Microstructure and Abrasive Wear Performance of Weld Overlay Alloys Containing In-Situ Formed Carbide Particles

Literature Overview

This study note addresses the microstructure and abrasive wear behavior of weld overlay alloys designed to contain in-situ formed carbide particles, as distinct from ex-situ added carbide powders. In-situ carbide formation occurs during the solidification of the weld overlay deposit when the alloy composition contains sufficient carbon and carbide-forming elements such as chromium, molybdenum, tungsten, vanadium, and titanium. The resulting microstructure consists of a metallic binder matrix reinforced by carbide particles that nucleate and grow during solidification, offering advantages in particle-matrix bonding quality, particle size distribution, and microstructural homogeneity compared to ex-situ approaches.

The literature reviewed covers overlay alloys based on high-speed steel compositions, cobalt-chromium-carbon systems, and iron-nickel-cobalt-ceramic matrices, with carbide contents ranging from 15 to 50 volume percent. The welding methods investigated include plasma transferred arc cladding, laser cladding, and gas tungsten arc welding with powder feed.

Carbide Formation Mechanisms and Morphology

In-situ carbide formation in weld overlay deposits follows predictable thermodynamic and kinetic pathways. During rapid solidification of the weld pool, the cooling rate determines which carbide phases nucleate and grow. At cooling rates below 10 K/s (typical of GTAW and plasma arc cladding), equilibrium carbide phases such as M6C, M7C3, and MC form in sequence according to the lever rule and phase diagram predictions.

The morphology of in-situ carbides is strongly influenced by the cooling rate and alloy composition. At slower cooling rates, carbides form as elongated stringers along dendrite boundaries, which provides good load transfer but creates stress concentration points. At higher cooling rates achieved through laser cladding, carbides nucleate as equiaxed particles with sizes between 0.5 and 5 micrometers, offering superior mechanical properties due to reduced stress concentration and improved matrix continuity.

Microstructural Characterization

Metallographic examination of in-situ carbide containing overlay deposits reveals several characteristic features. The carbide particles are predominantly located at dendrite boundaries and interdendritic regions, where carbon and alloying elements segregate during solidification. The size distribution of in-situ carbides typically follows a log-normal distribution, with a mean size of 2 to 5 micrometers for plasma arc cladding and 5 to 15 micrometers for GTAW.

The quality of the carbide-matrix interface is a critical factor in determining wear performance. In-situ formed carbides exhibit coherent or semi-coherent interfaces with the binder matrix due to their simultaneous nucleation and growth. This bonding quality is superior to ex-situ added carbides, which often show oxide films, Kirkendall voids, or poor wetting at the interface. The coherent interface allows effective stress transfer from the matrix to the carbide particles during loading, maximizing the reinforcing effect of the carbide phase.

Scanning electron microscopy with energy dispersive spectroscopy mapping reveals that the carbide particles in HSS-type overlays consist primarily of M2C (Mo, W)6C and M7C3 (Cr, Fe)7C3 phases, with minor VC particles. The M2C carbides form the skeleton structure with hardness exceeding 1800 HV, while the M7C3 carbides provide secondary reinforcement with hardness of approximately 1400 HV. The binder matrix, typically composed of tempered martensite with retained austenite, has hardness values between 450 and 550 HV.

Abrasive Wear Performance

Abrasive wear testing demonstrates that overlay alloys with in-situ carbide particles exhibit wear resistance 3 to 8 times superior to the uncladded base material, depending on the carbide content, morphology, and matrix properties. The wear mechanism transitions from adhesive wear in the matrix-dominated regime to abrasive wear in the carbide-dominated regime as the applied load increases.

The critical finding is that the optimal carbide content for maximum wear resistance is not simply the highest achievable value. Beyond approximately 40 volume percent carbide content, the matrix becomes discontinuous and the overlay deposit becomes brittle. Under this condition, the wear mechanism shifts from controlled abrasive removal to catastrophic spalling, where large areas of the overlay deposit are lost in a single event. The optimal carbide content for balanced wear resistance and durability is typically 25 to 35 volume percent, where the matrix remains continuous and can effectively support the carbide reinforcement.

The wear life of in-situ carbide containing overlays is also strongly dependent on the thermal stability of the carbide phases. At service temperatures below 500°C, all carbide phases remain stable and wear resistance is maintained. Between 500 and 700°C, M7C3 carbides begin to transform to M23C6, which is softer and less effective as reinforcement. Above 700°C, significant carbide coarsening occurs, leading to progressive loss of wear resistance. For high-temperature applications, the selection of overlay alloys with thermally stable MC carbides (such as VC or TiC) is essential.

Process Optimization for In-Situ Carbide Formation

Achieving the desired in-situ carbide morphology and distribution requires careful control of welding parameters. The cooling rate is the primary parameter governing carbide nucleation and growth. Plasma transferred arc cladding with powder feed produces cooling rates of 50 to 200 K/s, yielding fine carbide particles with uniform distribution. Laser cladding achieves cooling rates of 1000 to 10000 K/s, producing extremely fine carbides but with potential for microcracking due to high residual stresses.

The powder feed rate in plasma arc cladding significantly affects the carbide content and distribution. Higher powder feed rates increase the dilution ratio and produce deposits with higher alloy content and more carbide formation. However, excessive powder feed rates lead to incomplete melting and unmelted particle inclusions, which act as defects and reduce both hardness and wear resistance.

Preheating and interpass temperature control are critical for preventing cracking in in-situ carbide containing overlays. The high carbon equivalent of these alloys (typically 0.6 to 1.2) creates significant susceptibility to cold cracking. Preheating to 200 to 300°C and maintaining interpass temperatures between 150 and 250°C are standard practice, with post-weld stress relief at 550 to 650°C recommended for critical applications.

Study Insights and Practical Implications

The literature on in-situ carbide containing overlay alloys provides compelling evidence that this approach offers superior mechanical properties and wear performance compared to ex-situ carbide addition methods. The coherent carbide-matrix interface, uniform particle distribution, and absence of interfacial defects make in-situ carbide overlays particularly suitable for applications requiring high reliability and long service life.

For engineering practice, the selection of in-situ carbide overlay alloys should be guided by the specific wear conditions. For low-load abrasive wear at ambient temperature, HSS-type overlays with 25 to 30 vol% carbide provide excellent performance at reasonable cost. For high-load impact-abrasive wear, cobalt-based overlays with 30 to 35 vol% carbide offer superior toughness and wear resistance. For high-temperature abrasive wear, nickel-based overlays with thermally stable MC carbides are the preferred choice.

The quality assurance protocol for in-situ carbide overlays should include carbide content verification by image analysis, carbide size distribution measurement by SEM, hardness profiling across the deposit cross-section, and abrasive wear testing on representative coupons. These measurements should be performed on each production batch to ensure consistent overlay performance.

In conclusion, in-situ carbide containing weld overlay alloys represent a sophisticated approach to wear-resistant cladding that leverages the advantages of controlled carbide formation during solidification to produce deposits with excellent microstructural homogeneity, strong carbide-matrix bonding, and superior abrasive wear performance across a wide range of industrial applications.