Metallographic examination is one of the most practical ways to evaluate a hardfacing layer. Under an optical microscope, carbide type, morphology, size, spacing, and distribution can reveal how the deposited layer was formed and how it may behave under abrasive wear. However, optical identification should be treated as a morphological assessment rather than a final phase identification. M₇C₃, M₂₃C₆, MC and other hard phases can require SEM-EDS or XRD confirmation when phase identity is critical.

1. What Optical Metallography Can Tell You

Optical metallography is mainly used to examine the shape, size, volume fraction, orientation and spatial distribution of hard phases in a cross-section. These features are important because abrasive wear resistance is not controlled by hardness alone. Carbide morphology and the surrounding matrix can strongly affect crack initiation, carbide pull-out and wear resistance.

A properly prepared cross-section can also show the relationship between the deposited layer and the steel substrate. Important observations include metallurgical bonding, dilution, heat-affected regions, cracks, pores, dendritic solidification, eutectic structures and changes between individual overlay passes.

2. Common Carbide Types in Hardfacing Layers

Different alloying systems produce different hard phases. In Fe-Cr-C overlays, chromium-rich carbides are frequently observed. Additions of titanium, vanadium, niobium or tungsten can promote MC-type or complex carbides. The actual phase constitution depends on chemical composition, cooling rate and deposition conditions.

High-chromium hardfacing commonly contains M₇C₃ and M₂₃C₆ phases, while Ti- or V-containing alloys can develop MC-type carbides. Published metallographic studies also show that rapid solidification during welding can preserve non-equilibrium phase combinations.

3. How to Judge Carbide Morphology Under an Optical Microscope

3.1 Rod-Like and Elongated Carbides

Long, rod-like or elongated particles are often associated with chromium-rich carbide structures. The important point is not simply whether the particles are elongated, but whether they form a continuous network. A highly continuous carbide network may provide strong abrasion resistance, but excessive continuity can also create preferred paths for crack propagation.

3.2 Blocky and Polygonal Carbides

Blocky carbides are usually easier to identify at low magnification. Large isolated particles may provide strong resistance against abrasive particles, but very coarse carbides can become vulnerable to cracking or pull-out when the surrounding matrix cannot adequately support them.

3.3 Fine Eutectic Carbides

Fine carbide networks normally provide a more uniform distribution of hard phases. The microscope should be used to check whether the fine carbides are evenly distributed or concentrated along dendrite boundaries. A refined and relatively uniform structure is generally easier to support mechanically than a structure containing a few extremely large particles.

3.4 Coarse Carbide Networks

A continuous or semi-continuous network should be recorded separately from isolated carbides. During wear, cracks may develop along weak interfaces or through brittle carbide regions. Therefore, carbide continuity can be as important as average carbide size.

4. Carbide Size: What Should Be Measured?

Carbide size should not be reported only as “fine” or “coarse.” For quality control, the measurement method should be defined. Useful parameters include equivalent diameter, maximum length, aspect ratio, area fraction and particle number density.

For irregular carbides, equivalent circular diameter can be useful for statistical comparison, while maximum length and aspect ratio are more informative when elongated carbides are present. The measurement should be performed across several fields rather than from a single visually attractive region.

5. Carbide Distribution: The Most Easily Missed Factor

Two overlays can have similar carbide content but very different wear behavior if their distributions are different. A good metallographic report should therefore distinguish between uniform, clustered, dendritic, banded and networked distributions.

6. Sample Preparation for Reliable Metallography

A reliable metallographic result starts with sample preparation. The cross-section should normally be cut perpendicular to the overlay surface so that the coating thickness, fusion boundary and through-thickness microstructure can be evaluated together.

Grinding should be performed progressively to remove cutting damage. Polishing should produce a sufficiently smooth surface without pulling out large carbides. Excessive polishing can create artificial cavities around hard particles and may lead to an incorrect judgment of carbide distribution.

Etching should be selected according to the matrix and inspection objective. In some cases, an unetched polished surface is useful for observing hard-phase morphology, while etching can improve the contrast between the matrix and carbide-containing regions.

7. Optical Identification Is Not Final Phase Identification

Optical microscopy can suggest a carbide type from morphology, but it should not be used alone to establish chemical phase identity. Carbides with similar optical appearances may have different compositions. For a formal failure analysis or material qualification, optical microscopy can be combined with SEM-EDS, XRD or other phase-analysis methods.

For example, M₇C₃ and M₂₃C₆ may occur in related chromium-rich hardfacing systems, and phase transformation can modify carbide morphology during thermal exposure. Therefore, a report should clearly distinguish between “morphologically consistent with” and “confirmed as.”

8. A Practical Metallographic Acceptance Checklist

• Confirm the sampling position and overlay layer number.
• Check the bonding interface between the deposit and substrate.
• Record cracks, pores, inclusions and obvious metallurgical defects.
• Identify the dominant carbide morphology.
• Measure representative carbide dimensions.
• Evaluate carbide spacing and distribution.
• Check whether carbides form continuous networks.
• Compare different regions across the overlay thickness.
• Use SEM-EDS or XRD when phase identification must be confirmed.
• Keep microscope magnification and image scale consistent for comparison.

9. Why Carbide Size and Distribution Matter for Wear Resistance

Hardfacing performance depends on the combined behavior of hard phases and the matrix. Research on abrasive hardfacing has shown that carbide size, shape, distribution and chemical composition, together with matrix microstructure, can strongly influence abrasive wear. Large carbides can resist cutting, but excessively coarse particles may fracture or become detached. Conversely, a fine and well-supported carbide structure can provide a more stable wear surface.

This is why a metallographic inspection should never conclude simply that “more carbides are better.” The better engineering question is whether the carbide population, morphology and matrix provide an appropriate balance between hardness, abrasion resistance and resistance to carbide fracture or pull-out.

Bimetallic Hardfaced Steel Plate

Corrosion Resistant Overlay Plate

Cco Plate 12+12

Smooth Surface Overlay Plate

10. Metallography Support for Industrial Hardfacing Materials

For procurement and production teams, metallography is most useful when it is connected with the actual hardfacing material specification. Teda Ganghua can support industrial material sourcing with wear-resistant steel and related metal products, while metallographic evaluation can be used as part of incoming inspection, process verification and failure analysis.

For buyers who need material selection, specification matching or customized metal supply, see the wear-resistant steel products available for industrial applications. Metallographic requirements can be specified together with hardness, chemical composition, dimensions and other inspection requirements.

FAQ

What carbide type is most commonly observed in high-chromium hardfacing?

M₇C₃ is commonly observed in high-chromium Fe-Cr-C hardfacing systems. M₂₃C₆ can also occur depending on composition, cooling conditions and thermal history. Optical morphology alone should not be treated as definitive phase identification.

Can optical microscopy identify M₇C₃ with certainty?

No. Optical microscopy can provide strong morphological evidence, but phase confirmation may require SEM-EDS, XRD or another analytical technique.

Are larger carbides always better for abrasion resistance?

No. Large carbides can provide strong resistance to abrasive cutting, but excessively coarse or poorly supported particles may crack or pull out. Carbide size must be evaluated together with morphology, distribution and matrix structure.

What should be measured during quantitative metallography?

Useful parameters include carbide area fraction, equivalent diameter, maximum length, aspect ratio, particle number density and spacing. Multiple representative fields should be measured rather than relying on one microscope image.

Why should the matrix be evaluated together with carbides?

The matrix supports the hard phases and affects crack propagation, carbide pull-out and overall wear behavior. A high carbide fraction alone does not guarantee superior wear performance.