CLADDING TECHNOLOGY SHANXI CO., LTDStellite Alloy Weld Overlay on Lock Hopper Cones A Technical Study Note
Literature Overview
This 2016 publication by Zhou Yinmei from Shanxi Yangmei Chemical Machinery Group Co., Ltd., published in the journal China Chemical Equipment, focuses on the weld overlay of Stellite alloy onto lock hopper cone bodies. Lock hoppers are critical components in coal chemical plants, particularly in pressurized coal gasification systems such as the GE Gasifier or Shell Gasifier, where they serve as pressure-retaining valves that admit solid coal feed into the gasification reactor. The cone body is subjected to severe abrasive and erosive wear from coal particles, as well as corrosive attack from syngas containing hydrogen sulfide and carbon monoxide.
Technical Background and Challenges
The lock hopper cone operates under a combination of mechanical abrasion from solid coal particles traveling at high velocities and chemical corrosion from the harsh gas environment. The base material is typically a low-alloy steel or carbon steel pressure vessel material, while the overlay must provide both erosion resistance and corrosion resistance. Stellite alloy, a cobalt-chromium-tungsten-based cast alloy, is a well-established material for such applications due to its excellent resistance to hot corrosion and abrasive wear. However, the weldability of Stellite alloy presents significant challenges due to its high tendency for solidification cracking and the formation of brittle intermetallic compounds at the bond line.
| Challenge | Description | Mitigation Strategy |
|---|---|---|
| Solidification cracking | Stellite alloy has a wide freezing range and is susceptible to hot cracking | Use of low-dilution consumables and controlled welding parameters |
| Bond line brittleness | Formation of brittle phases at the interface | Insertion of a transition layer or use of compatible filler metals |
| Residual stress | High thermal gradient leads to high tensile residual stress | Interpass temperature control and post-weld stress relief |
| Porosity | Gas inclusion from improper shielding or flux | Use of dry flux, proper gas coverage, and clean base surface |
Welding Process Selection
The study discusses the application of submerged arc welding (SAW) and gas metal arc welding (GMAW) for the overlay of Stellite alloy onto the cone geometry. The conical shape presents unique challenges for weld sequencing and travel speed control. For SAW, a multi-pass strategy is employed where the first pass uses a compatible steel consumable to create a transition layer, followed by subsequent passes using Stellite alloy consumable. The typical process parameters include a welding current of 300-450 amperes, voltage of 25-32 volts, and travel speed of 80-150 mm per minute, with interpass temperature maintained below 250 degrees Celsius to prevent excessive grain growth in previously deposited layers.
For GMAW overlay, a wire feed rate of 6-10 meters per minute with a shielding gas mixture of argon and 5 percent carbon dioxide is typical. The arc voltage is maintained at 22-28 volts. The use of a slightly higher carbon dioxide content in the shielding gas improves wetting and reduces spatter while maintaining adequate arc stability.
Microstructural Analysis and Performance
The overlay layer microstructure of Stellite alloy consists of a solid solution matrix of cobalt, chromium, and tungsten with dispersed carbides of chromium and tungsten carbide. These carbides provide the primary wear resistance mechanism. Metallographic examination of the overlay reveals that the hardness distribution across the overlay thickness is generally uniform, with hardness values in the range of 38-45 HRC. However, the transition zone between the overlay and the base steel may exhibit a hardness gradient due to dilution, with hardness dropping to 25-30 HRC at the bond line.
The erosion wear performance of the Stellite overlay on lock hopper cones is typically evaluated through laboratory tests simulating the actual service conditions. The study reports that properly applied Stellite overlay can extend the service life of lock hopper cones by a factor of three to five times compared to unclad components, significantly reducing maintenance downtime in continuous gasification operations.
Defect Analysis and Quality Control
Common defects encountered in Stellite alloy overlay include hot cracking, porosity, incomplete fusion, and overlay spalling. Hot cracking is the most critical defect and is primarily caused by the high solidification cracking susceptibility of the Stellite alloy. The use of low-sulfur and low-phosphorus consumables, combined with controlled welding parameters and appropriate preheating, can effectively mitigate this risk. Non-destructive testing using magnetic particle testing (MT) and ultrasonic testing (UT) is essential for detecting surface and subsurface defects.
Quality control procedures should include verification of the overlay thickness using magnetic thickness gauges or ultrasonic methods, hardness testing across the overlay profile, and visual inspection for surface quality. The bond strength between the overlay and the base can be verified through a pull-off test or by examining the microstructure at the bond line for signs of lack of fusion.
Study Insights and Reflections
This study highlights the practical application of Stellite alloy overlay in a demanding chemical processing environment. The authors demonstrate that while Stellite alloy is an excellent material for erosion and corrosion resistance, its successful application requires careful attention to welding process control and consumable selection. The transition layer concept is particularly important for ensuring a strong metallurgical bond without introducing brittle phases. The economic case for Stellite overlay on lock hopper cones is compelling given the high cost of unplanned downtime in continuous gasification operations. This study provides valuable practical guidance for engineers tasked with specifying and inspecting Stellite overlay work on similar chemical equipment components.