Longitudinal cracks in steel

In the grand narrative of the steel industry, cracks are the most fatal “Achilles’ heel” of materials. Among the many forms of cracks, longitudinal cracks—these defects that extend along the direction of the steel, resembling a hideous scar—are undoubtedly the most insidious and destructive “invisible killers.” They are not only the culprit behind steel scrapping but also time bombs lurking in engineering structures. From the moment of solidification in the steel furnace to the plastic deformation under the forging hammer, and then to the rapid cooling and heating during heat treatment, longitudinal cracks, like ghosts, permeate the entire steelmaking process.

To overcome this intractable problem, we must, like surgeons, precisely dissect its causes, peeling away the layers of physical mechanisms and technological flaws behind continuous casting, ingots, forging, and heat treatment.

I. The Pain of Continuous Casting: “Tears” at the Solidification Front

In modern continuous casting production, longitudinal cracks in slabs and billets are the most common defects, essentially a violent struggle between “uniform solidification” and “thermal stress.”

The moment molten steel is poured into the crystallizer, a battle between speed and temperature begins. If the tundish nozzle is misaligned with the crystallizer, the steel flow will impact one side of the billet shell like an off-center bullet, causing localized slowing and thinning of the shell. Simultaneously, excessive slag buildup caused by excessive molten steel surface fluctuations disrupts the continuity of the shell. Both situations result in the billet shell being “damaged” before it enters the crystallizer.

Even more critical is uneven cooling. With prolonged use, wear and deformation of the copper tubes, especially a reduction in the inverted taper, can create air gaps between the billet shell and the vessel wall, hindering heat transfer. Poor performance or uneven spreading of the protective slag further exacerbates the problem. At this point, the billet shell resembles an unevenly heated piece of porcelain, with some areas cooling and shrinking rapidly while others cool slowly. The immense thermal stress directly tears the still-plastic shell, creating longitudinal cracks.

The solution lies in “precise control.” We must strictly adhere to the principle of “slow casting at high temperatures and fast casting at low temperatures,” controlling overheating to the lower-middle limit to avoid steel flow segregation. Simultaneously, we must ensure the crystallizer’s taper is appropriate, and the meltability and spreadability of the protective slag meet standards, allowing the billet shell to grow slowly and steadily in a uniform “gentle environment.”

II. The Tragedy of Steel Ingots: The “Original Sin” of Rolling Direction

For steel ingots, longitudinal cracks often originate from deeper metallurgical defects. Low-melting-point harmful impurities such as sulfur (S) and phosphorus (P) in molten steel will severely segregate along the rolling direction during solidification, forming fragile “grain boundary chains.” When the ingot is rolled or drawn, these impurity-rich areas act like pre-cut cracks, expanding instantly under external force.

Furthermore, a crude casting process is another major contributing factor. If the ingot mold design is unreasonable, the corner radius is too large, or the casting speed is too fast, the corner steel shell, due to insufficient cooling strength, cannot withstand tensile stress on both sides, resulting in corner cracks that then evolve into longitudinal cracks. Even more alarming is the “axial intergranular crack,” a spiderweb-like cracking caused by poor deoxidation of molten steel, excessive dendrite segregation, and the resulting internal stress during phase transformation.

To break this chain of “original sin,” the solution lies in addressing the root cause: thorough deoxidation and degassing. This can be achieved by strengthening slag formation during the reduction period, increasing the use of strong deoxidizers such as silicon-calcium and aluminum, and refining the grain size. Simultaneously, raw materials must be dry, and damp scrap steel must be prevented from entering the furnace to reduce hydrogen content and prevent white spot defects. For easily cracked steel grades, cap heating should be approached with caution; red-heat annealing or red-heat rolling is preferable, as it allows the ingot to release internal stress at red heat.

