Analysis of NDT Failure Causes in 4130X Continuous Casting Steel Pipe

Literature Overview

The paper by Zhang Xinwen and colleagues, published in Physical Testing in 2023 (Vol. 41, No. 1, pp. 40–45), presents a detailed metallurgical investigation into the root causes of non-destructive testing (NDT) rejection of 4130X seamless steel pipe produced from continuous cast round billets of 500 mm diameter. The authors, affiliated with Jiangsu Yonggang Group's Special Steel Company and its Product R&D Centre, conducted a systematic sampling study along the length of the cast billet, specifically examining the region near the pouring end (the "hook end" or "gou tou" in Chinese). Their findings reveal that secondary oxidation of molten steel during the initial pouring stage is the primary cause of elevated non-metallic inclusion (NMI) levels, which in turn lead to laminations and other internal defects detected by ultrasonic testing (UT) during pipe inspection.

Metallurgical Investigation Methodology

The authors adopted a rigorous sampling strategy that is instructive for any metallurgical root cause analysis. Samples were extracted at multiple positions along the length of the 500 mm round billet, at varying distances from the hook end (the first portion of the billet to be poured). For each sample position, the following parameters were measured: oxygen content, nitrogen content, aluminium consumption (as an indicator of deoxidiser depletion), and non-metallic inclusion ratings according to the GB/T 10561 or ASTM E45 classification system. The inclusion types examined were B-type (dendritic sulphides), D-type (chain-shaped silicates), and DS-type (mixed dendritic sulphide and chain silicate).

The results showed a clear and consistent trend: as the distance from the hook end decreased, oxygen content increased, nitrogen content increased, aluminium consumption increased, and the inclusion ratings for B, D, and DS types all increased. This pattern is entirely consistent with the metallurgical understanding of secondary oxidation during continuous casting. During the initial pouring stage, the tundish is not yet at thermal equilibrium, the ladle preheating may be insufficient, and the slag cover on the molten steel surface may be incomplete or unstable. Under these conditions, the molten steel is exposed to air and refractory materials, leading to re-oxidation. The deoxidiser (typically aluminium) added in the ladle is consumed by this secondary oxidation, leaving insufficient dissolved aluminium to form inclusions that can be removed by the ladle filter or the continuous casting tundish filter.

A particularly important finding was the relationship between distance from the hook end and the distribution density of aluminium-containing non-metallic inclusions with a size of 5 micrometres and larger. The authors found that the density of these larger inclusions increased significantly closer to the hook end. This size threshold is significant because inclusions larger than 5 micrometres are more likely to be detected by ultrasonic testing and are more likely to serve as crack initiation sites during mechanical working (rolling, piercing, and finishing of the seamless pipe). Inclusions in the 1 to 5 micrometre range may be present but are less likely to cause NDT rejection or service failure.

Connection to Seamless Pipe Production

The seamless pipe manufacturing process — whether by Mannesmann piercing, plug piercing, or plug-and-reamer piercing — involves substantial plastic deformation of the billet. During piercing and subsequent rolling, any internal defects present in the billet are elongated and aligned along the pipe axis. A lamination (a planar inclusion or crack) in the billet becomes a longitudinal defect in the finished pipe, which is easily detected by ultrasonic testing. The 4130X grade, a high-strength low-alloy steel designed for gas cylinder applications, is particularly sensitive to internal cleanliness because gas cylinders are pressure vessels subject to cyclic loading and hydrostatic testing. Any internal defect that reduces the effective cross-sectional area or serves as a stress concentrator can lead to premature failure under service conditions.

The NDT rejection rate for 4130X pipe from the first heat of a continuous casting campaign is typically higher than for subsequent heats. This is a well-known phenomenon in the seamless pipe industry, often referred to as the "first heat problem." The root cause, as demonstrated by this paper, is the metallurgical instability during the initial pouring stage. The practical solution involves several measures: extended ladle preheating, use of a properly conditioned tundish with a sufficient slag cover, addition of a desulfuriser (such as calcium silicate) to modify the inclusion morphology, and in some cases, rejection of the first heat or the first several metres of the billet.

Defect Classification and Countermeasures

Engineering Practice Integration

In my experience working with seamless pipe producers, the "first heat problem" is one of the most persistent quality challenges. The economic temptation to use every metre of the billet is strong, especially for high-value grades like 4130X where the raw material cost is significant. However, the cost of NDT rejection — including the time lost during investigation, the cost of the rejected pipe, and the risk of customer complaints — typically exceeds the cost of the billet material itself. The paper's recommendation to reject or downgrade the first heat is therefore economically justified, even if it is not always followed in practice.

A practical approach that I have seen implemented successfully is the use of a "sacrificial pour" — a small volume of molten steel is poured into the tundish before the actual casting begins, to stabilize the slag cover and allow the tundish to reach thermal equilibrium. This sacrificial pour is then discarded, and the actual billet casting begins with a much more stable metallurgical environment. This approach reduces the length of the affected billet section and can improve the NDT pass rate significantly.

Study Insights and Implications

This paper is a model of systematic metallurgical investigation. The authors did not simply observe a high NDT rejection rate and speculate about the cause; they designed a sampling plan, measured multiple parameters, identified clear trends, and arrived at a well-supported conclusion. The methodology is directly applicable to any metallurgical quality problem where the root cause is suspected to be related to steel cleanliness or casting conditions. The key insight for engineers is that NDT rejection is a symptom, not a disease. The disease is the metallurgical condition of the steel, and the cure must be applied at the steelmaking or casting stage, not at the NDT stage. This paper reinforces the principle that quality in seamless pipe production is determined upstream, during steelmaking and casting, and that downstream processes — piercing, rolling, and finishing — can only propagate or amplify the quality level established at the beginning of the process chain.