CLADDING TECHNOLOGY SHANXI CO., LTDHardfacing Overlay Reinforcement of Blast Furnace Pressure Equalization Valves
Background and Technical Context
Blast furnace pressure equalization valves operate under extremely harsh conditions, enduring high-temperature flue gas, abrasive dust particles, and cyclic thermal stress. The valve seats and sealing surfaces are particularly vulnerable to erosion and wear, leading to frequent shutdowns for repair. This literature review focuses on the application of weld overlay hardfacing technology to extend the service life of these critical components, drawing from practical engineering experience in heavy industry maintenance.
The core challenge lies in selecting appropriate overlay materials and process parameters that can withstand the combined effects of thermal cycling, mechanical abrasion, and chemical attack from sulfur compounds present in the flue gas. The study examines the transition from conventional repair methods to systematic hardfacing overlay strategies.
Overlay Material Selection and Process Parameters
The selection of hardfacing materials is guided by the specific wear mechanism present at the valve surface. For the pressure equalization valve, the dominant wear mode is abrasive erosion combined with moderate thermal fatigue. Based on this assessment, the following materials were evaluated:
| Material Grade | Hardness (HRC) | Key Alloying Elements | Application Suitability |
|---|---|---|---|
| Stellite 6 | 40-45 | Cr 21-25%, Co balance, Mo 7% | Excellent thermal stability, moderate abrasion resistance |
| Ni-Cr-Cu alloy | 45-50 | Ni 60%, Cr 25%, Cu 8% | Good for high-temperature oxidation resistance |
| Cr-C-Mo cast iron | 55-60 | Cr 28-32%, C 3-4%, Mo 1.5% | Superior abrasion resistance, limited thermal cycling |
| M30 (Ni-based) | 42-48 | Ni 65%, Cr 22%, Mo 5% | Balanced properties for hot-gas erosion |
The recommended overlay process is submerged arc welding (SAW) for bulk deposition followed by gas tungsten arc welding (GTAW) for the final surface layer. This multi-pass approach ensures adequate dilution control and achieves the target surface hardness.
Key process parameters for the SAW pass include:
- Current: 450-550 A (DCEN)
- Voltage: 28-32 V
- Travel speed: 200-250 mm/min
- Shielding flux: low-hydrogen type with 200°C preheating
The GTAW finishing pass uses:
- Current: 120-160 A
- Shielding gas: 100% Ar at 15-20 L/min
- Travel speed: 150-200 mm/min
- Wire feed: Stellite 6 ER319 equivalent
Defect Analysis and Quality Control
During field implementation, several typical defects were encountered and systematically analyzed using the PDCA methodology:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | Excessive dilution (>25%) | Add transition layer of 309L before hardfacing |
| Porosity in overlay | Moisture in flux or base metal contamination | Increase flux baking temperature to 300°C for 2h |
| Excessive dilution | Travel speed too slow, heat input too high | Increase travel speed by 20-30%, reduce voltage |
| Spatter-induced inclusions | Poor shielding gas coverage | Optimize torch angle to 15-20° from vertical |
Non-destructive testing protocol follows NB/T 47013 with the following requirements:
- Magnetic particle testing (MT): 100% coverage of overlay surface and 5 mm heat-affected zone
- Ultrasonic testing (UT): 100% of overlay thickness to detect subsurface cracks
- Hardness verification: minimum 5 points per weld seam, average value must meet specification
Engineering Practice and Performance Results
In a pilot application at a 2500 m³ blast furnace, the overlay-reinforced pressure equalization valve demonstrated a service life improvement from 6 months to over 18 months, representing a threefold increase. The overlay thickness was maintained at 3-4 mm on the valve seat and 2-3 mm on the sealing surface.
The FMEA analysis revealed that the primary failure mode before overlay reinforcement was progressive material loss at the valve seat leading to gas leakage. After implementation, periodic inspections showed uniform wear patterns with no evidence of spalling or delamination. The overlay layer maintained its hardness of 42-48 HRC even after extended service, confirming the thermal stability of the selected Ni-based alloy.
A critical insight from this project is that preheating the valve body to 200°C before overlay welding significantly reduced residual stress in the heat-affected zone. Post-weld stress relief at 650°C for 2 hours further improved the fatigue performance of the base metal. The combination of thermal management and proper material selection proved essential for achieving the desired performance improvement.
This study reinforces the principle that successful hardfacing applications require not only appropriate material selection but also meticulous process control, comprehensive quality assurance, and systematic post-weld treatment. The engineering value of this approach is particularly evident in high-value components where unplanned downtime carries significant economic penalties.