III. The Perils of Forging: “Overload” in Plastic Deformation

In the forging workshop, longitudinal cracks often appear disguised as “internal defects.” When the ingot’s cut is insufficient, secondary shrinkage cavities remain, or the heating rate is too rapid, leading to a significant temperature difference between the inside and outside, the internal low-plasticity regions will fracture instantly under hammering force.

Especially in low-plasticity materials, if the hammer feed is too large or the same area is repeatedly turned and stretched, the deformation exceeds the material’s ultimate elongation, creating tiny cavities inside the metal. These cavities connect to form terrifying internal longitudinal cracks. These cracks are extremely insidious, often appearing only during rough machining or even during service, with catastrophic consequences.

The key to prevention lies in “gentle forging.” The heating rate must be strictly controlled to ensure thorough heating before forging; a reasonable deformation amount must be set according to the steel’s characteristics to avoid “over-forging”; for alloy steels, special attention must be paid to the forging ratio, ensuring a sufficient forging ratio to weld internal pores, but not so large as to tear the matrix.

IV. The Calamity of Heat Treatment: The “Rampage” of Martensitic Phase Transformation

If the first three stages are “external injuries,” then longitudinal cracks during heat treatment are “internal injuries.” When the steel is fully quenched, the core transforms into quenched martensite, which has the highest specific volume, generating enormous tangential tensile stress. If the steel has a high carbon content or contains low-melting-point impurities such as S, P, Bi, and Pb, these impurities segregate along grain boundaries, severely weakening grain boundary bonding.

Under tensile stress, cracks propagate along grain boundaries like lightning. This risk increases exponentially, especially when the mold size falls within the “crack-sensitive range” (e.g., 8-15mm for carbon tool steel, 25-40mm for medium- and low-alloy steel), or when the cooling rate is too fast. Even more challenging are “arc-shaped cracks” and “peeling cracks,” which often occur at corners or at the interface between the hardened layer and the matrix, and are extreme products of the superposition of structural and thermal stresses.

To combat this problem, meticulous process design is required:

1. Purity control: Minimize the content of harmful impurities.

2. Cooling medium selection: Use media with adjustable concentrations, such as CL-1 quenching agent, and control the cooling rate near the critical quenching rate to avoid rapid cooling.

3. Timely Tempering: Tempering must be performed immediately after quenching to eliminate residual austenite and prevent secondary phase transformation cracking during use.

V. Flaw Detection and Redemption: “Judgment” Under Ultrasonic Waves

For longitudinal cracks, the naked eye is often powerless; ultrasonic flaw detection is essential.

When a 0° probe detects a bottom echo alarm, or a 37° probe captures an abnormal echo on the top surface of the flange, the crack has nowhere to hide. For internal cracks, the transverse wave method is the best tool: using the 6 dB attenuation method for defect wave height (half-wave height method) or the 6 dB attenuation method for bottom wave, the size and direction of crack extension can be accurately determined. If the defect wave is mobile and centrally symmetrical, it is irrefutable evidence of a crack.

Once a crack is discovered, welding is the last resort, but this is also fraught with risks. The transverse shrinkage during welding can easily trigger new cracks. Therefore, multi-layer, multi-pass welding is essential, with slag removal at each layer, temperature control, and segmented back-welding at the bevel to allow the base material to absorb shrinkage stress in a balanced manner. Post-weld stress-relief annealing is crucial; otherwise, the cracks are merely temporarily “sealed,” awaiting future eruption.

Conclusion

Longitudinal cracks in steel are a consequence of the combined efforts of metallurgy, thermodynamics, and mechanics. It serves as a warning: steel is not an invincible material; rather, it is a highly sensitive and precise system. From the purity of a single drop of molten steel to the millimeter-level temperature control in the crystallizer, and the varying pressure applied by the forging hammer, negligence in any环节 can sow the seeds of fracture within the steel.

Only by respecting materials, relying on data, and implementing “precise control” in every process can we tame this steel behemoth and make longitudinal cracks a thing of the past. This is not only a victory for technology but also a manifestation of the industrial spirit.