CLADDING TECHNOLOGY SHANXI CO., LTDHard Phase Characterization of NDG-2 Nickel-Based Wear-Resistant Cladding Alloy
Literature Overview
This study, published in 2004 by researchers at the Harbin Institute of Technology, focuses on the microstructural evolution and hard phase formation in the NDG-2 nickel-based wear-resistant cladding alloy system. The work was conducted at the Analysis and Testing Center of HIT and published in the journal Functional Materials. The NDG-2 alloy system belongs to the broader family of Ni-Cr-Mo-B-Si type hardfacing alloys, which are widely employed in high-temperature wear environments such as furnace components, petrochemical catalyst support rings, and industrial kiln linings. The research addresses a critical gap in understanding the relationship between heat treatment parameters, microstructure, and wear resistance in this specific alloy grade.
Core Technical Analysis
The NDG-2 alloy is a nickel-based hardfacing material typically deposited using submerged arc welding (SAW) or gas metal arc welding (GMAW) processes. The alloy contains approximately 40-50% nickel by weight, with significant additions of chromium (18-25%), molybdenum (10-15%), boron (2-4%), and silicon (3-5%). These alloying elements are strategically chosen to promote the formation of metastable hard phases during cooling or subsequent heat treatment.
The primary hard phases identified in the as-deposited condition include:
- M7C3 type carbides (Cr7C3, Mo2C) — providing primary wear resistance
- M23C6 type carbides — forming at grain boundaries and contributing to hardness
- B2-type intermetallic compounds (NiAl, NiSi) — reinforcing the matrix
- Sigma phase (Cr-rich) — which, if excessive, can embrittle the structure
The study employed metallographic examination, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and microhardness mapping to characterize these phases systematically. The researchers found that the cooling rate during welding deposition significantly influences the type and distribution of hard phases formed.
| Parameter | As-Deposited | After 800°C/2h Annealing | After 1000°C/1h Aging |
|---|---|---|---|
| Microhardness (HV0.3) | 650-720 | 580-650 | 700-780 |
| Dominant Carbide | M7C3 | M7C3 + M23C6 | Coarse M7C3 |
| Matrix Structure | Austenite + Ferrite | Recrystallized Austenite | Coarse Grained |
| Wear Index (Abrasion) | 1.8-2.1 | 1.5-1.8 | 2.2-2.5 |
Key Findings on Phase Transformation
The researchers demonstrated that a two-step heat treatment — first annealing at 800°C to relieve residual stresses and promote carbide homogenization, followed by aging at 1000°C for controlled precipitation — yields the optimal combination of hardness and toughness. The intermediate annealing step prevents the formation of brittle sigma phase that would otherwise develop during prolonged high-temperature exposure.
A critical insight from this work is that the boron content acts as a potent carbide former, preferentially forming NbB and TiB phases when niobium or titanium is present. These borides, while extremely hard (HV > 2500), are inherently brittle and should constitute less than 5 vol% of the total microstructure to avoid catastrophic brittle fracture during service.
Engineering Practice Implications
For engineers specifying NDG-2 cladding on industrial components, several practical considerations emerge from this research:
- Heat input control: Excessive welding heat input (>35 kJ/mm) leads to coarse grain growth and sigma phase formation, reducing toughness by 30-40%. Multi-pass deposition with controlled interpass temperature (150-250°C) is recommended.
- Dilution management: The base metal dilution should be maintained below 25% to preserve the designed alloy chemistry. Preheating to 200°C and using a backing strip can help control this parameter.
- Post-weld heat treatment: The optimal PWHT window is 750-850°C for stress relief, followed by optional aging at 950-1050°C for 1-2 hours, depending on the required hardness-toughness balance.
- Inspection criteria: Hardness verification should be performed at multiple depths (0.5mm, 1.0mm, 1.5mm from surface) to ensure adequate bond strength and uniform hard phase distribution throughout the cladding layer.
Study Insights and Reflections
This 2004 study remains highly relevant to contemporary cladding practice because the fundamental metallurgical principles governing hard phase formation in nickel-based alloys have not changed. What has evolved is our ability to predict and control these transformations through computational thermodynamics (CALPHAD) and advanced characterization techniques such as atom probe tomography. The practical wisdom embedded in this research — particularly regarding the delicate balance between boride content and toughness — continues to guide specification development for high-temperature wear-resistant cladding applications in power generation and petrochemical industries.
The study's emphasis on the synergistic effect of Mo and Cr in forming stable M7C3 carbides validates the design philosophy behind modern wear-resistant nickel alloys used in hydrogenation reactors and hot-gas filtration systems. Engineers working on bimetallic pressure vessels with nickel-based overlay layers should reference this work when establishing qualification procedures under NB/T 47014 or ASME IX requirements.