Grinding after welding

Welding, as a core process in metal processing, directly determines the safety, durability, and aesthetics of a product based on weld quality. The core requirements for post-weld grinding include ensuring a smooth and flat weld surface, removing weld slag and defects, controlling the amount of base material removed to no more than 5% or 0.2mm, ensuring the grinding pattern is consistent with the weld direction, and adjusting the specific operation according to the welding type (e.g., aluminothermic welding) and material (e.g., stainless steel).

A. Main Purposes of Post-Weld Grinding

1. Eliminating Surface Defects: Removing weld slag, spatter, porosity, and undercut, preventing stress concentration-induced cracking risks.

2. Optimizing Geometry: Reducing weld reinforcement height to minimize stress concentration (e.g., butt weld reinforcement height should be ≤0.5mm); ensuring smooth transitions in fillet welds to meet assembly accuracy requirements.

3. Enhancing Corrosion Resistance: Removing oxide and carburized layers (especially in stainless steel), restoring material corrosion resistance; obtaining a uniform surface to enhance coating adhesion.

4. Meeting Inspection Requirements: Providing a smooth surface for non-destructive testing (e.g., PT/MT) to avoid false defects interfering with judgment. 4. Improve Appearance Quality: Achieve product aesthetic requirements (e.g., decorative components).

B. Basic Principles and Operating Procedures for Grinding

1. Necessity Control: Weld strength is usually lower than the base material. Grinding is limited to geometric adjustment or defect repair to avoid excessive grinding that weakens strength. Special welds such as aluminothermic welds require strict control of the grinding range.

2. Surface Treatment Requirements: Remove weld slag, spatter, oxide layer, cracks, porosity, and other defects. Grind to a metallic luster.

3. Welds must be smooth and have a fish-scale pattern, without protrusions, burrs, or sharp edges. Welds on box-type workpieces must not be higher than the plane.

4. Dimension and Texture Specifications: The amount of base material removed should not exceed 5% of the thickness or 0.2mm. Grinding depth should be uniform. Texture must be parallel and consistent along the weld direction; irregular grinding is prohibited.

5. In the preparation stage, first check the area around the weld for spatter, porosity, and cracks. Mark the areas that need treatment with a marker.

Staged Grinding Requirements:

1. Pre-welding Grinding: Covering 20-50mm on both sides of the weld, remove the oxide film and grind to a metallic luster, with the grinding pattern parallel to the weld direction.

2. Post-welding Grinding.

3. Excess Height Grinding: Covering 20-50mm on both sides of the weld, use an angle grinder to remove excess height parallel to the weld, ensuring no damage to the base material in recessed areas.

4. Defect Handling: Scratches or spatter areas require directional grinding within a rectangular frame, removing no more than 0.2mm.

5. Joint Treatment: Use a straight grinder to remove defects at the arc initiation/exit points, ensuring no sharp edges.

6. Special Material Treatment: Stainless steel requires coarse grinding (80-120 grit), medium grinding (180-240 grit), fine grinding (400-600 grit), and polishing to restore luster.

7. Post-welding weld grinding (permissible grinding treatment of welding defects) specific requirements are as follows:

a. Grinding of welded joints. Post-weld grinding of defective weld joints is permitted, with a smooth transition on the weld surface.

b. Grinding treatment for excessive weld reinforcement, excessive weld leg, or asymmetrical fillet welds: Grinding direction should be parallel to the weld stress direction. After grinding with a grinding wheel, use a conical grinding wheel to smooth the weld surface.

c. Grinding treatment for slight undercut: Undercut depth not exceeding 0.5mm and confirmed to be within the standard acceptable range is permitted using a cloth grinding wheel, with a smooth transition during grinding.

3. Grinding Tool Selection

Tool Matching: Select a grinding wheel, angle grinder, or straight-handle grinder according to the weld type; stainless steel is preferred, use flocked sandpaper to avoid repeated operations and leaving marks.

a. Manual Grinding

Tools: Angle grinder (with grinding wheel/louvered disc), file, reamer, scraper, wire brush, and pneumatic chisel, etc.

b. Grinding Wheel Selection: Coarse Grinding (Removing Weld Beads): 60-80 grit alumina grinding wheel; Fine Grinding (Surface Treatment): 120-240 grit sandpaper/fiber disc.

Operating Procedures: Grind at a uniform speed along the weld direction, avoiding localized overheating (keeping stainless steel below 150℃); maintain a 15°-30° angle to prevent scratching the base material.

Specific grinding tool requirements are as follows:

1) Grinding Wheel Machine: Used for pre-weld cleaning and grinding of weld bead; removing rust, oil, and other impurities from the material surface; beveling before assembly; grinding weld joints; cleaning and grinding between weld layers and root cleaning; grinding beveling for weld repair.

2) Corner Grinding Machine: Used for grinding weld joints at corners; grinding beveling for weld repair; grinding for root cleaning.

