What are the specific impacts of the mechanical degreasing treatment process on the quality

The mechanical degreasing treatment process removes oil stains from the surface of stainless steel pipes through physical or mechanical actions. The process parameters and operation specifications have a direct impact on the quality of the pipes. The specific impacts are as follows:

1. Surface cleanliness and compatibility with subsequent processes

Harm of residual oil stains:

If degreasing is not thorough (oil residue > 10mg/m²), the passivation process will be hindered, resulting in an uneven passivation film (such as local areas without a film layer) and a decrease in corrosion resistance (rust spots appear earlier in salt spray tests).

Residual oil stains during welding will decompose at high temperatures, generating gas, which is prone to form pores (diameter > 0.5mm), reducing the strength of the weld seam (tensile strength may decrease by 15% - 20%).

Cleanliness standard:

Qualified state: continuous water film (rupture angle <10°), no oil stains found during wiping test, ensuring that subsequent passivation, welding, and other processes are not contaminated. 

II. Influence of Surface Roughness and Mechanical Properties

Physical effects of mechanical degreasing:

Sandblasting degreasing (e.g., Al₂O₃ abrasive, particle size 0.1 - 0.5mm):

Advantages: Can simultaneously remove oxide scale, increasing the surface roughness Ra to 1.6 - 3.2 μm, enhancing the adhesion of the passivation film (no peeling of scratches during the adhesion test).

Risks: If the abrasive particle size is too large (> 0.5mm) or the pressure is too high (> 0.6MPa), it may cause the surface scratch depth > 0.3mm, becoming a stress concentration source, reducing the fatigue strength (cycle number decreases by approximately 30%).

Ultrasonic degreasing (frequency 20 - 40kHz):

Advantages: Through cavitation effect, it can remove oil stains in crevices, having little impact on surface roughness (Ra variation <0.2μm), suitable for precision welded pipes (such as medical device pipes).

Note: Excessive power (> 1000W) may cause microcracks (especially in the sensitization temperature zone of 304 material). 

III. Influence of Material Intergranular Corrosion Sensitivity

Potential risks of alkaline degreasing agents:

Strongly alkaline degreasing agents (NaOH concentration > 15%, temperature > 70℃) may cause chromium deficiency at the grain boundaries of 304/316L and other austenitic stainless steels, increasing the tendency of intergranular corrosion (tested according to ASTM A262 E method, the corrosion rate after sensitization > 0.05mm/year).

Improvement measures: Use weakly alkaline degreasing agents (pH value 9~11), add corrosion inhibitors (such as silicates), control the temperature ≤ 60℃, and avoid the precipitation of carbides at the grain boundaries. 

IV. Dimension Accuracy and Appearance Impact

Precision impact of mechanical force:

For drum degreasing (rotation speed > 20 rpm), if the welded pipe is not fixed, it may result in excessive ellipticity (standard ≤ 0.5%), especially for thin-walled pipes (wall thickness <2mm), where the deformation risk is higher (ellipticity may reach 1% - 2%).

For high-pressure water degreasing (pressure > 50 MPa), the spray angle needs to be controlled (45° - 60°) to avoid local impact causing surface depression (depth > 0.1mm).

Appearance quality:

Qualified standard: No mechanical damage, color change on the surface, uniform metallic luster; cases： Inconsistent sandblasting results in surface pitting， insufficient ultrasonic cavitation leading to local oil residue (visible oil spots). 

V. Typical Defects and Process Improvement Cases

Sandblasting Degreasing

Common quality issues: surface scratches, uneven roughness

Improvement measures: Switch to glass bead abrasive (particle size 0.05 - 0.2mm)

Effect: Scratch depth <0.1mm, Ra control 1.6μm 

Alkaline drum degreasing

Common quality issue: Excessive ellipticity of welded pipes

Improvement measure: Add elastic support device, rotational speed ≤ 15 rpm

Effect: Ellipticity ≤ 0.3% 

Ultrasonic degreasing

Common quality issues: Oil residue remaining at the gaps

Improvement measures: Combined with spray rinsing (flow rate ≥ 10L/min)

Effect: Cleanliness improved to oil content <5mg/m² 

Summary of Key Control Parameters

Sanding pressure

Recommended parameter range: 0.3 - 0.5 MPa (for Al₂O₃ abrasive)

Core impact on quality: Uniformity of roughness, avoiding excessive damage 

Alkaline degreasing temperature

Recommended parameter range: 50 - 60℃ (pH value 9 - 11)

Core impact on quality: Prevent intergranular corrosion, ensure degreasing efficiency 

Ultrasonic power

Recommended parameter range: 500 - 800W (frequency 30kHz)

Core impact on quality: Moderate cavitation effect, avoiding material damage 

Rolling speed

Recommended parameter range: ≤ 15 rpm (for thin-walled tubes, ≤ 10 rpm)

Core impact on quality: Control dimensional accuracy, prevent deformation 

Conclusion: Mechanical degreasing requires a balance between cleaning efficiency and material protection. By optimizing parameters such as abrasive, temperature, and mechanical force, surface damage, intergranular corrosion, and dimensional deformation can be avoided, ensuring that the subsequent processes of the welded pipe meet the quality and corrosion resistance standards.