Analysis of Linear Surface Trace Defects in 20 Steel Pipe Sleeves

Literature Overview

This paper, published in Physical Testing (Vol. 43, No. 1, 2025, pp. 51-56) by Lv Fang, Huang Kaihua, Hao Yongjin, Yu Lei, Li Yuejie, and Ma Ding, investigates the root cause of longitudinal linear stripe defects found on the outer surface of multiple 20 steel pipe sleeve components during machining operations. The study was conducted across three institutions: Antai Tianlong Tungsten Molybdenum Technology Co., Ltd., Sichuan Aerospace Changzheng Equipment Manufacturing Co., Ltd., and the China Academy of Launch Vehicle Technology. The research employed metallographic examination, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to characterize the defect morphology and origin.

Core Findings

The investigation revealed that the longitudinal linear trace defects observed on the outer surface of the sleeves were not surface contamination or machining artifacts but rather internal material discontinuities that had been exposed through subsequent forming operations. The root cause was identified as micro-peeling defects present on the surface of the pipe blank prior to the drawing process. During the cold drawing operation, these micro-peeling areas were stretched and elongated in the direction of deformation, ultimately developing into full-length fold defects distributed along the longitudinal axis of the tube.

Technical Interpretation of Defect Mechanism

Origin of Micro-Peeling

Micro-peeling on pipe blanks typically originates from several sources:

Source Category	Typical Cause	Mechanism
Mill scale adhesion	Incomplete decarbonization or oxide scale detachment during hot rolling	Scale fragments embed into the surface during cooling
Casting defects	Surface porosity or segregation in the billet	Weak bonding between surface layer and substrate
Pickup contamination	Contact with contaminated rolls or guides	Foreign material transfer during rolling
Decarburization	Excessive exposure to oxidizing atmosphere	Surface composition change leading to brittle scale

Evolution During Cold Drawing

The cold drawing process imposes significant plastic deformation on the tube, with typical drawing reductions ranging from 5% to 30% for sleeve applications. The strain distribution during drawing is not uniform across the cross-section, creating differential stress states at the surface and core.

Elastic-plastic transition: As the drawing die applies compressive and tensile stresses, the surface layer containing embedded scale or peeling initiates micro-cracking.
Strain concentration: The micro-peeling area acts as a stress concentrator, with local strain reaching values 2-3 times the nominal drawing strain.
Crack propagation: Under sustained tensile stress in the drawing direction, the crack propagates parallel to the deformation axis, creating a longitudinal fold.
Surface expression: When the fold reaches the outer surface, it manifests as a visible linear trace with characteristic morphology distinguishable from machining marks.

Characterization Methodology

The study utilized a systematic approach to defect identification:

Optical metallography: Revealed the internal morphology of the fold, showing a continuous crack extending from the surface into the subsurface region, confirming the defect was not a surface-only phenomenon.
SEM examination: Provided high-magnification imaging of the crack faces, revealing features consistent with ductile tearing rather than brittle fracture. The crack surfaces showed evidence of plastic deformation and material flow along the drawing direction.
EDS analysis: Confirmed that the composition within the fold region matched the base material (20 steel, approximately 0.2% C), ruling out foreign material inclusion. The absence of significant oxygen, sulfur, or phosphorus enrichment indicated that the defect was a mechanical discontinuity rather than a chemical segregation zone.

Engineering Practice Implications

Quality Control Measures

Based on this analysis, several preventive measures should be implemented in the supply chain:

Stage	Control Measure	Acceptance Criterion
Blank procurement	Surface quality inspection of pipe blanks	No visible peeling, scale, or surface discontinuities exceeding 0.1 mm depth
Incoming inspection	Eddy current or magnetic particle testing	No indications of surface or near-surface defects
Drawing process	Monitor drawing force and temperature	Drawing force within ±10% of baseline; surface temperature below 100°C
Post-drawing inspection	Visual and penetrant testing	No longitudinal linear indications exceeding 5 mm length
Final machining	Dimensional and surface integrity check	Surface finish Ra ≤ 3.2 μm; no subsurface discontinuities

Key Engineering Lessons

This case study underscores a critical principle in precision component manufacturing: the quality of the final product is fundamentally limited by the quality of the initial material. In aerospace applications, where sleeve components serve as structural or sealing elements in launch vehicle systems, even microscopic surface defects can propagate during subsequent forming operations and compromise the structural integrity of the finished part.

The finding that micro-peeling defects can remain undetected through conventional visual inspection but become critical during forming highlights the need for advanced non-destructive testing (NDT) methods at the raw material stage. Magnetic particle testing (MT) or eddy current testing (ET) should be mandatory for high-specification pipe blanks intended for cold drawing applications.

FMEA Application

Applying Failure Mode and Effects Analysis (FMEA) to this scenario:

Failure Mode	Effect	Severity	Occurrence	Detection	RPN
Micro-peeling on blank	Fold defect after drawing	9 (structural failure)	4 (occasional)	3 (difficult to detect)	108
Incomplete surface cleaning	Contamination-induced cracking	7	3	4	84
Excessive drawing reduction	Surface tearing	8	2	2	32

The high RPN value for micro-peeling (108) indicates that this failure mode requires priority attention in the quality management system.

Study Insights and Reflections

This case study exemplifies the importance of traceability in defect analysis. The ability to link a final product defect back to a specific origin in the material supply chain requires rigorous metallurgical investigation. The conclusion that the defect originated from a pre-existing micro-peeling on the blank rather than from the drawing process itself has significant implications for liability determination and corrective action prioritization.

From a practical standpoint, this reinforces the need for collaborative quality management between material suppliers and component manufacturers. The pipe blank supplier must provide documented surface quality certification, while the component manufacturer must implement adequate incoming inspection protocols. In aerospace applications governed by standards such as ASTM A210 or GB/T 3077, the requirement for defect-free surface quality is well-established, yet the practical enforcement of these requirements remains challenging due to the microscopic nature of many surface defects.

The study also highlights a broader principle applicable across steel pipe manufacturing: plastic deformation operations such as cold drawing, cold forming, and tube rolling are highly sensitive to pre-existing surface and near-surface defects. Any defect present on the material surface before forming will be amplified and made visible during the deformation process. This principle should guide the development of incoming material specifications for all cold-forming applications in the industry.