Effect of Normalizing Temperature on Microstructure and Properties of NM360 Wear-Resistant Steel Overlay Layers

Literature Overview

The study by Deng Hanzhong and Meng Xiangfeng (2015), published in the journal "Mechanical Engineering Materials," investigates how normalizing temperature affects the microstructure and mechanical properties of overlay layers deposited on NM360 wear-resistant steel substrates. NM360 is a martensitic wear-resistant steel widely used in mining, construction, and material handling applications where severe abrasion is encountered. The overlay process, typically performed using submerged arc welding (SAW) or flux-cored arc welding (FCAW), introduces additional hard phases into the surface layer, but the resulting microstructure is highly sensitive to post-weld heat treatment conditions. Understanding the relationship between normalizing temperature and overlay performance is critical for optimizing service life in demanding industrial environments.

Core Technical Findings

Normalizing Temperature Range and Microstructural Evolution

The researchers examined normalizing temperatures spanning approximately 780°C to 920°C, which covers the austenitization range for martensitic high-carbon steels. At lower normalizing temperatures (around 780–820°C), the overlay layer retains a predominantly retained austenite and fine martensite structure with limited carbide transformation. As the normalizing temperature increases toward 860–900°C, the microstructure transitions toward a tempered martensite with increased carbide precipitation, primarily M7C3 and M23C6 type carbides. At the upper end of the range (920°C and above), excessive grain coarsening and potential decarburization of the overlay surface become concerns.

Mechanical Property Response

The key finding is that an optimal normalizing window exists around 840–860°C, where the overlay achieves the best balance between hardness and toughness. Below this range, insufficient carbide dissolution and re-precipitation results in a relatively soft matrix with limited work-hardening capacity. Above this range, while initial hardness may remain acceptable, the toughness degrades significantly due to grain coarsening and the formation of continuous carbide networks along grain boundaries, which become crack initiation sites under impact loading.

Dilution Effects Considered

The study acknowledges that NM360 substrate dilution into the overlay layer plays a role. NM360 typically contains 0.5–0.7% C, 1.5–2.5% Cr, and 0.5–1.0% Mo. The dilution from the base metal can modify the overlay chemistry, shifting the solidification sequence and affecting the final microstructure. The authors note that proper control of heat input during the overlay process is essential to minimize dilution to an acceptable level, generally below 30% for the first pass and below 15% for subsequent passes.

Engineering Practice Implications

Process Control Recommendations

Based on the literature findings, the following engineering practices should be adopted when overlaying NM360 substrates:

1. Pre-heat control: Maintain substrate pre-heat at 150–250°C to prevent cold cracking in the weld interface, especially given the high carbon equivalent of NM360 (typically Ceq > 0.5%).
2. Heat input management: Limit linear energy input to 8–14 kJ/mm during SAW overlay to control dilution and avoid excessive softening of the base metal.
3. Post-weld normalizing: Apply normalizing at 840–860°C for 30–60 minutes per 25 mm thickness, followed by furnace cooling or air cooling depending on the required toughness level.
4. Quench and temper alternative: For applications requiring higher toughness, a water quench followed by tempering at 500–550°C may achieve comparable or superior wear resistance with better impact properties.

Defect Analysis and Countermeasures

Key Reflections and Study Insights

The most significant insight from this study is that normalizing temperature is not merely a post-processing parameter but fundamentally determines the wear resistance mechanism of the overlay layer. In the optimal temperature window, the microstructure achieves a synergistic combination of hard carbide particles dispersed in a tough matrix, providing both abrasion resistance and resistance to impact-induced spalling. This has direct implications for equipment designers who must select normalizing parameters based on the specific loading conditions—whether predominantly sliding abrasion (favoring higher hardness) or impact-abrasion combined (favoring toughness).

Another important observation is the interaction between normalizing temperature and residual stress. Normalizing at 840–860°C partially relieves welding residual stresses, which is beneficial for fatigue life but may introduce dimensional changes in precision applications. Engineers should account for potential distortion of up to 0.3% in the normalizing direction when planning machining allowances.

The study also highlights a gap that warrants further investigation: the effect of cooling rate after normalizing on the final microstructure. Furnace cooling versus air cooling versus water quenching after normalizing can produce significantly different microstructures, and the literature does not fully address this variable. Future work should systematically evaluate the complete thermal cycle rather than isolating the normalizing temperature alone.

Reference Value and Outlook

This research provides a practical foundation for process engineers working on NM360 overlay applications. The identified optimal normalizing window of 840–860°C can be directly applied to production settings, provided that the specific chemistry of the overlay consumable and the dilution level are confirmed through metallographic analysis. For high-value components such as mining equipment, conveyor rollers, and crusher hammers, implementing this normalizing protocol can extend service life by 30–50% compared to as-welded conditions, representing significant economic benefit. The work also demonstrates the importance of integrating metallurgical understanding with process engineering to achieve reliable, repeatable overlay performance in industrial applications.