CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum on Microstructure and Tribological Properties of Cr27 High-Chromium Iron Weld Overlay
Literature Overview
This research investigates the influence of molybdenum (Mo) addition on the microstructure, mechanical properties, and tribological behavior of Cr27 high-chromium cast iron weld overlay layers. High-chromium white cast irons (HCrWCI) are widely used for wear-resistant cladding due to their high volume fraction of hard carbides and excellent abrasion resistance. The Cr27 designation indicates approximately 27 wt% chromium content, which is sufficient to stabilize a high volume fraction of Cr₇C₃ carbides. The study systematically examines Mo additions at levels of 0, 2, 4, and 6 wt% and evaluates their effects through metallographic examination, X-ray diffraction (XRD), hardness testing, and pin-on-disk wear testing.
Microstructural Evolution with Mo Addition
Phase Composition and Carbide Morphology
The as-cast microstructure of the Cr27 base alloy consists of a dendritic matrix of martensite with primary Cr₇C₃ carbides at the dendrite boundaries. With increasing Mo content, the following microstructural changes were observed:
| Mo Content (wt%) | Primary Carbide Type | Carbide Morphology | Matrix Structure | Hardness (HV0.3) |
|---|---|---|---|---|
| 0 | Cr₇C₃ | Coarse, irregular, 50–120 μm | Martensite + retained austenite | 850 |
| 2 | Cr₇C₃ + Mo₂C | Slightly refined, 40–100 μm | Martensite + retained austenite | 920 |
| 4 | Cr₇C₃ + Mo₂C + M₆C | Refined, 30–80 μm | Martensite + retained austenite | 980 |
| 6 | Cr₇C₃ + Mo₂C + M₆C | Further refined, 25–70 μm | Martensite + retained austenite | 1020 |
Mo Effects on Matrix Properties
Molybdenum plays multiple roles in the matrix phase of the Cr27 overlay:
- Solid solution strengthening: Mo atoms dissolved in the ferrite/austenite matrix create lattice distortion, increasing dislocation motion resistance. The strengthening contribution increases linearly with Mo content, contributing approximately 15–20 HV per wt% Mo.
- Retained austenite stabilization: Mo is an austenite-stabilizing element that increases the amount of retained austenite in the as-cast microstructure. At 4 wt% Mo, the retained austenite fraction increases from approximately 8% to 15%, which can be beneficial for impact resistance but may reduce hardness if excessive.
- Carbide modification: Mo preferentially partitions to carbides, forming Mo₂C and modifying the Cr₇C₃ to a (Cr,Mo)₇C₃ solid solution. The incorporation of Mo into the Cr₇C₃ lattice increases its hardness from approximately 1800 HV to 2000+ HV.
- Grain refinement: Mo additions refine the dendrite cell size from approximately 80 μm (0 wt% Mo) to 50 μm (6 wt% Mo), improving both strength and toughness per the Hall-Petch relationship.
Tribological Performance
Pin-on-Disk Wear Testing Results
Wear testing was conducted using a pin-on-disk tribometer with a WC-Co (80WC-10Co-10Cr) counterface under dry sliding conditions at 294 N normal load and 0.5 m/s sliding speed.
| Mo Content (wt%) | Wear Volume Loss (mm³) | Specific Wear Rate (×10⁻⁶ mm³/N·m) | Wear Mechanism |
|---|---|---|---|
| 0 | 12.5 | 85.2 | Abrasive + adhesive |
| 2 | 9.8 | 66.5 | Abrasive |
| 4 | 7.2 | 48.7 | Abrasive |
| 6 | 6.1 | 41.3 | Abrasive (slight oxidation) |
The wear resistance improved by approximately 52% with the addition of 6 wt% Mo compared to the base Cr27 alloy. The primary wear mechanism transitioned from combined abrasive-adhesive wear to predominantly abrasive wear as Mo content increased, indicating that the harder and more refined carbide structure effectively resisted material removal by abrasive particles.
Friction Coefficient
The coefficient of friction decreased from 0.62 (0 wt% Mo) to 0.48 (4 wt% Mo) and then slightly increased to 0.51 (6 wt% Mo). The minimum friction coefficient at 4 wt% Mo corresponds to the optimal balance between carbide hardness and matrix toughness, where the retained austenite fraction provides sufficient ductility to prevent carbide fracture while the hard carbides resist wear.
Practical Implications and Study Insights
The research demonstrates that Mo addition is an effective strategy for enhancing the wear resistance of Cr27 high-chromium iron overlays, with an optimal addition level of 4 wt% providing the best balance of hardness, toughness, and wear resistance. However, several practical considerations must be addressed when implementing Mo-containing overlays in production:
- Cost: Molybdenum is a relatively expensive alloying element, and adding 4–6 wt% Mo increases the overlay wire cost by 30–50%. The economic benefit must be justified by the extended service life and reduced maintenance frequency.
- Cracking susceptibility: Higher Mo content increases the hardenability of the matrix, potentially increasing the risk of cold cracking in thick overlay sections. Preheating to 200–250 °C and controlled cooling rates are recommended for Mo ≥ 4 wt% overlays.
- Dilution effects: During multi-pass overlay welding, dilution from the base metal reduces the effective Mo content in the final overlay. Compensation by increasing the Mo content in the consumable by 1–2 wt% is advisable.
- Standards compliance: The modified Cr27-Mo composition may not conform to standard consumable specifications (e.g., AWS A5.15), requiring custom qualification under NB/T 47014 or ASME IX.
The study provides a clear framework for Mo optimization in Cr27 overlays: for applications requiring maximum abrasion resistance with moderate impact loading, 4 wt% Mo is recommended; for applications with severe impact or where toughness is critical, 2 wt% Mo with austempering heat treatment offers a more balanced solution.