Root Cause Analysis of Outer Fold Defects in 34Mn5 Steel Pipes

Literature Overview

This paper, published in Tianjin Metallurgy (2014, Issue 4), by Wang Zhiyi, Ma Lijun, and Fu Changliang of Tianjin Steel Group Technical Center, investigates the root cause of outer fold defects appearing on the surface of 34Mn5 steel pipes after hot rolling of round billets. The analysis employs physical and chemical testing methods including chemical composition analysis, defect morphology examination, metallographic analysis, and hot acid etching of the cast billet.

Defect Description and Classification

Outer fold defects (also known as surface folds or lap defects) are surface discontinuities in which a portion of the pipe surface is folded over onto itself, creating a crevice-like opening that may extend into the pipe wall. These defects are classified as surface-breaking defects and are critical in pressure-containing applications because they can act as stress concentrators and crack initiation sites.

Defect Morphology Characteristics

Based on the metallographic analysis described in the paper, the outer fold defects exhibit the following characteristics:

Root Cause Analysis

Billet Surface Slag Groove (Zha Gou) Defect

The fundamental root cause identified is the presence of slag grooves (zha gou) on the surface of the round cast billet. Slag grooves are linear surface defects formed during the continuous casting process when molten protective slag flows into the liquid steel shell at the mold exit, creating a groove-like indentation in the solidifying shell surface.

The formation mechanism of slag grooves involves:

1. Protective slag flow: The molten protective slag in the mold and tundish is designed to prevent oxidation of the liquid steel surface. However, if the slag viscosity, temperature, or flow rate is not properly controlled, the slag can flow into the liquid shell surface.
2. Shell surface indentation: The flowing slag creates a groove in the semi-solid shell surface.
3. Shell healing: As casting continues, the groove may be partially or fully healed by subsequent solidification, but the healed groove retains a subsurface oxide film and a residual surface depression.

Evolution During Hot Rolling

During the hot rolling of the round billet into a seamless pipe, the surface slag groove is subjected to complex stress states:

1. Radial compression: The rolling forces compress the billet surface radially, causing the groove edges to deform.
2. Tangential shear: The rotation of the rolls induces tangential shear stresses at the surface.
3. Axial elongation: The longitudinal stretching of the pipe during rolling elongates the groove in the axial direction.

Under these combined stresses, the weakened surface at the slag groove location cannot accommodate the imposed deformation, and the material folds over, creating an outer fold defect. The oxide film at the root of the original slag groove acts as a stress concentrator and a preferred crack initiation site.

Supporting Evidence from Hot Acid Etching

The hot acid etching test on the cast billet revealed surface oxide spots and linear oxide inclusions that correspond to the locations of the slag grooves. These oxide inclusions are non-metallic inclusions formed by the interaction of the protective slag with the liquid steel. Their presence confirms the slag groove mechanism and provides a traceable link between the casting process and the final pipe defect.

Countermeasures and Quality Improvement

Protective Slag Optimization

The primary countermeasure is the adjustment of the protective slag physical and chemical properties:

Casting Process Control

Additional measures include:

• Mold design optimization: Ensuring proper mold geometry to minimize slag flow at the shell surface.
• Casting speed control: Maintaining a consistent casting speed to ensure uniform shell thickness and minimize surface turbulence.
• Tundish shroud design: Using proper shrouds to control the flow of protective slag in the tundish and prevent direct contact with the liquid steel surface.
• Billet surface inspection: Implementing 100% surface inspection of billets before hot rolling, using methods such as magnetic particle testing (MT) or ultrasonic testing (UT) to detect surface and near-surface defects.

Engineering Practice Implications

Quality Control Program for Seamless Pipe Production

Based on the findings of this study, a comprehensive quality control program for seamless pipe production should include:

1. Incoming material inspection: 100% surface inspection of cast billets before hot rolling. Defective billets should be removed or surface-treated (grinding, shot peeling) before processing.
2. In-process monitoring: Real-time monitoring of protective slag properties (temperature, viscosity, composition) during continuous casting.
3. Post-rolling inspection: 100% NDT of the finished pipe surface, including:

• Eddy current testing (ET) for surface and near-surface defect detection per ASTM E309 or ISO 17641
• Magnetic particle testing (MT) for surface defect detection per ASTM E1444 or ISO 9934
• Ultrasonic testing (UT) for subsurface defect detection per ASTM E309 or ISO 17640

1. Traceability: Maintaining a complete traceability system from cast billet heat number to finished pipe, enabling rapid identification and quarantine of defective material.

Impact on Welding and Fabrication

Outer fold defects in seamless pipes are particularly concerning when the pipe is used in welded fabrication:

• Weld preparation: The surface of the pipe at the weld location must be free of folds, laps, and other surface discontinuities. Any detected defect must be ground out with a proper repair groove preparation per ASME B31.3 Section 344.4 or API 5L Appendix Q.
• Weld quality: A residual fold at the weld root can act as a crack initiation site under cyclic loading or stress corrosion cracking conditions. The weld repair procedure must include full-penetration welding of the repair groove with 100% RT or UT of the repair weld.
• Fitness-for-service: If an outer fold defect is detected in a in-service pipe, a FFS assessment per API 579/ASME FFS-1 should be performed to determine whether the defect can be accepted or requires repair.

Key Reflections

The case study of 34Mn5 steel pipe outer fold defects illustrates the critical importance of upstream process control in seamless pipe manufacturing. The defect originates in the continuous casting process as a protective slag flow issue, propagates through the hot rolling process as a stress-induced surface fold, and ultimately manifests as a critical surface defect in the finished pipe. This chain of causation underscores the principle that quality is built into the product at each stage of manufacturing, and that defects introduced at an early stage are exponentially more difficult and costly to detect and prevent at later stages.

The systematic approach used in this investigation—combining chemical analysis, metallographic examination, and hot acid etching—is a model for defect root cause analysis in metallurgical engineering. The use of hot acid etching to reveal oxide inclusions that are invisible to conventional metallographic examination demonstrates the value of specialized testing techniques in defect analysis. For engineers involved in seamless pipe procurement and quality assurance, this case study serves as a reminder that the quality of the cast billet is the foundation of the quality of the finished pipe, and that any shortcut in billet quality control will ultimately manifest as costly defects in the final product.