What are the effects and removal processes of oxide scale on the surface of seamless steel pipe fittings

First, what is the core impact of oxide scale on the processing of seamless steel pipe fittings?

(1) Direct interference with the cutting of seamless steel pipe fittings

Increased tool wear: The hardness of the oxide scale is as high as HV 800-1000, far exceeding that of the pipe fitting substrate. During cutting, it is equivalent to "hard point grinding," leading to tool edge chipping and a 3-5 times faster wear rate. In particular, the integrity of the cutting edge of PCBN tools is easily damaged, shortening the life by 40%-60%.

Decreased processing accuracy: Uneven oxide scale thickness will generate uneven cutting resistance during cutting, causing tool wobble, which in turn leads to pipe size deviation and surface roughness deterioration. Thin-walled pipe fittings may also deform due to uneven stress.

Increased processing defects: The oxide scale is not tightly bonded to the substrate. During cutting, it is easy to fall off and embed into the processed surface, forming defects such as "indentations" and "pockmarks." Especially in thread processing and sealing surface processing, it will lead to excessive thread fit clearance and increased risk of sealing surface leakage.

(2) Indirect Impacts of Subsequent Processes on Seamless Steel Pipe Fittings

Potential Heat Treatment Quality Issues: Oxide scale hinders heat conduction, leading to uneven surface temperature and increased hardness dispersion (HRC deviation ≥ 4) during tempering. Impurities in the oxide scale may also penetrate the substrate, affecting material toughness.

Reduced Welding Reliability: Oxide scale can cause poor weld fusion, resulting in defects such as porosity and slag inclusions. Welding strength decreases by 20%-30%, especially in narrow-gap welding of small-diameter pipe fittings, where incomplete oxide scale removal can easily lead to weld cracks.

Surface Treatment Failure: Oxide scale affects the adhesion of subsequent treatments such as coating and electroplating, causing coating peeling, blistering of the electroplated layer, and reduced corrosion resistance of the pipe fittings.

(3) Efficiency Losses in Mass Production of Seamless Steel Pipes and Fittings. Oxide scale leads to frequent tool changes and workpiece rework (such as surface defect repair), extending the processing cycle time by 20%-30%. In mass production, the unit product processing cost increases by 15%-25%. Furthermore, the detached oxide scale may enter critical parts such as machine tool guideways and spindles, causing equipment accuracy drift.

Second, the cleaning process and technical details of mainstream seamless steel pipe fittings

(1) Mechanical cleaning process of seamless steel pipe fittings (pretreatment/post-processing cleaning)

1) Sandblasting/shot blasting

Principle: High-pressure airflow or shot impact is used to remove oxide scale. White corundum or cast steel shot with a particle size of 0.2-0.8mm is selected.

Process parameters: Sandblasting pressure 0.4-0.8MPa, spray angle 45°-60°, distance 100-150mm; shot blasting speed 60-80m/s, coverage ≥95%;

Advantages: High efficiency, can remove oxide scale from the inner and outer walls of pipe fittings, and simultaneously produce a surface strengthening effect.

Applicable scenarios: Batch pretreatment before rough processing, especially suitable for pipe fittings with a diameter ≥20mm and a wall thickness ≥3mm. Small-diameter pipe fittings need to be equipped with a special guide spray gun to avoid incomplete cleaning of the inner wall. 2) Mechanical Grinding/Polishing

Principle: Mechanical cutting using a grinding wheel, grinding belt, or wire brush to remove surface oxide scale and impurities;

Process Parameters: Grinding wheel speed 1500-2500 r/min, grinding pressure 0.1-0.3 MPa, feed speed 5-10 mm/s; wire brush made of stainless steel, wire diameter 0.1-0.2 mm;

Advantages: Low surface roughness after cleaning (Ra≤0.8μm), can also achieve surface shaping, suitable for local cleaning of residual oxide scale after finishing.

Precautions: Avoid over-grinding that could reduce the wall thickness of the pipe fitting (control the removal amount ≤0.05 mm). Thin-walled pipe fittings require the use of flexible grinding tools to prevent deformation.

3) High-Pressure Water Jet Cleaning

Principle: Utilizing a high-pressure water jet of 200-300 MPa (containing a small amount of abrasive, such as garnet powder) to impact the oxide scale. The abrasive particle size is 0.1-0.3 mm.

Process Parameters: Water flow velocity 200-300 m/s, spraying time 2-4 min/piece, abrasive concentration 5%-8%.

