CLADDING TECHNOLOGY SHANXI CO., LTDResearch on the Relationship Between Wear Resistance and Hardness of Overlay Metals
Literature Overview
This foundational study, published in 1991 by researchers from Tsinghua University, investigates the quantitative relationship between the wear resistance and hardness of various overlay (cladding) metals. The work was published in the journal "Welding" and represents an early systematic examination of one of the most fundamental questions in hardfacing technology: does higher hardness always translate to better wear resistance? The authors conducted extensive wear testing on multiple overlay materials and analyzed the microstructural features that govern tribological performance.
Core Technical Findings
The central finding of this research is that the relationship between hardness and wear resistance is not linear or universally proportional. Instead, the wear resistance of overlay metals is governed by a combination of factors including hardness, microstructure morphology, carbide type and distribution, matrix composition, and the specific wear mechanism involved.
| Overlay Material Type | Typical Hardness (HRC) | Wear Mechanism Dominance | Relative Wear Resistance |
|---|---|---|---|
| Martensitic (Cr-Mo) | 45–55 | Abrasion (sliding) | Moderate to High |
| High-Cr martensitic (28% Cr) | 55–60 | Abrasion + Impact | High |
| Austenitic (Ni-Cr) | 35–45 | Adhesive + Abrasive | Moderate |
| Cobalt-based (Stellite) | 40–50 | Abrasion + Thermal | High (elevated temp) |
| Carbide composite (Cr7C3) | 60–70 | Abrasion (severe) | Very High |
| High-vanadium martensitic | 55–62 | Abrasion + Impact | Very High |
The study identified several key relationships:
- For sliding abrasion against hard particles: Hardness is the dominant factor, and wear resistance increases approximately proportional to hardness raised to the 0.7–1.0 power, consistent with the Archard wear equation.
- For abrasive wear with impact loading: Toughness becomes equally important. Materials with very high hardness but low fracture toughness (such as some high-carbon martensitic alloys) may exhibit catastrophic spalling failure despite excellent static hardness values.
- For adhesive wear: The chemical affinity between the overlay material and the counterface plays a more significant role than hardness alone. Austenitic overlays with low chemical affinity to steel counterfaces can exhibit excellent wear resistance despite moderate hardness values.
- For erosive wear: The relationship between hardness and wear resistance is more complex, with optimum hardness values depending on the particle velocity, impact angle, and particle material.
Microstructural Analysis and Mechanisms
The authors conducted detailed metallographic analysis of the worn surfaces of various overlay materials. Key observations included:
- Martensitic overlays: Wear occurs primarily through micro-plastic deformation and micro-cracking of the martensitic matrix. The carbide particles (typically M7C3 or M23C6 type) act as wear-resistant inclusions, but if they are too large or too sparsely distributed, they can become crack initiation sites.
- Austenitic overlays: The face-centered cubic (FCC) austenite matrix provides excellent work-hardening capacity. Under sliding contact, the austenite undergoes strain-induced martensitic transformation (γ→α'), which increases local hardness and improves wear resistance progressively during service.
- Carbide composite overlays: The wear resistance is primarily governed by the volume fraction, size, shape, and bonding strength of the hard carbide particles (WC, Cr7C3, TiC) within the binder matrix. Poor bonding between carbides and the matrix leads to pull-out failure, which dramatically reduces effective wear resistance.
- Cobalt-based overlays: The combination of Co solid solution strengthening and M6C/M23C6 carbide precipitation provides excellent wear resistance, particularly at elevated temperatures where most other overlay materials soften significantly.
Engineering Implications and Practical Guidelines
The findings of this research have direct implications for overlay material selection in engineering practice:
- Do not select overlay materials based solely on hardness values. A material with 60 HRC hardness may perform worse in practice than a material with 50 HRC if the microstructure is inappropriate for the specific wear mechanism.
- Match the overlay material to the dominant wear mechanism. Sliding abrasion favors high-hardness martensitic or carbide composite overlays; erosive wear favors cobalt-based or high-toughness austenitic overlays; adhesive wear favors materials with low chemical affinity to the counterface.
- Consider the service temperature. Most martensitic overlays lose significant hardness above 400 °C due to tempering. Cobalt-based and some austenitic overlays maintain their properties at higher temperatures.
- Evaluate toughness requirements. In applications involving impact loading or cyclic stress, overlay materials with adequate fracture toughness must be selected, even if this means accepting somewhat lower hardness values.
Study Insights
This 1991 study from Tsinghua University represents an important contribution to the fundamental understanding of overlay material performance. Despite being published over three decades ago, its core findings remain highly relevant to modern engineering practice. The systematic approach of correlating microstructure with wear mechanism and performance provides a framework that can be applied to the selection and evaluation of any overlay material system. Modern engineers should build upon this foundational work by incorporating additional considerations such as thermal cycling resistance, corrosion-wear synergy, and the effects of welding process parameters on the final microstructure of the overlay deposit. The study reinforces the principle that overlay material selection is a multi-variable optimization problem, not a simple hardness-maximization exercise.