CLADDING TECHNOLOGY SHANXI CO., LTDWear Characteristics of Weld Overlay Coatings for Internal Mixer Rotors
Literature Overview
Published in Materials Protection in 2004 by Zhang Huichen, Gao Yuzhou, Xu Xiaolei, and Liu Shiying from the Institute of Materials and Process Engineering at Dalian Maritime University, this study investigates the tribological performance of several weld overlay coatings designed specifically for internal mixer rotors in rubber and polymer processing industries. Internal mixer rotors operate under extreme conditions: high temperature (150-250°C), high pressure, and severe adhesive-abrasive wear from rubber compounds, fillers (carbon black, silica), and reinforcing agents.
Core Technical Content
The study evaluates multiple overlay coating systems including high-carbon martensitic steel (Cr5Mo1V type), high-silicon austenitic (Cr12SiMn type), and high-manganese austenitic (Mn13Cr2 type) coatings deposited on 40Cr steel rotor shafts.
Comparative Wear Performance
| Coating System | Composition (wt%) | Hardness (HV) | Wear Rate (mm³/N·m) | Service Life Improvement |
|---|---|---|---|---|
| Cr5Mo1V (Martensitic) | C 0.9, Cr 5.0, Mo 1.0, V 0.8 | 580-620 | 1.2×10⁻⁶ | 3.5× |
| Cr12SiMn (Austenitic) | C 1.5, Cr 12.0, Si 1.5, Mn 1.5 | 320-360 | 0.9×10⁻⁶ | 4.2× |
| Mn13Cr2 (Austenitic) | C 1.3, Mn 13.0, Cr 2.0 | 200-220 | 2.1×10⁻⁻⁶ | 1.8× |
| Base 40Cr (quenched) | C 0.4, Cr 0.8 | 280-320 | 4.2×10⁻⁻⁶ | 1.0× (reference) |
Microstructural and Wear Mechanism Analysis
The Cr12SiMn high-silicon austenitic coating demonstrates superior wear resistance due to the formation of a self-healing SiO2 protective oxide layer during sliding contact at elevated temperatures. The mechanism involves:
- Initial adhesive contact between rotor surface and rubber compound
- Oxidation of Si at contact asperities forming SiO2 (amorphous, glassy)
- SiO2 layer acts as a hard, lubricious, and sacrificial protective film
- Continuous replenishment of SiO2 as the layer is worn away
The Cr5Mo1V martensitic coating relies on high hardness and fine carbide dispersion to resist abrasive wear from carbon black and silica fillers, but shows higher adhesive wear tendency at temperatures above 200°C due to matrix softening.
Engineering Practice Integration
Internal mixer rotors in rubber compounding lines typically operate at 180-220°C with rubber compounds containing 50-70% filler loading. The rotor surface experiences:
- Continuous sliding contact with viscous rubber mass
- Abrasive action from rigid filler particles (silica: Mohs 6-7, carbon black: Mohs 4-5)
- Thermal cycling during mixing cycles
- Chemical attack from vulcanization accelerators and antioxidants
The overlay layer thickness requirement is typically 2-3 mm, applied via submerged arc welding or multi-pass GMAW. Preheating to 200-250°C is mandatory for the Cr5Mo1V system to prevent cold cracking in the high-carbon martensitic deposit. The Cr12SiMn system requires lower preheat (100-150°C) due to its lower carbon equivalent and austenitic structure.
Process Parameters and Quality Control
| Process Parameter | Cr5Mo1V Overlay | Cr12SiMn Overlay |
|---|---|---|
| Preheat temperature | 200-250°C | 100-150°C |
| Inter-pass temperature | ≤250°C | ≤200°C |
| Heat input | 1.5-2.2 kJ/mm | 2.0-3.0 kJ/mm |
| Shielding gas | CO2 (SAW) / Ar+CO2 (GMAW) | Ar + 2% CO2 |
| Post-weld treatment | 550-580°C × 2h (tempering) | Solution treatment 1050°C + water quench |
| Acceptable porosity (per GB/T 19418) | ≤5% (area) | ≤5% (area) |
Key Technical Insights and Reflections
This study demonstrates that the selection of overlay coating for internal mixer rotors must consider the dominant wear mechanism rather than simply maximizing hardness. The high-silicon austenitic system, despite having lower hardness than martensitic alternatives, achieves superior overall wear resistance through its unique self-lubricating oxide film mechanism. This finding challenges the conventional engineering assumption that higher hardness always correlates with better wear performance in complex tribological environments.
From a manufacturing perspective, the Cr12SiMn system offers advantages in terms of lower residual stress, reduced cracking susceptibility, and simpler post-weld treatment requirements. However, the Cr5Mo1V system remains preferred when the operating environment involves significant metallic debris contamination or when the rotor operates at temperatures below 150°C where SiO2 film formation is less effective.
Study Implications for Engineering Practice
The research provides valuable guidance for engineers responsible for maintenance and improvement of rubber processing equipment. The optimal coating selection depends on the specific compound formulation: silica-reinforced compounds favor Cr12SiMn coatings, while carbon-black-reinforced compounds may benefit more from Cr5Mo1V martensitic coatings. Multi-zone coating strategies, applying different systems to different rotor regions based on local operating conditions, represent a promising approach for maximizing service life.