Zhuojin Pipe Fitting Co., LtdEffect of In-situ Generated Ceramic Hard Phases on Microstructure and Properties of Iron-based Surfacing Layer
Literature Overview
This paper by Liu Ke and Zhao Dongning, published in Hot Working Technology (2009, Vol. 38, No. 15, pp. 93-94), investigates the microstructure and wear properties of an iron-based surfacing layer produced by plasma arc surfacing with in-situ generation of ceramic hard phases. The approach uses a composite powder coating containing titanium iron, boron iron, silicon iron, and high-carbon chromium iron, applied to a low-carbon steel substrate, with the plasma arc process serving as the heat source for both melting and in-situ ceramic phase formation.
Technical Principles
The in-situ self-generation method (原位自生法) is a cost-effective alternative to directly adding pre-formed ceramic particles to the surfacing powder. Instead of incorporating expensive ceramic powders such as CrB₂, TiC, or SiC directly into the surfacing mixture, the authors utilize elemental precursors (Fe-Ti, Fe-B, Fe-Si, Fe-Cr) that react during the plasma arc melting process to form ceramic hard phases in situ.
Reaction Mechanisms
The key in-situ reactions occurring during plasma arc surfacing include:
| Reaction | Products | Hardness Contribution |
|---|---|---|
| Cr + 2B → Cr₂B | Chromium diboride | ~1800 HV |
| Ti + C → TiC | Titanium carbide | ~2400 HV |
| 5Si + 3C → Si₅C₃ | Silicon carbide | ~2000 HV |
| 7Cr + 3C → Cr₇C₃ | Chromium carbide | ~1600 HV |
| 4B + C → B₄C | Boron carbide | ~2500 HV |
The plasma arc process provides the necessary thermal energy (arc temperatures of 10000-30000 K) to drive these high-temperature reactions to completion within the short residence time of the molten pool (typically 0.1-0.5 seconds).
Process Parameters
| Parameter | Value | Effect |
|---|---|---|
| Arc current | 180-250 A | Controls heat input and pool size |
| Arc voltage | 25-35 V | Determines penetration depth |
| Travel speed | 10-20 cm/min | Affects cooling rate and phase formation |
| Powder feed rate | 150-300 g/min | Controls dilution and alloy composition |
| Powder layer thickness | 1.5-3.0 mm | Ensures complete coverage before melting |
| Arc-to-powder distance | 3-5 mm | Optimizes powder melting efficiency |
| Shielding gas | Ar or Ar/He mixture | Prevents oxidation of reactive elements |
Microstructural Analysis
The resulting surfacing layer exhibited a hardness exceeding 58 HRC, which is significantly higher than the base low-carbon steel (typically 120-180 HV). The microstructure consists of:
- Matrix phase: A martensitic or austenitic iron matrix, depending on the cooling rate and alloy composition.
- Ceramic hard phases: Dispersed particles of Cr₂B, TiC, Si₅C₃, Cr₇C₃, and B₄C, typically 2-10 μm in size.
- Carbide network: Chromium carbides (Cr₇C₃, Cr₂₃C₆) forming a continuous or semi-continuous network in the interdendritic regions.
Comparison with Direct Ceramic Addition
| Characteristic | In-situ Method | Direct Addition |
|---|---|---|
| Material cost | Low (elemental precursors) | High (pre-formed ceramics) |
| Particle distribution | Uniform (in-situ nucleation) | Potentially uneven (agglomeration) |
| Bond strength at interface | Excellent (co-formed with matrix) | May have weak interfaces |
| Process flexibility | High (composition tuning via precursor ratios) | Limited by available ceramic types |
| Hardness achieved | > 58 HRC | Comparable or higher |
| Wear resistance | Excellent | Excellent to superior |
Wear Performance and Engineering Applications
The wear resistance of the surfacing layer is attributed to the composite effect of hard ceramic phases dispersed in a tough metallic matrix. The in-situ formed ceramics benefit from:
- Fine dispersion: Nucleation during solidification produces uniformly distributed particles.
- Good interfacial bonding: Co-formed phases have coherent or semi-coherent interfaces with the matrix.
- Multiple hard phase types: The combination of different ceramic types provides synergistic wear resistance mechanisms.
Typical applications for this type of surfacing include:
- Mining equipment (shovel buckets, conveyor rollers)
- Cement industry (grinding balls, liner plates)
- Power generation (turbine blades, impeller vanes)
- Agricultural machinery (plowshares, harrow teeth)
- Industrial pumps (impellers, wear rings)
Key Insights and Study Reflections
The in-situ ceramic phase generation method represents a significant cost reduction strategy in hardfacing technology. Traditional approaches require the purchase of pre-formed ceramic powders (CrB₂, TiC, SiC), which can cost 5-20 times more than the elemental precursor alloys. The in-situ method achieves comparable or superior wear performance at a fraction of the material cost, making it economically attractive for large-scale industrial applications.
The critical process variable is the powder composition ratio, which determines the type and volume fraction of ceramic phases formed. For example, increasing the boron content favors Cr₂B and B₄C formation, while increasing titanium content promotes TiC formation. The optimal composition depends on the specific wear mechanism encountered in service: abrasion (requiring high hardness), adhesion (requiring low friction coefficient), or erosion (requiring toughness).
The plasma arc process is well-suited to this application because it provides concentrated heat input with minimal dilution of the base material (typically 10-20% dilution), ensuring that the surfacing layer composition remains close to the intended powder composition. This is in contrast to processes such as submerged arc surfacing, which may achieve dilution rates of 30-50% and thus require higher alloy content in the consumable to achieve the target deposit composition.
The technique also demonstrates the principle of process-material synergy: the same elemental precursors can produce different ceramic phase combinations depending on the thermal cycle characteristics of the welding process. This process sensitivity provides additional degrees of freedom for material design but also introduces process control challenges that must be addressed through rigorous process parameter optimization and monitoring.