CLADDING TECHNOLOGY SHANXI CO., LTDTransition Layer Cladding Process in 14Cr1MoR Welding
Literature Overview
This study focuses on the development and evaluation of a transition layer cladding process for 14Cr1MoR steel, a high-temperature low-alloy steel widely used in hydrogenation reactors, reformer tubes, and high-temperature pressure vessels. The transition layer serves as a critical intermediary between the 14Cr1MoR base material and the austenitic stainless steel or nickel-based alloy overlay, mitigating the risk of cracking caused by mismatched thermal expansion coefficients and metallurgical incompatibility.
Core Technical Points
The 14Cr1MoR steel contains approximately 1.25% Cr, 0.9% Mo, and 0.4% C, which gives it excellent high-temperature creep strength but also makes it highly susceptible to cold cracking during welding due to its high carbon equivalent (CE ≈ 0.65–0.75). The transition layer must therefore serve a dual purpose: providing adequate metallurgical compatibility with both the base metal and the overlay, and acting as a crack-arresting zone that accommodates the thermal strains generated during welding.
| Parameter | 14Cr1MoR Base Metal | Transition Layer (Typical) | Overlay (309L) |
|---|---|---|---|
| Carbon (wt%) | 0.38–0.48 | 0.05–0.08 | ≤0.02 |
| Chromium (wt%) | 1.05–1.35 | 22–25 | 22–25 |
| Molybdenum (wt%) | 0.85–1.05 | 0.5–0.8 | 0.5–0.8 |
| CE (IIW) | 0.65–0.75 | 0.40–0.45 | 0.35–0.40 |
| Preheat Temperature (°C) | 250–350 | 200–300 | 150–250 |
| Interpass Temperature (°C) | ≤350 | ≤300 | ≤250 |
Process Development and Optimization
The study employed a multi-pass submerged arc welding (SAW) approach for the transition layer, using E81T-Ni1Cr11Mo flux-cored wire with a two-layer configuration. The first transition layer pass used a lower-carbon, higher-nickel wire to ensure good wetting and bonding to the 14Cr1MoR surface, while the second pass employed a standard 309L-type wire to provide a smooth metallurgical transition to the austenitic overlay.
Preheat and Heat Input Control
- Preheat temperature was set at 300°C for the first pass to reduce the cooling rate below the critical threshold for hydrogen-induced cracking (HIC).
- Heat input was maintained between 1.8 and 2.5 kJ/mm for the transition layer, balancing sufficient grain refinement with adequate cooling rate control.
- Interpass temperature was strictly limited to 300°C to prevent excessive grain coarsening in the HAZ of the preceding pass.
- Post-weld heat treatment (PWHT) at 720°C for 2 hours was applied to relieve residual stresses and promote carbide spheroidization in the 14Cr1MoR HAZ.
Defect Analysis and Countermeasures
The most common defects observed during the transition layer welding were cold cracks in the HAZ of the 14Cr1MoR base metal and lack of fusion at the transition layer-substrate interface. The following countermeasures were identified as effective:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cold cracking in HAZ | High CE, excessive cooling rate, hydrogen presence | Increase preheat to 300°C, use low-hydrogen flux, apply PWHT |
| Lack of fusion at interface | Insufficient penetration, surface contamination | Increase arc current by 10%, clean substrate to SA2.5, use C-curve backing |
| Cracking in transition layer | Thermal mismatch between layers | Use nickel-rich wire for first pass, limit heat input to ≤2.5 kJ/mm |
| Porosity in overlay | Flux moisture, insufficient shielding | Dry flux at 200°C for 2h, use argon back-purging at 5–8 L/min |
Engineering Practice Integration
In the fabrication of hydrogenation reactors, the transition layer is often the most critical and failure-prone zone. The study's findings directly inform the design of welding procedures for large-diameter vessels where the 14Cr1MoR shell is clad with 316L stainless steel for corrosion resistance in the presence of hydrogen sulfide. The transition layer thickness of 3–5 mm is typically sufficient to prevent cracking while maintaining acceptable corrosion performance in the overlay.
Key Questions and Reflections
A significant practical concern is the effect of the transition layer on the overall vessel's creep performance at elevated temperatures. Since the transition layer contains austenitic microstructure with lower creep strength than the 14Cr1MoR base metal, it may become a weak link during long-term high-temperature service. This raises the question of whether thinner transition layers with optimized composition could better preserve the mechanical integrity of the vessel while still preventing cracking.
Study Insights and Implications
The study confirms that the transition layer is not merely a metallurgical convenience but a structural necessity in dissimilar material welding involving high-carbon-equivalent steels. The dual-purpose nature of the transition layer — providing both crack resistance and corrosion compatibility — demands a careful balance of composition and microstructure. Engineers should treat the transition layer design as a critical design element that requires dedicated welding procedure qualification and post-weld inspection, rather than as a routine intermediate step in the cladding process.