CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Strengthening of Blast Furnace Pressure Equalization Valves
Literature Overview
This 1992 paper by Ge Qixin and Li Chunyan from Ansteel Machinery Manufacturing Company, published in Ansteel Technology, reports on the application of overlay welding to strengthen the sealing surfaces of blast furnace pressure equalization valves. The work addresses a practical maintenance and reliability challenge in iron and steel production, where pressure equalization valves are subjected to severe wear and erosion from high-velocity gas flow containing abrasive dust particles.
Technical Background
Blast furnace pressure equalization valves are critical components in the blast furnace gas cleaning system. These valves regulate the pressure difference between the blast furnace top and the gas cleaning system, and they are exposed to high-temperature (200–400 °C), high-velocity gas flow containing abrasive iron oxide dust particles. The valve sealing surfaces, which are typically machined from carbon steel, suffer from rapid erosive wear, leading to frequent maintenance, gas leakage, and production downtime.
The overlay welding approach involves applying a hardfacing or wear-resistant alloy layer to the valve sealing surfaces to extend service life. The challenge is to select an appropriate overlay alloy and welding process that provides adequate wear resistance while maintaining the valve's sealing integrity and thermal stability.
Overlay Alloy Selection and Welding Process
The study evaluates several overlay alloys for the application:
| Alloy Type | Composition (wt%) | Hardness (HRC) | Wear Resistance | Cost |
|---|---|---|---|---|
| Stellite 6 (Co-Cr-W) | Co-27Cr-6W-5Fe | 40–45 | Excellent | High |
| High-speed steel type | W18Cr4V equivalent | 55–60 | Very good | Medium |
| Chromium carbide type | 60CrMoV (modified) | 50–55 | Good | Low |
| Nickel-based (Inconel 625) | Ni-22Cr-9Mo-3Nb | 30–35 | Moderate | High |
The study recommends the chromium carbide type overlay alloy for this application because it provides an optimal balance of wear resistance, cost, and weldability. The overlay is applied using a gas-shielded metal arc welding (GMAW) process with a flux-cored wire, which provides a high deposition rate suitable for large valve surfaces.
The welding parameters used are:
| Parameter | Value |
|---|---|
| Wire diameter | 1.6 mm |
| Shielding gas | CO2 or Ar + 2% CO2 |
| Current | 200–280 A |
| Voltage | 22–28 V |
| Travel speed | 200–350 mm/min |
| Number of passes | 2–3 |
| Interpass temperature | Below 150 °C |
| Preheat | Not required for carbon steel base |
Performance Evaluation
The overlay-strengthened valves were tested in service and compared with conventional carbon steel valves:
| Performance Metric | Carbon Steel Valve | Overlay-Strengthened Valve | Improvement |
|---|---|---|---|
| Service life between repairs | 3–6 months | 18–24 months | 3–4 times |
| Surface hardness after 6 months | 25–30 HRC | 45–50 HRC | Significant |
| Leakage rate after 12 months | High (requires repair) | Acceptable | Dramatic |
| Maintenance cost per year | High (frequent repairs) | Low (infrequent repairs) | 60–70% reduction |
The overlay layer provides a significant extension of service life, reducing the frequency of valve maintenance from several times per year to once per year or less. This translates into substantial cost savings and reduced production downtime.
Defect Analysis and Countermeasures
Despite the overall success of the overlay strengthening approach, several defects were observed during implementation:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking in overlay layer | High carbon equivalent of base metal, excessive heat input | Reduce current, use lower heat input per pass |
| Poor bond strength | Insufficient penetration into base metal | Increase first-pass current, ensure proper surface preparation |
| Spatter | Excessive arc length or gas flow | Optimize gas flow rate, maintain consistent arc length |
| Incomplete coverage | Uneven travel speed | Use automated welding or guide fixtures |
The most common defect is cracking in the overlay layer, which is caused by the high hardness and low ductility of the chromium carbide type alloy. To mitigate this, the study recommends using a transition layer of a more ductile alloy (such as nickel-based or austenitic stainless steel) between the base metal and the hardfacing overlay. This transition layer absorbs the residual stresses from the hardfacing and prevents cracking.
Study Insights
This paper demonstrates the practical value of overlay welding in extending the service life of critical industrial components. The blast furnace pressure equalization valve is a representative example of a component where the cost of failure (production downtime, gas leakage, environmental impact) far exceeds the cost of overlay strengthening. The approach is straightforward, cost-effective, and can be applied in the field without requiring specialized equipment.
The key lesson for engineers is that overlay welding is not merely a surface treatment but a system engineering solution that requires careful consideration of alloy selection, welding process parameters, and post-weld inspection. The transition layer concept, while adding complexity, is essential for preventing cracking in hardfacing applications and should be standard practice for high-stress components.