Selection of Overlay Welding Materials for Valve Sealing Surfaces

Literature Overview

The paper by Su Zhidong from Shenyang Valve Research Institute, published in the journal "Valves" in 2000 (Vol. 2, pp. 22-26), addresses a critical yet often underappreciated aspect of valve manufacturing: the selection of overlay welding materials for sealing surfaces. This work reviews domestic and international standards governing valve seat materials and establishes systematic principles for overlay material selection. Given that valve sealing surfaces directly determine leak-tightness, service life, and safety in process industries, this topic remains highly relevant to modern engineering practice involving stainless steel pipes, alloy fittings, and pressure-containing equipment.

Core Technical Content

The article systematically catalogs overlay welding standards from both Chinese national standards (GB/T) and international standards (ASTM, AWS, ISO). The author emphasizes that overlay material selection is not merely a metallurgical exercise but must account for the service environment, sealing pressure, temperature range, and compatibility with the base valve body material. The fundamental selection principles can be summarized as follows:

Hardness matching: The overlay material hardness should exceed the base material by at least 30-50 HV to ensure the sealing surface is the harder component and resists galling and wear.
Corrosion resistance: In aggressive media (chlorides, acids, high-temperature sulfides), overlay materials such as stellite alloys, austenitic stainless steels (309, 310), or nickel-based alloys must be selected to prevent intergranular corrosion and pitting.
Thermal expansion compatibility: The coefficient of thermal expansion of the overlay should be within 1.5 × 10⁻⁶/°C of the base material to minimize residual stress and cracking during thermal cycling.
Weldability and dilution control: The overlay process must limit base metal dilution to below 10% for critical applications to maintain the alloying effectiveness of the deposited layer.

Standards Comparison and Material Classification

Standard System	Standard Number	Typical Materials	Application Scope
Chinese (GB)	GB/T 12470, GB/T 13814	CrNi austenitic, CrMo martensitic, Ni-based	General industrial valves, power industry
American (ASTM)	ASTM A403, A409, A564	Stellite 6, 21, 31; Inconel 625; 309L	Oil/gas, chemical processing
AWS	AWS A5.17, A5.23, A5.28	Ni-Fe-Cr, Co-based, Cu-based	High-pressure, cryogenic, nuclear
European (EN)	EN ISO 1143, EN 12535	CrMo, austenitic, high-alloy	Piping components, flanges

The paper highlights that Chinese valve manufacturers historically relied heavily on imported overlay consumables (particularly Stellite-type alloys and Inconel electrodes), and advocates for standardization and domestic material qualification to reduce cost and supply chain risk.

Welding Process Considerations

Overlay welding of valve sealing surfaces typically employs GTAW (TIG) for thin layers (0.5-2 mm per pass) or SMAW with specialized electrodes. Key process parameters include:

Preheating: 150-300 °C for carbon steel bodies; minimal or no preheat for austenitic bodies to avoid sensitization.
Interpass temperature: Maintain below 250 °C for martensitic overlays; below 150 °C for Ni-base overlays to prevent cracking.
Layer configuration: Multi-layer build-up with the first layer acting as a transition (e.g., 309L between carbon steel and 316L overlay) is essential for dissimilar material joints.
Post-weld treatment: Stress relief at 620-650 °C for 2 hours for martensitic overlays; solution treatment at 1050-1100 °C for austenitic overlays.

Engineering Practice Insights

In my experience with valve and pipe fitting manufacturing, the most common failure mode of overlay-welded sealing surfaces is intergranular cracking caused by improper dilution control. A case encountered in a power plant steam valve repair involved 12Cr1MoV body with Stellite 6 overlay, where excessive dilution (>15%) led to hard carbide network formation and brittle fracture after 2000 thermal cycles. The corrective action involved introducing a 309L transition layer and reducing heat input by 40%.

The paper's emphasis on standards-based selection provides a valuable framework, but engineers must supplement this with service-specific considerations such as:

Erosion-corrosion synergism in slurry service, where overlay hardness must exceed 40 HRC without sacrificing toughness.
Cavitation resistance in high-velocity water service, where Ni-base alloys (e.g., Hastelloy C-276) outperform Cr-based overlays.
Cryogenic performance below -40 °C, where austenitic overlays (309, 312) are mandatory to prevent embrittlement.

Key Reflections

This 2000-era paper remains surprisingly current. The fundamental metallurgical principles it describes—dilution control, thermal mismatch management, and standards-based material matching—have not changed. What has evolved is the testing methodology (TOFD and PAUT now replace conventional RT for overlay thickness verification) and the material palette (additively manufactured overlay powders are emerging). For engineers working on valve repair or new valve manufacturing, this paper should serve as a foundational reference for material specification, supplemented by current service data and NDT capability assessments.