CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Two Medium-Chromium Cladding Alloys
Literature Overview
The 2019 study by Lei Sheng, Li Shuai, Ma Shihao, Sun Jun, and Zhu Jixiang from Anhui Jianzhu University's School of Mechanical and Electrical Engineering investigated the microstructure and properties of two medium-chromium cladding alloys. Medium-chromium cladding alloys, containing 8 to 12 percent chromium, occupy an important position in the cladding alloy classification system. They offer a balance between corrosion resistance and mechanical strength that is intermediate between low-chromium alloys (3 to 8 percent Cr) and high-chromium alloys (12 to 30 percent Cr). The study addressed a practical need in the cladding industry: understanding how subtle differences in alloy composition affect the microstructure, hardness, corrosion resistance, and wear resistance of the deposited cladding layer.
Classification and Characteristics of Medium-Chromium Cladding Alloys
Cladding alloys are commonly classified by their chromium content into three categories:
| Classification | Cr Content | Primary Application | Typical Microstructure |
|---|---|---|---|
| Low-chromium | 3–8% | Mild corrosion resistance, moderate wear | Ferrite + martensite |
| Medium-chromium | 8–12% | Moderate corrosion resistance, good wear | Martensite + austenite |
| High-chromium | 12–30% | High corrosion resistance | Austenite + carbides |
Medium-chromium cladding alloys are particularly useful in applications where the service environment involves moderate corrosion combined with moderate abrasive wear. Examples include chemical processing equipment, pulp and paper machinery, and mining equipment components exposed to mildly corrosive slurries. The medium chromium content provides sufficient chromium for the formation of protective chromium oxide films while maintaining adequate toughness and wear resistance.
The two alloys studied in this work differed in their minor alloying elements. One alloy contained additional molybdenum (Mo) for enhanced pitting corrosion resistance, while the other contained additional nickel (Ni) for austenite stabilization and improved toughness. Both alloys contained approximately 10 percent chromium, 2 to 3 percent carbon, and 1 to 2 percent manganese.
Microstructural Analysis
The microstructural evolution of the two alloys during GMAW cladding deposition was studied using optical microscopy and scanning electron microscopy (SEM). The results revealed distinct differences:
Alloy A (Mo-containing)
The Mo-containing alloy exhibited a microstructure consisting primarily of martensite with dispersed carbides. The martensite formed as a result of the rapid cooling rate during welding, which suppressed austenite transformation. The carbides were primarily M7C3 and M23C6 type, with chromium and molybdenum preferentially partitioning to the carbide phase. The carbide distribution was relatively uniform, with no large clusters observed. The molybdenum content increased the hardness of the martensite matrix by solid solution strengthening and also increased the hardness of the carbides by substitution of molybdenum for iron in the carbide lattice.
The microstructure showed some degree of grain refinement, with a grain size of approximately 20 to 40 μm. This refinement was attributed to the high cooling rate and the presence of carbide-forming elements that acted as nucleation sites. The absence of large grain boundaries reduced the risk of intergranular cracking during service.
Alloy B (Ni-containing)
The Ni-containing alloy exhibited a mixed microstructure of martensite and retained austenite. The nickel content stabilized the austenite phase, preventing complete transformation to martensite during cooling. The retained austenite content was approximately 15 to 25 percent, depending on the local cooling rate. The austenite was distributed as a network along the prior austenite grain boundaries and as islands within the martensite matrix.
The carbides in Alloy B were primarily M7C3 type, with a slightly lower volume fraction than in Alloy A. The lower carbide volume fraction was attributed to the nickel's effect on carbon activity: nickel reduces the activity of carbon in austenite, which can reduce carbide precipitation. The austenite phase in Alloy B provided a tougher, more ductile component to the microstructure, which improved the overall toughness of the cladding layer.
