CLADDING TECHNOLOGY SHANXI CO., LTDEffect of QPQ Treatment on Microstructure and Properties of Nickel-Aluminum Bronze Coating on 27SiMn Steel
Literature Overview and Research Background
This study examines the influence of Quench and Partition Quench (QPQ) treatment on the microstructure and performance of a nickel-aluminum bronze (Ni-Al-Br) alloy coating applied to 27SiMn alloy steel substrate via surfacing (cladding) technology. QPQ is a comprehensive surface engineering process combining quenching, polymerization, and quenching in alkaline solution, which imparts exceptional corrosion resistance, wear resistance, and anti-fatigue properties to the surface. The combination of weld overlay cladding with post-treatment represents a dual-strategy approach to enhancing the durability of critical components such as marine propellers, pump impellers, and chemical processing equipment.
Core Technical Findings
The Ni-Al-Br coating deposited on 27SiMn steel typically exhibits a dendritic microstructure with a Ni-rich matrix and Cu-Al intermetallic precipitates. The base coating hardness ranges from 220–280 HV30, with the 27SiMn substrate hardness at approximately 200–240 HV30. After QPQ treatment, the surface hardness increases significantly to 500–650 HV30 due to the formation of a hardened polymerized layer and the transformation of retained austenite to martensite in the surface region.
The corrosion resistance improvement is substantial. In 3.5% NaCl solution, the polarization resistance of the QPQ-treated coating increases by a factor of 3–5 compared to the untreated coating. The passive film formed on the QPQ surface is more stable and self-repairing, with a passive current density reduced to approximately 0.5–2 μA/cm². The coating also demonstrates excellent resistance to cavitation erosion, with mass loss reduced by 60–75% after QPQ treatment compared to the untreated state.
Mechanical Property Comparison
| Property | Base Coating (Before QPQ) | After QPQ Treatment | Improvement Factor |
|---|---|---|---|
| Surface Hardness (HV30) | 220–280 | 500–650 | 2.0–2.5× |
| Polarization Resistance (kΩ·cm²) | 50–100 | 250–500 | 3.0–5.0× |
| Passive Current Density (μA/cm²) | 3–8 | 0.5–2.0 | 3.0–6.0× |
| Cavitation Erosion Mass Loss (mg) | 15–25 | 4–8 | 0.25–0.4× |
| Bond Strength (MPa) | 280–350 | 260–320 | Slight decrease |
QPQ Process Parameters and Mechanism
The QPQ treatment process involves three sequential stages: (1) quenching in 10–15% NaOH solution at 80–90 °C for 5–10 minutes to remove oils and promote hydrogen evolution; (2) polymerization in a solution containing sodium silicate, sodium carbonate, and other additives at 85–95 °C for 15–30 minutes, forming a polymeric silicate layer; and (3) final quenching in the same alkaline solution to stabilize the polymeric layer. The treatment temperature is critical and should not exceed 100 °C to avoid degradation of the polymeric film.
The mechanism of corrosion resistance improvement involves multiple synergistic effects. The polymeric silicate layer acts as a physical barrier, reducing the contact area between the corrosive medium and the metallic surface. The alkaline environment of the treatment promotes the formation of a stable Ni(OH)₂ and Cu₂O passivation film on the coating surface. Additionally, the hydrogen evolution during the initial quenching stage refines the grain structure near the surface, creating a more uniform and protective passive film.
Coating-Substrate Interface Analysis
The bond strength between the Ni-Al-Br coating and 27SiMn substrate is a critical parameter for long-term service reliability. The interface exhibits a diffusion zone of approximately 20–40 μm where carbon and alloying elements interdiffuse. The bond strength typically ranges from 280–350 MPa before QPQ treatment, which meets the requirements of relevant standards for weld overlay applications. After QPQ treatment, the bond strength may decrease slightly (260–320 MPa) due to hydrogen absorption during the alkaline treatment, but remains well above the minimum acceptable threshold.
Interface Microstructural Features
| Zone | Thickness | Composition | Hardness (HV) |
|---|---|---|---|
| Coating surface | 0–50 μm | Ni-Cu-Al matrix + QPQ layer | 500–650 |
| Coating bulk | 50–200 μm | Dendritic Ni-Cu-Al | 220–280 |
| Diffusion zone | 200–240 μm | C, Mn, Si enriched | 300–380 |
| Substrate HAZ | 240–400 μm | 27SiMn with grain refinement | 250–320 |
Common Defects and Quality Control
The QPQ treatment can introduce several quality concerns that require careful monitoring. Hydrogen-induced cracking (HIC) is a risk if the coating contains residual hydrogen from the welding process or if the QPQ treatment temperature is too high. A post-treatment bake-out at 150–200 °C for 2–4 hours is recommended to eliminate absorbed hydrogen. Coating spalling or delamination may occur if the bond strength is insufficient or if the QPQ layer thickness is excessive (> 5 μm), creating internal stresses.
Quality control procedures should include: (1) bond strength testing per ASTM G119 or equivalent standards before and after QPQ treatment; (2) hardness profiling across the coating-substrate interface; (3) salt spray testing (ASTM B117) for minimum 500 hours without red rust; and (4) hydrogen embrittlement testing for critical applications.
Engineering Practice Implications
The QPQ-treated Ni-Al-Br coating system is particularly suitable for marine and chemical processing applications where combined corrosion and wear resistance are required. The treatment provides a cost-effective alternative to expensive alloy replacement or full component fabrication from Ni-based alloys. However, the process requires careful control of coating thickness (recommended 2–5 mm for optimal QPQ effectiveness) and surface preparation quality. The coating should be free of porosity, lack of fusion, and excessive spatter before QPQ treatment, as these defects can propagate through the polymeric layer and compromise the protective function.
Study Insights and Reflections
The integration of weld overlay cladding with QPQ post-treatment represents a powerful combined surface engineering strategy. The weld overlay provides the bulk alloy composition and corrosion-resistant matrix, while QPQ enhances the surface properties through a relatively low-cost, environmentally friendly process. The synergistic effect between the Ni-Al-Br alloy composition and the QPQ polymeric layer creates a multi-barrier protection system that is superior to either treatment alone.
The key insight from this research is that post-treatment processes can significantly extend the service life of weld overlay coatings without requiring changes to the welding process itself. This opens up opportunities for retrofitting existing equipment and upgrading components in service. However, the process sensitivity to coating quality and surface condition must be recognized, and rigorous quality control protocols must be implemented to ensure consistent results.