CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of High-Chromium Cast Iron Cladding Deposited with Self-Shielded Flux-Cored Wire
Literature Overview
This 2009 study by Li Da, Hao Feifei, Liu Qingfeng, Tang Lifang, and Yang Qingxiang, conducted under the Hebei Province Science and Technology Project (04212201D), investigates the microstructure and mechanical properties of high-chromium cast iron weld overlay deposited using self-shielded flux-cored wire (FCAW). The research was supported by the State Key Laboratory of Metastable Materials Preparation Technology at Yanshan University and involved industrial validation through the Dalian Boiler and Pressure Vessel Inspection Institute and Dalian Tianyi Natural Gas Company. This work addresses a practically significant problem: the economical repair and surface hardening of high-chromium cast iron components used in abrasive service environments such as mining, cement, and natural gas processing.
Core Technical Content and Process Parameters
High-chromium cast irons (typically 12 to 30 wt% Cr, with 2 to 4 wt% C) are valued for their exceptional resistance to abrasive wear and oxidation at elevated temperatures. However, these materials are notoriously difficult to weld due to the high carbon equivalent, extensive martensitic transformation during cooling, and susceptibility to cracking. The use of self-shielded flux-cored wire offers advantages over solid wire processes in this application, including the ability to deposit high-carbon, high-chromium compositions without the need for external shielding gas, and the provision of a slag layer that promotes controlled solidification.
| Parameter | Typical Range | Notes |
|---|---|---|
| Wire diameter | 1.2–2.0 mm | Self-shielded FCAW wire |
| Arc voltage | 22–30 V | Lower than solid wire to maintain arc stability |
| Current | 150–250 A | DCEN polarity preferred for deep penetration |
| Travel speed | 100–200 mm/min | Slower than conventional welding for adequate deposition |
| Preheat temperature | 200–400°C | Critical for preventing cold cracking |
| Interpass temperature | ≤300°C | Maintained to control cooling rate |
| Post-weld cooling | Controlled (furnace or insulating blanket) | Avoids rapid quenching |
The self-shielded flux-cored wire typically contains a flux coating rich in iron oxide, calcium fluoride, and silicon carbide. During welding, the flux decomposes to produce shielding gases (CO₂, CO) and forms a protective slag over the weld pool. The carbon and chromium content of the deposited metal is controlled by both the wire core composition and the flux chemistry.
Microstructure Analysis
The microstructure of the high-chromium cast iron cladding layer deposited by self-shielded FCAW is characterized by a complex mixture of phases. The primary phases include:
- Martensite: Forms during rapid cooling from the austenite region; provides high hardness (700 to 900 HV) but limited toughness.
- Chromium carbides (Cr₇C₃, Cr₂₃C₆, Cr₃C): The type and morphology of chromium carbides depend on the Cr/C ratio. In high-chromium irons (Cr > 20%), Cr₇C₃ is the dominant carbide, which is more thermodynamically stable than Cr₂₃C₆.
- Residual austenite: Present in amounts of 5 to 25% depending on the alloy composition and cooling rate; contributes to toughness but may transform to brittle martensite during service.
- Free graphite: May appear in slowly cooled regions; reduces hardness but improves machinability.
The interface between the cladding layer and the base high-chromium cast iron substrate is a critical region. Dilution from the base metal affects the final composition of the first pass, potentially reducing the chromium content below the threshold required for full austenite stabilization. This can lead to excessive martensite formation and increased cracking susceptibility in the first pass. Subsequent passes, deposited on the already-clad surface, experience less dilution and develop a more homogeneous microstructure.
Mechanical Properties and Wear Performance
The hardness profile across the cladding layer typically shows the first pass at 600 to 750 HV and subsequent passes at 750 to 900 HV, reflecting the increasing chromium and carbon content as dilution decreases. The tensile strength of the cladding layer is generally lower than the base material (approximately 500 to 700 MPa versus 800 to 1000 MPa for the base cast iron), but the combination of hardness and residual austenite content provides adequate toughness for most abrasive service applications.
Wear resistance testing (typically using ASTM G65 pin-on-disk or dry sand rub methods) demonstrates that the high-chromium cast iron cladding deposited by self-shielded FCAW achieves wear rates 2 to 5 times lower than conventional medium-carbon steel overlays. The wear mechanism transitions from abrasive ploughing (on the base material) to micro-cutting and oxidation (on the cladding layer), with chromium carbides serving as the primary wear-resistant phase.
Engineering Practice and Quality Control
For industrial implementation of this technology, the following quality control measures are essential:
- Preheat and interpass temperature monitoring using infrared thermometers or embedded thermocouples to prevent cold cracking in the high-carbon base material.
- Metallographic examination of cross-sections to verify adequate bond strength, absence of cracks, and acceptable dilution levels at the interface.
- Hardness mapping across the cladding surface and depth to ensure uniformity and identify any regions of excessive softening or over-hardening.
- Impact testing of coupon specimens to verify that the cladding retains sufficient toughness for the intended service conditions.
- Visual and magnetic particle inspection of the completed cladding surface to detect surface cracks, porosity, and undercut.
Study Insights and Reflections
This research demonstrates that self-shielded FCAW is a viable and economical alternative to solid wire processes for high-chromium cast iron cladding, particularly in field repair applications where the availability of shielding gas equipment may be limited. The key advantage is the ability to deposit compositions with higher carbon and chromium content than achievable with solid wire, due to the flux chemistry contributing additional alloying elements. However, the variability inherent in flux-cored wire processes—related to flux coating uniformity, wire straightness, and gas pocket formation—requires stricter process control and more frequent quality checks compared to solid wire welding. Engineers should weigh the economic benefits of self-shielded FCAW against the need for rigorous quality assurance when selecting the process for critical components.