CLADDING TECHNOLOGY SHANXI CO., LTDElectroslag Welding Overlay of High-Chromium Cast Iron Interface Temperature Field and Microstructure Properties
Literature Overview
This paper, published in the Welding Journal in 2023 and supported by a self-initiated research project (2022501110) from the Naval Engineering University, addresses a critical challenge in the repair and overlay welding of high-chromium cast iron components. The authors — Wang Hao, Hu Hui'e, Chi Junhan, Chen Ze, and Feng Zijian — conducted a systematic investigation into the thermal field distribution at the interface between the high-chromium cast iron substrate and the electroslag weld overlay deposit, along with the resulting microstructural evolution and mechanical properties. This work is particularly significant given the widespread use of high-chromium cast irons (such as Cr26, Cr30, and Cr40 grades) in marine, mining, and chemical processing applications where severe abrasive and corrosion resistance are demanded.
Core Technical Content
Interface Temperature Field Analysis
The electroslag welding (ESW) overlay process is characterized by its high thermal input, which creates a complex temperature distribution at the substrate-overlay interface. The study employs finite element simulation to map the transient thermal field during the ESW overlay process. The key findings regarding the thermal profile include:
| Parameter | Typical Value | Engineering Significance |
|---|---|---|
| Peak interface temperature | 1350–1500 °C | Determines dilution ratio and microsegregation |
| Cooling rate at interface | 5–15 °C/s | Influences carbide morphology and hardness |
| Thermal input | 40–80 kJ/mm | Higher than SAW or GTAW overlay |
| Heat affected zone width | 8–15 mm | Affects substrate property degradation |
The high thermal input of ESW results in a broader heat-affected zone (HAZ) compared to other overlay processes. This is a double-edged sword: while the slower cooling rate reduces the risk of cracking in the brittle high-chromium cast iron substrate, it also increases the dilution of the overlay material and can lead to undesirable microstructural changes in the substrate.
Microstructural Evolution
The interface region between the high-chromium cast iron and the ESW overlay deposit exhibits a complex microstructural transition. The base metal typically contains a matrix of martensite with dispersed primary chromium carbides (M7C3 and M23C6 type). In the overlay deposit, the microstructure is governed by the cooling rate, composition, and dilution level.
Key microstructural features identified in the study include:
- Overlay deposit: A mixture of austenite, martensite, and chromium-rich carbides (M7C3, M23C6) depending on the chromium content and cooling conditions.
- Interface region: A transition zone where the carbide morphology changes from coarse primary carbides in the base metal to finer secondary carbides in the overlay.
- HAZ: Partially austenitized martensite with possible formation of brittle carbide networks along grain boundaries.
Mechanical Property Assessment
The hardness distribution across the overlay interface reveals a characteristic gradient. The base metal hardness typically ranges from 55–65 HRC for high-chromium cast irons, while the overlay deposit hardness can reach 58–70 HRC depending on the specific alloy composition and heat treatment condition. The interface hardness often shows a slight depression due to the softening effect of the thermal cycle on the base metal carbide structure.
Process Analysis and Engineering Implications
Comparison with Alternative Overlay Processes
| Process | Thermal Input (kJ/mm) | Dilution (%) | Interface Cracking Risk | Production Rate |
|---|---|---|---|---|
| ESW overlay | 40–80 | 15–30 | Medium | Very High |
| SAW overlay | 15–35 | 10–25 | Low–Medium | High |
| GTAW overlay | 5–15 | 5–15 | Low | Low |
| PTA cladding | 8–20 | 3–10 | Very Low | Medium |
The ESW overlay process offers superior deposition rates, making it economically attractive for thick overlay layers (greater than 3 mm). However, the high thermal input necessitates careful control of preheat and interpass temperature to prevent excessive grain growth in the HAZ and to minimize the risk of thermal cracking in the brittle cast iron substrate.
Key Process Parameters for ESW Overlay on High-Chromium Cast Iron
Based on the study's findings and engineering practice, the following process window is recommended:
- Preheat temperature: 200–300 °C to reduce thermal gradients and minimize cracking risk.
- Interpass temperature: Maintain above 150 °C to avoid excessive cooling rates.
- ESW voltage: 35–45 V depending on electrode diameter and travel speed.
- Current: 800–1500 A with DCEN polarity for adequate penetration.
- Flux composition: Rutile-type flux with controlled moisture content (below 0.5%) to ensure stable slag pool operation.
Defect Analysis and Countermeasures
The primary defects encountered in ESW overlay of high-chromium cast iron include:
- Hot cracking: Caused by low melting point eutectics at grain boundaries in the HAZ. Countermeasures include controlling sulfur and phosphorus content in the base metal and using appropriate preheat.
- Cold cracking: Hydrogen-induced delayed cracking in the high-strength martensitic structure. Mitigated through low-hydrogen flux selection and post-weld heat treatment.
- Delamination at interface: Poor bonding due to oxide inclusions or excessive dilution. Addressed by thorough surface preparation and control of the thermal cycle.
- Excessive dilution: Leading to reduced overlay hardness and corrosion resistance. Managed by optimizing ESW parameters and using multiple passes with controlled penetration.
Study Insights and Reflections
This research provides valuable quantitative data on the thermal field at the ESW overlay interface, which is essential for process optimization. From a practical standpoint, the findings reinforce the importance of understanding the thermal-microstructure-property relationship when selecting overlay processes for high-chromium cast iron components. The high thermal input of ESW, while beneficial for deposition rate, demands rigorous control of the thermal cycle to prevent degradation of the substrate properties.
In pressure vessel repair applications, where high-chromium cast iron components may be encountered in high-wear zones, the ESW overlay process can be advantageous for building up significant thickness. However, the broader HAZ and higher dilution must be carefully evaluated against the service requirements. For critical pressure boundary applications, a hybrid approach combining ESW for bulk deposition followed by PTA or laser cladding for the final surface layer may offer the best compromise between productivity and quality.
The study's emphasis on interface temperature field analysis represents a methodological advancement, as understanding the precise thermal conditions at the bonding interface is fundamental to predicting long-term joint integrity under cyclic thermal and mechanical loading. This approach aligns with modern digital twin methodologies for welding process optimization and should be encouraged in future research programs.