Property Comparison
The mechanical and corrosion properties of the two alloys were compared through hardness testing, wear testing, and corrosion testing.
| Property | Alloy A (Mo) | Alloy B (Ni) | Significance |
|---|---|---|---|
| Surface hardness (HV) | 750–850 | 600–700 | Alloy A harder due to higher carbide content |
| Core hardness (HV) | 500–600 | 400–500 | Gradient from surface to core in both alloys |
| Wear resistance (wear loss) | 0.05–0.08 mg | 0.08–0.12 mg | Alloy A superior in dry sliding wear |
| Pitting corrosion resistance | Superior | Moderate | Mo enhances pitting resistance |
| General corrosion resistance | Good | Good | Both alloys perform similarly |
| Bond strength (% of base) | 85–92% | 88–95% | Alloy B slightly better bond strength |
| Impact toughness | Moderate | Good | Alloy B tougher due to retained austenite |
The results clearly demonstrate the trade-offs inherent in medium-chromium cladding alloy selection. Alloy A (Mo-containing) offers superior hardness and wear resistance, making it suitable for applications where abrasive wear is the dominant degradation mechanism. Alloy B (Ni-containing) offers better toughness and slightly better bonding strength, making it suitable for applications where impact loading or thermal cycling is significant.
Corrosion Behavior Analysis
The corrosion behavior of the two alloys was evaluated through potentiodynamic polarization tests and pitting corrosion tests in 3.5 percent NaCl solution. The results showed that Alloy A had a higher corrosion potential and a lower corrosion current density, indicating better general corrosion resistance. The pitting potential of Alloy A was also higher, confirming the beneficial effect of molybdenum on pitting corrosion resistance.
The mechanism of molybdenum's beneficial effect on pitting corrosion resistance is well established in the stainless steel literature. Molybdenum increases the passivity of the chromium oxide film by promoting the formation of a more stable, less defective passive layer. In the cladding context, this is particularly important because the dilution zone between the cladding layer and the substrate may have a lower chromium content, making it more susceptible to pitting corrosion. The molybdenum in Alloy A helps to maintain corrosion resistance even in the dilution zone.
Engineering Practice Guidelines
Based on the study results, the following guidelines can be offered for medium-chromium cladding alloy selection:
- For abrasive wear applications: Select Alloy A (Mo-containing) when the primary degradation mechanism is dry or semi-dry abrasive wear. The higher hardness and carbide content provide superior wear resistance. Typical applications include slurry pump impellers, valve seats, and crusher components.
- For combined wear and corrosion applications: Select Alloy A when pitting corrosion is a concern, or Alloy B when general corrosion is the primary concern. The molybdenum in Alloy A provides superior pitting resistance, while the nickel in Alloy B provides adequate general corrosion resistance with better toughness.
- For impact-loaded applications: Select Alloy B (Ni-containing) when the component is subject to impact loading or thermal cycling. The retained austenite provides strain-induced transformation toughening, which improves resistance to crack initiation and propagation.
- For thick cladding layers: Both alloys can be deposited in multiple passes, but the dilution zone composition should be controlled by adjusting the first-pass welding parameters. A lower current and faster travel speed on the first pass reduces dilution and ensures that the cladding layer composition approaches the intended alloy composition.
Key Questions and Reflections
The study raises several important questions about medium-chromium cladding alloy design. First, can the beneficial effects of both molybdenum and nickel be combined in a single alloy, and if so, what is the optimal combination? Second, how does the cooling rate during welding affect the microstructure and properties of these alloys, and can controlled cooling be used to optimize the microstructure? Third, what is the effect of post-weld heat treatment on the microstructure and properties of medium-chromium cladding alloys?
The work also highlights the importance of understanding the microstructure-property relationship in cladding alloys. The same chromium content can produce very different properties depending on the minor alloying elements and the resulting microstructure. Engineers should not select cladding alloys based solely on chromium content; the full composition and the expected microstructure must be considered.
Study Insights and Implications
This study provides a clear demonstration of how minor alloying elements can significantly influence the properties of medium-chromium cladding alloys. The molybdenum-nickel comparison is particularly instructive because both elements are common in stainless steel and cladding alloys, yet they produce fundamentally different microstructures and property profiles. For engineers selecting cladding alloys for specific applications, the key insight is that alloy selection should be based on a thorough understanding of the service environment and the dominant degradation mechanism. The study reinforces the principle that there is no single "best" cladding alloy; the optimal alloy depends on the specific combination of wear, corrosion, mechanical, and thermal requirements. The systematic approach taken by the authors—comparing two alloys with a single variable changed—provides a model for future alloy development studies and demonstrates the value of controlled experimentation in cladding alloy optimization.