4130 Round Billet Internal Cracks: The 900–750 °C Straightening Mistake, Not the Melt

A 4130 round billet’s internal cracks are decided on the caster, not in the melt

When a buyer rejects a AISI 4130 or Chinese 30CrMo round billet for “internal cracks,” the reflex is to blame the steel — wrong chemistry, dirty melt, bad deoxidation. A 2018 study on a 350 mm ASTM 4130 continuous-casting round billet shows the opposite. The billet chemistry was fully on-spec. The cracks came from one place only: the billet was straightened while its solidified structure sat inside the third brittle temperature zone (Zone III, roughly 900–750 °C, where hot ductility collapses to a minimum). Nothing in the ladle caused it. Everything in the straightening stand did. The practical message for anyone who sources Cr-Mo billet or forging stock: a clean chemical certificate is not proof of a sound billet, and the straightening temperature window is a quality parameter you should be asking about.

Why 4130 turns brittle in the 900–750 °C window

Steel does not lose ductility smoothly as it cools. Its high-temperature ductility curve has three zones, and Zone III is the dangerous one. Above it (around 1300–900 °C) the steel is in the single-phase austenite region and is highly ductile. Below it (under ~700 °C) a ferrite film thickens at the prior-austenite grain boundaries and ductility partly recovers. But in the pocket of about 900–750 °C, the γ→α transformation begins while carbonitrides (AlN, Nb/V carbides) precipitate at the grain boundaries. The grain boundaries weaken, deformation localizes there, and a small applied stress opens an intergranular crack.

That mechanism is generic to low-alloy steels. What the study added for 4130 specifically is the measured boundary. Using a Gleeble-1500 thermal simulator on as-cast samples, the team mapped RA against temperature and pinned the trough precisely to 900–750 °C, with the worst point at 850 °C. The widely used rule of thumb “keep the straightening surface temperature above 900 °C” is exactly what this curve demands for 4130.

What the high-temperature tensile test proved

The RA criterion for the brittle zone is RA = 50 %: below that, the steel is considered crack-sensitive. The measured curve falls off a cliff inside Zone III.

Above the trough, fracture surfaces were dimpled and ductile. Inside Zone III the fracture turned intergranular: at 900–800 °C the structure was martensite, and at 775–750 °C ferrite precipitated along the grain boundaries. That microstructural evidence is the smoking gun — the cracks are a straightening-temperature artifact, not a melting or segregation artifact.

The caster setup that was failing

The 350 mm billet was produced on a full-arc caster, and two original parameters put the billet straight into Zone III under load.

With that cooling load the billet surface dropped into the 900–750 °C band before it reached the 3-point straightener, and the 4-stand arrangement concentrated bending stress at discrete points. The combination — low ductility plus high local force — is the textbook recipe for internal cracks.

The fix: lift the straightening temperature above 900 °C

The countermeasures are a direct response to the RA curve. Cool the billet less aggressively so it arrives at the straightener still above the brittle pocket, and spread the bending force over more stands.

After the change the internal cracks were eliminated, the defect rate fell, and the mill reported an annual saving of about 1.272 million RMB. The chemistry of the steel never changed — only the path it took through the caster.

Same melt, different certificate: 4130 / 30CrMo / 25CrMo4

The grade in this study is ASTM 4130, and its chemistry matches the international family. If your drawing calls a different name, it is very likely the same low-alloy Cr-Mo steel — provided the heat-treatment condition is also matched.

Note the study’s P ≤ 0.020 % and S ≤ 0.015 % are tighter than the commercial ASTM A29 limits (P ≤ 0.035 %, S ≤ 0.040 %). That cleaner composition is what a continuous-cast special-steel route is expected to deliver, and it is why the crack problem here was process-driven, not impurity-driven.

What a buyer should actually check

If you specify 4130, 30CrMo, or 25CrMo4 billet or forging stock, do not stop at the chemistry line of the MTR. Ask three casting-process questions: (1) what is the surface temperature entering the straightener, and is it held above 900 °C; (2) is straightening continuous (multi-stand) rather than 3-point; (3) is the secondary-cooling water ratio controlled so the surface temperature drop stays under ~150 °C/m. A supplier who can answer those has controlled the third brittle zone. One who only shows you a perfect chemical certificate may be shipping billets that crack the moment they are bent cold — or, worse, that hide internal cracks which surface only after you forge or machine them.

The grade was never the problem. The temperature window was.

By Harris — Technical Team, SHUNFU METAL