Advantages: No dust pollution, no damage to the substrate, precise control of the cleaning range, suitable for pre-treatment before finishing processes with high surface quality requirements, especially suitable for cleaning the inner walls of small-diameter, complex-structured pipe fittings.

(2) Chemical Cleaning Process for Seamless Steel Pipe Fittings (Pretreatment Stage)

1) Pickling Cleaning

Principle: A mixed solution of hydrochloric acid and sulfuric acid is used to dissolve the oxide scale, combined with a corrosion inhibitor to prevent over-corrosion of the substrate.

Process Parameters: Hydrochloric acid concentration 15%-20%, sulfuric acid concentration 5%-10%, corrosion inhibitor addition 0.3%-0.5%, temperature 40-60℃, soaking time 10-20min;

Advantages: Thorough cleaning, can remove oxide scale from dead corners of complex pipe fittings, and has a lower cost.

Precautions: After pickling, rinse immediately with clean water, then neutralize and passivate to prevent secondary oxidation; not suitable for pre-machined surfaces to avoid corrosion leading to dimensional deviations.

2) Combined Alkaline Washing and Acid Washing

Process: First, alkaline washing removes grease and some loose oxide layer from the oxide scale; then, acid washing dissolves the remaining dense oxide scale; finally, neutralization and passivation are performed.

Advantages: Suitable for pipe fittings with thick oxide scale or heavy surface oil contamination; the cleaning effect is more stable, and the corrosion of the substrate can be controlled within 0.01-0.03mm.

Applicable Scenarios: Pre-treatment of batch pipe fittings after heat treatment, especially high-hardness pipe fittings, avoiding over-corrosion of the substrate caused by single acid washing.

(3) Electrolytic Cleaning Process for Seamless Steel Pipe Fittings (Adapted for High-Precision Machining)

Principle: Using the pipe fitting as the anode and an inert material as the cathode, direct current is applied to an electrolyte solution. The oxide scale dissolves and detaches through electrolysis, while a passivation film forms on the substrate surface.

Process Parameters: Current density 10-20 A/dm², voltage 6-12 V, temperature 30-50℃, processing time 5-10 min;

Advantages: High cleaning precision, surface roughness can be controlled within Ra≤0.4μm, does not damage the substrate dimensions, suitable for pipe fittings after finishing or with high precision requirements.

Limitations: Higher equipment investment, single-piece processing cost is 2-3 times that of pickling, and it is not suitable for mass production at low cost.

Third, the scenario-based selection strategy for cleaning processes of seamless steel pipe fittings

(1) Adaptation according to the processing stage

Pre-treatment before rough machining: Sandblasting/shot blasting or alkaline washing-acid washing combined process is preferred to remove most of the oxide scale and reduce the wear of subsequent cutting tools;

Cleaning before finishing: High-pressure water jet or electrolytic cleaning is used for small-diameter, high-precision pipe fittings to avoid dimensional deviations caused by mechanical cleaning.

Residual cleaning after machining: Local residual oxide scale is cleaned by mechanical grinding or ultrasonic cleaning. Ultrasonic cleaning parameters: frequency 20-40kHz, power 500-1000W, cleaning fluid is an alkaline degreasing and rust removing agent, temperature 40-60℃, time 10-15min.

(2) Adaptation to Pipe Fitting Characteristics

High-hardness thick-walled pipe fittings: A combined shot blasting and pickling process is adopted. Shot blasting removes the loose oxide scale on the surface, while pickling dissolves the dense oxide layer, balancing efficiency and cleaning effect.

High-hardness thin-walled pipe fittings: Low-pressure sandblasting and electrolytic cleaning are selected to avoid deformation caused by high-pressure impact or excessive corrosion.

Small-diameter complex pipe fittings: A combined high-pressure water jet and mechanical grinding process is adopted to ensure no residue in key areas.

(3) Key Points for Process Optimization

Cleaning Timing Control: Oxide scale should be cleaned as soon as possible after formation, and the storage time should not exceed 72 hours to avoid further bonding between the oxide scale and the substrate or moisture absorption and rusting.

Residual Detection Standards: The residual oxide scale area on the surface after cleaning should be ≤5%, and the surface roughness should be measured with a roughness meter Ra≤0.8μm. Surface defects should be checked using magnetic particle testing or penetrant testing.

Environmental Requirements: Pickling and electrolytic processes require wastewater treatment equipment. The pH value after neutralization should be controlled at 6-9. Sandblasting/shot blasting should be equipped with dust collection devices to meet environmental emission requirements.