CLADDING TECHNOLOGY SHANXI CO., LTDResearch on 15CrMo Tube Sheet Cladding Process
Literature Overview
The study by Zou Yuqing and Zhang Shu, published in 2015 in the journal Hot Working Technology by Jilin Electronic Information Vocational and Technical College and China Petroleum Group Northeast Refining and Chemical Engineering Co., Ltd., investigates the cladding process for 15CrMo tube sheets used in heat exchangers and pressure vessels. This research addresses a critical engineering challenge in the petrochemical and power generation industries, where 15CrMo tube sheets must provide both structural strength and corrosion resistance in high-temperature, high-pressure service environments.
Core Technical Content
The 15CrMo steel is a low-alloy martensitic steel containing approximately 1.1–1.4% Cr and 0.40–0.65% Mo, providing excellent high-temperature strength and resistance to hydrogen attack and sulfidation corrosion. However, in service environments involving chloride-containing fluids, acidic media, or reducing conditions, 15CrMo exhibits limited corrosion resistance, necessitating a corrosion-resistant overlay layer.
The cladding process must address several metallurgical challenges:
- Carbon equivalent: 15CrMo has a relatively high carbon equivalent (Ceq ≈ 0.45–0.55%), increasing susceptibility to cold cracking.
- Dilution control: Excessive dilution with the base material reduces the corrosion resistance of the overlay layer.
- Residual stress management: The thermal expansion mismatch between the base material and overlay can induce significant residual stresses.
- Microstructural compatibility: The overlay must bond metallurgically with the base while maintaining its corrosion-resistant properties.
| Parameter | 15CrMo Base Material | Overlay Material (316L) | Overlay Material (Inconel 625) |
|---|---|---|---|
| Carbon (wt.%) | 0.30–0.40 | ≤0.08 | ≤0.10 |
| Chromium (wt.%) | 0.90–1.20 | 16.0–18.0 | 20.0–23.0 |
| Molybdenum (wt.%) | 0.40–0.65 | 2.0–3.0 | 8.0–10.0 |
| Hardness (HV) | 250–350 | 180–220 | 250–300 |
| Thermal expansion (10⁻⁶/K) | 12.0 | 17.0 | 13.0 |
| Corrosion resistance | Moderate | Excellent | Outstanding |
Process Analysis and Parameter Optimization
The study evaluates multiple cladding processes for 15CrMo tube sheets, with particular emphasis on submerged arc welding (SAW) and gas metal arc welding (GMAW) overlay techniques.
Preheat and interpass temperature requirements:
| Process | Preheat (°C) | Interpass (°C) | Post-weld Heat Treatment |
|---|---|---|---|
| SAW overlay | 250–350 | 250–350 | 600–650°C, 2 h |
| GMAW overlay | 200–300 | 200–300 | 600–650°C, 2 h |
| GTAW overlay | 150–250 | 150–250 | 550–650°C, 2 h |
| Laser cladding | 100–200 | 100–200 | 550–650°C, 1–2 h |
The preheat temperature is critical for preventing cold cracking in the 15CrMo base material and the heat-affected zone. The interpass temperature must be maintained to avoid excessive hardness in multi-pass welds, while the post-weld heat treatment (PWHT) relieves residual stresses and tempers any martensitic structures that may have formed.
Welding parameter recommendations:
| Parameter | SAW | GMAW | GTAW |
|---|---|---|---|
| Current (A) | 400–600 | 200–350 | 80–150 |
| Voltage (V) | 30–40 | 22–30 | 10–20 |
| Travel speed (mm/min) | 200–400 | 150–300 | 50–150 |
| Shielding gas | Flux (rutile/basic) | Ar + 5% CO₂ | Ar |
| Wire/powder feed | 5–8 mm | 1.2–1.6 mm | 1.6–2.4 mm |
| Heat input (kJ/mm) | 1.0–2.5 | 0.5–1.5 | 0.2–0.8 |
Defect Analysis and Countermeasures
The study identifies several critical defects specific to 15CrMo tube sheet cladding:
| Defect Type | Root Cause | Detection Method | Prevention and Remediation |
|---|---|---|---|
| Cold cracking in HAZ | High Ceq, rapid cooling | MT, PT, UT | Preheat 250–350°C, control cooling rate, PWHT |
| Cracking at overlay/boundary | Thermal mismatch, residual stress | MT, PT | Reduce heat input, use multi-pass with lower current |
| Excessive dilution | High heat input, improper wire position | Metallography, hardness | Reduce current, increase travel speed, use smaller wire |
| Lack of fusion | Low current, improper technique | UT, RT | Increase current, correct electrode/wire angle |
| Porosity | Flux contamination, moisture | RT, UT | Dry flux, clean base metal, control gas flow |
| Excessive hardness in HAZ | Martensitic transformation | Hardness mapping | PWHT at 600–650°C, slow cooling |
The most challenging defect is cracking at the overlay/base material boundary, which results from the thermal expansion mismatch between the austenitic overlay and the ferritic-martensitic base. This mismatch creates tensile stresses at the interface during cooling, which can exceed the fracture strength of the base material. The countermeasure is to use a multi-pass approach with the first pass being a transition layer of lower carbon equivalent composition, followed by the final corrosion-resistant overlay.
Engineering Practice Implications
The study provides practical guidance for the manufacturing of cladded 15CrMo tube sheets in the petrochemical and power generation industries:
- Design considerations: The cladding thickness must be sufficient to provide corrosion protection (typically 1.5–3.0 mm for severe service) while maintaining tube sheet structural integrity. The tube hole drilling must account for the overlay thickness to ensure proper tube-to-tubesheet fit.
- Welding procedure qualification: Qualification per NB/T 47014 or ASME Section IX is required, including mechanical property testing, metallographic examination, and non-destructive examination of the overlay layer.
- Non-destructive examination: Surface examination by MT or PT is required for the overlay layer, while UT or RT is required for the base material welds. The bond strength between the overlay and base must be verified by shear testing or ultrasonic examination.
- Post-weld heat treatment: PWHT at 600–650°C for 2 hours is essential to relieve residual stresses, temper martensitic structures, and reduce the risk of delayed cracking.
- Hydrostatic testing: After fabrication, the pressure vessel must undergo hydrostatic testing at 1.25 times the design pressure to verify weld integrity and structural soundness.
The study also addresses the challenge of tube hole drilling in cladded tube sheets. The overlay material must be removed from the tube hole area to ensure proper tube insertion and sealing. This requires careful control of the cladding thickness and the use of precision drilling or reaming operations.
Study Insights and Conclusions
The research by Zou Yuqing and Zhang Shu provides a comprehensive framework for the cladding of 15CrMo tube sheets, addressing the metallurgical challenges, process parameters, defect prevention, and quality control measures required for reliable manufacturing. The key insight is that the cladding process must be designed to manage the thermal mismatch between the base material and overlay while maintaining the corrosion resistance of the overlay layer. For engineers in the petrochemical and power generation industries, this work provides a practical guide for consumable selection, process qualification, and quality assurance in the manufacturing of cladded 15CrMo tube sheets for high-temperature, high-pressure applications.