Fabrication of Bimetal Turbine Upper Crown Using Stainless Steel Strip Cladding Technology

Technical Background and Application Context

The upper crown of a turbine rotor in power generation systems is subjected to alternating thermal cycling, centrifugal forces, and corrosion from steam and water media. Traditional monolithic construction using stainless steel for the entire rotor is prohibitively expensive due to the large volume of nickel and chromium content required. The bimetal approach, combining a carbon or low-alloy steel core with a stainless steel overlay layer, offers an economical solution that maintains corrosion resistance where needed while leveraging the superior mechanical properties of the structural steel. Strip cladding technology, particularly using electroslag welding (ESW) or submerged arc welding (SAW) processes, provides the means to achieve uniform and reliable overlay layers on large turbine components.

Core Technical Approach: Strip Cladding Process

The fabrication of the bimetal upper crown involves several critical stages. The base material is typically a low-carbon steel such as Q345R or 16MnR, which provides adequate strength and toughness for the structural requirements. The cladding layer consists of austenitic stainless steel strips, commonly 304 or 316 grade, applied using one of two primary methods: electroslag welding for thick overlay layers and submerged arc welding for thinner layers or edge preparation.

The ESW strip cladding process operates by feeding a stainless steel strip and flux into an electroslag pool, where the slag's high temperature melts the strip and the base metal surface simultaneously. The process is inherently stable due to the electromagnetic stirring effect of the current flowing through the slag, which ensures uniform melting and penetration. Typical ESW parameters for turbine crown cladding include current of 3000-5000 A, voltage of 40-55 V, and travel speed of 150-300 mm/min, with strip thickness of 6-12 mm.

Key Technical Challenges and Solutions

The primary technical challenge in turbine upper crown cladding is achieving adequate bond strength between the dissimilar metals while minimizing dilution. Excessive dilution leads to a transition zone with reduced corrosion resistance, while insufficient penetration results in incomplete bonding and potential delamination. The literature identifies several strategies for optimizing this balance:

1. Surface Preparation: The base metal surface must be machined to a roughness of Ra 12.5-25 μm to promote mechanical interlocking and increase the effective bonding area. Surface contamination such as oil, rust, and scale must be completely removed through grinding and solvent cleaning.
2. Welding Parameter Optimization: The current density and travel speed must be adjusted to achieve a penetration depth of 1-3 mm into the base metal, ensuring metallurgical bonding without excessive dilution. Higher current with lower travel speed increases penetration but also increases dilution.
3. Multi-Layer Deposition: For overlay thicknesses exceeding 6 mm, multiple layers are applied with each subsequent layer reducing the dilution effect. The first layer typically has the highest dilution (20-30%), while subsequent layers drop to 5-10%.
4. Interpass Temperature Control: Maintaining interpass temperature between 200-300°C prevents excessive cooling that could cause cold cracking in the HAZ while avoiding overheating that could lead to grain coarsening.

Quality Assurance and Inspection Requirements

Quality control for bimetal turbine upper crowns is critical due to the safety implications of turbine failure. The overlay layer must be inspected for bond quality using ultrasonic testing (UT) in accordance with NB/T 47013 or equivalent standards. The acceptance criteria typically require no lack of bond defects exceeding 50 mm in length or 10 mm in width for critical areas. Surface quality of the overlay must meet Ra 3.2-6.3 μm after machining, with no visible porosity, cracks, or inclusions.

Mechanical testing of the overlay layer includes tensile testing of coupon specimens to verify yield strength exceeding 205 MPa and elongation above 30%. Hardness testing across the fusion line should show a gradual transition without sharp discontinuities. Intergranular corrosion testing per ASTM A263 or equivalent must confirm that the overlay layer meets the corrosion resistance requirements for the intended service environment.

Study Reflections and Practical Implications

The study of strip cladding technology for turbine upper crowns highlights the importance of process optimization in achieving reliable bimetal fabrication. The ESW method offers superior efficiency and consistency for thick overlay layers, making it the preferred choice for large turbine components. However, the initial setup complexity and equipment requirements must be weighed against the production volume and schedule constraints. In my experience, the transition from traditional welding methods to strip cladding represents a significant productivity improvement, with deposition rates 3-5 times higher than conventional SAW processes. The key to successful implementation lies in thorough process qualification testing and strict adherence to qualified welding procedures during production.