| Base Material | Confirm that the material is 304 or 304L stainless steel and identify the thickness before selecting the joint design. | Common fabrication range: approximately 1.5–12 mm sheet or plate. | Material grade and thickness affect filler selection, heat input, distortion, and the required welding procedure. |
| Joint Cleaning | Remove oil, grease, paint, moisture, oxides, and shop dirt from both sides of the joint. Use a dedicated stainless-steel brush or clean abrasive that has not contacted carbon steel. | Clean at least 25–50 mm on each side of the joint. | Prevents porosity, contamination, arc instability, and embedded iron that can later cause surface rust. |
| Edge Preparation: Thin Material | For thin sheet, use a square-butt joint when full penetration can be achieved without excessive burn-through. | Typically up to about 3 mm thickness; joint gap commonly 0–1 mm. | Keep the fit-up uniform and use low heat input. Confirm penetration on a test coupon when the joint is critical. |
| Edge Preparation: Medium Thickness | Use a single-V groove for thicker material when a square edge cannot provide reliable full penetration. | Typical included angle: 60–70°; root face: 1–2 mm; root opening: 1–3 mm. | These are practical starting dimensions. Final values should follow the qualified welding procedure and applicable code. |
| Edge Preparation: Heavy Section | For thicker sections, use a double-V, U-groove, or another balanced preparation when access is available from both sides. | Often considered above approximately 10–12 mm; use balanced grooves to reduce weld volume and distortion. | Back-gouge or back-grind the first side to sound metal before welding the second side. |
| Root Gap | Set the root opening with clean, consistent spacers or calibrated fit-up tools. Avoid forcing parts together during welding. | Common starting range: 1–3 mm for a manually welded open root, depending on thickness and process. | A gap that is too small can cause incomplete penetration; an excessive gap increases burn-through, filler use, and distortion. |
| Root Face | Keep the root face even around the joint and remove sharp burrs after cutting or machining. | Typical root face: 1–2 mm for a single-V groove. | Uniform land thickness helps control penetration and reduces the risk of a locally excessive weld pool. |
| Joint Alignment | Align the parts before tacking and prevent high-low mismatch. Use temporary fixtures that allow thermal expansion. | Keep mismatch as low as practical; a common fabrication target is about 10% of the thinner wall thickness, subject to the governing specification. | Excessive offset creates stress concentration and uneven penetration. Measure with a straightedge, bridge gauge, or suitable fit-up gauge. |
| Joint Restraint | Use balanced clamping and tack-weld sequencing rather than excessive restraint. Do not use galvanized or contaminated clamps at the weld area. | Use evenly spaced tacks sized for the thickness; remove cracked, porous, or contaminated tacks completely. | Balanced restraint limits distortion while still allowing controlled shrinkage during cooling. |
| Filler Metal | For standard 304 stainless-steel joints, select a compatible low-carbon austenitic stainless filler. | Common choice: ER308L for TIG or MIG welding; use the filler specified by the qualified procedure for the service conditions. | The “L” designation limits carbon in the deposited weld metal and helps reduce sensitization risk during thermal exposure. |
| TIG Shielding Gas | Use clean, dry argon with stable coverage. Keep the torch angle low and maintain a short, consistent arc length. | Typical argon flow: approximately 7–12 L/min, adjusted for torch, cup size, draft, and joint geometry. | Too little gas can cause oxidation and porosity; excessive flow can create turbulence and draw air into the shielding zone. |
| Back Purging | For open-root pipe, tube, or full-penetration joints where the reverse side will be exposed, purge the root with argon. | Typical purge flow: approximately 5–12 L/min after the enclosure is displaced; maintain positive, low-turbulence coverage. | Back purging prevents heavy oxidation, known as sugaring, which can reduce corrosion resistance and impair root quality. |
| MIG Shielding Gas | Use a qualified stainless-steel GMAW procedure with a low-oxidizing shielding-gas mixture appropriate for the transfer mode. | Common starting range: 12–18 L/min; exact composition and flow depend on wire diameter, transfer mode, and equipment. | Do not use an unqualified gas or excessive flow. Verify bead shape, arc stability, and porosity on a test piece. |
| Heat Input | Use the lowest heat input that provides complete fusion and penetration. Prefer multiple controlled passes over one excessively hot pass on thick material. | Keep travel speed, current, voltage, and pass sequence consistent; avoid prolonged arc stops on thin sections. | Lower heat input reduces distortion, discoloration, carbide precipitation risk, and loss of corrosion performance. |
| Interpass Temperature | Allow the joint to cool between passes and monitor the temperature near the weld area. | A common control limit is below 150°C between passes unless the qualified procedure specifies otherwise. | Low interpass temperature helps limit sensitization, distortion, and excessive heat accumulation. |
| Tack Welds | Place clean, sound tacks symmetrically and fuse their ends into the final weld. Grind out any tack containing cracks or visible contamination. | Typical tack spacing: about 50–150 mm, adjusted for joint length, thickness, and distortion risk. | Proper tack placement maintains root gap and alignment without creating hard restraint or trapped defects. |
| Welding Sequence | Use a skip, back-step, or alternating sequence where appropriate. Weld from the center outward on long joints when practical. | Use short, controlled weld segments on thin sheet; allow cooling between opposing passes. | Sequencing distributes shrinkage and helps maintain straightness, root opening, and dimensional accuracy. |
| Interpass Cleaning | Remove slag, oxides, and discoloration between passes using stainless-only tools. Do not weld over visible contamination. | Clean until the previous bead and fusion boundaries are clearly visible. | Prevents inclusions and lack of fusion, especially when using processes or consumables that produce surface oxides. |
| Post-Weld Cleaning | Remove heat tint and surface contamination by an appropriate mechanical or chemical method, then rinse and dry thoroughly. | For corrosion-critical service, restore a clean, passive surface after welding. | Heat tint can reduce the corrosion resistance of stainless steel near the weld. Follow the approved safety procedure for any pickling or passivation chemicals. |
| Visual Inspection | Inspect the completed weld for cracks, undercut, overlap, excessive concavity or convexity, arc strikes, porosity, and incomplete fusion indicators. | Acceptance limits must come from the applicable drawing, code, or qualified welding procedure. | Visual inspection should be completed after cleaning and before painting, insulation, pressure testing, or service. |
| Technical note: The dimensions and settings above are typical fabrication starting points, not universal acceptance criteria. Final joint preparation, tolerances, filler metal, shielding, heat input, inspection, and acceptance requirements must be established by a qualified welding procedure and the applicable project standard. |
