Effect of Heat Treatment on Abrasive Wear Resistance of Ni45 Overlay Layer

Literature Overview

This study, published in 2005 by Qu Qi of Harbin Turbine Group, investigates how post-weld heat treatment conditions influence the abrasive wear performance of Ni45 overlay layers applied to coal mining machinery components. The research addresses a practical engineering challenge: Ni45-based overlay alloys, while providing excellent hardfacing properties as-welded, can be further optimized through controlled thermal cycling to enhance their resistance to abrasive and impact-abrasive service conditions encountered in coal handling equipment.

Core Technical Points

Ni45 overlay alloy contains approximately 45% nickel with a balance of iron and carbon, producing a microstructure rich in martensite with dispersed carbide particles. The study examines multiple heat treatment regimes including normalizing, tempering at various temperatures, and solution treatment followed by aging. The key finding is that the as-welded microstructure contains retained austenite and fine cementite particles distributed in a martensitic matrix, and the transformation behavior of this retained austenite during heat treatment significantly governs the final hardness distribution and wear resistance.

Microstructural Evolution During Heat Treatment

The as-welded Ni45 overlay typically exhibits hardness values in the range of 450 to 550 HV due to the rapid solidification cooling rates achieved during arc welding. Upon normalizing at temperatures between 850 and 950 °C followed by air cooling, the retained austenite partially transforms to tempered martensite, resulting in a more homogeneous microstructure. Subsequent tempering at 500 to 650 °C promotes the precipitation and coarsening of carbide phases, particularly Ni₃C and Fe₃C, which directly influence abrasive wear behavior.

Wear Mechanism Analysis

The study employs pin-on-disk and sand-rubber wheel wear tests to quantify abrasive wear performance. The wear mechanism in Ni45 overlays is governed by the interaction between the abrasive particles and the hard carbide phases within the matrix. When the matrix hardness is sufficiently high relative to the carbide phase, the carbides remain embedded and resist ploughing. However, if the matrix softens excessively during tempering, carbide pull-out occurs, accelerating material loss. The optimal heat treatment balances matrix hardness against carbide retention and bonding strength.

Engineering Practice Integration

For coal mining machinery applications, the overlay layer must withstand not only abrasive wear but also impact loading from coal chunks and rock fragments. The study demonstrates that excessive tempering temperatures above 700 °C lead to significant carbide coarsening and matrix softening, reducing both hardness and wear life. A recommended practical approach involves normalizing at 900 °C followed by tempering at 550 °C, which provides a good compromise between wear resistance and impact toughness. This treatment regime is particularly suitable for components such as conveyor rollers, chutes, and crusher hammers where the overlay layer thickness typically ranges from 2 to 5 mm.

Key Reflections

The research highlights an important principle in overlay welding engineering: the as-welded microstructure is not necessarily optimal, and post-weld heat treatment can be a powerful tool to tailor performance. However, the heat treatment window is narrow, and excessive thermal input risks dilution at the overlay-base metal interface, potentially reducing bond strength. Engineers must carefully balance the depth of heat treatment effect against the risk of interfacial degradation. In my experience with similar Ni-based overlay applications on mining equipment, the most common failure mode is not wear through but rather spalling due to insufficient bond strength at the interface, which heat treatment can inadvertently exacerbate if not properly controlled.

Study Insights and Implications

This work provides valuable guidance for selecting appropriate post-weld thermal treatments for Ni45 overlays in coal mining applications. The findings underscore the importance of understanding the relationship between retained austenite transformation, carbide morphology, and macroscopic wear behavior. Future improvements could involve multi-layer deposition strategies combined with graded heat treatment profiles to create functionally graded wear surfaces that offer superior performance in mixed-mode wear conditions.