Scientific and standardized grinding process after workpiece welding
Welding is a fundamental process in metal processing that significantly impacts the safety, durability, and appearance of a product, all of which depend on the quality of the weld. However, issues such as residual weld slag, spatter, excess weld material, and potential defects can create “hidden dangers” that compromise product performance. Fortunately, these problems can be effectively addressed through scientifically standardized grinding processes.
Grinding serves not only as a method for fixing imperfections but also as a vital step in optimizing weld geometry, enhancing corrosion resistance, and meeting the demands of precision applications. But how should post-weld grinding be precisely carried out? What specific techniques are applicable to various materials and types of welds?
The essential requirements for post-weld grinding include: ensuring a smooth and flat weld surface, removing weld slag and defects, limiting base material removal to no more than 5% or 0.2 mm, maintaining a grinding pattern consistent with the weld direction, and tailoring the operation based on the specific welding type (e.g., aluminothermic welding) and material (e.g., stainless steel).
1. Main Purposes of Post-Weld Grinding
Eliminating Surface Defects:
Removes weld slag, spatter, porosity, and undercut, preventing stress concentration-induced cracking.
Optimizing Geometry:
Minimizes weld reinforcement to reduce stress concentration (e.g., butt weld reinforcement should be ≤0.5 mm) and ensures smooth transitions in fillet welds to meet assembly accuracy requirements.
Improving Corrosion Resistance:
Eliminates oxide and carburized layers, particularly on stainless steel, while restoring corrosion resistance. It achieves a uniform surface that improves coating adhesion.
Meeting Flaw Detection Requirements:
Provides a smooth surface for non-destructive testing (e.g., PT/MT), avoiding false defects from interfering with judgment.
Improving Appearance Quality:
Achieves product aesthetic requirements (e.g., decorative components).
2. Basic Principles and Operating Procedures for Grinding
Necessity Control
Weld strength is usually lower than the base material. Grinding is limited to geometric adjustments or defect repair to avoid excessive grinding that weakens the strength. For special welds such as aluminothermic welds, the grinding range must be strictly controlled.
Surface Treatment Requirements
Remove weld slag, spatter, oxide layer, cracks, porosity, and other defects. Grind to a metallic luster.
The weld should be smooth and have a fish-scale pattern, without protrusions, burrs, or sharp edges. For box-type workpieces, the weld should not be higher than the plane.
Dimensions and Texture Specifications
The amount of base material removed should not exceed 5% of the thickness or 0.2mm. Grinding depth should be uniform.
The texture must be parallel and consistent along the weld direction; irregular grinding is prohibited.
In the preparation stage, first check the area around the weld for spatter, porosity, and cracks. Mark the areas that need to be treated with a marker.
Phased Grinding Requirements:
Pre-welding grinding should cover 20-50 mm on both sides of the weld. Remove the oxide film and grind to achieve a metallic luster, ensuring that the texture is parallel to the direction of the weld.
Post-welding Grinding:
Excess Height Grinding:
Grind excess height parallel to the weld, covering 20-50mm on both sides, while ensuring not to damage the base material in recessed areas.
Defect Handling:
Scratches or spatter areas require directional grinding within a rectangular frame, removing no more than 0.2mm.
Joint Treatment:
Use a straight grinder to remove defects at the arc starting/ending points, ensuring no sharp edges.
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.
Post-welding weld grinding (permissible grinding 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. Treatment for Excessive Weld Reinforcement, Excessive Weld Leg, or Asymmetrical Fillet Welds:
The grinding direction should be parallel to the weld’s stress direction. Begin by using a disc grinding wheel to grind the weld. After that, switch to a conical grinding wheel to smooth the weld surface.
c. Grinding treatment for slight undercut: An undercut depth of up to 0.5mm, confirmed to be within the standard acceptable range, is permitted using a cloth grinding wheel, ensuring a smooth transition during the grinding process.
3. Grinding Tool Selection
Tool Matching:
Select a grinding wheel, angle grinder, or straight-handle grinder based on the type of weld. For stainless steel, use flocked sandpaper to prevent repeated operations and avoid leaving marks.
1. Manual Grinding
Tools: Angle grinder (with grinding wheel/louvered disc), file, reamer, scraper, wire brush, and pneumatic chisel, etc.
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 consistently along the weld direction to avoid localized overheating, keeping stainless steel below 150℃. Maintain an angle of 15° to 30° to prevent scratching the base material.
The specific requirements for using grinding tools are as follows:
- Grinding wheel machine: Used for pre-weld cleaning, grinding of weld beads, removal of rust, oil, and other impurities from material surfaces, beveling before assembly, grinding of weld joints, interlayer cleaning, root cleaning of welds, and beveling for weld repair.
- Corner grinding machine: Used for grinding weld joints at corners; beveling for weld repair; root cleaning.
- Mill cutters: Select the appropriate mill cutter according to the different grinding spaces; used for root cleaning and grinding.
- Steel wire grinding wheel: Used to remove rust from welds or material surfaces.
