Manufacturing Bimetal Runner Crown Using Stainless Steel Strip Electrode Cladding Technology

Literature Overview

This 1993 study by Du Bing, Li Yan, Xia Weimin (Harbin Welding Institute) and Wang Aimin, Li Lin (Harbin Electric Machinery Factory) documents the application of stainless steel strip electrode cladding technology for manufacturing the bimetal upper crown of a hydroelectric turbine runner. This represents an early Chinese engineering application of strip cladding (also known as strip surfacing or band cladding) technology for large-scale power generation equipment.

Application Background

Hydroelectric turbine runners operate under extreme conditions:

• High-velocity water flow (10–30 m/s)
• Cavitation erosion
• Abrasive wear from suspended sediment
• Corrosive attack from dissolved gases
• Cyclic loading from runner rotation

The upper crown (or upper band) of the runner is particularly susceptible to cavitation damage due to its position in the high-velocity flow region near the guide vanes. Traditional monolithic stainless steel runners are prohibitively expensive for large-scale applications, motivating the development of bimetallic construction.

Strip Electrode Cladding Technology

Process Description

Strip electrode cladding involves:

1. A continuous strip of overlay material (typically 3–5 mm thick)
2. A consumable electrode (covered or bare)
3. Flux coverage for atmosphere protection
4. Submerged arc welding (SAW) or electroslag welding (ESW) process

The strip is fed simultaneously with the electrode, and the molten pool forms a bond between the strip and the base metal.

Process Parameters

Bimetal Construction Design

Material Selection

Design Considerations

The bimetallic construction must address:

• Thermal expansion mismatch (stainless steel vs. carbon steel)
• Stress concentration at the bond interface
• Potential for interfacial cracking under cyclic loading
• Cavitation resistance of the overlay surface
• Weldability of dissimilar materials

Metallurgical Analysis

Bond Interface Microstructure

The bond interface between carbon steel base and stainless steel overlay exhibits:

1. Austenite phase — from stainless steel solidification
2. Ferrite phase — from carbon steel dilution
3. Mixed microstructure — in the transition zone
4. Potential brittle phases — if dilution is excessive

Dilution Control

For strip cladding, dilution is typically lower than conventional welding because:

• The strip provides a large volume of overlay material
• Heat input is distributed over a wider area
• The process is inherently more stable

Typical dilution levels:

• First layer: 15–25%
• Second layer: 5–10%
• Final surface: < 5%

Quality Control Requirements

Non-Destructive Testing

Mechanical Testing

• Tensile bond strength: ≥ 200 MPa (typical requirement)
• Hardness of overlay: HV 180–250 (austenitic stainless steel)
• Impact toughness of HAZ: ≥ 27 J at service temperature

Engineering Practice and Lessons Learned

Production Challenges

The manufacturing of bimetal runner crowns presents several unique challenges:

1. Large component size — runner crowns may have diameters exceeding 3 meters
2. Curved geometry — requires specialized equipment for strip cladding on curved surfaces
3. Positional welding — all positions must be achievable with strip cladding equipment
4. Distortion control — thermal stresses from cladding can distort the runner shape
5. Surface quality — overlay surface must be smooth for hydraulic efficiency

Distortion Management

To control distortion during strip cladding:

• Apply preheat uniformly across the entire component
• Use symmetric welding sequences to balance thermal input
• Apply mechanical restraint during welding
• Perform stress relief treatment after cladding (550–650°C for 2 hours)

Study Reflection

This 1993 work represents a pioneering application of strip cladding technology in Chinese hydropower equipment manufacturing. The successful implementation of bimetallic runner crowns demonstrated that:

• Strip cladding technology could be applied to large-scale power generation equipment
• Bimetallic construction provided significant cost savings compared to monolithic stainless steel
• The bonding quality was sufficient for demanding cavitation and erosion service
• The technology could be transferred from research institute to manufacturing plant

The Harbin Welding Institute and Harbin Electric Machinery Factory collaboration exemplifies the effective partnership between research institutions and manufacturing enterprises. The technical knowledge developed in laboratory conditions was successfully transferred to production environments, resulting in commercially viable bimetallic products.

For contemporary engineers, this work provides valuable historical perspective on the evolution of cladding technologies in China. The fundamental principles established in this study — dilution control, bond quality assurance, distortion management — remain directly applicable to modern cladding operations, whether using conventional strip cladding or advanced technologies such as laser cladding or plasma transferred arc welding.