3) Mill Cutters: Select the appropriate mill cutter according to the different grinding spaces; used for root cleaning and grinding. 4) Steel wire grinding wheel: Used to remove rust from welds or material surfaces.

5) Air chipper: Used for interlayer cleaning of welds, removing oxide scale or flux residue. Also used for surface cleaning, removing oxide scale and spatter.

2. Mechanical Automated Grinding

Equipment: Robotic grinding system, CNC belt sander, etc.

Applicable Scenarios: Mass production or high-precision requirements (e.g., track welded joint tread surface straightness ≤ 0.3mm/m).

Advantages: High consistency, controllable dust.

3. Special Material Processing

1) For stubborn oxide stains, try stainless steel-specific pickling paste. Apply, let stand for two minutes, then rinse. For slag inclusions from multi-layer welding, first use a tungsten carbide scriber for positioning, then use a 0.5mm thin grinding wheel for local trimming. Color differences after grinding are normal, especially at dissimilar steel welded areas; sandblasting is more effective than repeated grinding.

2) After aluminum alloy welding, a layer of weld black (the key component being metal oxide volatilized at high temperatures) usually remains on the weld surface. This weld black should be treated immediately to prevent the metal oxide from absorbing moisture from the air and corroding the weld surface, thus affecting weld quality. Additionally, after welding, the arc crater at the weld initiation and termination points, as well as the weld joint, should be ground to ensure a smooth transition and prevent stress concentration, thereby ensuring the quality of the workpiece welding. Special attention should be paid to grinding aluminum; the sandpaper grit should be above 240 grit, otherwise scratches that are difficult to see with the naked eye will appear. To check for defects, shine a strong flashlight at an angle; a continuous light band on the reflective surface indicates acceptance.

3) Grinding after titanium alloy welding is a crucial step in ensuring weld quality and improving the performance and aesthetics of the parts. Use a stainless steel wire brush or sandpaper (grit ≥ 120 grit) to remove spatter, slag, and temporary protective agents (such as glass powder) around the weld. Avoid using carbon steel tools to prevent iron ion contamination of the titanium alloy surface.

Oxide Removal

Mechanical Grinding: Use carbide (YG8) or ceramic grinding wheels with a coolant (such as an emulsion) to prevent overheating and oxidation.

Chemical Pickling: For complex structures or precision parts, a short immersion (≤5 minutes) in a mixed solution of hydrofluoric acid (HF) and nitric acid (HNO₃) (ratio 1:3~1:5) can be used to remove the oxide layer. Immediately rinse with water and dry.

Rough Grinding Stage: Use an electric angle grinder with a carbide grinding head or fiber-reinforced resin grinding wheel (80~120 mesh) to remove weld excess and obvious defects.

Key Point: Control the feed rate to avoid localized overheating that could degrade material properties.

Fine Grinding Stage: Switch to 240~400 mesh sandpaper or an elastic grinding disc, gradually reducing the surface roughness to Ra≤0.8μm. Use a cross-grinding method (first longitudinal, then transverse) to ensure surface uniformity.

Edge treatment involves chamfering the base material on both sides of the weld (0.5~1mm × 45°) to eliminate sharp edges and prevent stress concentration.

Immediately after grinding, perform pickling and passivation (e.g., immersion in a 5% nitric acid + 2% hydrofluoric acid solution at room temperature for 10~20 minutes) to form a dense oxide film. After passivation, rinse with deionized water and dry to avoid corrosion from residual acid.

Titanium alloys are easily oxidized above 400℃; continuous cooling is required during grinding to avoid localized overheating. After grinding, surface hardness (HV≥300), roughness (Ra≤0.8μm), and corrosion resistance (no rust after ≥500 hours of salt spray test) must be tested.

IV. Grinding Safety Measures

1. Wear appropriate personal protective equipment during grinding and cutting operations; mandatory wearing of safety goggles and dust masks (to prevent metal dust);

2. Install protective covers on equipment to prevent grinding wheel fragments from flying.

3. When smoothing local weld seams, avoid excessive grinding of the base material.

4. Strictly follow the operating instructions when using machine tools.

5. When using mechanical grinding tools, avoid bringing ties/scarves near the tools and accessories to prevent entanglement and suffocation, and avoid hair getting caught in the tools and accessories.

6. Standing in the grinding direction is strictly prohibited to prevent injury from sparks and metal shavings.

7. Before grinding, ensure the workpiece is securely clamped; never grind small parts by hand.

8. Clean the work area immediately after completion and keep the workstation tidy.

V. Quality Verification After Grinding

Surface roughness inspection (Ra≤6.3μm); Clean surface oil before penetrant testing (especially container welds).

Avoid excessive grinding: Base material thinning ≤0.5mm (e.g., rail grinding); Grooves must not form in the fusion line area (depth ≤0.1mm).

Specific post-grinding quality requirements shall be implemented according to the specific project requirements.