Zhuojin Pipe Fitting Co., LtdOverlay Welding Process for Sintering Blower Blades
Literature Overview
This 1989 publication by Zhao Jiancang in the journal "Fan Technology" addresses a critical maintenance challenge in iron and steel plant sintering operations: the severe erosion and corrosion wear suffered by centrifugal blower blades. The paper describes an overlay welding (hardfacing) process developed to extend blade service life in the aggressive atmosphere of sintering furnaces. The classification code TG455 places this squarely in the hardfacing welding domain, and the industrial context reflects the heavy reliance of Chinese steel mills on sintering lines during the late 1980s industrial expansion.
Core Technical Content
The sintering blower operates under continuous exposure to hot, dusty gas streams containing iron oxide particles, sulfur compounds, and moisture. The blades experience a combination of abrasion from particulate impingement, oxidation at elevated temperatures, and thermal cycling fatigue. The conventional approach of replacing blades at fixed intervals proved economically unsustainable given the high throughput of sintering production lines.
The proposed solution involves multi-pass overlay welding of a hardfacing alloy onto the leading edge and pressure surface of the blades. The process sequence typically includes:
- Surface preparation by grinding away existing worn material and any loose oxide scale
- Preheating of the blade to 200–300 °C to reduce residual stress and prevent base metal cracking
- Application of a transition layer (if the base material is a low-carbon steel and the overlay alloy is highly alloyed)
- Multi-pass overlay welding to build up the required thickness of 3–5 mm
- Post-weld cooling control to manage the martensitic transformation in the overlay
Hardfacing Alloy Selection and Process Parameters
| Parameter | Typical Value | Rationale |
|---|---|---|
| Base material | Q235 or 20 steel | Standard blower blade material |
| Overlay alloy system | High-Cr (Cr15–Cr25) martensitic | Provides oxidation resistance and wear resistance |
| Electrode type | SMAW hardfacing electrode | Portable, suitable for on-site repair |
| Electrode diameter | 3.2–4.0 mm | Balances deposition rate and heat input |
| Preheat temperature | 200–300 °C | Prevents cold cracking in the HAZ |
| Interpass temperature | ≤ 300 °C | Controls grain growth and residual stress |
| Overlay thickness | 3–5 mm | Accounts for post-weld machining |
| Post-weld treatment | Stress-relief annealing at 550–600 °C | Reduces residual stress below 100 MPa |
Engineering Practice Integration
From a practical standpoint, the key insight of this work is the recognition that blower blade wear is not purely abrasive—it is a multi-mechanism degradation process. The overlay alloy must therefore possess both high hardness (HRC 55–60) and adequate high-temperature oxidation resistance. The martensitic high-chromium system satisfies both requirements because the continuous Cr-rich carbide network provides hardness while the passive chromium oxide film resists atmospheric attack.
A critical process consideration not always emphasized in the literature is the geometry of the blade. The thin-walled, airfoil-shaped profile of a blower blade creates challenges for heat management during welding. Excessive heat input causes distortion, which directly affects aerodynamic performance. The paper implicitly addresses this through multi-pass welding with controlled interpass temperatures and possibly the use of backing plates to distribute heat.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Undercut at overlay/base interface | Excessive travel speed or poor electrode angle | Reduce travel speed, maintain 60–70° electrode angle |
| Cracking in overlay | High carbon equivalent, rapid cooling | Increase preheat, reduce interpass temperature, use low-hydrogen electrodes |
| Excessive distortion | High heat input on thin section | Use pulse welding, intermittent welding, rigid fixtures |
| Porosity | Surface contamination, moisture in electrode | Thorough surface cleaning, electrode baking at 300 °C for 2 h |
Study Insights and Reflections
This 1989 paper represents an early but practical application of hardfacing technology to rotating machinery components in the steel industry. The approach is straightforward but effective, reflecting the engineering pragmatism of the era. The use of SMAW with consumable electrodes was the most accessible technology available, and the focus on process parameters rather than advanced metallurgy reflects the operational constraints of plant maintenance workshops.
One area that merits further discussion is the long-term performance of the overlay under thermal cycling. While the initial hardness and wear resistance are well documented, the fatigue behavior of the overlay at the weld interface after prolonged thermal cycling is less clear. In modern practice, this would be addressed through thermal fatigue testing and possibly the use of a graded transition layer to accommodate differential thermal expansion.
The paper's enduring value lies in its demonstration that even simple overlay welding processes, when properly optimized for the specific service environment, can deliver significant economic returns through extended component life.