CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Inconel 625 on 12Cr2Mo1 Forging Process Study
Literature Overview
This study investigates the strip cladding (electroslag welding, ESW) process for depositing Inconel 625 overlay layers onto 12Cr2Mo1 steel forgings, which are commonly used as base materials for high-pressure hydrogenation reactor shells and other severe-service pressure vessel components. The research addresses the metallurgical compatibility challenges between a Cr-Mo low-alloy base metal and a Ni-based superalloy overlay, including dilution control, bond strength assurance, and defect prevention. The study adopts a systematic approach combining process parameter optimization with microstructural characterization and mechanical property evaluation.
Core Technical Points
The fundamental challenge in this application lies in the significant mismatch between the base metal and the overlay material. 12Cr2Mo1 is a Cr-Mo alloy steel with moderate creep strength designed for elevated-temperature service, while Inconel 625 offers exceptional corrosion resistance in aggressive chemical environments. The coefficient of thermal expansion difference and the dilution zone chemistry directly determine the long-term reliability of the clad interface.
The ESW strip cladding process was selected for its high deposition rate and deep penetration characteristics, which are essential for achieving the required cladding thickness in a single or double pass. The process parameters studied included:
| Parameter | Typical Range | Optimized Value |
|---|---|---|
| Arc voltage | 28-36 V | 32-34 V |
| Current | 800-1200 A | 1000-1100 A |
| Travel speed | 150-300 mm/min | 200-250 mm/min |
| Flux type | ESW flux for Ni-based | Low-silica, low-fluoride |
| Preheat temperature | 200-350 °C | 250-300 °C |
| Interpass temperature | ≤350 °C | ≤300 °C |
The study demonstrates that controlling the dilution ratio to below 25% is critical for maintaining the Ni-based alloy's corrosion resistance. When dilution exceeds this threshold, chromium carbide precipitation at the interface becomes increasingly pronounced, leading to potential intergranular corrosion susceptibility.
Process Analysis and Microstructural Interpretation
Interface Microstructure
Metallographic examination reveals a distinct diffusion zone at the cladding-base metal interface, typically 10-40 micrometers in thickness. This zone exhibits a gradient in carbon, chromium, and molybdenum concentrations. In the optimized process window, the diffusion zone is characterized by fine, dispersed carbides without continuous grain boundary networks. When preheat is insufficient or travel speed is excessive, coarse intermetallic phases form along the interface, compromising bond integrity.
Dilution Control Strategy
The study proposes a two-pass strategy: the first pass uses a transition filler material (such as a Ni-Cr transition alloy) to moderate the dilution effect, followed by the final Inconel 625 pass. This approach effectively limits the carbon content in the final overlay to below 0.08%, which is essential for resisting intergranular corrosion in chloride-containing environments.
Defect Analysis
Common defects identified include:
- Surface cracks in the overlay layer, primarily caused by high hydrogen content from improper flux conditioning
- Lack of fusion at the cladding interface, associated with insufficient current or excessive travel speed
- Excessive porosity, linked to inadequate flux coverage or moisture contamination
- Undercutting at strip edges, mitigated by adjusting the strip alignment and backing gas flow
Standards and Code Compliance
The fabrication and inspection requirements reference GB/T 150 and NB/T 47002 for design, ASME IX or NB/T 47014 for WPS/PQR qualification, and JB/T 4730 for NDT procedures. The Inconel 625 overlay must achieve a minimum bond strength of 250 MPa (transverse tensile test per ASTM A263) and pass 100% surface magnetic particle inspection (MT) for the final layer.
Engineering Practice Integration
In practice, this process has been successfully applied to hydrogenation reactor shells where the base metal must withstand high internal pressure and temperature while the overlay resists hydrogen attack and corrosion. The key engineering insight is that the ESW process parameters must be qualified specifically for the forging geometry, as curvature and section thickness variations significantly affect heat input distribution and solidification behavior. Post-weld heat treatment (PWHT) at 720-760 °C for 2-4 hours per 25 mm of thickness is mandatory to relieve residual stresses and stabilize the microstructure.
Study Insights and Reflections
The most valuable finding from this study is the quantitative relationship between preheat temperature and dilution ratio. A preheat of 250-300 °C provides an optimal balance between reducing thermal gradient-induced cracking and minimizing excessive dilution. The study also highlights that flux conditioning is often overlooked but is critical for hydrogen control in Ni-based overlay welding. For future work, I believe integrating real-time arc monitoring with adaptive parameter adjustment could further improve process consistency, particularly for large-diameter forgings with variable wall thickness.