CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Tribological Properties of Weld Overlay Repair Layer on 45CrNiMoVA Steel
Literature Overview
45CrNiMoVA is a high-strength low-alloy steel widely used in demanding applications such as automotive crankshafts, aircraft landing gear components, and heavy machinery shafts. When such components suffer surface damage from wear, fatigue, or corrosion, weld overlay repair is a viable alternative to complete component replacement. This literature investigates the microstructure and tribological performance of weld overlay repair layers deposited on 45CrNiMoVA steel, addressing the critical challenge of matching overlay properties to the base material while achieving superior surface performance.
Core Technical Content
Base Material Characteristics
45CrNiMoVA steel has the following typical composition and properties:
| Parameter | Value |
|---|---|
| C | 0.43-0.48% |
| Cr | 0.80-1.10% |
| Ni | 1.40-1.70% |
| Mo | 0.15-0.25% |
| V | 0.04-0.10% |
| Hardness (quenched and tempered) | 35-42 HRC |
| Yield strength | ≥ 980 MPa |
| Tensile strength | ≥ 1080 MPa |
| Impact energy (20 °C) | ≥ 47 J |
The high strength and toughness of 45CrNiMoVA make it suitable for critical structural applications but also create challenges for weld overlay repair, as the base material is susceptible to cold cracking during welding due to its high carbon equivalent (CE = 0.55-0.62).
Overlay Material Selection and Microstructure
The overlay material selected for 45CrNiMoVA repair is a Ni-Cr alloy system (similar to Stellite 6 or Inconel 625 family), chosen for its excellent corrosion resistance, wear resistance, and compatibility with the high-strength base material.
The microstructure of the overlay layer exhibits:
- Columnar dendrites growing from the fusion boundary, with dendrite arm spacing of 20-40 μm
- γ-Ni solid solution matrix with dissolved Cr, Mo, and Co
- MC-type carbides (Cr₂₃C₆, Mo₂C) precipitated at dendrite boundaries
- ε-Mo₃C carbides dispersed within the matrix
- Retained austenite of 5-15% in some regions
The dilution from the base material introduces additional alloying elements into the overlay, modifying the microstructure. The carbon from the base material (0.45%) combines with Cr and Mo to form additional carbides, which can increase hardness but may also reduce toughness.
Tribological Performance
The tribological testing was conducted using a pin-on-disc apparatus with the overlay as the pin and GCr15 bearing steel as the disc. Testing parameters included normal load of 5-20 N, sliding speed of 0.5-2.0 m/s, and test duration of 30-60 minutes.
| Parameter | Overlay Layer | Base Material |
|---|---|---|
| Hardness | 380-420 HV | 350-380 HV |
| Friction coefficient (mean) | 0.45-0.55 | 0.60-0.72 |
| Wear rate (10⁻⁶ mm³/N·m) | 8-15 | 25-40 |
| Wear mechanism | Adhesive + abrasive | Severe abrasive |
| Surface roughness (post-wear) | Ra 0.8-1.2 μm | Ra 2.5-4.0 μm |
The overlay layer demonstrates approximately 3-fold improvement in wear resistance compared to the base material, with a significant reduction in friction coefficient. The wear mechanism transitions from severe abrasive wear on the base material to a combination of mild adhesive and abrasive wear on the overlay, indicating that the overlay surface forms a protective tribofilm during sliding.
Process Analysis and Defect Control
Welding Process Parameters
The overlay was deposited using gas tungsten arc welding (GTAW/TIG) with a filler wire matching the Ni-Cr alloy composition. The process parameters were carefully controlled to minimize dilution and ensure sound metallurgical bonding.
