 WCB carbon steel, were clad with Inconel 625 on the sealing surface to resist corrosion from the treated cooling water. After two years of continuous service, the valves showed no evidence of sealing surface degradation, and the overlay thickness remained above the minimum required 1.0 mm.
In another application, valves used in a pharmaceutical clean steam system were clad with 316L stainless steel to meet hygienic requirements and resist corrosion from steam sterilization cycles. The cladded valves passed all required pressure tests and bacteriological inspections after installation.
Key Reflections and Study Insights
The study underscores the importance of process integration in cladding applications. The success of the cladding process depends not only on the welding parameters but also on the preceding machining quality, surface preparation, and subsequent finishing operations. The sealing surface must be machined to precise geometric tolerances before cladding, and the overlay must be finished to the required surface roughness after cladding, which may involve additional machining or grinding.
A critical insight from the study is the recognition that the dilution ratio must be carefully controlled in the base pass. For nickel-based overlays on carbon steel, the dilution ratio in the first pass can be as high as 40 to 50 percent, which can significantly alter the microstructure and properties of the overlay. The study recommends using a lower current and slower travel speed for the base pass to minimize penetration, or alternatively, using a pre-deposited transition layer of austenitic stainless steel before applying the nickel-based overlay.
The study also highlights the importance of post-weld machining. After cladding, the sealing surface must be machined to the final geometry, which removes the top layer of the overlay. This means that the overlay thickness must be sufficient to accommodate the machining allowance, typically 0.5 to 1.0 mm. If the overlay is too thin, the machining process may expose the base metal, compromising the corrosion resistance of the sealing surface.
In conclusion, this study provides a thorough and practical guide for cladding bellows globe valve sealing surfaces, covering material selection, process design, quality control, and engineering application. The emphasis on process integration and the detailed defect analysis make this a valuable reference for engineers working in the valve manufacturing and maintenance industry. The documented applications in nuclear and pharmaceutical sectors demonstrate the reliability and versatility of the cladding approach for critical sealing applications.