CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Process Research on 15CrMo Tube Sheets
Literature Overview
This study, authored by Zou Yuqing and Zhang Shu (2015), investigates the hot working process parameters for weld overlay on 15CrMo tube sheets, a critical component in high-pressure heat exchangers and reactors used in petrochemical applications. The authors, affiliated with Jilin Electronic Information Vocational and Technical College and CNPC Northeast Refining and Chemical Engineering Co., Ltd., address a practical challenge encountered in the fabrication of large-diameter heat exchanger tube sheets where corrosion resistance is required at the tube-to-tubesheet joint. The publication appears in the context of thermal processing technology literature and reflects the industrial demand for reliable overlay solutions on low-alloy steel substrates.
Core Technical Content and Process Analysis
15CrMo steel is a normalized low-alloy steel containing approximately 1.25–1.65 wt% Cr and 0.40–0.65 wt% Mo, providing excellent high-temperature strength and moderate oxidation resistance. However, in aggressive chemical environments, the base material itself is insufficiently corrosion resistant, necessitating a weld overlay of austenitic stainless steel or nickel-based alloy on the tube sheet surface. The primary challenge lies in achieving a sound metallurgical bond between the ferritic/martensitic substrate and the austenitic overlay while minimizing cracking susceptibility due to the significant difference in thermal expansion coefficients and carbon diffusion behavior.
Key Process Parameters
| Parameter | Typical Range | Rationale |
|---|---|---|
| Preheat temperature | 200–250 °C | Reduces cooling rate, limits martensite formation in HAZ |
| Interpass temperature | ≤250 °C | Controls hydrogen diffusion and residual stress |
| Overlay thickness | 3–6 mm | Provides adequate corrosion reserve while limiting dilution |
| Welding current (SAW) | 400–600 A | Optimized for single-layer deposition efficiency |
| Travel speed | 100–150 mm/min | Balances penetration depth and dilution ratio |
| Post-weld heat treatment | 620–680 °C × 2–4 h | Stress relief and microstructure stabilization |
The study likely employed submerged arc welding (SAW) with E8018 or equivalent consumables for the transition layer followed by austenitic stainless steel filler (such as ER309L or E309L) for the final overlay layers. The choice of a 309-type filler for the transition is critical because its high nickel content (≥23%) ensures full austenite in the weld metal despite dilution with ferritic base metal, thereby preventing the formation of brittle delta ferrite or martensite at the interface.
Dilution Control Strategy
A major engineering concern in overlaying 15CrMo is the dilution of the base alloy into the overlay layer. Chromium and molybdenum from the substrate can lower the equivalent carbon content of the overlay and potentially reduce corrosion resistance. The study likely addressed this through:
- Multi-pass overlay strategies with decreasing dilution ratios in successive layers
- Use of low-carbon filler metals (C ≤ 0.03%) to minimize intergranular sensitization
- Post-overlay solution treatment at 1050–1100 °C followed by water quenching to re-dissolve carbides
Defect Analysis and Countermeasures
The most common defects encountered in 15CrMo tube sheet overlay include:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking in overlay | Sulfur/phosphorus segregation, high delta ferrite | Use low-S/P consumables; control cooling rate |
| Cold cracking in HAZ | Hydrogen embrittlement in martensitic HAZ | Maintain preheat ≥200 °C; limit hydrogen in shielding |
| Lack of fusion at interface | Insufficient heat input | Increase current or reduce travel speed |
| Excessive dilution | High heat input, single-layer strategy | Multi-layer approach; reduce current per pass |
| Surface porosity | Flux moisture, base contamination | Dry flux storage; thorough surface preparation |
Engineering Practice Integration
In practice, the tube sheet overlay process must be qualified per NB/T 47014 or ASME IX Section IX, Category 1 welding procedure qualification. The qualification coupon should replicate the actual geometry as closely as possible, including the tube holes and the curved surface of the tube sheet. Post-overlay inspection should include:
- Visual examination (VT) of the entire overlay surface for undercut, porosity, and lack of fusion
- Magnetic particle testing (MT) or penetrant testing (PT) for surface-breaking defects
- Ultrasonic testing (UT) for subsurface defects and bond quality
- Hardness mapping across the overlay-to-base transition to verify the absence of brittle phases
- Chemical analysis of the overlay layer to confirm Cr ≥ 18% and Ni ≥ 8% after dilution
Study Insights and Reflections
The significance of this work lies in its practical orientation toward a specific industrial application rather than a generic process study. The collaboration between an academic institution and an industrial fabrication company reflects the transfer of laboratory findings into shop-floor practice. However, several areas warrant further investigation: the long-term creep performance of the overlay at elevated temperatures (above 400 °C), the effect of repeated thermal cycling on the overlay-substrate interface, and the impact of tube drilling after overlay on the residual stress state. Engineers working on similar projects should pay particular attention to the interaction between the overlay process and subsequent machining operations, as excessive mechanical stress during tube drilling can initiate cracking in the overlay layer, particularly if the post-weld heat treatment has not been properly executed.