What are the key points of the polishing process for common industrial precision steel pipe fitting

First, what are the key points of the pretreatment process before polishing precision steel pipe fittings?

Pretreatment is the prerequisite for ensuring the polishing effect and avoiding secondary damage. The core objective is to thoroughly remove surface impurities and repair basic defects, laying a uniform and clean surface foundation for subsequent polishing processes. The quality of this pretreatment directly affects the final polishing accuracy.

(I) Surface Cleaning Treatment of Precision Steel Pipe Fittings.

After machining, precision steel pipe parts are prone to the adhesion of cutting fluid residue, iron filings, dust, oil, and oxide scale. If cleaning is not thorough, impurities can easily embed into the surface during polishing, causing secondary scratches or affecting the uniformity of contact between the polishing medium and the workpiece surface. The preferred method is a combination of "neutral cleaning + ultrasonic strengthening": using a neutral cleaning agent with a pH of 7-8, combined with ultrasonic cleaning equipment, setting the frequency to 28-40kHz, the cleaning temperature to 40-60℃, and the cleaning time to 15-20 minutes. The impact force of the microbubbles generated by ultrasonic vibration thoroughly removes impurities from crevices, deep holes, and other dead corners. After cleaning, immediately rinse thoroughly with deionized water, then dry with hot air (80-100℃, 5-8 minutes) to ensure no water accumulation or cleaning agent residue on the surface, preventing surface oxidation caused by moisture residue.

(II) Surface Defect Inspection and Repair of Precision Steel Pipe Fittings.

Polishing can only remove shallow surface defects. Defects exceeding the standard must be inspected and addressed in advance to prevent magnification after polishing. Use a 10-20x magnifying glass to comprehensively inspect the surface of the parts, focusing on micro-scratches, pits, and oxide spots. Shallow scratches with a depth ≤0.005mm can be removed by subsequent fine polishing without additional treatment. For pits and scratches exceeding the standard depth or with a large area, local repair using a grinding process is necessary. Use fine-grained grinding paste (W10-W15) with a grinding rod for light grinding. After repair, the surface roughness must be controlled at Ra≤0.8μm to ensure a smooth transition between the repaired area and the surrounding surface, without any step-like appearance.

(III) Stress Relief Treatment for Precision Steel Pipe Fittings.

Precision steel pipe fittings are prone to residual stress during deep-hole machining. Direct polishing may cause deformation due to stress release, affecting dimensional accuracy and polishing uniformity. Low-temperature stress relief treatment is necessary before polishing: place the parts in a heat treatment furnace, heat to 180-220℃, hold for 1-1.5 hours, and then slowly cool to room temperature with the furnace. This gentle heating method gradually releases internal residual stress while avoiding changes in material properties caused by high temperatures. For thin-walled precision steel pipes (wall thickness ≤ 2mm), the holding temperature should be appropriately reduced (180-200℃) and the holding time extended (1.5-2 hours) to further enhance stress relief and prevent deformation during polishing.

Second, what are the key points for selecting and adapting polishing processes for precision steel pipe fittings?

The polishing process for precision steel pipe fittings needs to be selected based on differences in hole diameter, surface quality requirements, batch size, and material characteristics, balancing polishing effect, processing efficiency, and production cost. The core process types and application points are as follows.

(I) Honing and Polishing: Preferred Process for Medium-Deep Hole Parts.

Honking and polishing are suitable for medium-deep hole precision steel pipe parts with a depth-to-diameter ratio (L/D) of 5-15. They are particularly suitable for medium-to-high strength steel pipes such as 45# steel and 40Cr. Honing and polishing improve surface finish and simultaneously corrects dimensional errors such as roundness and cylindricity. Surface roughness can be controlled within Ra=0.2-0.8μm, making it the preferred solution for polishing batches of medium-deep hole parts.

Key process points:

(A) Selection of honing tools and media: Use fine-grained honing stones (grit size W10-W20) and extreme pressure honing fluid (kinematic viscosity 8-12 mm²/s at 20℃). The honing fluid needs to contain extreme pressure additives and oiliness agents to balance lubrication, cooling, and chip removal capabilities, preventing the honing stone from wearing out too quickly or its surface from being scratched.

