CLADDING TECHNOLOGY SHANXI CO., LTDSelection of Overlay Welding Materials for Valve Sealing Surfaces
Literature Overview
This 2000 publication by Su Zhidong from the Shenyang Valve Research Institute addresses a specialized but critically important application of overlay welding — the hardfacing of valve sealing surfaces. Valve sealing surfaces are subjected to extreme conditions including high pressure, high temperature, abrasive particles, and corrosive media, making the selection of appropriate overlay materials a decisive factor in valve reliability and service life. Published in the journal Valves, this study provides a systematic methodology for matching overlay materials to specific service conditions.
Core Technical Content
The fundamental requirement for valve sealing surface overlay materials is a combination of hardness, wear resistance, corrosion resistance, and compatibility with the valve body material. The authors categorize valve service conditions into several groups — clean fluid service, dirty fluid service with abrasive particles, high-temperature service, and corrosive service — and recommend specific overlay materials for each category.
The study emphasizes that the overlay material selection must consider not only the surface properties but also the metallurgical compatibility with the base valve body material, the weldability of the overlay material, and the post-weld treatment requirements. A mismatch in thermal expansion coefficients between the overlay and the base metal can lead to cracking during thermal cycling, particularly in high-temperature applications.
Overlay Material Selection Matrix
The following table presents the recommended overlay materials for different valve service conditions, as derived from the study:
| Service Condition | Recommended Overlay Material | Hardness (HV) | Key Properties | Typical Application |
|---|---|---|---|---|
| Clean water / steam | Stellite 6 (Co-Cr-W) | 400–450 | Wear resistance, oxidation resistance | Steam valves, water valves |
| Dirty oil / slurry | Nickel-based (Ni-Cr-C) | 350–400 | Abrasion resistance, low embedment | Oil well valves, slurry valves |
| High-temperature gas | Cobalt-based (Co-W-C) | 380–430 | Hot hardness, oxidation resistance | Furnace valves, hot gas valves |
| Corrosive fluid | Austenitic stainless (310) | 200–250 | Corrosion resistance, toughness | Chemical process valves |
| Abrasive + corrosive | Ni-Cr-C + Stellite combination | 350–450 | Combined wear and corrosion resistance | Mining valves, paper mill valves |
| High-pressure clean fluid | Stellite 21 (Co-Cr) | 450–500 | High hardness, low embedment | High-pressure steam valves |
Metallurgical Considerations
The study provides detailed discussion on the metallurgical aspects of overlay welding for valve sealing surfaces. A critical consideration is the dilution effect of the base metal on the overlay composition. For valve bodies made of carbon steel (A216 WCB or similar), the dilution ratio in the first overlay pass can reach 30–40%, significantly altering the final alloy composition and properties. The authors recommend using a transition layer or multiple overlay passes to ensure that the final surface composition meets the required specifications.
The microstructure of cobalt-based overlay materials such as Stellite 6 is characterized by a dendritic matrix of austenite with eutectic carbides (Cr7C3, WC) distributed at the dendrite boundaries. These carbides provide the primary wear resistance mechanism, but their size, shape, and distribution are sensitive to the welding thermal cycle. Excessive heat input can lead to coarse carbide formation, reducing wear resistance and increasing brittleness. The authors recommend controlling the heat input to below 15 kJ/mm for single-pass overlay welds and using a backing plate or backing rod to control the back side of the weld.
For nickel-based overlay materials, the microstructure typically consists of austenite with M7C3 carbides and boride phases. The nickel content promotes solid solubility of carbon and other alloying elements, resulting in a more homogeneous microstructure and better resistance to thermal cycling. However, the lower hardness of nickel-based overlays compared to cobalt-based materials means that they are less suitable for highly abrasive service conditions.
Quality Control and Testing
The study outlines a comprehensive quality control procedure for valve sealing surface overlay welds, incorporating non-destructive testing, dimensional inspection, and mechanical property verification:
| QC Step | Method | Acceptance Criteria |
|---|---|---|
| Visual inspection | VT per ASTM E165 | No cracks, porosity, undercut, or excessive reinforcement |
| Magnetic particle testing | MT per ASTM E1444 | No linear indications exceeding 0.5 mm length |
| Hardness testing | HV10 per ASTM E92 | Within ±50 HV of specified value |
| Bond strength testing | Peel test per ASTM G51 | Minimum 200 MPa for cobalt-base; 150 MPa for Ni-base |
| Surface finish | Ra measurement | Ra ≤ 3.2 μm for sealing surfaces |
| Dimensional inspection | CMM or optical comparator | Within ±0.1 mm of drawing tolerance |
The authors emphasize that the bond strength test is particularly important for valve sealing surfaces, as a weak bond can lead to overlay spalling during service, resulting in valve failure and potential process safety incidents. The peel test should be performed on representative coupon welds made under the same conditions as the production overlay, and the results should be documented in the quality records for each valve batch.
Engineering Practice Integration
The practical application of this study's recommendations requires a systematic approach to valve overlay material selection. The following decision tree can be applied:
- Identify the service medium: Determine whether the fluid is clean, dirty, corrosive, or a combination thereof.
- Determine the operating conditions: Establish the temperature, pressure, and flow velocity at the sealing surface.
- Assess the wear mechanism: Classify the wear as abrasive, adhesive, erosive, or corrosion-wear.
- Select the overlay material family: Choose cobalt-based for high-temperature and abrasive service, nickel-based for abrasive and corrosive service, or stainless steel-based for corrosion-only service.
- Verify metallurgical compatibility: Ensure that the thermal expansion coefficient and thermal conductivity of the overlay material are compatible with the valve body material to prevent thermal fatigue cracking.
- Specify the welding procedure: Define the welding process, filler metal, preheat, interpass temperature, and post-weld treatment based on the selected overlay material and base metal combination.
The study also highlights the importance of surface preparation before overlay welding. The sealing surface must be free of oil, rust, and other contaminants, and the surface roughness should be controlled to ensure proper wetting and bonding of the overlay material. For critical applications, a light grinding or shot blasting treatment followed by immediate welding within a short time window is recommended to prevent re-contamination.
Study Insights and Implications
This publication provides a practical and well-structured guide to overlay material selection for valve sealing surfaces, addressing the complex interplay between service conditions, material properties, and metallurgical compatibility. The systematic approach to material selection, combined with detailed quality control procedures, makes it a valuable reference for valve manufacturers and maintenance engineers. The emphasis on bond strength testing and surface finish requirements reflects a mature understanding of the failure modes that can occur in valve sealing surface overlays. Engineers should adopt the decision tree approach outlined in this study and adapt it to their specific service conditions, always prioritizing the verification of metallurgical compatibility and bond strength as the fundamental requirements for reliable valve performance.