In recent years, high-temperature and high-pressure corrosion-resistant pressure vessels used in petrochemical and coal chemical industries have become increasingly large. For major equipment requiring comprehensive consideration of strength and corrosion resistance, the inner walls often require surfacing with austenitic stainless steel or nickel-based alloys. To enhance production efficiency and product quality, submerged arc surfacing technology has been widely applied for large-area inner wall surfacing. However, due to the unique shape of conical sections, successful submerged arc surfacing is rare in China. Based on the factory's actual conditions, the focus was placed on the feasibility and reliability of production, leading to research and development of specialized equipment. By formulating reasonable processes and parameters, the use of submerged arc surfacing improved productivity while ensuring product quality.
**1. Overview**
A factory was contracted to produce lock cylinder containers for a coal chemical project. The container belonged to Class II (SAD), with a base material of 16MnR and a corrosion-resistant layer of 00Cr17Ni14Mo2. Design parameters are listed in Table 1.
The inner diameter of the container was 2190 mm, with a length of approximately 8586 mm. A 5 mm thick corrosion-resistant composite layer was required, making the welding process challenging. The cone section ranged from DN2200 to DN350 mm, with a length of 2170 mm. The quality of the surfacing weld was strictly controlled, requiring a smooth surface after surfacing. Any depression between adjacent welds should not exceed 1 mm, and the unevenness of the weld joint must be ≤ 1 mm. The device structure is shown in Figure 1.
(1) Main Surfacing Technology Requirements: Before surfacing, the base metal surface must be 100% magnetic tested for cracks or defects. Each weld overlay layer must be 100% penetrant tested, and 100% ultrasonic tested after surfacing. After the transition layer surfacing, stress relief heat treatment is performed before depositing the surface layer. The chemical composition within 3 mm of the surfacing layer must match the surface layer. Before heat treatment, the ferrite content of the weld overlay layer must be measured, with a requirement of 4% to 10%.
(2) Features of Electrode Surfacing: Electrode surfacing is a cost-effective and fast method for surface modification. It offers a lower dilution rate and higher deposition rate compared to other surfacing techniques, resulting in better surfacing properties.
Key advantages include:
- High current can be used due to low resistance heat, increasing production efficiency.
- Penetration depth can be controlled within 1 mm, resulting in a low dilution rate and high-quality weld overlay.
- The flux-to-solder ratio is 0.4–0.5, saving flux and reducing costs.
- Minimal alloy element loss and higher plasticity and toughness compared to submerged arc welding.
Since the carbon diffusion layer is narrow and the martensite zone is small, the joint performance is superior to that of silica electrode submerged arc surfacing.
**2. Selection and Chemical Composition of Surfacing Materials**
According to the design requirements, the transition layer material was E309MoL-15, and the surface layer material was E316L-15. The transition layer used H309LMo, and the surface layer used H316L welding wire with a specification of 60 mm × 0.5 mm, along with SJ304 flux. Their chemical compositions are shown in Tables 2 and 3.
**3. Surfacing Process Evaluation**
Electrode stacking welding uses high current and voltage to ensure the thickness of the surfacing layer, improving productivity. However, increasing current and voltage raises the dilution rate. Similarly, increasing welding speed also increases the dilution rate. To maintain a low dilution rate, the welding parameters should not be too high.
During surfacing, if the pressure is too low, the arc directly acts on the base metal, increasing the dilution rate and forming stress concentration. If the pressure is too high, the weld surface becomes uneven and wastes material. After multiple adjustments, the most stable and well-formed pressure was found to be 8–10 mm.
The surfacing process was evaluated according to JB 4708-2000, with a test piece size of 300 mm × 600 mm × 25 mm. The best welding parameters and assessment results are shown in Table 4 and Table 5.
**4. Cone Surfacing Design**
Due to the weight of the container and the inclusion of a cone, the cone often "slips" when rotated on standard tires, affecting the build-up and quality of the weld. To ensure uniform rotation, appropriate surfacing parameters were determined, and specialized welding equipment was developed. A tire for cone-stack welding was designed as shown in Figures 2 and 3. Due to the smaller diameter at the small end, the welding parameters were gradually reduced. The welding current and voltage were decreased by 20A and 3V per stage, and the welding speed was slowed to reduce heat input. After 3–5 passes, the strip was repositioned to maintain a 30–40 mm dry end. The taper at the strip end was cut to 120° for easier arc ignition. The tire design was patented under ZL200820111544.5.
**5. Surfacing Process Development**
Based on the above evaluation, the following surfacing process was developed:
(1) Magnetic powder inspection was conducted on the base metal surface before surfacing to ensure no cracks were present. The base metal surface was smoothed and qualified via radiographic testing before surfacing. The surfacing surface must be clean, free of oil and impurities.
(2) Preheating of the base metal was carried out at 150°C to control cooling rates and reduce hydrogen diffusion. An integral heating box was used for the entire cone, followed by placing the tire on the welding area. During welding, continuity was maintained to ensure sufficient interlayer temperature. For joints, a Harvard-type heating sheet was used for simultaneous heating and welding.
(3) In the transition layer surfacing, H309LMo welding tape and SJ304 flux were used, with strict adherence to assessed parameters. After transition layer surfacing, dehydrogenation treatment was performed at 300–350°C for 2 hours, followed by PT testing. Stress relief treatment was then applied at (620 ± 20)°C for 2.5–3 hours.
(4) Surface layer surfacing used H316L welding tape and SJ304 flux, with strict adherence to assessed parameters. After surfacing, PT testing was performed, followed by UT detection.
(5) Ferrite content was tested at 18 points, with results ranging from 4% to 10%, meeting the requirements.
**6. Welding Defects and Preventive Measures**
(1) Strict implementation of welding parameters to avoid excessive current and slow welding speeds, which cause high dilution rates and poor hardness.
(2) Strict insulation measures to prevent temperature fluctuations that may lead to cracks. Post-weld heat treatment was performed promptly after transition layer surfacing.
(3) Thorough cleaning before welding to remove surface impurities, slag, and coatings that could cause defects.
**7. Conclusion**
(1) Following the described process and conducting non-destructive testing according to JB4730 ensured surfacing quality.
(2) The welded products met all standards and requirements, proving the effectiveness of the process.
(3) A reasonable cone tire was designed to ensure surfacing quality and successful completion of the process.
(4) Using H309LMo and H316L welding strips with SJ304 flux, along with proper welding parameters, preheating, and post-weld heat treatment, were key to ensuring surfacing quality.
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