(B) Precise parameter control: Strictly control the expansion and contraction of the honing head within 0.01-0.03 mm to prevent excessive expansion and contraction. For holes with out-of-tolerance diameters, the linear velocity is set to 80-120 m/min, and the feed rate is 0.01-0.03 mm/r. Multiple honing passes are used to gradually reduce surface roughness. After each honing pass, the hole wall and honing stone are cleaned to prevent grinding residue from affecting subsequent polishing.

(C) Adaptive adjustments: When machining high-strength steel pipes, the feed rate is appropriately reduced, and the honing fluid flow rate is increased to reduce honing stone wear. When machining thin-walled parts, the honing pressure is controlled to ≤0.1 MPa to avoid part deformation.

(II) Grinding and Polishing: A Dedicated Process for Ultra-High Precision Parts.

For ultra-deep hole parts with L/D > 15 and ultra-high precision scenarios requiring Ra ≤ 0.2 μm surface roughness, grinding and polishing achieve surface refinement through ultra-micro removal, reducing surface roughness to Ra ≤ 0.05 μm while maximizing the preservation of part dimensional accuracy, adapting to customized high-end part machining.

Key process points:

(A) Grinding media selection: Select appropriate grinding paste according to material characteristics. Use chromium oxide or aluminum oxide grinding paste (grit size W5-W10) for carbon steel and alloy steel, and diamond grinding paste for stainless steel to ensure grinding efficiency and surface quality.

(B) Grinding method optimization: For ultra-deep hole parts, adopt a segmented grinding strategy, with each segment length controlled at 50-80mm. Proceed gradually and clean grinding residue promptly to avoid impurities accumulating and scratching the machined surface; control grinding pressure at 0.1-0.3MPa, using reciprocating uniform motion to ensure grinding uniformity;

(C) Post-grinding cleaning: Immediately after grinding, use anhydrous ethanol for ultrasonic cleaning to thoroughly remove residual grinding paste and prevent the paste from hardening and affecting surface finish. (III) Electrochemical Polishing: A Process for Mass Production of Complex Structural Parts.

Electrochemical polishing relies on electrolytic reactions to dissolve protruding parts on the surface of parts, achieving rapid mirror-like finishes with a surface roughness of Ra = 0.1-0.4 μm. It also improves the corrosion resistance of the part surface and is suitable for mass production of precision steel pipe parts with complex internal structures. Its processing efficiency is significantly higher than that of mechanical polishing. Key process points:

(A) Electrolyte preparation and control: Use a phosphoric acid-sulfuric acid composite electrolyte (60%-70% phosphoric acid, 20%-30% sulfuric acid), control the temperature at 50-70℃, the current density at 10-20A/dm², and the polishing time at 3-5 minutes. Regularly monitor the electrolyte concentration and replenish the components as needed to avoid uneven polishing due to concentration imbalance.

(B) Pre-treatment strengthening: Before polishing, thoroughly degrease the parts to ensure the surface is free of oil; uneven electrolytic reaction may occur, resulting in defects such as localized darkening and spots.

(C) Post-treatment protection: Immediately after polishing, rinse thoroughly with clean water, then immerse in a 5%-8% nitric acid solution for passivation treatment for 5-10 minutes to improve surface oxidation resistance. Finally, dry and seal for storage to prevent secondary oxidation. Note that this process should be used with caution for stainless steel precision pipe parts to avoid intergranular corrosion.

(iv) Mechanical Polishing: A process for small-batch shallow-hole/external surface parts.

Mechanical polishing is convenient to operate and has low equipment modification costs. It is suitable for small-batch production of shallow-hole and precision steel pipe parts with external surfaces. Surface refinement is achieved through friction between the polishing tool and the part. The polishing precision can be flexibly adjusted according to requirements, adapting to multi-variety, small-batch processing scenarios.

