Zhuojin Pipe Fitting Co., LtdMicrostructure and Property Analysis of Overlay Welded Wear-Resistant Alloy on Intensive Mixer Rotor Surface
Literature Overview
This paper, published in Rubber and Plastics Technology and Equipment (2022, Vol. 48, No. 6, pp. 61-64), presents a metallurgical and tribological investigation of overlay welding applications on intensive mixer (internal mixer) rotor surfaces. The author, from Dalian Rubber and Plastics Machinery Co., Ltd., applied open-arc overlay welding to deposit two types of high chromium cast iron filler wires onto a ZG35 steel base material. The study examines the effect of niobium (Nb) addition on the microstructure, hardness, and wear resistance of the overlay metal.
Technical Background
Intensive mixers are critical equipment in rubber and plastic processing industries, where rotors are subjected to severe abrasive and adhesive wear from high-friction mixing of polymer compounds filled with abrasives such as silica, carbon black, and mineral fillers. The rotor surface must withstand continuous high-temperature, high-shear contact with these materials, leading to rapid wear of unprotected steel surfaces. Overlay welding of wear-resistant alloys is a well-established practice for extending rotor service life.
Base Material and Filler Selection
| Component | Material | Key Properties |
|---|---|---|
| Base material | ZG35 (cast steel, 0.35% C) | Yield strength ~245 MPa; good toughness |
| Filler wire A | High Cr cast iron (no Nb) | ~25-30% Cr; high hardness |
| Filler wire B | High Cr cast iron with Nb addition | ~25-30% Cr + 0.3-0.5% Nb |
| Welding process | Open-arc overlay welding (SAW or FCAW) | High deposition rate; good penetration |
The ZG35 base material is a medium-carbon cast steel with adequate strength and toughness for rotor applications. Its relatively low carbon content (0.35%) provides good weldability with high chromium filler metals, though the carbon content difference between base and filler creates a significant dilution challenge.
Microstructural Analysis
The metallographic examination of the overlay metal reveals important microstructural differences between the two filler wire compositions:
Without Nb Addition
The original high chromium cast iron overlay exhibits:
- Primary M7C3 carbides with large size (5-15 μm) and relatively coarse distribution
- Ferritic matrix with some pearlite formation
- Grain size in the range of 100-200 μm
- Hardness: approximately 55-58 HRC
The M7C3 carbides (Cr7C3) are the primary wear-resistant phase in high chromium cast irons. Their size, distribution, and volume fraction directly influence the wear resistance of the overlay. Coarse carbides, while providing some abrasive resistance, can act as crack initiation sites under impact or cyclic loading conditions.
With Nb Addition
The Nb-modified overlay exhibits:
- NbC hard phases forming preferentially at grain boundaries and within the matrix
- Significantly refined primary M7C3 carbides (reduced in both number and size)
- Grain refinement effect: NbC particles impede grain boundary migration during solidification
- Hardness: approximately 60-63 HRC (approximately 5 HRC increase over unmodified overlay)
- Improved toughness due to finer carbide distribution
The refinement mechanism is well explained by classical nucleation and growth theory: NbC particles act as heterogeneous nucleation sites for M7C3 carbides and simultaneously impede grain boundary migration during solidification. This results in a finer, more uniform microstructure with improved mechanical properties.
Hardness and Wear Performance Comparison
| Property | Without Nb | With Nb | Improvement |
|---|---|---|---|
| Hardness (HRC) | ~55-58 | ~60-63 | ~5 HRC |
| Wear volume loss (relative) | 1.0 (baseline) | 0.6 | 40% reduction |
| Primary M7C3 size | 5-15 μm | 2-8 μm | ~50% reduction |
| Grain size | 100-200 μm | 50-120 μm | ~40% refinement |
| NbC phase presence | None | Present (fine, dispersed) | New hard phase |
The 40% reduction in wear volume loss is a substantial improvement, translating directly into extended rotor service life and reduced maintenance intervals in intensive mixing operations.
Engineering Practice and Application Considerations
The application of Nb-modified high chromium cast iron overlay on intensive mixer rotors presents several practical considerations:
- Dilution control: The dilution rate between ZG35 base and high Cr filler is significant. With open-arc welding processes, dilution rates of 20-40% are typical. This dilution reduces the effective chromium content in the weld metal, potentially affecting carbide formation and hardness. The Nb addition helps compensate by providing additional hard phases even at reduced chromium levels.
- Thermal cycling effects: Intensive mixer rotors experience cyclic thermal loading during operation, with surface temperatures reaching 200-350°C during mixing. The overlay microstructure must retain its properties under these thermal conditions. NbC is thermodynamically stable at these temperatures and maintains its hardening effect, unlike some other carbide phases that may coarsen or dissolve.
- Post-weld treatment: A stress relief treatment at 550-650°C for 2-4 hours is recommended to reduce residual stresses without significantly affecting the overlay hardness. Higher temperature treatments should be avoided as they may promote carbide coarsening and hardness degradation.
- Surface finish requirements: The overlay surface must be machined or ground to the required dimensional tolerance and surface roughness (typically Ra ≤ 6.3 μm for rotor surfaces). The machining process should be optimized to avoid work hardening or surface damage that could initiate cracking.
Study Insights and Implications
This study demonstrates the effectiveness of alloying modification (Nb addition) as a strategy for enhancing the wear resistance of hardfacing overlays. The mechanism—refinement of primary carbides and formation of additional NbC hard phases—is well-established in the metallurgy of wear-resistant alloys, but its application to intensive mixer rotor overlays provides valuable practical data. The 40% improvement in wear resistance is significant from an economic perspective, as it directly translates into extended equipment availability and reduced downtime for rotor replacement or repair.
The study also highlights an important principle in hardfacing alloy design: the relationship between microstructure refinement and wear resistance is not always straightforward. While finer carbides generally improve wear resistance through increased hardness and reduced crack initiation, the optimal carbide size and distribution depend on the specific wear mechanism (abrasive, adhesive, erosive, or impact). For intensive mixer rotors, where wear is predominantly abrasive and adhesive, the refined microstructure provided by Nb addition is particularly effective. Engineers selecting overlay alloys for similar applications should consider the specific wear mechanism and service conditions when evaluating the potential benefits of alloying modifications. The findings of this study provide a useful data point for alloy selection and process optimization in rubber and plastic processing equipment maintenance.