CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Normalizing Temperature on Microstructure and Properties of NM360 Wear-Resistant Steel Weld Overlay Layer
Literature Overview
The paper authored by Deng Hanzhong and Meng Xiangfeng, published in 2015 in the journal "Mechanical Engineering Materials," investigates the influence of normalizing (austenitizing and air-cooling) temperature on the microstructure evolution and mechanical properties of weld overlay layers deposited on NM360 wear-resistant steel. NM360 is a high-carbon, high-chromium martensitic wear-resistant steel widely used in mining, cement, and material handling applications where severe abrasive wear is the dominant failure mechanism. The study addresses a critical engineering question: how post-weld heat treatment parameters, specifically normalizing temperature, affect the hardness profile, microstructure composition, and overall wear resistance of the overlay layer, which is essential for optimizing service life in high-wear environments.
Core Technical Content and Key Findings
The research systematically examined the effect of varying normalizing temperatures on the overlay microstructure and hardness distribution. The overlay layers were deposited using conventional arc welding processes with appropriate filler materials, and subsequent normalizing was carried out at different temperature levels to evaluate the resulting transformations.
Microstructural Evolution with Normalizing Temperature
| Normalizing Temperature (°C) | Dominant Phase Composition | Hardness (HRC) | Microstructural Features |
|---|---|---|---|
| 800 | Retained austenite + tempered martensite | 48–52 | Coarse carbides, incomplete transformation |
| 850 | Mixed martensite + retained austenite | 55–58 | Uniform carbide distribution, moderate grain size |
| 900 | Fine martensite + dispersed carbides | 58–62 | Optimal carbide refinement, minimal retained austenite |
| 950 | Coarse martensite + coarse carbides | 52–56 | Over-tempered tendency, grain coarsening begins |
| 1000 | Coarse martensite + agglomerated carbides | 46–50 | Significant grain growth, carbide coarsening |
The study demonstrates that there exists an optimal normalizing temperature window, typically in the range of 850–900 °C, where the overlay microstructure achieves the best combination of hardness, toughness, and wear resistance. Below this range, incomplete austenitization results in retained austenite and coarse primary carbides that reduce overall hardness. Above this range, excessive grain growth and carbide coarsening lead to reduced hardness and increased brittleness.
Hardness Gradient Analysis
A critical finding is the establishment of a controlled hardness gradient from the overlay layer through the fusion zone into the base metal. The normalizing process creates a transition zone where hardness gradually decreases from the overlay surface toward the base metal. This gradient is beneficial because it reduces residual stresses at the interface and minimizes the risk of cracking during service. The optimal normalizing temperature produces a hardness profile where the overlay surface hardness exceeds 58 HRC while the base metal remains in the 35–40 HRC range, providing adequate toughness in the substrate.
Engineering Practice Integration
Application in Mining Equipment
NM360 steel is extensively used in bucket wheel excavators, conveyor rollers, and crusher components. The weld overlay layer is typically applied to restore worn surfaces or to pre-protect critical areas. The normalizing temperature selection must consider the following engineering constraints:
- Thermal distortion control: Large components with thick overlay layers require careful temperature selection to minimize distortion. Temperatures above 950 °C can cause unacceptable warping in thin-walled components.
- Residual stress management: The normalizing process serves a dual purpose of microstructure refinement and residual stress relief. The optimal temperature window ensures both objectives are met simultaneously.
- Service life optimization: Based on field data from mining operations, overlay layers normalized at 870–900 °C exhibit 30–50% extended service life compared to as-welded conditions or improperly normalized layers.
Process Recommendations
For industrial application, the following process parameters are recommended based on the study findings:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Normalizing temperature | 870–900 °C | Optimal carbide refinement and hardness |
| Holding time | 2–4 hours | Complete transformation without grain growth |
| Cooling method | Controlled air cooling | Avoids quench cracking while achieving martensitic transformation |
| Post-normalizing tempering | 200–250 °C, 2 hours | Relieve residual stresses, improve toughness |
| Maximum overlay thickness | 3–5 mm per pass | Control dilution and thermal input |
Key Questions and Reflections
Several important technical questions emerge from this study that warrant further investigation. First, the interaction between normalizing temperature and the carbon/chromium content of the specific filler material used is not fully explored. Different filler compositions may shift the optimal normalizing temperature window. Second, the effect of multi-pass overlay welding on the normalizing response is complex, as each subsequent pass acts as a localized heat treatment on the previous pass. This creates a heterogeneous thermal history that may affect the final microstructure differently at various depths within the overlay layer.
From a practical standpoint, the study highlights the importance of process control in weld overlay applications. Many industrial failures attributed to premature wear are actually caused by improper post-weld heat treatment. The normalizing step is often neglected in field repairs, where convenience takes precedence over metallurgical optimization. This represents a significant opportunity for improving equipment reliability through better process discipline.
Study Insights and Implications
The research by Deng and Meng provides valuable guidance for optimizing the post-weld heat treatment of wear-resistant overlay layers on NM360 steel. The key insight is that normalizing temperature is not merely a stress-relief parameter but a critical microstructure-controlling variable that directly determines the service performance of the overlay. The optimal window of 870–900 °C represents a balance between carbide refinement and grain growth control.
For engineers involved in the repair and maintenance of mining and material handling equipment, this study underscores the importance of integrating metallurgical knowledge with practical welding operations. The systematic approach to determining optimal normalizing parameters, combined with the understanding of the resulting microstructure-property relationships, provides a foundation for developing standardized repair procedures. Future work should address the scalability of these findings to large industrial components and the influence of welding sequence on the final overlay properties. The practical value of this research lies in its direct applicability to extending the service life of critical wear components, thereby reducing maintenance costs and improving operational availability in demanding industrial environments.