Zhuojin Pipe Fitting Co., LtdEffect of Niobium Content on Plasma Surfacing of Nickel-Based Alloys
Literature Overview
Published in Hot Working Technology in 2019, this study by Geng Yanchao and colleagues from Dalian University of Technology and Shenyang Blower Works Group investigates the effect of niobium powder addition on the microstructure and properties of nickel-based alloy overlays deposited by plasma surfacing onto 304L stainless steel. Funded by the National 973 Program (Project 2011CB013402) and the National Natural Science Foundation of China (Project 11072045), this research addresses a critical issue in plasma surfacing: the optimization of alloy powder composition to achieve desired mechanical properties. The study examined five different Nb powder ratios and analyzed the resulting microstructure, elemental distribution, and microhardness.
Core Technical Points
Niobium as an Alloying Element in Nickel-Based Systems
Niobium is a powerful alloying element in nickel-based superalloys and coatings. It serves multiple functions:
- Carbide formation: Nb forms NbC carbides with a melting point of 3520°C, which are extremely hard (2500-3000 HV) and thermally stable.
- Gamma prime precipitation: In Ni-Base superalloys, Nb promotes the formation of Ni3(Nb,Al) gamma prime precipitates that provide high-temperature strength.
- Grain refinement: Nb acts as a grain refiner by forming fine NbC particles that pin grain boundaries during solidification.
- Corrosion resistance: Nb improves resistance to pitting and crevice corrosion in chloride environments.
In the context of plasma surfacing, the addition of Nb powder to the nickel-based alloy powder blend creates a composite coating with NbC particles dispersed in a nickel-based matrix. The challenge is to optimize the Nb content to maximize hardness and wear resistance without compromising the coating's adhesion, ductility, or corrosion resistance.
Microstructural Findings
The study examined five different Nb powder ratios and found that the overlay microstructure consists of:
- γ-Ni dendrites: The primary solidification phase, forming a dendritic network that provides the matrix for the coating.
- Interdendritic eutectic structure: Formed at the dendrite tips and boundaries during the final stages of solidification.
- Chromium borides: Formed from the interaction of Cr and B elements present in the nickel-based alloy powder.
- Dispersed NbC particles: Fine carbide particles distributed throughout the matrix, providing particle strengthening.
The NbC particles are the key reinforcement phase. They form in situ during the plasma melting process when Nb reacts with carbon present in the powder blend. The size and distribution of these particles are critical for the coating's mechanical properties.
Hardness Enhancement
| Nb Content (wt%) | Relative Hardness Increase | Primary Mechanism |
|---|---|---|
| 0 (baseline) | 0% | γ-Ni matrix only |
| 5 | ~40% | Optimal NbC dispersion |
| Higher | Diminishing returns | Possible NbC coarsening or agglomeration |
The study found that 5 wt% Nb powder addition produced the highest hardness, with approximately 40% improvement over the pure nickel-based alloy overlay. This is a significant enhancement that is directly attributable to the formation of fine, uniformly distributed NbC particles.
The 40% hardness increase at 5 wt% Nb is consistent with the rule of mixtures for particle-reinforced composites. For a coating with approximately 10-15 vol% NbC (assuming complete reaction of Nb with carbon), the expected hardness increase based on the rule of mixtures would be approximately 25-35%, which is close to the observed 40%. The slightly higher observed value suggests that there may be additional strengthening mechanisms, such as grain refinement or solid solution strengthening from dissolved Nb in the γ-Ni matrix.
Process Analysis: Plasma Surfacing
Plasma Surfacing Process Characteristics
Plasma surfacing (also known as plasma transferred arc surfacing or PTAS) uses a high-velocity plasma arc to melt both the base material surface and a powder stream simultaneously. The molten pool solidifies rapidly, producing a refined microstructure with minimal dilution from the base metal.
| Parameter | Typical Range | Effect |
|---|---|---|
| Plasma current | 150-400 A | Controls melt pool size and heat input |
| Arc voltage | 15-25 V | Controls arc length and stability |
| Powder feed rate | 50-200 g/min | Controls deposition rate and dilution |
| Travel speed | 50-300 mm/min | Controls cooling rate and bead geometry |
| Shielding gas | Ar or Ar/He mix | Controls arc characteristics |
The rapid cooling rates achievable in plasma surfacing (100-1000 K/s) are beneficial for producing fine microstructures and suppressing the formation of coarse intermetallic phases. This is particularly important for Nb-containing coatings, where rapid cooling helps to maintain fine NbC particles rather than allowing them to coarsen during prolonged solidification.
Niobium Content Optimization
The optimization of Nb content involves balancing several competing factors:
- Hardness: Increases with Nb content up to an optimum, then may decrease due to NbC agglomeration.
- Ductility: Decreases with increasing NbC content as the brittle carbide phase reduces the coating's ability to deform plastically.
- Adhesion: Excessive NbC can increase residual stress and reduce coating adhesion to the substrate.
- Corrosion resistance: Nb generally improves corrosion resistance, but excessive NbC can create galvanic couples with the matrix.
The 5 wt% optimum identified in this study represents a balance between these competing factors. At lower Nb contents, there are insufficient NbC particles to provide significant strengthening. At higher Nb contents, the carbide particles may begin to agglomerate, creating stress concentrations and reducing the coating's overall performance.
Phase Equilibrium Considerations
The formation of NbC in the coating depends on the thermodynamic stability of the phase relative to other possible Nb-containing phases. In the Ni-Nb-C system, the possible phases include:
- NbC (hexagonal, very stable)
- Nb2C (hexagonal, less stable)
- Ni3Nb (intermetallic)
- NiNb (intermetallic)
The plasma surfacing process, with its rapid heating and cooling, often produces non-equilibrium phases. The study's observation of NbC particles suggests that the process conditions favored the formation of this thermodynamically stable carbide. The presence of chromium borides indicates that the powder blend contained boron, which reacted with chromium to form CrB or CrB2 phases. These borides can also contribute to hardness but are generally less stable than NbC at high temperatures.
Engineering Practice Integration
Application to 304L Stainless Steel
The selection of 304L stainless steel as the substrate is significant. 304L is the most widely used austenitic stainless steel in industrial applications, and the addition of a hardfacing overlay can dramatically extend its service life in abrasive or erosive environments. The 40% hardness increase achieved with 5 wt% Nb addition transforms a standard 304L surface (approximately 150-200 HV) into a surface with hardness exceeding 250-300 HV, which is comparable to many dedicated hardfacing alloys.
Comparison with Conventional Hardfacing
| Property | 304L Base |
|---|