CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Normalizing Temperature on Microstructure and Properties of NM360 Wear-Resistant Steel Overlay Layer
Literature Overview and Research Background
NM360 is a widely used wear-resistant steel grade characterized by high hardness (typically 360 HV or above), excellent impact toughness, and good weldability, making it a popular choice for overlay applications in mining, construction, and material handling equipment. The post-weld normalizing treatment is a critical heat treatment step that significantly influences the final microstructure and mechanical properties of the overlay layer. This study systematically examines how varying normalizing temperatures affect the microstructure, hardness, toughness, and wear resistance of the NM360 overlay layer, providing valuable guidance for optimizing post-weld heat treatment (PWHT) protocols in engineering practice.
Core Technical Points and Microstructural Evolution
The NM360 overlay layer is typically deposited using submerged arc welding (SAW) or flux-cored arc welding (FCAW) with a matching or slightly higher-carbon consumable. The as-welded microstructure generally consists of a mixture of martensite, bainite, and retained austenite, with hardness values often exceeding 400 HV in the weld zone due to the rapid cooling rates associated with welding.
The study investigates normalizing temperatures ranging from 750 °C to 920 °C, with holding times of 30 to 90 minutes followed by air cooling. The key findings are summarized below:
| Normalizing Temperature (°C) | Microstructure | Hardness (HV) | Impact Energy (J) | Wear Resistance Index |
|---|---|---|---|---|
| As-welded (no PWHT) | Martensite + retained austenite | 420-460 | 15-25 | Baseline |
| 750 | Fine bainite + residual martensite | 380-410 | 25-35 | Moderate improvement |
| 800 | Coarse bainite + some pearlite | 340-370 | 35-50 | Good |
| 850 | Pearlite + ferrite (optimal) | 320-350 | 50-70 | Best balance |
| 900 | Coarse pearlite + ferrite | 300-330 | 60-80 | Reduced hardness |
| 920 | Coarse pearlite + spheroidized cementite | 280-310 | 65-85 | Poor wear resistance |
The optimal normalizing temperature of 850 °C produces a fine pearlite and ferrite microstructure with a hardness of approximately 330 HV and impact energy of 60 J, representing the best balance between wear resistance and toughness. At lower temperatures (750 to 800 °C), residual martensite and bainite phases remain, providing higher hardness but at the cost of reduced toughness. At temperatures above 900 °C, excessive grain growth and cementite spheroidization reduce the hardness below the NM360 specification requirement, compromising wear resistance.
Process Analysis and Heat Treatment Optimization
The normalizing temperature selection must be considered in the context of the entire overlay fabrication sequence. For thick multi-pass overlay builds, the thermal history experienced by lower passes differs significantly from upper passes, leading to microstructural non-uniformity through the overlay thickness. The normalizing treatment serves to homogenize this microstructure, but the effectiveness depends on achieving adequate austenitization throughout the section thickness.
Several practical considerations arise from this study:
- The austenitization temperature must be sufficient to dissolve the hard carbides and retained austenite present in the as-welded microstructure, but not so high as to cause excessive grain growth.
- The cooling rate after normalizing significantly affects the final microstructure. Air cooling (the standard normalizing practice) produces a fine pearlite structure, while furnace cooling (annealing) would produce a coarser pearlite with lower hardness.
- For overlay layers on thick base plates, the thermal mass of the base material can slow the heating rate, potentially requiring longer holding times or higher temperatures to ensure complete austenitization at the weld root.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Incomplete austenitization | Normalizing temperature too low or holding time too short | Increase temperature by 20-30 °C or extend hold time |
| Grain coarsening | Normalizing temperature too high | Reduce temperature to 830-860 °C range |
| Hardness below specification | Excessive cementite spheroidization at high temperature | Reduce normalizing temperature; verify furnace calibration |
| Cracking during PWHT | Excessive residual stress from welding combined with thermal shock | Implement preheating before PWHT; use controlled heating rates (max 150 °C/h for thick sections) |
| Uneven hardness through thickness | Insufficient heat penetration in thick sections | Increase holding time; consider induction heating for localized treatment |
Integration with Engineering Practice
In engineering practice, the NM360 overlay layer is commonly applied to equipment such as excavator bucket teeth, conveyor chutes, crusher hammers, and slurry pump impellers. The selection of normalizing temperature must be made in conjunction with the service requirements of the specific application. For applications where maximum wear resistance is prioritized over impact resistance (such as slurry pump impellers operating in non-abrasive but erosive service), a slightly lower normalizing temperature (800 to 820 °C) that retains some bainitic character may be appropriate. For applications subject to impact loading (such as crusher hammers), the optimal 850 °C normalizing temperature provides the best combination of hardness and toughness.
Study Insights and Reflections
The most valuable contribution of this study is the establishment of a clear quantitative relationship between normalizing temperature and the resulting microstructure-property combination in NM360 overlay layers. The identification of 850 °C as the optimal normalizing temperature provides a practical target for fabrication specification and quality control. Furthermore, the study reinforces the principle that post-weld heat treatment is not merely a stress-relief step but a critical microstructure-modifying process that can significantly enhance the service performance of the overlay. For quality assurance purposes, the hardness profile through the overlay thickness should be verified after normalizing, with a minimum of three test points per 100 mm of overlay width to ensure uniformity of treatment.