| Parameter | Value |
|---|---|
| Process | GTAW (TIG) |
| Filler wire | Ni-Cr-Mo alloy, 1.6 mm diameter |
| Current | 120-160 A |
| Voltage | 12-15 V |
| Travel speed | 40-60 mm/min |
| Shielding gas | Ar (99.99%) |
| Gas flow rate | 15-20 L/min |
| Preheat | 250-300 °C |
| Interpass temperature | ≤ 200 °C |
| Dilution rate | 15-25% |
Defect Analysis and Prevention
The primary defects encountered during overlay repair on 45CrNiMoVA include:
| Defect | Root Cause | Prevention |
|---|---|---|
| Cold cracking in HAZ | High CE of base material, hydrogen embrittlement | Preheat 250-300 °C, low-hydrogen consumables, post-weld bake |
| Overlay cracking | Thermal stresses, high carbon from dilution | Control interpass temp, reduce carbon dilution |
| Poor fusion | Contamination, insufficient heat input | Thorough cleaning, increase current |
| Porosity | Gas entrapment, insufficient shielding | Ensure gas coverage, clean surfaces |
| Excessive dilution | High heat input, single large pass | Use multiple small passes, reduce heat input |
Post-Weld Treatment
Post-weld heat treatment is critical for the overlay repair on 45CrNiMoVA. A tempering treatment at 580-620 °C for 2 hours reduces residual stresses in both the overlay and the HAZ while maintaining the hardness of the overlay layer. The treatment also stabilizes the microstructure of the base material HAZ, which may have undergone partial martensitic transformation during welding.
Engineering Practice Integration
The weld overlay repair of 45CrNiMoVA components has been applied to:
- Crankshaft repair: Overlay repair of worn journal surfaces, extending crankshaft life by 2-3 replacement cycles.
- Landing gear shaft repair: Overlay of worn bearing surfaces, meeting airworthiness requirements.
- Gear shaft repair: Overlay of worn tooth surfaces, restoring dimensional accuracy and surface hardness.
FMEA Application
A Failure Mode and Effects Analysis (FMEA) was conducted for the overlay repair process:
| Failure Mode | Effect | Severity | Occurrence | Detection | RPN | Action |
|---|---|---|---|---|---|---|
| Cold cracking | Component failure | 10 | 4 | 5 | 200 | Increase preheat, use low-H consumables |
| Excessive dilution | Reduced overlay hardness | 7 | 6 | 4 | 168 | Multi-pass strategy, reduce heat input |
| Poor fusion | Overlay delamination | 9 | 3 | 6 | 162 | Improve surface prep, increase penetration |
| Hardness variation | Uneven wear | 5 | 7 | 3 | 105 | Uniform parameter control, hardness survey |
Key Reflections and Technical Insights
The most important insight from this study is the critical role of dilution control in overlay repair of high-strength steels. The 45CrNiMoVA base material has a high carbon equivalent, and excessive dilution into the overlay layer introduces carbon that forms brittle carbides and increases the cracking susceptibility of the overlay. The optimal dilution rate of 15-25% represents a balance between achieving sufficient bonding strength and maintaining the desired overlay microstructure and properties.
Another significant finding is the tribological improvement achieved by the overlay layer. The reduction in friction coefficient from 0.65 to 0.50 represents not only improved wear resistance but also reduced energy consumption in sliding applications. This has implications for the overall efficiency of mechanical systems where friction losses are significant.
The microstructural evolution during post-weld tempering is also instructive. The tempering treatment dissolves some of the fine carbides formed during welding, redistributing them as coarser particles that provide more stable hardening. This is analogous to the tempering of quenched steels, and the analogy helps engineers understand the process without requiring advanced metallurgical knowledge.
A practical consideration not fully addressed in the literature is the dimensional accuracy of the overlay repair. For precision components such as crankshafts and landing gear shafts, the overlay must be machined to final dimensions after deposition. The machinability of the Ni-Cr overlay material is significantly different from the base steel, requiring different cutting parameters and tool materials.
Summary
The weld overlay repair of 45CrNiMoVA steel components demonstrates that Ni-Cr alloy overlay materials can effectively restore and enhance the surface performance of high-strength low-alloy steel components. The overlay layer provides 3-fold improvement in wear resistance with reduced friction, while maintaining adequate bonding strength and toughness. The key success factors are careful control of dilution rate, appropriate preheat and post-weld treatment, and thorough quality verification. Engineers working on repair of critical high-strength steel components should consider weld overlay as a viable and cost-effective alternative to component replacement, provided that rigorous process qualification and quality control are maintained throughout the repair procedure.