CLADDING TECHNOLOGY SHANXI CO., LTDResearch Status of Nitrogen Alloyed Wear-Resistant Overlay Welding
Literature Overview
This 2012 paper by Liu Yue, Zhang Guoshang, and Wei Shizhong from Henan University of Science and Technology, published in Welding Technology, provides a comprehensive review of the research status of nitrogen alloyed wear-resistant overlay welding. The work was supported by a Henan Provincial Science and Technology Project (No. 112102213117). Nitrogen alloying is a relatively recent approach to improving the wear resistance of overlay weld deposits, offering a cost-effective alternative to traditional hardfacing alloys based on cobalt, chromium, or tungsten carbide. The review covers the metallurgical mechanisms, process parameters, microstructure, and wear performance of nitrogen alloyed overlay welds.
Core Technical Content and Metallurgical Mechanisms
Nitrogen alloying improves wear resistance through several mechanisms:
- Nitride formation: Nitrogen reacts with alloying elements such as Cr, Mo, V, Nb, and Ti to form hard, fine, and uniformly distributed nitrides (CrN, Mo2N, VN, NbN, TiN) with hardness values of 1500–2500 HV. These nitrides provide excellent resistance to abrasive wear.
- Solid solution strengthening: Interstitial nitrogen in the austenitic or ferritic matrix provides significant solid solution strengthening, increasing the yield strength and hardness of the matrix.
- Microstructure refinement: Nitrogen promotes the formation of a fine, cellular, or dendritic microstructure with a high volume fraction of hard phases, which enhances the wear resistance.
- Work hardening resistance: The nitrogen-strengthened matrix maintains its hardness under severe deformation, providing good resistance to impact and abrasion.
The review summarizes the key research findings on nitrogen alloyed overlay welding:
| Alloy System | Typical Composition (wt%) | Hardness (HV) | Wear Mechanism | Application |
|---|---|---|---|---|
| Fe-Cr-N | Cr 20–30, N 0.5–2.0 | 800–1200 | Abrasive | Mining equipment |
| Fe-Cr-Mo-N | Cr 15–25, Mo 5–10, N 0.5–1.5 | 900–1400 | Abrasive + impact | Crusher components |
| Fe-Cr-V-N | Cr 20–30, V 3–8, N 0.5–1.5 | 1000–1500 | Abrasive | Slurry pumps |
| Co-Cr-N | Co 50–60, Cr 20–30, N 0.5–2.0 | 1200–1800 | Abrasive + corrosive | Chemical equipment |
| Fe-Ni-Cr-N | Ni 20–40, Cr 15–25, N 0.5–1.5 | 900–1300 | Abrasive + impact | Power plant components |
The nitrogen content is a critical parameter that must be carefully controlled. Too low a nitrogen content results in insufficient nitride formation and reduced hardness, while too high a nitrogen content can cause porosity, excessive brittleness, and hot cracking. The optimal nitrogen content is typically in the range of 0.5–2.0 wt% for iron-based alloys and 0.5–1.5 wt% for cobalt-based alloys.
Process Parameters and Welding Methods
Nitrogen alloyed overlay welding can be performed using several welding methods, each with distinct advantages and challenges:
- Submerged arc welding (SAW): The most common method for nitrogen alloyed overlay welding. The flux acts as a nitrogen carrier, releasing nitrogen during the welding process. The nitrogen pickup is controlled by the flux composition, the welding current, and the travel speed. Typical nitrogen pickup is 0.3–1.0 wt%.
- Gas metal arc welding (GMAW): Nitrogen is introduced through a nitrogen-containing flux or through a mixed shielding gas containing nitrogen. The nitrogen pickup is lower and less controllable than in SAW.
- Plasma transferred arc welding (PTA): Nitrogen powder can be added to the feedstock or introduced through a side torch. The nitrogen pickup is highly controllable and can be varied during the welding process.
- Laser cladding: Nitrogen-containing powder can be fed into the laser melt pool. The rapid cooling rate and high energy density produce a fine microstructure with high hardness, but the nitrogen pickup is limited by the powder composition.
The key process parameters for nitrogen alloyed SAW overlay welding include:
| Parameter | Typical Range | Effect |
|---|---|---|
| Welding current | 400–700 A | Higher current increases nitrogen pickup |
| Travel speed | 200–400 mm/min | Slower speed increases nitrogen pickup |
| Flux composition | CaF2, SiO2, Al2O3, MnO, Fe | Flux composition determines N2 release rate |
| Wire composition | Fe-Cr-Mo or Fe-Cr-V base | Base composition affects nitride type |
| Preheat temperature | 100–200 °C | Reduces cooling rate and improves nitrogen solubility |
Engineering Practice Integration
The application of nitrogen alloyed overlay welding in engineering practice requires careful consideration of several factors:
- Wear condition matching: The selection of the alloy system and nitrogen content must be matched to the specific wear condition. For dry sliding abrasion, high nitrogen content with Cr-V nitrides is preferred. For wet or corrosive abrasion, a Co-Cr-N system with moderate nitrogen content is more appropriate.
- Toughness requirements: Nitrogen alloyed welds tend to be brittle, and the impact toughness can be very low (below 5 J). For applications requiring impact resistance, a multi-pass approach with a tough transition layer between the base metal and the hard overlay layer is recommended.
- Welding procedure development: The welding procedure must be developed to control the nitrogen pickup and minimize porosity. The flux composition and welding parameters must be optimized to achieve the desired nitrogen content without excessive gas porosity.
- Quality control: The nitrogen content in the weld metal should be measured by combustion analysis or inert gas fusion analysis. The hardness and microstructure should be verified by metallographic examination. The wear performance should be evaluated by standardized wear tests (e.g., ASTM G99, ASTM G65).
- Repair and maintenance: Nitrogen alloyed overlay layers can be ground and re-welded, but the repair weld must be matched to the original composition and nitrogen content. The repair area should be inspected by MT or PT for cracks and porosity.
Key Questions and Reflections
The nitrogen alloying approach offers a cost-effective solution to the wear resistance challenge, but several open questions remain. The long-term wear performance of nitrogen alloyed overlay welds under severe industrial conditions is not well documented, and more field trials are needed to validate the laboratory findings. The interaction between nitrogen and other alloying elements during the welding process is complex, and the nitrogen pickup is sensitive to process variations, which can lead to inconsistency in the weld properties.
Another important consideration is the effect of nitrogen on the weldability of the overlay layer. High nitrogen content increases the hot crack susceptibility and can cause porosity, which must be carefully controlled through process optimization. The brittleness of nitrogen alloyed welds is also a concern, and the impact toughness must be evaluated for applications involving impact loading.
Study Insights and Implications
This review highlights the significant potential of nitrogen alloying as a strategy for improving the wear resistance of overlay weld deposits. The approach offers several advantages over traditional hardfacing alloys: lower cost, better availability of raw materials, and the ability to tailor the wear properties through nitrogen content adjustment. For engineering practice, the key insight is that nitrogen alloyed overlay welding requires a systematic approach to process development, quality control, and wear performance evaluation. The selection of the alloy system, nitrogen content, and welding method must be matched to the specific wear condition and service requirements. The approach is particularly promising for mining, cement, and power generation industries where abrasive wear is a major concern and cost-effective solutions are needed.