CLADDING TECHNOLOGY SHANXI CO., LTDStellite Cobalt-Based Alloy GTAW Cladding Process Research - A Study Note
Introduction to Stellite Alloy Cladding
Stellite alloys, particularly Stellite 6, are cobalt-chromium-tungsten-based superalloys renowned for their exceptional combination of wear resistance, corrosion resistance, and hot hardness. These alloys are widely used in aerospace, oil and gas, mining, and power generation industries for components subjected to extreme conditions. Gas tungsten arc welding (GTAW), also known as TIG welding, is one of the most precise and widely used methods for depositing Stellite alloys, offering excellent control over heat input and dilution.
Metallurgical Characteristics of Stellite 6
Stellite 6 contains approximately 59% cobalt, 21% chromium, 5% tungsten, 3% molybdenum, and 1% iron. The microstructure consists of a face-centered cubic (FCC) austenitic matrix with dispersed carbide particles, primarily Cr7C3 and W2C. This microstructure provides excellent resistance to abrasive wear, cavitation erosion, and corrosion at elevated temperatures. The solubility of carbon in the cobalt matrix allows for controlled carbide precipitation, which is critical for achieving the desired hardness and wear properties.
GTAW Process Parameters for Stellite 6 Cladding
| Parameter | Value | Notes |
|---|---|---|
| Shielding gas | Argon (99.99%) | Flow rate 15-20 L/min |
| Back purge | Argon | Flow rate 5-8 L/min |
| Current | 120-180 A | DC electrode positive |
| Travel speed | 60-100 mm/min | Depends on wire diameter |
| Wire diameter | 1.6-2.4 mm | ERCoCr-A equivalent |
| Electrode | WC-10 tungsten | 2.4-3.2 mm diameter |
| Preheat | 150-250 °C | For thick sections |
| Interpass temperature | Below 250 °C | Critical for carbide control |
The key to successful Stellite 6 GTAW cladding is maintaining low dilution, ideally below 20%. High dilution leads to the formation of brittle intermetallic compounds and reduced hardness in the cladding layer. The use of a tungsten-copper (WC-10) electrode provides a stable arc and good penetration characteristics. The travel speed must be carefully controlled to balance deposition rate with heat input; excessive travel speed leads to poor fusion, while insufficient travel speed causes excessive heat input and dilution.
Microstructural Evolution and Phase Analysis
Metallographic examination of Stellite 6 GTAW cladding layers reveals a complex microstructure consisting of:
- An equiaxed austenitic matrix with grain size of 50-80 μm
- Primary carbide particles (Cr7C3 and W2C) distributed along grain boundaries and within grains
- Secondary carbide precipitation during cooling, contributing to overall hardness
- A narrow heat-affected zone (HAZ) in the base material with minimal microstructural change
The hardness profile typically shows a gradient from 45-50 HRC at the surface to 35-40 HRC near the fusion boundary, reflecting the dilution effect. The surface hardness is primarily determined by the carbide content and distribution, which can be optimized by controlling the cooling rate and interpass temperature.
Common Defects and Quality Control
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking | High carbon activity, rapid cooling | Increase preheat, reduce travel speed |
| Porosity | Gas entrapment, inadequate shielding | Increase gas flow, clean wire surface |
| Excessive dilution | High heat input, poor technique | Reduce current, increase travel speed |
| Hardness non-uniformity | Inconsistent parameters | Use automated wire feeding |
| Surface oxidation | Inadequate back purge | Increase back purge flow rate |
Non-destructive testing (NDT) of Stellite 6 cladding layers requires special consideration due to the high density and low neutron absorption of cobalt alloys. Magnetic particle testing (MT) is ineffective due to the non-magnetic nature of the cobalt matrix. Ultrasonic testing (UT) and radiographic testing (RT) are the preferred methods, with UT being the most practical for field applications.
Engineering Applications and Performance
Stellite 6 GTAW cladding has been successfully applied to:
- Steam turbine blades and valve seats in power plants
- Pump impellers and wear rings in oil and gas applications
- Injection molding tool inserts in plastics processing
- Mining equipment components such as drill bits and bucket teeth
In a documented case involving the repair of a steam turbine valve seat, Stellite 6 cladding extended the service life from 8,000 hours to over 25,000 hours, representing a 3-fold improvement. The cladding layer maintained its hardness and microstructural integrity throughout the extended service period, demonstrating the long-term reliability of GTAW-deposited Stellite 6.
Study Insights and Conclusions
The study of Stellite 6 GTAW cladding technology reveals several fundamental principles that are applicable to all hard-facing overlay applications. First, dilution control is paramount; the properties of the deposited layer are critically dependent on the dilution level, and maintaining dilution below 20% is essential for achieving the full benefit of the Stellite alloy. Second, the cobalt-based matrix provides excellent hot hardness and corrosion resistance, but the alloy is susceptible to intergranular cracking if the carbon activity is too high or the cooling rate is too fast. Third, the use of appropriate shielding gas and back purge is critical to prevent oxidation of the hot cobalt surface, which can lead to surface degradation and reduced wear resistance. This study reinforces the importance of process parameter optimization and quality control in achieving reliable and durable Stellite cladding layers, and underscores the value of understanding the metallurgical behavior of cobalt-based alloys under welding conditions.