CLADDING TECHNOLOGY SHANXI CO., LTDElectroslag Cladding of High-Chromium Cast Iron Interface Temperature Field and Microstructure
Literature Overview
This 2023 study by Wang Hao, Hu Hui'e, Chi Junhan, Chen Ze, and Feng Zijian from the Naval University of Engineering investigates the electroslag welding (ESW) cladding process applied to high-chromium cast iron substrates. Funded under project 2022501110, the work focuses on the thermal field evolution at the interface between the cladding layer and the base material, and how this thermal history governs microstructural development and mechanical performance. High-chromium cast irons (typically 12–27% Cr) are widely used in marine propeller shafts, pump impellers, and naval propulsion components due to their exceptional corrosion resistance and cavitation erosion resistance.
Core Technical Content
Thermal Field Characteristics
The electroslag cladding process produces a fundamentally different thermal cycle compared to arc-based methods. The molten slag pool acts as a heat reservoir, creating a sustained, relatively uniform heating environment. Key thermal characteristics include:
| Thermal Parameter | ESW Cladding | Comparison with SAW |
|---|---|---|
| Peak interface temperature | 1150–1250°C | 1350–1450°C |
| Cooling rate (800→500°C) | 15–35°C/s | 50–120°C/s |
| Heat input density | 25–40 kJ/mm | 10–18 kJ/mm |
| Thermal gradient at interface | 15–25°C/mm | 40–80°C/mm |
The lower cooling rates and reduced thermal gradients in ESW cladding result in coarser but more equiaxed microstructures with reduced residual stress. This is advantageous for high-chromium cast iron substrates that are inherently susceptible to cracking during welding due to their high carbon equivalent and low ductility.
Interface Microstructural Evolution
The interface region between the high-chromium cast iron substrate and the deposited overlay undergoes complex phase transformations. The study identifies several critical zones:
- Base metal heat-affected zone (HAZ): Exhibits carbide dissolution and grain boundary coarsening
- Interface transition zone (100–300 μm): Mixed microstructure with partial melting and resolidification
- Overlay base: Fully resolidified microstructure with characteristic eutectic ledeburite
The high-chromium cast iron's characteristic M₇C₃ carbides partially dissolve during the ESW thermal cycle, redistributing carbon and chromium into the liquid. This creates a dilution zone where the composition gradient can span several hundred micrometers. The slower cooling rates allow for more complete precipitation of secondary carbides (M₂₃C₆, M₆C) in the overlay, contributing to hardness values of 650–750 HV₀.₂.
Process-Property Relationships
The study establishes clear correlations between thermal parameters and resulting properties:
| Process Parameter | Effect on Hardness | Effect on Bond Strength |
|---|---|---|
| Increased current | Decreases (coarser microstructure) | Increases (better fusion) |
| Increased travel speed | Increases (faster solidification) | Decreases (incomplete melting) |
| Increased slag basicity | Minimal effect | Increases (better wetting) |
| Preheating to 200°C | Decreases by 30–50 HV | Increases by 20–30% |
The optimal process window identified requires a current of 400–550 A, travel speed of 80–120 mm/min, and preheating of the substrate to 150–200°C. Under these conditions, bond strength tests (per ASTM G128 or equivalent) demonstrate shear strengths exceeding 450 MPa, which exceeds the requirement for most naval applications.
Engineering Practice Implications
For naval applications involving propeller shafts and rudder stock components, ESW cladding offers distinct advantages over other overlay methods. The process is inherently suited to large-diameter cylindrical geometries, producing uniform cladding layers with minimal geometric distortion. The lower thermal stresses reduce the risk of cracking in the brittle high-chromium cast iron substrate—a critical consideration given the consequences of component failure in marine environments.
The study's emphasis on the thermal field at the interface provides valuable guidance for process qualification under NB/T 47014 or ASME IX. Understanding the interface thermal history enables more accurate prediction of HAZ properties and supports the development of reliable acceptance criteria for production components.
Summary and Reflections
This research demonstrates that the electroslag cladding process, while often considered a legacy technology, remains highly relevant for large-scale naval applications involving high-chromium cast iron substrates. The detailed thermal field analysis provides the mechanistic understanding necessary for rational process optimization rather than empirical trial-and-error. The relatively slow cooling rates inherent to ESW actually represent an advantage for materials with limited crack resistance, as they allow for more controlled solidification and stress relief during cooling. This work contributes meaningfully to the qualification and standardization of ESW cladding for critical naval components.