CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Inconel 625 on 12Cr2Mo1 Forging Process Research
Literature Overview
This 2024 study published in Chemical Equipment and Piping represents current industrial practice in the cladding of nickel-based alloys onto low-alloy steel forgings. Conducted by the Hebei Special Equipment Supervision and Inspection Research Institute and the Hebei Pressure Vessel Inspection and Evaluation Technology Innovation Center, the research addresses a highly practical and widely encountered engineering challenge: the application of Inconel 625 overlay to 12Cr2Mo1 (12Cr1Mo) steel forgings using strip cladding (electroslag welding) technology. The work reflects the ongoing demand for corrosion-resistant overlays in chemical processing equipment, particularly in hydrogenation reactors and other high-pressure, high-temperature vessels.
Technical Background and Process Parameters
12Cr2Mo1 is a chromium-molybdenum low-alloy steel widely used in high-temperature pressure vessel applications due to its excellent creep strength and oxidation resistance up to approximately 550 degrees Celsius. However, its corrosion resistance in aggressive chemical environments is limited, necessitating overlay with a nickel-based alloy such as Inconel 625. Strip cladding, also known as electroslag welding overlay, offers high deposition rates (typically 30-80 kg/h) and excellent metallurgical bond quality, making it the preferred method for thick overlay layers on large forgings.
Typical Process Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Stripping current | 250-400 A | Balances deposition rate and heat input |
| Stripping voltage | 28-36 V | Controls slag pool stability |
| Stripping speed | 80-150 mm/min | Maintains consistent bead profile |
| Preheat temperature | 200-300 C | Reduces cracking susceptibility in base metal |
| Interpass temperature | 250-350 C | Controls cooling rate and grain growth |
| Strip composition | Inconel 625 (UNS N06625) | Ni-22Cr-13Mo-9Nb system |
| Layer thickness per pass | 3-5 mm | Typical ESW deposition thickness |
| Number of layers | 2-4 | Achieves total overlay of 6-15 mm |
Metallurgical Bond Quality
The metallurgical bond between Inconel 625 and 12Cr2Mo1 is primarily a diffusion bond with some intermetallic phase formation at the interface. The key concerns include:
- Cracking at the bond line: The coefficient of thermal expansion mismatch between the nickel-based overlay (approximately 13 x 10^-6 /K) and the ferritic base metal (approximately 12.5 x 10^-6 /K) generates significant residual stresses during cooling.
- Intermetallic phase formation: Hard, brittle phases such as sigma phase (CrMo-rich) and Laves phase (Mo-rich) can form at the interface, reducing bond strength and crack resistance.
- Dilution effects: Excessive dilution of base metal into the first overlay layer can alter the corrosion resistance of the Inconel 625, particularly if carbon and chromium levels are elevated.
The study reported bond strength values in the range of 250-350 MPa when proper process parameters were maintained, with interpass temperatures controlled to prevent excessive grain coarsening in the overlay.
Quality Control and Inspection Requirements
For Inconel 625 strip cladding on pressure vessel forgings, the following inspection regime is essential:
- Visual inspection of all overlay surfaces for uniform coverage, absence of undercut, and proper bead profile.
- Ultrasonic testing (UT) of the bond line per ASME V or GB/T 11345 to detect lack of fusion, cracks, and delaminations.
- Magnetic particle testing (MT) of the overlay surface to identify surface-breaking defects.
- Chemical analysis of the first and last overlay layers to verify dilution levels remain within acceptable limits (typically less than 15 percent base metal dilution in the first layer).
- Mechanical testing including tensile and bend tests on weld coupons to verify the overlay meets the mechanical requirements of ASME IX or the applicable Chinese standard.
Engineering Practice Insights
In practical fabrication, several lessons emerge from this type of work:
- The stripping electrode must be preheated and maintained at a consistent temperature to ensure stable slag pool formation and uniform deposition.
- Flux composition is critical - a low-silica, low-fluoride flux should be used to minimize sulfur and phosphorus pickup, which can promote hot cracking in the Inconel 625.
- The base metal surface must be thoroughly prepared, with any scale, oxide, or contamination removed by grinding to a bright metal finish immediately before cladding.
- Post-cladding stress relief at 620-650 degrees Celsius for 2-4 hours is recommended to reduce residual stresses, but this must be followed by re-inspection for any new defects.
The research underscores that while strip cladding is a well-established technology, achieving consistent quality on complex forging geometries requires careful attention to process parameters and thorough inspection protocols. The growing use of Inconel 625 in hydrogenation reactor applications continues to drive refinement of these fabrication practices.