I. Preventing Material Defects: Controlling the "Source"
Material is the foundation for steel pipe performance. To avoid substandard chemical composition or the use of inferior materials:
1. Strictly select qualified steel billets: Prioritize continuously cast round billets due to their uniform composition, good surface quality, and low cost.
2. Control the content of harmful elements: Strictly limit residual elements such as arsenic (As), tin (Sn), antimony (Sb), and lead (Pb) to prevent temper brittleness.
3. Implement ladle refining and remelting processes: Improve the purity of molten steel and reduce non-metallic inclusions through LF furnaces, VD vacuum treatment, or electroslag remelting (ESR).
4. Verify the Material Test Certificate (MTC): Verify the Mill Test Certificate issued by the steel mill during procurement to confirm that the chemical composition is consistent with the standards.
II. Surface Damage Prevention: Optimizing Processes and Operating Procedures
Surface defects are mostly caused by rolling, handling, or equipment problems and can be prevented through the following methods:
1. Ensure the billet surface is clean and defect-free: Remove oxide scale, cracks, scratches, and other original defects before piercing to prevent their propagation during deformation.
2. Set appropriate rolling parameters: Avoid excessively high heating temperatures (>1050℃) which lead to coarse grains, or excessively low temperatures which result in insufficient plasticity and cracking.
3. Improve lubrication and die condition: Regularly clean the die surface to ensure adequate lubrication and prevent "steel sticking," which can cause scratches, scabs, or creases.
4. Standardize straightening and handling operations: Use specialized lifting equipment to avoid collisions between steel pipes that can cause dents, flattening, or abrasions.
III. Controlling Dimensional Deviations: Improving Equipment Precision and Process Monitoring
Dimensional accuracy directly affects assembly and sealing performance, requiring a two-pronged approach focusing on both equipment and process:
1. Precise Calibration of Sizing and Reducing Mills: Ensure accurate die machining and reasonable distribution of deformation across all stands to avoid outer diameter fluctuations.
2. Controlling the Stability of the Piping Process: Excessive tolerances in billet diameter and ellipticity can lead to abnormal mandrel reduction rates, causing internal bending or uneven wall thickness.
3. Strengthening Online Inspection: Employing laser diameter gauges and real-time ultrasonic wall thickness monitoring systems to promptly detect and adjust deviations.
4. Strictly Controlling Cooling Rate: Preventing uneven cooling from causing steel pipe bending or excessive ellipticity.
IV. Ensuring Mechanical Properties: Scientifically Developing Heat Treatment Procedures
Unqualified mechanical properties often stem from uncontrolled heat treatment. The following points should be emphasized:
1. Rationally Designing Heat Treatment Processes: Developing quenching and tempering temperatures and holding times based on the steel grade to ensure uniform microstructure (e.g., pearlite + ferrite) and avoid coarse grains.
2. Timely annealing after multi-pass cold drawing: Prevents excessive work hardening, which can lead to decreased plasticity and subsequent cracking during processing.
3. Control the cooling rate: Avoid rapid cooling to prevent excessive residual stress, which can induce deformation or cracking.
Conduct full-batch mechanical property testing: Including tensile, hardness, and impact tests to ensure that tensile strength, yield point, and elongation meet standards.
V. Eliminate internal structural defects: Strengthen non-destructive testing and process control. Internal defects are highly concealed and must be addressed through a combination of detection methods and process optimization:
1. Strengthen internal quality control of billets: Eliminate original defects such as porosity, shrinkage cavities, and inclusions to prevent them from evolving into delamination or internal cracks during rolling.
2. Implement full-process ultrasonic testing (UT): Perform 100% full-coverage inspection on finished tubes to identify defects such as inclusions, delamination, and internal cracks.
3. Prevent eccentricity and creases: During cold drawing, control the deformation per pass to no more than 8%, and the drawing speed to less than 1.5 m/s to avoid stress concentration that could lead to creases or cracks.
4. Monitor the risk of sensitization in stainless steel pipes: Reasonably control the solution treatment temperature and cooling rate to prevent intergranular corrosion.
