CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay of Cobalt-Based Hardfacing Alloys
Literature Overview
Cobalt-based hardfacing alloys, commonly known as Stellite alloys, are widely employed in demanding applications requiring exceptional resistance to wear, corrosion, and high-temperature oxidation. This study investigated the weld overlay of cobalt-based hardfacing alloys (Stellite 6, Stellite 21, and Stellite 25) on various base metals including carbon steel, austenitic stainless steel, and nickel-based superalloys. The research covered multiple overlay processes including oxy-fuel welding, gas tungsten arc welding (GTAW), plasma transferred arc (PTA) welding, and laser cladding, with emphasis on the microstructure, hardness, and wear resistance of the overlay layers.
Material Properties and Microstructure
Cobalt-based hardfacing alloys are characterized by a high cobalt content (typically 55–65 wt%), with alloying additions of chromium, tungsten, molybdenum, and carbide-forming elements such as carbon and chromium. The following table presents the key properties of the three Stellite alloys studied:
| Property | Stellite 6 | Stellite 21 | Stellite 25 |
|---|---|---|---|
| Co Content (wt%) | 58–65 | 58–65 | 58–65 |
| Cr Content (wt%) | 24–30 | 24–30 | 24–30 |
| W Content (wt%) | 11–13 | 4.0–5.5 | — |
| C Content (wt%) | 0.35–0.45 | 1.80–2.20 | — |
| Hardness (HV) | 200–230 (as-cast) | 350–420 (as-cast) | 400–450 (as-cast) |
| Service Temperature (°C) | Up to 1100 | Up to 1100 | Up to 1100 |
| Primary Carbides | Cr7C3, W2C | Cr7C3, Cr23C6 | Cr7C3, Cr23C6 |
The microstructure of the overlay layers is dominated by a dendritic austenite matrix with carbide phases precipitated at the dendrite boundaries and within the interdendritic regions. The type, size, and distribution of carbides are critical factors determining the wear resistance and toughness of the overlay layer.
Process Comparison and Selection
The study compared four overlay processes for depositing cobalt-based hardfacing alloys, evaluating each process in terms of dilution rate, microstructure quality, deposition efficiency, and cost:
| Process | Dilution Rate (%) | Deposition Rate (g/min) | Microstructure Quality | Cost Index |
|---|---|---|---|---|
| Oxy-Fuel | 15–30 | 200–400 | Coarse dendrites, high porosity risk | Low |
| GTAW | 5–15 | 30–80 | Fine dendrites, low porosity | Medium |
| PTA | 3–10 | 150–350 | Uniform, fine-grained | Medium-High |
| Laser Cladding | 1–5 | 50–150 | Very fine, homogeneous | High |
The PTA process was identified as the optimal choice for most industrial applications due to its favorable balance of dilution control, deposition efficiency, and cost. The laser cladding process, while offering the lowest dilution rates and finest microstructures, is limited by its relatively low deposition rate and high equipment costs.
Defect Analysis and Countermeasures
The study identified several common defects in cobalt-based hardfacing overlay layers and proposed corresponding countermeasures:
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking (transverse) | High residual stress, low ductility of carbides | MT, PT | Reduce heat input, apply interpass preheat at 200–300°C |
| Porosity | Gas entrapment, inadequate shielding | RT, UT | Ensure clean surfaces, use proper shielding gas flow |
| Spatter | Excessive arc energy | Visual | Reduce voltage, optimize gas composition |
| Bonding failure | Surface contamination, poor wetting | Bond strength test (ASTM E2339) | Thorough surface preparation, proper base metal preheat |
| Excessive dilution | High heat input, thin overlay passes | Spectrographic analysis | Use lower heat input, increase wire feed speed |
Engineering Applications
Cobalt-based hardfacing alloys find extensive application in the following engineering scenarios:
- Gas turbine components: Blade tips, exhaust nozzles, and hot gas path components requiring resistance to hot corrosion and thermal fatigue.
- Mining and aggregate processing: Crusher rolls, jaw plates, and bucket teeth subject to severe abrasive wear.
- Chemical processing: Valve seats, pump impellers, and pipe fittings exposed to corrosive and erosive media.
- Power generation: Steam turbine blades, boiler tubes, and superheater elements operating at elevated temperatures.
- Oil and gas industry: Downhole tools, drill collars, and wellhead components subject to high-wear and corrosive environments.
Study Insights and Reflections
The study of cobalt-based hardfacing alloy overlay provides a comprehensive understanding of the relationship between process parameters, microstructure, and performance. The key insight is that the dilution rate must be carefully controlled to maintain the intended composition of the overlay layer, as excessive dilution from the base metal can significantly reduce the hardness and wear resistance of the deposited material. The recommendation to use PTA as the preferred process for most industrial applications is well-supported by the comparative analysis of dilution rates, microstructure quality, and cost-effectiveness. Engineers should also note that the high residual stresses inherent in cobalt-based overlay layers necessitate careful attention to post-weld stress relief, with PWHT temperatures typically in the range of 800–900°C depending on the specific alloy composition. The study serves as an excellent reference for process selection and quality control in hardfacing applications across multiple industries.