Overlay Welding of Copper Sealing Surfaces on Valve Bodies

Literature Overview

The 1990 paper by Deng Hongli in the journal "Valves" addresses a specialized welding challenge: the application of copper alloy overlay to sealing surfaces of valve bodies. The classification TG455 confirms the hardfacing/overlay welding focus, while the keywords—valve body, copper alloy, sealing surface, overlay welding—identify the precise application. This work represents an important contribution to the understanding of dissimilar metal welding in pressure-containing components.

Core Technical Challenges

Valve sealing surfaces must achieve a tight seal under pressure while resisting galling, corrosion, and thermal degradation. The base material is typically carbon steel or low-alloy steel (e.g., WCB, A216 WCB, or equivalent Chinese grades), while the overlay material is a copper alloy such as CuSn10 (leaded tin bronze), CuAl10Fe5, or a nickel-aluminum bronze variant. The fundamental challenge is the vast difference in thermal expansion coefficients between the steel base and the copper overlay:

Material Property	Carbon Steel Base	Copper Alloy Overlay
Thermal expansion (10⁻⁶/K)	11–13	16–18
Melting point (°C)	1420–1520	900–1050
Thermal conductivity (W/m·K)	45–55	200–380
Typical hardness (HB)	120–180	80–150 (as-cast)
CTE mismatch factor	—	~1.5×

This mismatch creates significant thermal residual stresses at the interface, which can lead to delamination, cracking, or loss of sealing integrity during pressure cycling.

Process Development and Key Parameters

The paper describes a multi-step overlay welding process designed to manage the metallurgical incompatibility between steel and copper. The critical process elements include:

Surface preparation: Machining of the sealing surface to remove scale, oxide, and any previous coating, followed by thorough cleaning with acetone or similar solvent.
Transition layer: Application of a nickel-based or nickel-copper intermediate layer to act as a diffusion barrier and reduce the CTE gradient at the steel-copper interface.
Copper overlay deposition: Multi-pass welding of the copper alloy using either SMAW with coated copper electrodes or OAW (oxy-acetylene welding) with copper alloy filler wire.
Post-weld stress relief: Controlled annealing to reduce residual stresses without softening the copper overlay below its minimum hardness specification.

Recommended Process Parameters

Process Parameter	Recommended Value	Purpose
Preheat temperature	150–250 °C	Reduce thermal gradient
Interpass temperature	≤ 200 °C	Prevent excessive grain growth in copper
Welding current (SMAW)	150–220 A	Moderate heat input
Travel speed	150–250 mm/min	Control penetration depth
Electrode angle	30–40° from travel direction	Promote uniform bead profile
Number of passes	2–3	Build up required thickness
Final overlay thickness	2–4 mm	After machining allowance
Stress relief temperature	400–450 °C	Below copper recrystallization temperature
Cooling rate	Furnace cool or air cool	Avoid thermal shock

Metallurgical Considerations

The steel-copper interface is metallurgically complex. Without an intermediate layer, direct welding of copper to steel produces a brittle Fe-Cu intermetallic phase (FeCu) that is prone to cracking. The nickel-based transition layer serves several functions:

It acts as a diffusion barrier, limiting the formation of brittle intermetallics
It provides a metallurgical bridge with intermediate CTE between steel and copper
It improves wetting and adhesion of the copper overlay to the steel substrate
It accommodates differential thermal contraction during cooling

The microstructure of the copper overlay itself is typically dendritic with eutectic constituents. For CuSn10, the microstructure consists of α-Cu solid solution dendrites with Cu₆Sn₅ and Cu₃Sn eutectic phases in the interdendritic regions. These hard intermetallic phases provide the wear resistance required for the sealing surface.

Quality Assurance Requirements

Test Method	Acceptance Criteria	Purpose
Visual inspection (VT)	No cracks, porosity, undercut	Surface integrity
Magnetic particle testing (MT)	No linear indications	Detect surface/subsurface cracks
Hardness test	CuSn10: HB 120–180	Verify alloy composition
Sectioning and metallography	No interfacial cracking	Confirm bonding quality
Pressure test	1.5× design pressure, 30 min hold	Verify sealing integrity
Dimensional check	Surface flatness ≤ 0.05 mm	Ensure proper sealing

Engineering Practice Integration

In valve manufacturing, the overlay welding of copper sealing surfaces is particularly important for valves operating in high-temperature steam service, where the copper alloy provides thermal conductivity and resistance to steam-side corrosion. The sealing surface must be machined after welding to achieve the required surface finish (typically Ra 0.8–1.6 μm) and flatness.

A common failure mode observed in service is progressive delamination of the copper overlay, starting at the edge of the sealing face where the constraint from the surrounding steel is least effective. This is directly related to the thermal mismatch and is mitigated by ensuring adequate bond strength through the transition layer and by controlling the geometry of the overlay to avoid sharp transitions.

Study Insights and Reflections

This paper highlights a fundamental principle in overlay welding: the interface between dissimilar materials is often the weakest link, and process development must focus as much on the transition zone as on the overlay itself. The use of a nickel-based intermediate layer is a classic solution that remains valid in modern practice. The paper's emphasis on post-weld machining and surface finish requirements also reminds us that overlay welding is not merely a metallurgical process—it is an enabling process for subsequent machining operations that determine the final functional performance of the component.