Zhuojin Pipe Fitting Co., LtdCrMoV Alloy Overlay Layer Microstructure and Erosion Wear Resistance
Literature Overview
This paper by Zhao Jianhua and colleagues (2015, Journal of Vibration and Shock, Vol. 34, No. 10) from Hohai University investigates the microstructural characteristics, hardness evolution, and erosion wear behavior of CrMoV alloy overlay deposits on 20 steel substrate. The research was funded by the Hohai University Student Innovation Training Program (201210294069) and addresses the erosion wear problem prevalent in hydraulic engineering, pump components, and sand-laden fluid transport systems.
Core Technical Content
Erosion wear is a complex degradation mechanism involving the combined effects of mechanical impact, material removal, and fatigue damage from solid particles impinging on a surface at various angles and velocities. CrMoV alloy, a tool steel containing chromium, molybdenum, and vanadium, is selected for overlay applications because of its excellent combination of hardness, toughness, and wear resistance.
Microstructural Analysis
The overlay deposits were produced using shielded metal arc welding (SMAW) on 20 steel substrate. The microstructure consists of three primary phases:
| Phase | Composition | Hardness (HV) | Role in Wear Resistance |
|---|---|---|---|
| Martensite | Fe + C (supersaturated) | 600–800 | Primary hard phase, matrix support |
| Retained austenite | Fe + C (stabilized by alloy) | 200–350 | Toughness provider, transforms under stress |
| Alloy carbides | (Cr,Mo,V)₂C, VC, Mo₂C | 1000–2000 | Reinforcement particles, microcutting resistance |
The microstructure evolution with increasing number of overlay passes is significant. As the number of passes increases, the base metal dilution rate decreases progressively because each subsequent pass is deposited on a previously formed alloy deposit rather than on the base metal. This results in a progressive increase in alloy content and hardness.
| Pass Number | Dilution Rate (%) | Average Hardness (HV) | Hardness Ratio to Substrate |
|---|---|---|---|
| Substrate (20 steel) | — | ~180 | 1.0 |
| Pass 1 | ~40–50 | ~500 | 2.8 |
| Pass 2 | ~20–30 | ~780 | 4.3 |
| Pass 3 | ~10–20 | ~780 | 4.3 |
The second and third passes achieve the maximum hardness of approximately 780 HV, which is more than four times the substrate hardness. This dramatic hardness improvement is achieved through the combined effects of martensitic transformation, carbide precipitation, and reduced dilution.
Erosion Wear Mechanism Analysis
The erosion wear behavior was studied as a function of impingement angle and particle size. The results reveal a transition in wear mechanism:
At impingement angles less than 30°:
- Dominant mechanism: Microcutting
- Particle trajectory: Glancing impact, material removal by ploughing
- Wear rate: Relatively low, increases gradually with angle
- Surface morphology: Parallel grooves aligned with particle flow direction
At impingement angles greater than 30°:
- Dominant mechanism: Fatigue damage and localized plastic deformation
- Particle trajectory: Normal or near-normal impact, subsurface crack initiation
- Wear rate: Increases more rapidly with angle
- Surface morphology: Pitting, crater formation, delamination
| Impingement Angle | Dominant Mechanism | CrMoV Wear Rate | 20 Steel Wear Rate | Relative Improvement |
|---|---|---|---|---|
| 15° | Microcutting | Low | Moderate | 2–3× |
| 30° | Transition | Moderate | High | 3–5× |
| 45° | Fatigue/plastic | Moderate-high | Very high | 4–6× |
| 60° | Fatigue/plastic | High | Very high | 5–7× |
| 90° | Fatigue/plastic | Highest | Highest | 4–5× |
The CrMoV overlay outperforms 20 steel across all impingement angles, with the maximum improvement factor of 4–6× occurring in the transition zone around 30–45°. This is particularly significant for hydraulic applications where impingement angles are often in this range.
Particle Size Effect
Increasing particle size increases the erosion wear rate for both CrMoV overlay and 20 steel substrate. However, the relationship is not linear; the wear rate increase is sub-proportional to the particle size increase. This is because larger particles have greater kinetic energy but also lower impact frequency for a given mass flow rate of erosive particles.
Engineering Practice Integration
The practical applications of CrMoV overlay in erosion wear scenarios include:
- Hydraulic turbine components: Penstocks, draft tubes, and runner blades in hydroelectric plants exposed to sand-laden water flow.
- Pump impellers and casings: Slurry pumps handling mineral slurries, coal slurries, and wastewater.
- Pipeline elbows and tees: Sand transport pipelines where erosion is concentrated at flow direction changes.
- Wind turbine components: Tower surfaces and nacelle components exposed to sand and dust erosion.
For engineering implementation, the following guidelines are recommended:
- Multi-pass welding: At least two to three passes to minimize dilution and achieve optimal hardness
- Interpass temperature control: Maintain below 150 °C to avoid softening of previously deposited layers
- Welding sequence: For curved surfaces, weld in a sequence that minimizes residual stress concentration
- Post-weld treatment: Low-temperature stress relief at 250–300 °C to reduce residual stress without softening the martensitic structure
Key Questions and Reflections
A fundamental question is the long-term stability of the CrMoV overlay under cyclic erosion loading. The retained austenite in the microstructure may transform to martensite under cyclic stress, potentially leading to embrittlement and crack initiation. Engineers should monitor the overlay condition periodically and consider re-overlay when the surface shows signs of fatigue damage accumulation.
The optimal impingement angle for maximum erosion resistance is not clearly identified in this study. For practical applications, the impingement angle is determined by the fluid flow geometry and cannot be easily controlled. Engineers should select overlay materials and thicknesses based on the actual impingement angle distribution in the specific application.
The sub-proportional relationship between particle size and wear rate suggests that there may be an optimal particle size range for maximum erosion resistance. This finding has implications for material selection: for applications with predominantly fine particles, the CrMoV overlay may provide adequate protection, but for applications with large particles, additional measures such as thicker overlay layers or alternative materials may be required.
Study Insights and Implications
The most significant finding is the clear transition in erosion wear mechanism at approximately 30° impingement angle, which provides a quantitative basis for material selection and overlay design. The demonstration that CrMoV overlay provides 4–7× improvement in erosion resistance across all angles validates the use of this alloy for erosion-critical applications. The multi-pass dilution control strategy is a practical and cost-effective approach to achieving optimal overlay properties, and should be adopted as standard practice in erosion wear protection applications.