CLADDING TECHNOLOGY SHANXI CO., LTDStudy Notes on Weld Overlay Strengthening of Blast Furnace Equalizing Valve
Service Environment and Failure Mechanisms
The blast furnace equalizing valve is a critical component in the ironmaking process that regulates the pressure equalization between the blast furnace top and the gas cleaning system. It operates under conditions of high temperature (300–450 °C), high pressure (up to 0.3 MPa gauge), and exposure to abrasive dust-laden gas streams containing SiO₂, Al₂O₃, Fe₂O₃, and other oxides. The valve seat and sealing surfaces are subjected to severe erosive wear, which is the dominant failure mechanism.
The literature describes the wear mechanism as a combination of abrasive erosion from solid particles impinging on the valve surface at high velocity, and thermal fatigue from cyclic temperature fluctuations. The uncladded valve seat, typically made of 16Mn or Q345 carbon steel, exhibits wear rates of 0.5 to 1.5 mm per 6-month campaign, necessitating frequent shutdowns for repair or replacement. The literature quantifies the economic impact: each unplanned shutdown costs approximately 50,000 to 100,000 RMB in lost production, making the extension of valve life through weld overlay cladding highly economical.
Overlay Material Selection and Process Design
The literature evaluates several overlay material options for the blast furnace equalizing valve, including high-chromium cast irons (Cr20, Cr26), Stellite 6 (Co-Cr-W alloy), and martensitic stainless steels (410, 440C). The selection is based on a balance of hardness, wear resistance, and weldability with the carbon steel substrate.
| Overlay Material | Hardness (HV) | Wear Resistance Index | Dilution Sensitivity | Cost Factor |
|---|---|---|---|---|
| Cr20 (D2) | 550–650 | Excellent | Moderate | Low |
| Cr26 (D3) | 600–700 | Excellent | Moderate | Low |
| Stellite 6 | 400–450 | Excellent (high temp) | Low | High |
| 410 Martensitic SS | 350–400 | Good | Low | Low |
| 440C Martensitic SS | 500–550 | Very Good | Low | Moderate |
The literature recommends Cr20 or Cr26 high-chromium cast iron for the valve seat surface due to their exceptional abrasion resistance and reasonable cost. The overlay is applied using multi-layer SAW with a flux-cored wire, followed by machining to the required surface finish and geometry.
The process design involves a multi-pass approach: the first pass is a bond coat using a nickel-based or austenitic stainless steel filler to ensure metallurgical compatibility with the carbon steel substrate. Subsequent passes use the high-chromium cast iron filler to build up the wear-resistant surface. The dilution in the first pass is typically 30% to 50%, while the dilution in subsequent passes decreases to 5% to 15% as the overlay thickness increases.
Process Parameters and Heat Treatment
The welding process parameters described in the literature are optimized to minimize dilution while ensuring complete fusion and sound metallurgical bonding. The preheat temperature is set at 200 to 250 °C to prevent cold cracking in the carbon steel substrate. The interpass temperature is maintained at 200 to 300 °C.
| Parameter | Bond Coat Pass | Overlay Passes |
|---|---|---|
| Process | SAW (flux-cored) | SAW (flux-cored) |
| Wire Type | ENi-CrMo / E309L | High-Cr cast iron (Cr20) |
| Wire Diameter | 1.6 mm | 2.0 mm |
| Current | 280–340 A | 320–400 A |
| Voltage | 30–36 V | 32–38 V |
| Travel Speed | 200–300 mm/min | 250–350 mm/min |
| Layers | 1–2 | 3–5 |
| Post-Weld Treatment | Stress relief 550–600 °C × 2 h | — |
The post-weld stress relief treatment is critical for the blast furnace equalizing valve because the component is subjected to cyclic thermal loading during operation. Residual tensile stresses from welding can initiate thermal fatigue cracks. The stress relief at 550 to 600 °C reduces residual stresses to below 100 MPa and also tempers the martensitic structure in the high-chromium overlay, improving toughness without significantly reducing hardness.
Performance Evaluation and Field Results
The literature reports field performance data from multiple blast furnace campaigns. The Cr20 overlay-cladded valve seats exhibited wear rates of 0.1 to 0.2 mm per 6-month campaign, compared to 0.5 to 1.5 mm for uncladded valves. This represents a 4- to 8-fold improvement in service life. The overlay surface after one campaign showed a characteristic "glazed" appearance with a thin layer of oxide that actually provided additional protection against further wear.
Metallographic examination of the worn overlay surface revealed a two-layer structure: a thin oxide layer (20 to 50 μm) on the surface and a work-hardened layer (100 to 200 μm) beneath it. The hardness of the work-hardened layer was 700 to 800 HV, indicating significant strain hardening during service. The unexposed overlay material retained its original hardness of 550 to 650 HV.
The literature also discusses the importance of the dilution gradient at the overlay-to-substrate interface. A gradual transition in composition and hardness minimizes stress concentrations and improves fatigue resistance. The bond coat layer serves this purpose by providing a compositionally intermediate zone between the carbon steel substrate and the high-chromium overlay.
Engineering Reflections and Practical Recommendations
The most important lesson from this case study is that weld overlay strengthening of blast furnace equalizing valves is not merely a matter of applying a hard overlay but requires careful consideration of the entire process chain: material selection, process design, heat treatment, and quality control. The multi-layer approach with a bond coat is essential for achieving both metallurgical compatibility and functional performance.
The literature also highlights the importance of surface preparation and geometry control. The valve seat must be machined to a precise geometry before cladding, and the overlay must be machined back to the original dimensions after welding. Any geometric deviation can affect the valve's sealing performance and lead to gas leakage.
Another practical insight is the economic analysis. The cost of overlay cladding is approximately 15% to 25% of the cost of replacing the valve seat, while the service life extension is 4 to 8 times. This makes the investment in overlay cladding highly justified, with a payback period of less than one campaign.
In summary, the weld overlay strengthening of blast furnace equalizing valves using high-chromium cast iron overlays represents a proven and cost-effective solution to the severe abrasive wear problem in ironmaking. The key success factors are proper material selection, multi-layer process design with a bond coat, appropriate heat treatment, and rigorous quality control. The field performance data presented in the literature provides strong evidence for the widespread adoption of this technology in the iron and steel industry.