A pressure vessel manufactured in a factory is constructed using SUS316L stainless steel with a plate thickness of 16mm. As 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, generally requiring no special process measures. However, when using submerged arc welding (SAW), the heat input tends to be high. Due to the low thermal conductivity of stainless steel and its high coefficient of linear expansion, the welded joint may experience significant tensile stress during cooling. Austenitic stainless steels are also prone to forming columnar grain structures with strong directional growth, which can lead to impurity segregation, especially sulfur and phosphorus, increasing the risk of hot cracking. Therefore, it is 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**: Due to the unique physical properties of 316L stainless steel, the melting rate of the wire and base metal is significantly higher than that of carbon steel under similar welding conditions. Adjustments must be made to the welding parameters to achieve the desired joint quality.
(2) **Hot Cracking in the Weld Center**: The low thermal conductivity of 316L means that the weld joint remains at high temperatures for longer periods. This leads to higher tensile stress in the center area, which contributes to hot cracking. Additionally, the formation of columnar grains enhances the risk of cracking due to their directional nature.
(3) **Corrosion Resistance Reduction**: The heat-affected zone (HAZ) of 316L can suffer from reduced corrosion resistance due to the precipitation of chromium carbides along grain boundaries, leading to chromium depletion. High heat input from SAW and poor cooling rates exacerbate this issue.
**3. Selection of Welding Materials**
(1) **Welding Wire**: The selected wire is H00Cr19Ni12Mo2, an ultra-low carbon austenitic type. It has a composition close to the base material and ensures a fully austenitic structure in the weld, enhancing toughness. The chromium content is slightly higher to compensate for any loss during welding.
(2) **Flux Selection**: Two types of fluxes are commonly used: smelting and sintered. Sintered fluxes like SJ601 are preferred because they allow better control over ferrite content and reduce the risk of hot cracking by promoting a dual-phase (A + 5%F) structure. They also help prevent intergranular corrosion by transferring elements like Cr, Ti, and Nb to the weld metal. Smelting fluxes, such as HJ260, are less effective in this regard. SJ601 should be dried at 350°C for two hours before use.
**4. Welding Process**
(1) **Groove Preparation**: The groove form is shown in the drawing. The surface within 50mm of the groove and its sides is cleaned with an electric stainless steel brush to expose metallic luster. White chalk powder is applied to prevent spatter.
(2) **Welding Parameters**: Primary welding current ranges from 550–570A, arc voltage 36–38V, and speed 46–48cm/min. Back welding uses 600–650A, 38–40V, and 48–50cm/min.
**5. Key Precautions**
(1) Avoid random arcing on the workpiece to prevent surface damage and corrosion.
(2) The first layer is manually arc-welded using A022 electrodes (φ4.0mm). Keep the arc short and move forward in straight lines to prevent thermal cracking.
(3) Use minimal flux to avoid poor bead formation and surface defects.
(4) Follow the WPS strictly, and weld quickly to keep interpass temperatures below 100°C, reducing overheating and improving crack resistance.
(5) For circumferential seams, weld the inner side upward and the outer side downward to ensure proper solidification.
(6) Weld areas in contact with corrosive media last to avoid overheating.
(7) Use a welding fan to accelerate cooling if needed.
(8) Clean slag and spatter after welding with a stainless steel brush.
(9) Ensure the arc-start and arc-end plates match the base material in thickness and have no gaps.
(10) Fill craters completely to prevent solidification cracks.
(11) If reworking is required, use carbon arc gouging carefully and clean thoroughly.
(12) Maintain alignment and adjust offsets as needed during welding.
(13) Address unstable arcs promptly to prevent defects like burn-through or slag inclusion.
(14) Keep the weld width within 20mm and depth between 1–2mm for a neat appearance.
**6. Conclusion**
After completing the welding, radiographic testing was performed according to JB4730.2-2005. A 20% inspection rate was applied, with all joints meeting grade III standards. The A and B seam acceptance rate exceeded 98%, and the deformation was minimal. This process has been proven to be practical and reliable.
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