Microstructure and Wear Resistance of High-Chromium Open-Arc Overlay Welding Alloys with B4C Addition

Literature Overview

This paper published in the Welding Journal (2012, Vol. 33, Issue 11) by Gong Jianxun and Xiao Yifeng from Xiangtan University investigates the microstructure and wear resistance of high-chromium open-arc overlay welding alloys. The study focuses on the effect of B4C (boron carbide) addition on the primary M7C3 carbide morphology and its influence on abrasive wear performance. The research is supported by the Hunan Provincial Natural Science Foundation, the Xiangtan Municipal Natural Science Joint Fund, the Hunan Provincial Department of Education Key Project, and the National Natural Science Foundation of China.

Core Technical Findings

The overlay alloy was produced using metal-powder flux-cored wire self-shielded open-arc welding (SAW with flux-cored wire in open arc configuration). The base composition contains 21-23% Cr, 3.5-4.2% C, 1.4-1.6% Si, and variable B4C content (0-1.8%). The open-arc process eliminates the need for shielding gas, making it suitable for outdoor and field applications where gas shielding is impractical.

The key findings regarding B4C addition are:

1. Si5C3 (silicon carbide from the flux) serves as heterogeneous nucleation sites for primary M7C3 carbides during solidification.
2. As B4C content increases, the volume fraction and size of primary M7C3 carbides significantly increase.
3. The morphology of M7C3 carbides transitions from dispersed distribution to clustered arrangement with increasing B4C content.
4. Wear resistance depends on the size and distribution morphology of primary M7C3 carbides.
5. Micro-ploughing (micro-ploughing) is the primary wear mechanism under wet sand abrasion conditions.

Interpretation of Technical Points

The role of B4C as a nucleation agent for M7C3 carbides is metallurgically significant. During the rapid solidification of the open-arc weld pool, the cooling rate is typically 10-100°C/s, which favors the formation of primary carbides. B4C particles, with their high melting point (2450°C) and crystallographic compatibility with M7C3, provide effective nucleation sites that promote early carbide precipitation.

The transition from dispersed to clustered M7C3 morphology with increasing B4C content represents a critical microstructural evolution. Dispersed carbides provide uniform hardness distribution and good toughness, while clustered carbides create local hard spots that are susceptible to debonding and spalling under abrasive loading. This explains why there is an optimal B4C content beyond which wear resistance may deteriorate despite increased carbide volume fraction.

The open-arc welding process introduces unique metallurgical features compared to gas-shielded welding:

• The flux provides additional deoxidation through Si and Al reactions
• The slag layer influences the solidification sequence and carbide morphology
• The absence of gas shielding allows for higher deposition rates but introduces oxide inclusions
• The self-shielded configuration enables field application without gas supply infrastructure

The Si5C3 phase identified in the study is formed from the interaction between Si in the flux and C in the alloy during the high-temperature welding process. This phase acts as a pre-existing nucleation site for M7C3, demonstrating the complex interplay between flux chemistry and weld metal microstructure in flux-cored wire welding.

Wear Mechanism Analysis

The wet sand abrasion test results indicate that micro-ploughing is the dominant wear mechanism. This mechanism involves:

1. Abrasive particles ploughing into the surface, creating micro-grooves
2. Material displacement rather than removal (ploughing vs. cutting)
3. Progressive surface roughening through repeated ploughing events
4. Potential initiation of micro-cracks at carbide-matrix interfaces

The dependence of wear resistance on M7C3 carbide size and distribution can be explained by the following relationships:

Engineering Practice Integration

High-chromium overlay alloys are extensively used in the following industrial applications:

• Mining equipment: excavator buckets, conveyor rollers, crusher hammers
• Cement industry: kiln liners, mill liners, slide surfaces
• Power generation: ash handling systems, coal mill components
• Construction: concrete pump pistons, wear plates
• Agricultural machinery: plowshares, auger flights

The open-arc self-shielded process is particularly advantageous for:

1. Field repair operations where gas shielding equipment is unavailable
2. Large-scale component overlay requiring high deposition rates
3. Outdoor applications subject to wind and weather conditions
4. Cost-sensitive operations where shielding gas supply adds significant expense

The B4C addition strategy provides a means to tailor the wear resistance of the overlay alloy for specific service conditions. For applications requiring maximum wear resistance with good toughness (such as rotating components), a moderate B4C content that produces dispersed M7C3 carbides is preferred. For applications where extreme wear resistance is paramount and some toughness sacrifice is acceptable (such as static wear plates), higher B4C content may be used.

Process parameters for the open-arc flux-cored wire overlay welding should be carefully controlled:

Key Questions and Reflections

The study focuses on wet sand abrasion testing, which represents a specific wear condition. The wear behavior under dry sliding, impact loading, and corrosive-wear combined conditions may differ significantly. The transition from micro-ploughing to micro-cutting or adhesive wear mechanisms under different loading conditions should be investigated.

The clustered morphology of M7C3 carbides at high B4C content raises concerns about the long-term durability of the overlay. Under cyclic loading, the interfaces between clustered carbides and the matrix may become preferential crack initiation sites. Fatigue wear resistance, which is critical for many industrial applications, was not evaluated.

The open-arc process, while offering practical advantages, introduces oxide inclusions and potential porosity that may affect the overlay's mechanical properties. The balance between deposition efficiency and metallurgical quality is a key consideration in process selection.

Study Insights and Implications

This research demonstrates that B4C addition is an effective method for controlling the microstructure and wear resistance of high-chromium overlay alloys produced by open-arc flux-cored wire welding. The identification of Si5C3 as a nucleation site for M7C3 carbides provides fundamental insight into the solidification metallurgy of these alloys. For engineering applications, the optimal B4C content should be selected based on the specific wear conditions, with moderate additions preferred for balanced performance. The open-arc self-shielded process remains a practical choice for field applications, and the microstructural control achieved through B4C addition enhances its versatility. Future research should extend the wear testing to multi-mode conditions and evaluate the long-term performance of these overlays in representative service environments.