- Air chipper: Used for interlayer cleaning of welds, removing oxide scale or flux residue; used for surface cleaning, removing oxide scale and spatter.
2. Automated Mechanical Grinding
Equipment: Robotic grinding systems, CNC belt sanders, etc.
Applicable Scenarios: Mass production or high-precision requirements (e.g., track welding joint tread straightness ≤ 0.3mm/m).
Advantages: High consistency, controllable dust.
3. Special Material Processing Technology
1) For stubborn oxide stains, consider using stainless steel pickling paste. Apply the paste, let it sit for two minutes, and then rinse it off. If you encounter slag inclusions from multi-layer welding, start by using a tungsten carbide scriber to locate them. Next, utilize a 0.5 mm thin abrasive wheel to locally smooth those areas. It’s normal to see color differences after grinding, especially when working with dissimilar steel welds; sandblasting tends to be more effective than repeated grinding for achieving a uniform finish.
2) After welding aluminum alloys, a layer of black residue, known as weld black, typically remains on the surface. This residue is primarily composed of metal oxides that have vaporized at high temperatures. It is important to treat the weld black promptly to prevent these metal oxides from absorbing moisture from the air, which could lead to corrosion and negatively impact the quality of the weld.
Additionally, the arc crater at both the initiation and termination points of the weld, as well as the weld joint itself, should be ground down to ensure a smooth transition. This step is crucial in preventing stress concentration, ultimately helping to maintain the quality of the welded machining parts.
Special attention should be paid to grinding aluminum materials; the sandpaper grit should be above 240 grit, otherwise scratches invisible to 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) Post-weld grinding of titanium alloys is an essential process to ensure high weld quality, enhance part performance, and improve aesthetics. To remove spatter, slag, and temporary protective agents (like glass powder) around the weld, use a stainless steel wire brush or sandpaper with a grit of 120 mesh or higher. It is important to avoid using carbon steel tools, as they can introduce iron ion contamination to the surface of the titanium alloy.
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: To remove the oxide layer from complex structures or precision parts, immerse them for a short duration (≤5 minutes) in a mixed solution of hydrofluoric acid (HF) and nitric acid (HNO₃) at a ratio of 1:3 to 1:5. 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 to 400 mesh sandpaper or an elastic grinding disc, gradually reducing the surface roughness to a level of Ra ≤ 0.8 μm. Use a cross-grinding method—first grinding longitudinally, then transversely—to ensure a uniform surface.
For edge treatment, chamfer the base material on both sides of the weld at dimensions of 0.5 to 1 mm with a 45° angle to eliminate sharp edges and prevent stress concentration.
Immediately after grinding, perform pickling and passivation. This can be done by immersing the material in a solution of 5% nitric acid and 2% hydrofluoric acid at room temperature for 10 to 20 minutes to form a dense oxide film. After passivation, rinse the material with deionized water and dry it thoroughly to avoid corrosion due to residual acid.
Be aware that titanium alloys are prone to oxidation at temperatures above 400°C. Therefore, continuous cooling is essential during grinding to prevent localized overheating. After grinding, ensure that the surface hardness is measured (HV ≥ 300), surface roughness is maintained (Ra ≤ 0.8 μm), and corrosion resistance is tested (no rust after 500 hours of salt spray testing).
4. Grinding Safety Measures
- Always wear appropriate personal protective equipment (PPE) during grinding and cutting operations. This includes mandatory safety goggles and dust masks to protect against metal dust.
- Install protective covers on equipment to prevent flying fragments from grinding wheels.
- When smoothing local weld seams, avoid excessive grinding of the base material.
- Follow the operating instructions carefully when using mechanical tools.
- Keep ties and scarves away from mechanical grinding tools to prevent entanglement and suffocation. Also, ensure that long hair is secured to avoid getting caught in tools and accessories.
- Do not stand in the grinding direction to prevent injury from sparks and metal shavings.
- Before grinding, make sure the workpiece is securely clamped. Never grind small parts by hand.
- Clean the work area immediately after finishing and keep the workstation tidy.
5. Post-Grinding Quality Verification
Surface roughness inspection should ensure that Ra is 6.3μm or lower. Before conducting penetrant testing, make sure to clean any oil stains on the surface, particularly around container welds. Avoid excessive grinding; the reduction in base material thickness must not exceed 0.5mm (for example, during track grinding). Additionally, grooves in the fusion line area should not exceed a depth of 0.1mm. Specific quality requirements after grinding should be followed according to the project’s specific needs.
6. Typical Cases
A five-axis CNC grinding machine, along with a vacuum dust extraction system, provides high-precision, low-dust grinding, ensuring a seamless transition between the weld and the base material.
Medical Device Implants: Ultrasonic-assisted grinding, combined with electrolytic polishing, achieves biocompatibility requirements (surface roughness Ra≤0.1μm).
7. Welding and Grinding Summary
By carefully selecting grinding tools, precisely controlling process parameters, and using post-processing techniques, the performance of various alloy welded joints can be significantly enhanced, meeting the high standards of reliability, durability, and aesthetics demanded in high-end fields.
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