Embedded M23C6 Carbides in Open-Arc Cladding Alloys and Wear Resistance

Literature Overview

This 2013 study from Xiangtan University's College of Mechanical Engineering, authored by Gong Jianxun, Yang Boxiang, Xiao Yifeng, and Xu Jiqing, investigates the formation, distribution, and mechanical effects of embedded M23C6 type carbides in open-arc cladding alloys. Supported by the National Natural Science Foundation of China (Grant No. 51271158), the Hunan Provincial Natural Science Foundation and Xiangtan Municipal Government Joint Fund (Grant No. 11JJ9015), and the Hunan Provincial Department of Education Key Project (Grant No. 11A114), this research provides valuable insights into the role of carbide morphology and distribution in determining the wear resistance of cladding layers.

Published in Powder Metallurgy Materials and Technology, this study addresses a fundamental aspect of cladding alloy design: the relationship between carbide phase characteristics and wear performance. The authors demonstrate that the morphology, size, and distribution of M23C6 carbides embedded within the cladding matrix significantly influence the wear resistance of the overlay layer, and that these carbide characteristics can be controlled through alloy composition and process parameter optimization.

Carbide Formation and Morphology Control

The authors investigate the formation mechanism of M23C6 carbides in open-arc cladding alloys through thermodynamic calculations and microstructural analysis. M23C6 is a chromium-rich carbide that forms preferentially in high-chromium austenitic and ferritic alloys during solidification and subsequent cooling. The authors identify several factors that influence M23C6 carbide formation:

• Chromium content: Higher chromium content promotes M23C6 carbide precipitation, with carbide volume fraction increasing linearly with chromium content above 20 percent.
• Carbon content: Carbon is the primary carbide-forming element, and the authors find that carbon content in the range of 1.0 to 2.5 percent yields optimal M23C6 carbide volume fraction for wear-resistant applications.
• Cooling rate: Faster cooling rates suppress M23C6 carbide precipitation and promote the formation of finer, more uniformly distributed carbides.
• Alloying additions: Elements such as molybdenum, vanadium, and tungsten can modify carbide morphology and distribution, influencing wear resistance.

The authors employ metallographic analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy to characterize the M23C6 carbides in the cladding layers. The results reveal that M23C6 carbides typically exhibit blocky or irregular morphology with sizes ranging from 5 to 50 micrometers, depending on the alloy composition and solidification conditions.

The authors find that the addition of molybdenum and vanadium promotes the formation of finer M23C6 carbides with more uniform distribution. This refinement is attributed to the interaction between these alloying elements and the carbide precipitation process, which modifies the nucleation and growth kinetics of M23C6 carbides.

Wear Resistance Mechanisms

The authors conduct comprehensive wear testing to evaluate the wear resistance of cladding layers with different M23C6 carbide characteristics. The wear tests are performed under both dry sliding and abrasive wear conditions, simulating typical service environments for wear-resistant cladding applications.

The results demonstrate that wear resistance is strongly influenced by M23C6 carbide morphology and distribution:

• Fine, uniformly distributed carbides: Cladding layers with fine (5–20 micrometer) and uniformly distributed M23C6 carbides exhibit the highest wear resistance. The fine carbides provide numerous hard points that resist abrasive wear, while the uniform distribution ensures consistent performance across the cladding surface.
• Large, clustered carbides: Cladding layers with large (30–50 micrometer) and clustered M23C6 carbides exhibit lower wear resistance due to stress concentration at carbide-matrix interfaces and the formation of wear debris from debonded carbides.
• Carbide volume fraction: An optimal volume fraction of 20–30 percent provides the best wear resistance. Below this range, the carbide reinforcement effect is insufficient, while above this range, the matrix becomes too brittle to support the carbides effectively.

The authors identify three primary wear mechanisms operating in the cladding layers:

1. Abrasive wear: Hard M23C6 carbides plow through the counterface material, creating wear grooves and removing material from the cladding surface.
2. Adhesive wear: Localized bonding between the cladding surface and the counterface leads to material transfer and surface damage.
3. Fatigue wear: Cyclic loading at the cladding surface leads to microcrack initiation and propagation, resulting in surface spalling.

The authors find that fine, uniformly distributed M23C6 carbides primarily resist abrasive wear by providing hard points that resist material removal. The metallic matrix between the carbides provides toughness and supports the carbides against debonding. This synergistic interaction between hard carbides and tough matrix is the key to achieving high wear resistance.

Process Optimization for Optimal Carbide Morphology

The authors systematically optimize the open-arc cladding process parameters to achieve the desired M23C6 carbide morphology and distribution. The key process parameters investigated include welding current, arc voltage, travel speed, and powder feed rate.