Effect of Annealing Temperature on NM360 Wear-Resistent Steel Surfacing Layer Microstructure and Properties

Literature Overview

This study by Deng Hanzhong and Meng Xiangfeng from Liaoning Technical University and Liaoning Engineering Vocational College investigates the influence of post-weld annealing temperature on the microstructure and mechanical properties of surfacing layers deposited on NM360 wear-resistant steel using D507MoNb electrode. The research was published in Materials for Mechanical Engineering in 2015 (Vol. 39, No. 3, pp. 50-54). The work addresses the practical challenge of optimizing the balance between hardness, toughness, and wear resistance in surfacing repairs of wear-resistant steel components.

Core Technical Findings

The as-welded surfacing layer consists of martensite and carbides. Post-weld annealing at 200°C, 400°C, and 600°C produces different microstructural transformations: tempered martensite at 200°C, tempered troostite at 400°C, and tempered sorbite at 600°C. As the annealing temperature increases, the hardness of the surfacing layer decreases, impact toughness increases, and wear resistance decreases. The coating annealed at 200°C exhibits the best wear resistance, with a relative wear resistance of 1.327 times that of the base metal.

The D507MoNb electrode is a low-hydrogen type electrode with typical composition containing 1.0-1.5% C, 0.5-1.0% Mo, and trace Nb additions. The NM360 base metal is a high-strength wear-resistant steel with typical hardness of 360-400 HBW, containing microalloying elements such as Nb, V, and Ti for grain refinement and precipitation hardening.

Interpretation of Key Technical Points

The microstructural evolution with increasing annealing temperature follows classical tempering behavior:

1. 200°C (tempered martensite): At this low temperature, only fine carbide precipitation occurs within the martensite laths. The martensite retains its high dislocation density and the overall structure remains hard. The tempered martensite consists of a supersaturated ferrite matrix with fine, dispersed carbide precipitates. This microstructure provides excellent wear resistance due to the combination of high matrix hardness and hard carbide particles.
2. 400°C (tempered troostite): At this intermediate temperature, the martensite begins to decompose more significantly. The carbides coarsen and the dislocation density decreases. Troostite (a fine lamellar structure of ferrite and cementite) forms, with a finer lamellar spacing than pearlite. The hardness decreases as the structure softens, but some hard carbides remain.
3. 600°C (tempered sorbite): At this higher temperature, the martensite fully decomposes into a coarser lamellar structure. Sorbite (a coarser lamellar structure of ferrite and cementite) forms with larger lamellar spacing. The carbides are significantly coarsened and the dislocation density is substantially reduced. The hardness is lowest but the toughness is highest.

The wear resistance decrease with increasing annealing temperature is directly related to the hardness decrease and carbide coarsening. In abrasive wear, the hardness of the coating relative to the abrasive particles is the primary factor determining wear life. As the coating hardness decreases below the abrasive particle hardness, the wear mechanism transitions from micro-cutting to plowing and material removal, significantly reducing wear life.

Process Analysis and Standards Considerations

The D507MoNb electrode specification follows the Chinese GB/T 5117 standard for carbon steel welding electrodes. The "507" designation indicates a low-hydrogen, rutile-type electrode with a minimum tensile strength of 500 MPa, while "MoNb" indicates the alloying elements. The NM360 steel specification follows GB/T 24186 for wear-resistant steel.

For welding repair of NM360 components, the following considerations are important:

The selection of 200°C annealing as the optimal temperature is based on the specific application requirements. For applications where maximum wear resistance is the priority (such as conveyor rollers, crusher hammers, and grinding surfaces), the 200°C annealing provides the best balance. However, for applications involving impact loading or thermal cycling (such as excavator buckets or shovel teeth), a higher annealing temperature (300-400°C) may be more appropriate to improve toughness at the expense of some wear resistance.

Engineering Practice Integration

In practice, the selection of annealing temperature for surfacing repairs should be guided by a systematic analysis of the service conditions. A decision matrix approach can be applied:

The relative wear resistance of 1.327× the base metal at 200°C annealing is a modest improvement. This suggests that the D507MoNb electrode, while providing some improvement, may not be the optimal choice for severe wear applications. Engineers should consider alternative electrodes or welding consumables with higher carbon and alloy content for more significant wear life improvements.

A practical FMEA for this application would include:

Key Questions and Reflections

One important question is whether the 200°C annealing temperature represents the absolute optimum or whether temperatures slightly below (150°C) or above (250°C) might provide comparable or better performance. The study only investigates three discrete temperatures, and the actual optimum may lie between these points. A more detailed investigation with finer temperature increments would provide more precise optimization data.

Another consideration is the effect of annealing time. The study does not appear to investigate the time-temperature interaction, but from a metallurgical perspective, longer annealing times at a given temperature will promote further carbide coarsening and softening. The practical annealing time should be sufficient to achieve uniform temperature throughout the component but not so long as to cause excessive tempering.

The role of the NM360 base metal in the welding process deserves attention. The high strength and wear resistance of NM360 means that the weld dilution into the coating is significant, and the base metal composition affects the final coating properties. The Nb and other microalloying elements in NM360 can form fine precipitates in the weld and HAZ, potentially contributing to wear resistance but also affecting the heat treatment response.

Study Insights and Implications

This research provides practical guidance for optimizing post-weld heat treatment of surfacing repairs on high-strength wear-resistant steels. The key insight is that the annealing temperature is a critical parameter that must be carefully selected based on the specific service requirements.

For production implementation, the following recommendations are made:

• For maximum wear resistance applications, use 200°C annealing for 1-2 hours.
• For applications requiring balanced properties, use 300°C annealing.
• Always verify the coating hardness after annealing to confirm the achieved properties.
• Consider multi-layer approaches with different electrode types for graded property coatings.
• Implement proper surface preparation (grinding to bare metal, cleaning) to ensure good bonding.

The study also highlights the importance of post-weld heat treatment in surfacing applications. The as-welded structure, while hard, contains high residual stresses that can lead to cracking during service. Even when maximum wear resistance is desired, some form of post-weld heat treatment is recommended to relieve stresses without significantly reducing hardness. The 200°C annealing serves this dual purpose effectively.

Future work should investigate the long-term stability of the coatings under actual service conditions, including thermal cycling, impact loading, and corrosion exposure. The laboratory wear test results provide valuable data but may not fully represent the complex degradation mechanisms encountered in real-world applications. Additionally, the investigation of alternative welding consumables (such as hardfacing electrodes with higher carbon and alloy content) could provide more significant wear life improvements for severe applications.