A pressure vessel manufactured in a factory is constructed using SUS316L stainless steel with a plate thickness of 16mm. Since this is the first time this specific thickness of SUS316L material is being used, extensive research was conducted on relevant materials, and a suitable welding process was developed through welding procedure qualification tests.
**1. Welding Analysis of 316L Stainless Steel**
SUS316L is an austenitic stainless steel known for its excellent weldability, typically requiring no special measures during welding. However, when using submerged arc welding, the heat input tends to be high. Due to the low thermal conductivity of stainless steel and its high coefficient of thermal expansion, localized heating and cooling can lead to significant tensile stress during the cooling phase. Austenitic stainless steels are prone to forming columnar grain structures, which have a directional nature that can promote the segregation of impurities like sulfur and phosphorus, increasing the risk of hot cracking. To mitigate this, it's recommended to use lower power settings and faster welding speeds to avoid overheating and improve crack resistance.
**2. Characteristics of Submerged Arc Welding for 316L Stainless Steel**
(1) **Material Behavior**: The melting speed of the stainless steel wire and base metal is significantly higher than that of carbon steel under similar welding conditions. Therefore, adjustments must be made to the welding parameters to achieve the desired joint quality.
(2) **Hot Cracking in the Weld Center**: Due to the low thermal conductivity of 316L stainless steel, the weld area remains at high temperatures for longer periods, leading to increased tensile stress and strain, which contributes to hot cracking. Additionally, the tendency to form columnar grains enhances this risk.
(3) **Corrosion Resistance in Heat-Affected Zone (HAZ)**: The reduction in corrosion resistance in the HAZ is often caused by the precipitation of chromium carbides at grain boundaries, depleting chromium from the area. High heat input from submerged arc welding, combined with the material’s properties, can exacerbate this issue.
**3. Selection of Welding Materials**
(1) **Welding Wire**: The selected wire is ultra-low carbon austenitic type H00Cr19Ni12Mo2, matching the composition of the base material. It provides a fully austenitic structure with high toughness. Slight increases in chromium content help compensate for any loss during welding.
(2) **Flux Selection**: Two main types of fluxes are available—smelting and sintered. Smelting fluxes like HJ260 are less ideal due to difficulties in adjusting ferrite content and slag removal. Sintered fluxes like SJ601 offer better control over alloy elements, allowing for a dual-phase microstructure that reduces hot cracking. SJ601 also helps prevent intergranular corrosion by transferring elements like Cr, Ti, and Nb to the weld metal. It is dried at 350°C for two hours before use.
**4. Welding Process**
(1) The groove design is shown in the drawing. The surface within a 50mm range around the groove is cleaned using an electric stainless steel brush to expose the metallic luster, and white chalk powder is applied to prevent spatter.
(2) Welding parameters include a current of 550–570A, voltage of 36–38V, and a speed of 46–48 cm/min for the main weld, and 600–650A, 38–40V, and 48–50 cm/min for back welding.
**5. Key Precautions**
(1) Avoid striking arcs randomly on the workpiece to prevent surface damage and corrosion.
(2) The first layer should be welded with electrode-arc welding using A022 (φ4.0mm) rods. Keep the arc short and move straight to reduce cracking risk.
(3) Use minimal flux to avoid poor bead formation and surface defects.
(4) Follow the welding procedure specifications strictly, using fast welding to keep interpass temperatures below 100°C and reduce overheating.
(5) For circumferential seams, the inner joint is welded upward and the outer joint downward to ensure proper solidification.
(6) Welds in contact with corrosive media should be done last to prevent intergranular corrosion.
(7) A welding fan can be used to accelerate cooling and reduce heat effects.
(8) Clean the weld thoroughly after welding to remove slag and spatter.
(9) Ensure the arc-start and arc-end plates match the workpiece in thickness and composition.
(10) Fill the crater as much as possible to prevent solidification cracks.
(11) When reworking with carbon arc gouging, ensure uniform cuts and clean the area thoroughly.
(12) Maintain alignment throughout the welding process.
(13) If the arc becomes unstable or "pulls," stop immediately and investigate the cause.
(14) Keep the weld width under 20 mm and depth between 1–2 mm for a better appearance.
**6. Conclusion**
After completing the welding, radiographic testing was performed according to JB4730.2-2005, with 20% of each weld joint inspected. All results were rated as Grade III, meeting the required standards. The welding efficiency for A and B joints exceeded 98%, with minimal deformation. This process has proven to be practical and effective for welding 16mm thick SUS316L stainless steel.
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