First, what are the preparatory steps for identifying cracks in cold-drawn seamless steel pipes?
Environmental requirements: Choose a well-lit location, avoiding strong light reflection or shadows; the pipe surface must be clean, removing oil, scale, and residual cold-drawing lubricant.
Auxiliary tools: Equipped with a 5-10x magnifying glass, flashlight, and ruler. No complex testing equipment is needed; it is suitable for rapid on-site screening in the workshop.
Second, what are the core techniques for identifying cracks in cold-drawn seamless steel pipes?
(I) Longitudinal surface cracks in cold-drawn seamless steel pipes.
(most common in cold-drawing processes, mostly distributed along the length of the pipe)
Visual characteristics:
The cracks are straight or slightly curved, ranging in length from a few millimeters to tens of centimeters, and the width is usually greater than 0.1mm (a noticeable gap is visible to the naked eye).
There are no obvious protrusions on both sides of the crack, and the edges are relatively sharp. A slight “stutter” can be felt when lightly scratched with a fingernail (distinguishing it from the “smoothness” of oxide scale cracks).
When illuminated by sidelight (with the flashlight at a 30°-45° angle to the pipe surface), a “black line-like shadow” will appear at the crack, forming a strong contrast with the smooth surface of the pipe.
Key identification points: Focus on inspecting the “weak areas of machining marks” on the outer surface of the pipe: the pipe area corresponding to the cold drawing die inlet, and areas with uneven wall thickness (such as the transition section of the shock absorber sleeve with varying wall thickness); Eliminate interference: “pseudo-cracks” formed by oxide scale peeling (rough edges, lacking sharpness, disappearing or becoming shallower after wiping), and “striped color differences” formed by residual cold drawing lubricant (seamless, no sticking when scratched with a fingernail).
(II) Surface transverse cracks in cold-drawn seamless steel pipes.
(mostly caused by stress concentration after cold drawing or insufficient annealing)
Visual characteristics: Cracks are perpendicular to the length of the pipe, appearing discontinuous or continuous ring-shaped, and are relatively short (usually 1-5mm). Some cracks extend circumferentially.
When the crack depth is shallow, the surface appears as “fine hairline,” requiring a magnifying glass for observation. When the crack depth is deep, obvious “groove-like” depressions can be seen.
Transverse cracks are mostly concentrated at the pipe ends (near the cold-drawn blanking cut) or in the weld heat-affected zone (if the pipe has undergone pre-treatment welding), requiring focused inspection.
Key Identification Points:
Using the “Segmented Rotation Observation Method”: Fix the pipe and slowly rotate it. Illuminate each segment with a flashlight from the side. Transverse cracks will show alternating light and dark lines during rotation, easily distinguishable from surface scratches (scratches are mostly unidirectional with no light-dark changes).
Distinguishing Scratches from Cracks: Scratches have smooth edges without branching, while cracks have fine branching or “serrated” edges. The length of scratches is usually consistent with the processing direction (transverse cracks are perpendicular to the processing direction).
(III) Internal Hidden Cracks in Cold-Drawn Seamless Steel Pipes.
(Can be indirectly identified with the naked eye, but requires judgment based on process characteristics)
Visual Characteristics:
The pipe surface has no obvious gaps, but there are “local color differences” (slight traces of cracks extending to the surface), or “local bulges/depressions” appear after cold drawing (stress release deformation caused by internal cracks).
After end-face cutting of the pipe end, “radial fine seams” or “dot-like dark spots” (exposed traces of internal cracks on the end face) can be observed. Key Identification Points:
Focus on “abnormal deformation zones” after cold drawing: such as irregular “wavy” protrusions on the pipe surface, or sudden thinning of the wall thickness in some areas (possibly due to material loss caused by internal cracks);
Consider the processing technology: if the cold drawing speed is too fast or the annealing temperature is insufficient, the pipe is prone to internal cracks. Such pipes may have “brittle oxide scale” on the surface (easily detached upon tapping), which can aid in identification.
(IV) Cracks in the weld area of cold-drawn seamless steel pipes (for shock absorber sleeve end connection processing)
Visual characteristics: Cracks are mostly distributed along the weld edge or weld center, appearing “serrated” or “dendritic,” and some cracks extend to the base material.
If there are “dense porosity areas” or “undercut” on the weld surface, cracks are likely to accompany them (fine seams extending next to the pores are visible to the naked eye).
Key identification points: Observe the transition zone between the weld and the base material using a magnifying glass. Cracks will appear as “fine black seams,” distinguishing them from the “fish-scale” pattern of the weld. Gently tap the pipe (using a rubber mallet to avoid damaging it). Cracks will produce a “crisp abnormal sound” (the sound in crack-free areas is deep), aiding in the identification of hidden cracks.
Third, what are the key areas for checking cracks in cold-drawn seamless steel pipes?
Raw material stage: Focus on checking the “annealed surface” of the seamless steel pipe before cold drawing to avoid residual cracks due to insufficient annealing.
After cold drawing: Focus on the “critical functional areas” of the shock absorber sleeve—the transition section with varying wall thickness, the area around the lightweight hole (heat-affected zone after laser cutting), and the end-flanging forming area.
Post-processing storage stage: Inspect the cut end face of the cold-drawn seamless steel pipe (after cold drawing) and the stacking contact areas (to avoid secondary cracks caused by compression).
Fourth, what are the criteria for judging cracks in cold-drawn seamless steel pipes?
Acceptable range: Crack length ≤ 3mm, width ≤ 0.05mm, and not concentrated in the load-bearing area (such as the load-bearing area in the middle of the sleeve, or at the installation interface), with no branching extension;
Prohibited criteria: Longitudinal crack length ≥ 5mm or transverse crack ≥ 3mm; Cracks concentrated in the transition section of variable wall thickness or around lightweight holes (easily leading to increased stress concentration); Any visible cracks in the weld area (directly affecting connection strength and fatigue life).
Post time: Feb-25-2026