SUP13Cr Billet Selection: Forged vs Continuous-Cast Tubing

In Chinese specifications, the super-martensitic 13Cr-5Ni-2Mo stainless used for CO₂ oilfield tubing is called SUP13Cr. The same alloy is UNS S41426 in the US system and, in the European super-martensitic family, EN 1.4414. The 110-grade tube (110 ksi min yield, ~758 MPa) sits inside the API 5CRA / ISO 13680 Group 1, Category 13-5-2 family. Test methods map across the three systems: chemistry by GB/T 4336 (China), ASTM E1086 (US) and EN 10351 (EU); tensile by ASTM A370 / EN ISO 6892-1; impact by ASTM E23 / EN ISO 148-1; Rockwell hardness by ASTM E18 / EN ISO 6508; CCT by YB/T 5128 (close to ISO 22077); metallography by ASTM E407 / ISO 4967. With the naming and standards aligned, the question for a tubing buyer is simple: when the same heat of steel can be made into a tube from a forged billet or a continuous-cast billet, does the billet source actually matter?

A recent production study at TPCO on 110-grade SUP13Cr ran the same chemistry, the same 900 °C × 23 min quench + 610 °C × 37 min temper cycle, and the same piercing/continuous-mill route on the two billet origins side by side. The result is unambiguous, and it is the subject of this article.

Same chemistry, different billet DNA

The two billets — a continuous-cast bloom and a forged bloom — were taken from production heats and finished into ϕ244.48 × 13.84 mm tubes. Heat-treated tube chemistry (three positions, averaged by OES) shows the two are effectively the same steel:

CCT curves from the same study are equally close. Ac₁ = 665 °C and Ms = 240 °C for both billets; Ac₃ = 905 °C for the continuous-cast billet, 910 °C for the forged billet — a 5 °C shift well within normal cast-to-cast variation. In the industrial cooling range of 1–40 °C/s both transform to martensite only; no pearlite, no bainite. A buyer reading only the mill test certificate and the CCT plot would reasonably conclude the two are interchangeable. The microstructure says otherwise.

The banded structure heat treatment cannot erase

The decisive difference is the as-solidified structure. The continuous-cast bloom carries coarse columnar grains and dendritic segregation that locally concentrate Cr and Mo, which in turn produces clusters of δ-ferrite in bands. Hot rolling breaks up the cast grains but cannot erase the long-range segregation, so the banded δ-ferrite carries through to the finished tube. The forged bloom, by contrast, has already been through a large plastic reduction that smears and refines the segregation; δ-ferrite ends up as small, evenly dispersed islands. Metallography (Table 4 and Figs 2–3 of the source) shows it cleanly:

Two facts deserve underlining. First, the banding survives the quench-and-temper cycle; the heat-treated tube from the continuous-cast billet still shows localised bands. Second, the grain size inverts the usual expectation: the continuous-cast tube is finer (7.5 vs 7.0), but grain size alone is not the lever that controls performance in this steel — the distribution of δ-ferrite is. This is the genetic difference the rest of the data traces downstream.

Room-temperature properties hide the problem

At room temperature the two tubes look almost identical, which is precisely why the difference is so often missed. Tensile and hardness data from the same ϕ244.48 × 13.84 mm tubes:

Rm differs by 14 MPa, Rp0.2 by 15 MPa, elongation by 1 % — inside the same lot of test scatter. The forged billet’s hardness range, however, is tighter (0.8–0.9 HRC vs 1.1–1.3 HRC), and that tighter range is the first measurable echo of the cleaner microstructure. None of this is enough, on its own, to disqualify a continuous-cast tube against a given tender. The impact data is.

The −40 °C transverse impact is where the billet is decided

Charpy V-notch at 0, −10, −20 and −40 °C, longitudinal and transverse, all specimens 100 % shear:

Both billets pass the API 5CRA impact floor with a wide margin (≥80 J longitudinal, ≥60 J transverse at −10 °C). The longitudinal gap is small: 14 J or less across the whole sub-zero range. The transverse gap is not. At −40 °C the continuous-cast tube delivers 139 J transverse against 176 J for the forged tube — the cast tube is 79.0 % of the forged tube. That is the headline number. The mechanism is straightforward: bands of δ-ferrite run along the rolling direction and act as short-circuit paths for cracks; under transverse loading the crack plane intersects those bands head-on, so the tube loses the toughness its longitudinal test would suggest it has.

The hot-workability window the mill forgets to mention

The same microstructural feature shows up in the high-temperature ductility, where it is arguably more dangerous. Hot tensile tests at 1100–1300 °C, strain rate 0.43 /s, soak 128 s:

Three observations the data forces. First, the forged billet’s reduction of area is 88.8–99.0 % across the entire window — a 10.2 percentage-point spread, all of it comfortably above the 80 % rule-of-thumb for safe hot working. Second, the continuous-cast billet has a ductility trough at 1180 °C (89.4 %) and a near-collapse at 1300 °C (79.8 %); the 19.1 percentage-point spread signals that any small drift in reheat temperature or soak time can drop a continuous-cast billet into its own cracking zone. Third, the forged billet’s hot tensile strength declines smoothly from 99 MPa to 50 MPa, while the continuous-cast billet’s strength zig-zags (117 → 119 → 92 → 94 → 100 → 76 MPa) because its microstructure cannot deform uniformly at every temperature. For a mill piercing 110-grade SUP13Cr on a continuous mandrel mill, that zig-zag is exactly the behaviour that produces the occasional surface and through-thickness cracking on billets pushed to the upper end of the reheat envelope.

The takeaway for a tubing buyer

If your well is shallow, the CO₂ partial pressure modest, and the low-temperature design margin generous, the continuous-cast SUP13Cr billet is a defensible cost-down choice — both billet origins clear the API 5CRA 13Cr-5Ni spec on chemistry, tensile, hardness and impact at −10 °C. The data in this study does not contradict that.

If the well is deep — 6000 m and beyond (with 8000 m+ increasingly routine in fields such as Tarim), CO₂/Cl- in the produced fluid, and operating temperatures that swing sub-zero in winter or in shut-in — the same data changes the answer. The continuous-cast billet tube retains 79.0 % of the forged billet tube’s transverse Charpy at −40 °C, its hot ductility troughs at 1180 °C, and its hot tensile strength is non-monotonic with temperature. None of these facts are visible on the MTC. They are written into the billet before the tube exists, and they cannot be erased by heat treatment.

That is the case for buying forged-billet SUP13Cr tubing for any application where transverse impact and hot-mill reliability are on the critical path. The same case, conversely, is the boundary that should keep continuous-cast SUP13Cr tubing inside the envelope it actually fits.