CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Wear-Resistant Layer Hardfacing Test and Technology Application for Coal Chemical Equipment
Literature Overview
The study by Chen Sunyi (2015) from Maoming Gravity Petrochemical Machinery Manufacturing addresses the hardfacing of wear-resistant layers on coal chemical equipment. Coal chemical plants process coal-derived products such as coal gas, coal tar, methanol, and synthetic fuels, and the equipment used in these processes is subjected to severe wear, corrosion, and erosion. The literature describes the development and application of hardfacing technology to extend the service life of critical equipment components in coal chemical plants.
Technical Background
Coal chemical equipment operates under extremely harsh conditions:
- Abrasive wear: Coal particles and ash entrained in the process streams cause severe abrasive wear on equipment surfaces.
- Corrosive environments: The process streams contain various corrosive agents, including H2S, CO2, organic acids, and chloride ions.
- Thermal cycling: The equipment is subjected to repeated heating and cooling cycles, causing thermal fatigue.
- Impact loading: The equipment may be subjected to impact loading from falling coal particles or slurry.
The hardfacing of wear-resistant layers on coal chemical equipment is a cost-effective approach to extend the service life of critical components. The hardfacing layer must provide adequate wear resistance while maintaining compatibility with the base material and resisting the corrosive environment.
Common Hardfacing Alloys for Coal Chemical Equipment
| Alloy Type | Composition | Hardness | Wear Resistance | Corrosion Resistance | Application |
|---|---|---|---|---|---|
| High-carbon chrome alloy | C 2.5–3.5%, Cr 20–25% | 55–60 HRC | Excellent | Moderate | Abrasive wear surfaces |
| Nickel-based alloy | Ni 60%, Cr 15%, Mo 5% | 40–45 HRC | Good | Excellent | Corrosive-abrasive environments |
| Tungsten carbide composite | WC 50–60%, Fe/Ni binder | 70–80 HRA | Outstanding | Low | Severe abrasive wear |
| Stellite alloy (Co-Cr-W) | Co 60%, Cr 25%, W 7% | 45–50 HRC | Excellent | Excellent | High-temperature abrasive-corrosive |
Hardfacing Process Selection and Parameters
The selection of the hardfacing process depends on the geometry of the component, the required overlay thickness, and the production volume. The following processes are commonly used for coal chemical equipment:
Submerged Arc Welding (SAW) Overlay
SAW is the most commonly used process for hardfacing large, flat or cylindrical surfaces of coal chemical equipment. The process offers high deposition rates, low spatter, and good control over the microstructure.
| Parameter | Typical Value |
|---|---|
| Current | 500–700 A |
| Voltage | 30–35 V |
| Travel speed | 250–350 mm/min |
| Wire feed speed | 7.0–9.0 m/min |
| Flux | Rutile-basic composite flux |
| Preheat | 200–300 °C |
Gas Metal Arc Welding (GMAW) Overlay
GMAW is used for hardfacing smaller components or areas that are difficult to access with SAW. The process offers good versatility and can be performed in various positions.
| Parameter | Typical Value |
|---|---|
| Current | 150–250 A |
| Voltage | 22–28 V |
| Travel speed | 150–250 mm/min |
| Wire feed speed | 5.0–7.0 m/min |
| Shielding gas | CO2 or Ar/CO2 mixture |
| Preheat | 150–250 °C |
Hardfacing Layer Microstructure and Wear Mechanisms
The microstructure of the hardfacing layer depends on the alloy composition and the welding process. For high-carbon chrome alloys, the microstructure consists of martensite with dispersed carbides (Cr7C3, Fe3C). The carbides provide the primary wear resistance, while the martensitic matrix provides the necessary toughness.
The wear mechanisms in coal chemical equipment include:
- Abrasive wear: The primary wear mechanism, caused by hard particles in the process stream. The hardfacing layer resists this wear through the presence of hard carbides.
- Adhesive wear: Occurs when the hardfacing surface is in contact with another surface under load. The hardfacing layer resists adhesive wear through its high hardness and low adhesion to the counterface.
- Erosive wear: Caused by the impact of particles or liquid droplets on the hardfacing surface. The hardfacing layer resists erosive wear through its high hardness and toughness.
- Corrosive wear: Occurs when the hardfacing surface is exposed to a corrosive environment. The hardfacing layer must have adequate corrosion resistance to prevent accelerated wear.
Quality Control and Inspection
The quality of the hardfacing layer is critical for the long-term reliability of coal chemical equipment. The following quality control measures are essential:
Non-Destructive Testing
| Test Method | Purpose | Acceptance Criteria |
|---|---|---|
| Magnetic Particle Testing (MT) | Detect surface and near-surface cracks | No cracks > 1 mm |
| Ultrasonic Testing (UT) | Detect internal defects (porosity, lack of fusion) | No defects > 3 mm |
| Dye Penetrant Testing (PT) | Detect surface cracks | No linear indications |
Mechanical Property Testing
| Test | Requirement |
|---|---|
| Hardness | As specified by the alloy type (e.g., 55–60 HRC for high-carbon chrome) |
| Bond strength | Minimum 300 MPa (tensile test on coupon) |
| Wear resistance | Minimum 2x that of the base material (pin-on-disk test) |
Engineering Application Cases
The hardfacing technology has been successfully applied to the following coal chemical equipment components:
- Coal slurry pump impellers: Hardfaced with high-carbon chrome alloy, the impellers show a 3–4x extension in service life compared to unhardfaced impellers.
- Coal feeder chutes: Hardfaced with tungsten carbide composite, the chutes resist the severe abrasive wear from falling coal particles.
- Gasifier internals: Hardfaced with Stellite alloy, the internals resist the combined effects of high-temperature abrasive and corrosive wear.
- Heat exchanger tubes: Hardfaced with nickel-based alloy, the tubes resist the corrosive-abrasive wear in the coal gasification process.
Service Life Improvement
| Component | Original Service Life | Hardfaced Service Life | Improvement Factor |
|---|---|---|---|
| Coal slurry pump impeller | 6 months | 24 months | 4x |
| Coal feeder chute | 3 months | 12 months | 4x |
| Gasifier internals | 12 months | 36 months | 3x |
| Heat exchanger tubes | 18 months | 36 months | 2x |
Study Reflections
This study provides a comprehensive overview of the hardfacing technology for coal chemical equipment, covering the selection of hardfacing alloys, the welding processes, the quality control measures, and the engineering applications. The key insight is that the hardfacing layer must be designed to address the specific wear and corrosion mechanisms present in the coal chemical environment.
The study also highlights the importance of quality control in hardfacing operations. The hardfacing layer must be free of defects (cracks, porosity, lack of fusion) to ensure long-term reliability. The use of non-destructive testing and mechanical property testing is essential to verify the quality of the hardfacing layer.
The economic evaluation of hardfacing must consider not only the upfront cost of the hardfacing consumables and labor but also the extended service life and the reduced downtime costs. In many coal chemical applications, the hardfacing investment is recovered within the first replacement cycle, making it a highly cost-effective approach to equipment maintenance.
The findings of this study are directly applicable to engineers working in the coal chemical industry, providing practical guidelines for the selection of hardfacing alloys, the optimization of welding parameters, and the implementation of quality control measures. The technology has significant potential for further development, particularly in the areas of advanced hardfacing alloys (e.g., high-entropy alloys, nanocomposite alloys) and advanced welding processes (e.g., laser cladding, cold spray cladding).