Zhuojin Pipe Fitting Co., LtdStrip Cladding Inclination Effect on Dilution and Ferrite in Stainless Steel Overlay Welds
Literature Focus and Core Result
The 2020 paper in Pressure Vessel examines how specimen inclination during stainless steel strip cladding affects dilution, ferrite content, and process stability. The important observation is that dilution is the main quality concern, and the angle of the workpiece strongly influences dilution. The study also notes that post weld heat treatment can increase ferrite content, which means the as welded condition is not always the final service condition.
This is highly relevant to pressure vessel, piping, and fitting overlay work. Strip cladding is efficient for large area corrosion resistant overlays, but it is sensitive to gravity, weld pool flow, flux behavior, and base metal melting. If the part is inclined, the molten pool may shift, the strip may not melt symmetrically, and the interface can become uneven. These effects change the chemical dilution and therefore the ferrite number of the overlay weld metal.
Dilution and Ferrite Mechanism
Dilution is the fraction of base metal mixed into the deposited weld metal. In stainless steel cladding, dilution is not only a thickness issue. It changes the final weld metal composition and can shift the microstructure toward higher ferrite, lower corrosion resistance, or increased sensitivity to sensitization depending on the base metal and filler metal combination.
| Factor | Observation | Engineering consequence |
|---|---|---|
| Specimen inclination | Dilution increases noticeably | Overlay chemistry becomes less predictable. |
| Weld pool position | Gravity changes pool shape and wetting | Bead shape and fusion line become uneven. |
| Post weld heat treatment | Ferrite content increases | Final microstructure must be evaluated after PWHT. |
The study suggests that weld strip position and inclination angle are the dominant process variables for dilution in this case. This is consistent with practical experience on flat, inclined, and overhead strip cladding. When the part is not horizontal, the slag layer, flux, and weld pool can move differently. The result may be excessive penetration on the lower side and insufficient fusion on the upper side.
Engineering Practice and Quality Control
For stainless steel pipe and fitting overlays, dilution should be controlled by a combination of joint preparation, cladding sequence, electrical parameters, and workpiece orientation. A qualified procedure should specify the maximum allowable inclination angle or require a separate procedure qualification for inclined positions. Ferrite measurement should be performed on the overlay surface and near the fusion zone, because ferrite can vary through the thickness.
When post weld heat treatment is required, ferrite testing should include both as welded and heat treated conditions. Some austenitic weld metals can experience ferrite transformation or redistribution during PWHT, and the service condition may therefore be more critical than the initial production condition. For sour service or chloride service, the final microstructure should also be considered together with corrosion testing and hardness limits.
The study reinforces the need to treat strip cladding as a system. The strip, flux, current, voltage, speed, tilt, and preheat are not independent. In a pressure vessel or pipe spool shop, a simple change in fixture angle can change dilution enough to affect ferrite and corrosion performance. The practical conclusion is that cladding angle should be controlled, measured, and qualified as a real welding variable, not treated as a minor shop convenience.