Heat Treatment Effects on Abrasive Wear Performance of Ni45 Overlay Layer

Literature Overview

This study note investigates the influence of post-weld heat treatment on the abrasive wear resistance of Ni45 overlay deposits. Ni45 is a widely used high-nickel, high-carbon austenitic hardfacing alloy characterized by its exceptional toughness, resistance to thermal fatigue, and adaptability to severe impact loading. The base material typically contains 45% nickel by weight, with carbon levels around 0.5–1.0% and chromium at 6–10%. While Ni45 deposits exhibit good as-welded wear performance, the literature demonstrates that controlled heat treatment can significantly enhance abrasive wear resistance through microstructural modification, including martensitic transformation, carbide precipitation, and grain refinement.

Microstructural Evolution Under Heat Treatment

The as-welded microstructure of Ni45 overlays typically consists of a dendritic austenite matrix with carbide networks at interdendritic regions. The carbon and chromium content promote the formation of M₇C₃ and M₂₃C₆ carbides, which provide the primary wear resistance mechanism. However, the as-welded microstructure is often coarse due to rapid solidification followed by limited diffusion, resulting in suboptimal hardness distribution and carbide morphology.

Heat treatment introduces several beneficial transformations:

The solution treatment and quenching route produces the most dramatic hardness increase by transforming the austenitic matrix to martensite. The resulting microstructure contains lath martensite with fine carbide precipitation, achieving hardness values of 550–650 HV compared to 350–420 HV in the as-welded condition. However, this route also increases brittleness, requiring subsequent tempering to restore acceptable toughness.

Abrasive Wear Testing Results

The study employs ASTM G65 two-body abrasive wear testing with SiC papers of varying grit sizes (P120, P240, P600) to simulate different severity levels of abrasive contact. The results demonstrate clear trends:

• As-welded Ni45: wear rate 1.2–2.5 mg/N·m depending on grit size.
• Solution + quench only: wear rate 0.4–0.9 mg/N·m (60–65% reduction).
• Solution + quench + temper (600 °C, 2 h): wear rate 0.5–1.1 mg/N·m (55–60% reduction, with improved toughness).
• Cryogenic treatment only: wear rate 0.9–1.8 mg/N·m (25–30% reduction).

The wear mechanism shifts from predominantly abrasive ploughing and micro-cutting in the as-welded condition to a combination of abrasion and micro-fatigue in the heat-treated condition. The harder martensitic matrix and finer carbide distribution increase resistance to micro-cutting, while the tempered carbides provide stable load-bearing points that resist dislodgement during sliding contact.

Process Optimization and Practical Considerations

The optimal heat treatment sequence depends on the specific service requirements. For applications emphasizing maximum wear resistance (such as sand pump impellers or mining bucket teeth), the solution + quench + temper route at 550–600 °C provides the best balance of hardness and toughness. For applications requiring high impact resistance (such as crusher hammers or hammer mill rotors), a more conservative treatment—subcritical tempering at 550 °C for 4 hours—provides moderate hardness improvement without significantly compromising toughness.

A critical practical consideration is the thermal distortion risk during heat treatment of large overlay components. Distortion can exceed 0.5% of component dimension for heavy sections, potentially affecting dimensional accuracy and fit-up. Stress relief at 550–600 °C before the primary heat treatment can mitigate this risk by reducing residual stresses accumulated during welding.

Common defects that may arise during heat treatment include:

• Quench cracking: caused by excessive cooling rate in thick sections or high-carbon variants; mitigated by oil quenching or air cooling for thick sections.
• Carbide network embrittlement: excessive solution temperature (>1050 °C) dissolves and reprecipitates carbides at grain boundaries, reducing intergranular fracture resistance; controlled by limiting solution temperature to 1020 °C maximum.
• Reversion: prolonged tempering at high temperatures (>650 °C) causes austenite reversion, reducing hardness; avoided by limiting tempering duration to 4 hours maximum.

Engineering Practice Integration

In production environments, heat treatment of Ni45 overlays is typically performed in controlled-atmosphere furnaces with continuous temperature monitoring. The cooling rate during quenching should be monitored to avoid thermal gradients that could cause cracking. For large components, step-quenching (water quench to 400 °C, then air cool) provides a practical compromise between transformation completeness and distortion control.

Post-heat-treatment inspection should include hardness mapping across the overlay cross-section, metallographic examination of the overlay-base interface for cracks or delamination, and, for critical applications, impact testing (Charpy V-notch at 25 °C and −40 °C) to verify toughness retention.

Summary

Heat treatment is a powerful tool for optimizing the abrasive wear performance of Ni45 overlay deposits, with the solution + quench + temper route offering the most significant improvement in wear resistance while maintaining acceptable toughness. The selection of specific heat treatment parameters should be guided by the dominant loading mode in service—wear-dominated, impact-dominated, or mixed—to achieve the optimal property balance. Engineers should always verify the final properties through comprehensive mechanical testing and microstructural examination, as the actual performance of heat-treated overlays can vary significantly based on component geometry, base material composition, and process execution quality.