Key Process Points:

(A) Tool and Media Selection: In the rough polishing stage, use yellow wax with a wool wheel to remove surface cutting marks, reducing surface roughness to Ra≤1.6μm; in the fine polishing stage, switch to white wax with a microfiber polishing cloth to further improve the smoothness until the design requirements are met.

(B) Parameter Control: Set the polishing speed to 1500-2500 r/min, and uniformly control the hand pressure at 0.05-0.1 MPa to avoid excessive local pressure leading to surface overheating, oxidation, or deformation. Simultaneously, keep the polishing tool parallel to the part surface to prevent over-polishing in certain areas.

(C) Adaptation and Adjustment: When processing thin-walled parts, reduce the polishing speed (1500-2000 r/min) and reduce hand pressure. When processing high-strength steel pipes, the speed can be appropriately increased to shorten the polishing time.

Third, What are the Quality Control and Precautions for the Polishing Process of Precision Steel Pipes and Fittings?

A full-process quality control system needs to be established for the polishing process to accurately control dimensional accuracy, surface condition, and process parameters. At the same time, material compatibility issues must be avoided to ensure stable and reliable polishing results and prevent quality defects.

(I) Dimensional and Surface Condition Control.

The polishing allowance is strictly controlled within 0.01-0.03mm. Over-polishing can easily lead to dimensional deviations such as reduced hole diameter and uneven wall thickness. During batch processing, samples are taken every 10-15 pieces for inspection. A roughness tester is used to accurately measure the surface roughness, and a coordinate measuring machine is used to verify core dimensions such as hole diameter and roundness to ensure compliance with design requirements. The condition of polishing tools is checked regularly. Honing stones are replaced immediately when the wear exceeds 0.02mm. Polishing cloths and wool wheels are replaced promptly when damaged or frayed to prevent tool wear from causing surface scratches and uneven polishing.

(II) Environmental and Media Control.

The polishing area must be kept clean, dry, and dust-free to prevent dust and oil contamination of the part surface and polishing media. The processing environment temperature is controlled at 20±2℃ to reduce thermal expansion and contraction of parts caused by temperature differences, which can affect polishing accuracy. Regularly change the polishing media. Honing slurry and polishing paste must be stored in sealed containers to prevent moisture and contamination. The contamination level of the honing slurry must be maintained below NAS level 8. The concentration and pH value of the electrolyte should be tested every 8-10 hours of use, and replenished and adjusted promptly to ensure stable media performance.

(III) Material Compatibility and Operational Restrictions.

Different materials of precision steel pipes require different polishing processes: High-strength steel pipes (hardness > 260HB) require tempering and softening treatment before polishing to reduce polishing difficulty and tool wear; avoid electrochemical polishing for stainless steel parts, and prioritize grinding or mechanical polishing to prevent surface corrosion; thin-walled steel pipes (wall thickness ≤ 2mm) should prioritize grinding or gentle mechanical polishing, strictly controlling polishing pressure and speed to avoid deformation risks. During operation, avoid keeping polishing tools in the same position for extended periods to prevent localized overheating and surface oxidation and discoloration; after polishing, clean, dry, and seal promptly to avoid secondary contamination and oxidation.

Fourth, what are the application value and optimization directions of polishing technology for precision steel pipe fittings?

Scientific and reasonable polishing treatment can not only significantly improve the surface finish of precision steel pipe parts but also enhance surface wear resistance, corrosion resistance, and sealing performance, effectively extending the service life of parts and expanding their application scenarios in high-end manufacturing. By optimizing the polishing process and parameters, the surface defect rate of parts can be controlled within 0.5%, while improving polishing efficiency by more than 20%, balancing quality and economy.

In the future, as high-end manufacturing continues to demand higher surface quality from parts, polishing processes can be optimized towards intelligence and precision: combining online monitoring technology to detect surface roughness and dimensional changes in real time during polishing, achieving automatic adjustment of polishing parameters; developing specialized polishing media and tools, and customizing polishing solutions for precision steel pipe parts with special materials and complex structures; integrating green manufacturing concepts, selecting environmentally friendly polishing media, reducing waste liquid and residue emissions, and achieving a high-efficiency, environmentally friendly, and precise upgrade of the polishing process.