Zhuojin Pipe Fitting Co., LtdOverlay Welding of Stellite Alloy on Supercritical Main Steam Inlet Insert Pipe
Literature Overview
The paper by Chen Lijuan, Sun Zhongmin, Zhang Liyan, and Li Quanhua from Harbin Turbine Works Co., Ltd., published in Turbine Technology (Vol. 49, No. 4, 2007, pp. 314-316), addresses a critical engineering challenge in supercritical 600 MW turbine units. The main steam inlet insert pipe is subjected to severe erosion from high-velocity steam flow combined with mechanical vibration, leading to premature failure. The design requirement mandates overlay welding of Stellite alloy on the insert pipe surface to enhance erosion resistance and vibration-impact tolerance, thereby extending service life. The study demonstrates successful completion of the overlay welding using Tungsten Inert Gas (TIG/GTAW) welding.
Core Technical Context
Supercritical power units operating above the critical pressure of water (22.1 MPa) present unique metallurgical challenges for component protection. The main steam inlet insert pipe typically operates at temperatures ranging from 540°C to 620°C with steam velocities exceeding 100 m/s. Under these conditions, the base material—usually a Cr-Mo-V alloy steel such as 12Cr1MoV or similar grades—experiences progressive material loss through a combination of erosion, thermal fatigue, and vibration-induced fretting.
Stellite alloy, originally developed by Stellite Corporation, is a cobalt-chromium-tungsten-based alloy renowned for its exceptional resistance to erosion, corrosion, and wear under high-temperature conditions. Common grades include Stellite 6 (Co-Cr-W with approximately 21% Cr, 5% W, 5% Mo) and Stellite 21 (higher Cr content at approximately 29%). The alloy forms a protective Cr₂O₃ oxide film at elevated temperatures and maintains hardness above 500°C where most iron-based materials soften significantly.
Welding Process Analysis
The selection of GTAW (Tungsten Inert Gas Arc Welding) for this application is technically sound and warrants detailed discussion. The following factors justify this process choice:
| Process Parameter | Typical Value | Rationale |
|---|---|---|
| Welding process | GTAW (TIG) | Low dilution, precise heat input control, no slag interference |
| Shielding gas | Argon (99.99% purity) | Inert atmosphere prevents oxide inclusion in Co-Cr-W weld metal |
| Tungsten electrode | WC-20 or WC-30, 2.4-3.2 mm | High current density with minimal electrode consumption |
| Filler metal | Stellite 6 or 21 rod, 2.0-3.0 mm diameter | Matched chemistry for overlay composition |
| Current range | 120-200 A (DCEN) | Direct current electrode negative for deep penetration with controlled heat |
| Travel speed | 30-60 mm/min | Sufficient overlap between adjacent passes for full coverage |
| Preheat temperature | 150-250°C | Reduce thermal gradient, minimize cracking susceptibility |
| Interpass temperature | Below 300°C | Control microstructure coarsening in previous passes |
Key Technical Challenges
- Base metal dilution: The high-temperature supercritical insert pipe base material contains significant Cr, Mo, and V alloying elements. During overlay welding, dilution from the base metal into the weld metal can alter the microstructure and properties of the Stellite overlay. The first pass typically experiences 40-60% dilution, which must be compensated by subsequent passes to achieve the target overlay composition.
- Hot cracking susceptibility: Cobalt-based alloys are susceptible to solidification cracking due to their narrow freezing range and high liquidus-solidus temperature spread. The presence of sulfur and phosphorus impurities in the base metal further exacerbates this tendency.
- Thermal stress management: The coefficient of thermal expansion mismatch between the Cr-Mo-V base steel and the Co-Cr-W overlay creates residual stresses upon cooling. This can lead to cracking or delamination if not properly managed through preheating and post-weld heat treatment.
Engineering Practice Integration
In practice, the overlay welding of Stellite alloy on supercritical turbine components follows a systematic approach:
- Surface preparation: The base metal surface must be ground to a minimum depth of 3 mm to remove any decarburized or contaminated layers. The prepared surface should be free of oxide, scale, and oil contamination.
- Multi-pass strategy: Typically 2-4 passes are applied, with the first pass serving as a transition layer and subsequent passes building up the full overlay thickness. The total overlay thickness is generally 3-5 mm for erosion-critical applications.
- Post-weld heat treatment: A stress-relief treatment at 850-900°C for 2-4 hours in a controlled atmosphere furnace is essential to reduce residual stresses without compromising the overlay properties.
- Quality verification: Hardness testing (minimum 300 HV for Stellite 6), chemical analysis of the overlay, and non-destructive examination (MT or PT for surface cracks) are mandatory acceptance criteria.
Key Reflections and Study Insights
The significance of this work extends beyond the specific component. It establishes a replicable methodology for applying cobalt-based overlay alloys to high-temperature, high-velocity flow components in power generation equipment. The GTAW process, while slower than submerged arc or flux-cored wire methods, offers superior control over dilution and microstructure, which is paramount when the overlay composition directly determines service performance.
A critical insight from this literature is that overlay welding success depends not only on the selection of the overlay material but equally on the process parameters that control dilution, cooling rate, and residual stress. In supercritical applications where the operating temperature exceeds 540°C, the overlay material must maintain its protective properties at these temperatures, making the Co-Cr-W system uniquely suitable compared to iron-based alternatives that would soften and lose oxidation resistance.
The engineering implication is clear: for components subjected to combined erosion, thermal cycling, and vibration, overlay welding with cobalt-based alloys using GTAW provides a reliable, proven solution. Future developments should focus on optimizing the transition layer composition to minimize dilution effects and exploring multi-layer schemes with graded compositions to further reduce residual stress at the overlay-base metal interface.