Analysis of Ultrasonic Testing Failure Causes in 4130X Continuous Casting Steel Pipe

Literature Overview

This study by Zhang Xinwen and colleagues from Jiangsu Yonggang Group's Special Steel Company and Product R&D Center, published in Physical Testing (Vol. 41, No. 1, 2023, pp. 40-45), presents a systematic investigation into the root causes of ultrasonic testing (UT) rejection in 4130X seamless steel pipes used for gas cylinders. The paper focuses on a specific production scenario: the first heat (tapping furnace) of a continuous casting round billet with a diameter of 500 mm. Through sampling at different positions relative to the tapping point, the authors demonstrate that secondary oxidation during the tapping stage is the primary cause of non-metallic inclusion contamination leading to UT failure.

Research Methodology and Sampling Strategy

The research employs a well-designed sampling approach that provides clear spatial correlation between the position along the billet and the severity of inclusion contamination. Samples were taken from different locations along the length of the 500 mm continuous casting round billet, with positions defined by their distance from the tapping end (the hook end).

The following indicators were analyzed for each sample position:

Oxygen content (O)
Nitrogen content (N)
Aluminum burn-off (Al loss)
Non-metallic inclusion grades (B-type, D-type, DS-type)
Distribution density of Al-containing inclusions >= 5 micrometers

Key Findings and Technical Analysis

The results reveal a clear gradient pattern: samples taken closer to the tapping end exhibit progressively worse metallurgical quality. This finding is consistent with the well-known phenomenon of secondary oxidation during the initial pouring phase of continuous casting.

During the tapping furnace operation, when the ladle is first opened, the molten steel is exposed to air for a longer duration as the stream establishes itself. This exposure leads to significant oxidation of the deoxidizer (aluminum) and absorption of nitrogen from the atmosphere. The consequences are:

Increased oxygen content: As aluminum is consumed by oxidation, the residual oxygen level in the steel rises.
Increased nitrogen content: Nitrogen absorption from the air increases the total gas content.
Accelerated Al burn-off: The deoxidizing capacity is depleted more rapidly near the tapping end.
Elevated inclusion grades: B-type (silicate) and D-type (oxide) inclusions increase in both size and count. DS-type (total) inclusion ratings are correspondingly higher.
Higher density of large Al-containing inclusions: Inclusions of 5 micrometers or larger are more prevalent near the tapping end, indicating that secondary oxidation preferentially generates coarse oxide particles.

The following table summarizes the expected trends:

Distance from Tapping End	Oxygen Content	Nitrogen Content	Al Burn-off	B-type Inclusion Grade	D-type Inclusion Grade	DS-type Grade	Al Inclusion Density (>=5 μm)
Closest to tapping end	Highest	Highest	Greatest	Highest	Highest	Highest	Highest
Mid-length	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate
Farthest from tapping end	Lowest	Lowest	Least	Lowest	Lowest	Lowest	Lowest

Root Cause Analysis Using 5W2H Framework

Applying the 5W2H analytical framework to this failure scenario:

What: UT rejection due to laminations and non-metallic inclusions in 4130X seamless pipe.
Where: First heat (tapping furnace) of 500 mm continuous casting round billet, particularly near the tapping end.
When: During the initial pouring phase when the steel stream is establishing.
Why: Secondary oxidation of molten steel during ladle tapping causes excessive aluminum burn-off and inclusion formation.
Who: Production personnel responsible for ladle handling and tapping operations.
How: Improper ladle preheating, insufficient cover slag, or extended tapping duration.
How much: Inclusion grades and oxygen levels are significantly elevated compared to steady-state pouring conditions.

Engineering Countermeasures

Based on the findings, several practical countermeasures can be recommended:

Ladle preheating optimization: Ensure the ladle is properly preheated to minimize heat loss and secondary oxidation during tapping.
Cover slag application: Apply a high-quality cover slag immediately upon tapping to shield the molten steel surface from atmospheric contact.
Tapping duration control: Minimize the time between ladle opening and stream establishment by optimizing the ladle design and pouring system.
Increased deoxidizer addition: Compensate for expected Al burn-off by adding extra deoxidizer during the tapping phase.
Sampling and quality verification: Implement enhanced sampling protocols for the first heat of each casting campaign, with particular attention to the tapping-end section.
Process monitoring: Install online monitoring systems for oxygen and nitrogen content in the molten steel during tapping.

Study Insights

This paper exemplifies a rigorous approach to quality failure analysis that is often lacking in industrial practice. The decision to sample at multiple positions along the billet and correlate the results with specific metallurgical indicators provides unambiguous evidence for the root cause. The finding that secondary oxidation preferentially affects inclusions of 5 micrometers or larger is particularly significant from a UT perspective, as these larger particles are more readily detected by ultrasonic testing and are more likely to initiate fatigue cracks under cyclic loading in gas cylinder service. For seamless pipe manufacturers producing 4130X or similar grades, this study underscores the importance of controlling the tapping phase of continuous casting and highlights the value of position-dependent sampling in quality investigations.