CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Characteristics of Wear-Resistant Overlay Layer and Its Relationship with Wear Resistance
Overview and Significance
This study systematically investigates the microstructural evolution of wear-resistant overlay layers produced by submerged arc welding (SAW) and gas metal arc welding (GMAW), and establishes a quantitative correlation between microstructural features and tribological performance. The research is significant because wear resistance is the primary performance criterion for overlay applications in mining, cement, and material handling industries, where components are subjected to severe abrasive and erosive conditions.
The investigation employed a comprehensive methodology including optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers hardness testing, and pin-on-disc wear testing under standardized conditions. Multiple overlay alloys were evaluated, including high-chromium cast iron types, cobalt-chromium-tungsten alloys, and martensitic stainless steel types.
Microstructural Characterization
The microstructure of the overlay layers was found to be governed by several key factors including cooling rate, alloy composition, and welding process parameters. The high-chromium cast iron overlay exhibited a ledeburitic microstructure consisting of a martensitic matrix with eutectic carbides distributed throughout. The carbides were identified as M7C3 type chromium carbides with a hardness of approximately 1800–2000 HV, providing the primary wear resistance contribution.
| Overlay Alloy Type | Matrix Structure | Carbide Type | Vickers Hardness (HV) | Wear Volume Loss (mm³/N·m) |
|---|---|---|---|---|
| High-Cr cast iron (28% Cr) | Martensite + retained austenite | M7C3 | 850–950 | 4.2×10⁻⁶ |
| Co-Cr-W (Stellite type) | Austenite + carbides | M6C | 400–450 | 2.8×10⁻⁶ |
| Martensitic SS (440C type) | Martensite | M23C6 | 600–680 | 6.5×10⁻⁶ |
| Ni-Cr-C alloy (Alloy 5) | Austenite + Ni3(Fe,Cr)C | Ni3(Fe,Cr)C | 550–620 | 3.1×10⁻⁶ |
The cooling rate, which varied from approximately 5°C/s at the top of the overlay to 50°C/s at the interface, produced a gradient microstructure. Near the surface, slower cooling promoted the formation of larger eutectic carbides and coarser martensite, while near the interface, rapid cooling produced finer microstructural features with higher hardness but reduced toughness.
Quantitative Correlation Between Microstructure and Wear Resistance
A key finding of the study is the establishment of a quantitative model relating microstructural parameters to wear resistance. The wear volume loss was found to be inversely proportional to the product of matrix hardness and carbide volume fraction, modified by a carbide size factor. Specifically:
- Carbide volume fraction: Increasing the carbide volume fraction from 15% to 35% reduced wear volume loss by approximately 60%, demonstrating the dominant role of hard second-phase particles in abrasive wear resistance.
- Carbide hardness: M7C3 carbides (1800 HV) provided superior wear resistance compared to M23C6 carbides (1500 HV) under the same testing conditions, confirming that carbide type and intrinsic hardness are critical factors.
- Matrix hardness: Higher matrix hardness contributed to improved resistance to micro-ploughing and micro-cutting mechanisms, particularly in the early stages of wear.
- Carbide size and distribution: Uniformly distributed fine carbides (1–3 μm) outperformed coarse carbides (5–10 μm) due to reduced stress concentration and more uniform load-bearing capacity.
Process Parameters and Microstructural Control
The study demonstrated that welding process parameters have a profound influence on the resulting microstructure and, consequently, on wear performance. The following parameter windows were identified as optimal for maximizing wear resistance:
| Parameter | Optimal Range | Effect on Microstructure |
|---|---|---|
| Current density (SAW) | 80–100 A/mm² | Controls dilution and cooling rate |
| Travel speed | 250–350 mm/min | Governs heat input and grain size |
| Preheat temperature | 100–200°C | Reduces thermal gradient and cracking |
| Electrode flux coverage | 8–12 mm | Controls cooling rate and alloy partitioning |
The dilution rate, typically 15–30% for single-pass SAW overlay, was found to be a critical parameter. Excessive dilution (>35%) introduced carbon and manganese from the base material into the overlay, altering the carbide chemistry and reducing the volume fraction of the desired wear-resistant phase. Conversely, insufficient dilution (<10%) resulted in poor metallurgical bonding and interface cracking.
Engineering Practice and Application Guidance
The findings of this study have direct implications for the selection and application of wear-resistant overlay alloys in industrial settings. For severe sliding abrasion conditions such as those encountered in cement mill liners and mining equipment, high-chromium cast iron overlays with 25–30% Cr content and controlled carbide morphology are recommended. For combined abrasive and corrosive environments, cobalt-chromium-tungsten alloys offer superior performance due to their combination of hardness, corrosion resistance, and thermal stability.
In practice, engineers should pay particular attention to the following:
- Pre-weld preparation: Base material surface preparation should ensure a minimum roughness of Ra 12.5 μm to promote mechanical interlocking and reduce dilution.
- Multi-pass strategy: For thick overlays (>3 mm), a multi-pass approach with controlled interpass temperatures (200–300°C) produces a more uniform microstructure and reduces residual stress.
- Post-weld heat treatment: Temper treatment at 500–600°C for 2–4 hours is recommended to relieve residual stresses and transform retained austenite to tempered martensite, improving dimensional stability and fatigue resistance.
- Quality verification: Post-overlay hardness mapping, microstructure verification via metallographic examination, and wear testing of coupon specimens are essential to confirm that the as-built overlay meets specification requirements.
Study Insights and Conclusions
This study provides valuable insights into the complex relationship between microstructure and wear resistance in overlay alloys. The quantitative model developed offers a predictive tool for alloy design and process optimization, enabling engineers to select appropriate overlay compositions and process parameters for specific service conditions. The key takeaway is that wear resistance is not a single-parameter property but a system-level characteristic governed by the synergistic interaction of matrix hardness, carbide characteristics, and microstructural uniformity. Future work should focus on extending these models to multi-body wear conditions and incorporating environmental factors such as temperature and corrosive media into the wear resistance prediction framework. The practical value of this research lies in its ability to bridge the gap between fundamental metallurgical understanding and industrial overlay welding practice, ultimately leading to improved component life and reduced maintenance costs.