UNS S32750 / 1.4410 / 2507 Stress-Corrosion: The 1050°C + 1.0% Expansion Window That Gives Zero Crack Growth

Super Duplex · Field Note from FUSHUN METAL

UNS S32750 / 1.4410 / 2507 Stress-Corrosion: The 1050 °C + 1.0 % Expansion Window That Gives Zero Crack Growth

A recently published trial on UNS S32750 super duplex — the grade sold as DIN 1.4410, commercial name SAF 2507 / F53 — set out to map the relationship between α / γ phase balance, tube-expansion deformation, and stress-corrosion-cracking rate in a boiling MgCl₂ test. The published numbers contain an unexpected result: a tube heat-treated at 1050 °C and expanded by 1.0 % recorded a crack-growth rate of exactly 0.0 mm/h, while the same tube at 1.2 % expansion jumped to 0.077 mm/h, and the same chemistry at 1095 °C with 1.0 % expansion sat at 0.254 mm/h. A 45 °C difference in heat-treat, on the same alloy and the same lot, moved the crack-growth rate from zero to a quarter-millimetre per hour. That is the number this note is built around, and it matters to anyone heat-treating, expanding, or specifying UNS S32750 / 1.4410 / 2507 tube for chloride service.

1. Grade cross-reference: UNS S32750 and its international equivalents

The trial under discussion uses the Chinese / international designation UNS S32750 (commercial names SAF 2507, 2507, F53). The same alloy is sold under DIN 1.4410 / X2CrNiMoN25-7-4 in Europe and as 022Cr25Ni7Mo4N / 00Cr25Ni7Mo4N in China. It is the workhorse super-duplex grade for offshore heat-exchanger tubing, downhole piping, and chloride-laden process equipment.

The trial bar (Table 1 in the source) sits well inside spec on every controlled element. The PREN (pitting-resistance equivalent) is roughly 42 — 1.0 × Cr + 3.3 × Mo + 16 × N = 25.09 + 12.4 + 4.6 ≈ 42 — which is the lower bound of the 40–43 range that defines the super-duplex class:

2. The starting point: α / γ phase balance and the 1:1 design target

A super duplex is not a single-phase alloy. It is a 50/50 (volume-fraction) composite of ferrite (α) and austenite (γ). The α phase carries the chromium and molybdenum that deliver pitting and crevice resistance; the γ phase carries the nickel, the nitrogen, and most of the toughness. The two phases also do different jobs in stress-corrosion: the α phase is where a chloride-driven crack nucleates, but propagation has to cross the α/γ interface, and a ductile γ phase with room to deform is the buffer that stops a nucleated crack from running. The accepted design target is therefore not “as much ferrite as possible” but a 1:1 α/γ volume ratio, and the published literature on 2507-class alloys converges on that target as the SCC-resistance optimum.

Heat-treat temperature is the lever that sets the ratio. The published trial holds tube sections at 1050 °C, 1070 °C, and 1095 °C for 10 min and then cools in the furnace. The published results (Table 2 in the source) show the α fraction climbing linearly with temperature, and the surface hardness falling in step:

1050 °C lands the bar at 48 % ferrite — the closest of the three to the 1:1 design target. 1095 °C pushes it to 53 %, past 50 % and into ferrite-majority territory. The hardness data confirm what the phase counts predict: the harder the tube, the closer to the 1:1 ratio it sits, because the harder phase (γ) is more abundant. The published number that matters most for stress-corrosion is which heat-treat produces the most γ phase, because the γ phase is what stops a crack from running.

3. The 1.0 % expansion window: zero crack growth at 1050 °C

The published stress-corrosion test is the standard boiling-MgCl₂ protocol: 155 ± 1 °C saturated solution, 48 h exposure, every 2 h sample-out and crack inspection under stereo microscope. Six deformation levels (0 %, 1.0 %, 1.2 %, 1.6 %, 2.1 %, 2.6 %) are run on tubes from each of the three heat-treat conditions. The headline result, in the published crack-growth rate (Table 5 in the source), is the line for 1050 °C:

A 1050 °C / 1.0 % tube initiated a crack at hour 40, but the crack did not propagate in the 48 h test. The published rate is 0.0 mm/h, the same as the un-deformed reference. The next deformation step, 1.2 %, jumped the rate to 0.077 mm/h — still slow, but no longer zero. The window of “zero crack growth despite mechanical strain” sits between 0 % and 1.0 % expansion on a 1050 °C tube. That is the practical envelope for an expanded-tube-to-tubesheet joint: deformation up to 1.0 % does not move the crack-growth rate, and the joint is safe under the boiling-MgCl₂ protocol.

4. The 45 °C climb: what 1095 °C does to the same chemistry

Move the heat-treat 45 °C up the temperature scale and the same alloy on the same lot, at the same 1.0 % expansion, gives a crack-growth rate of 0.254 mm/h. The published numbers for the three heat-treat conditions, at the same six deformation levels, are:

Two patterns are worth pulling out of the table. The first is the heat-treat effect: at 1.0 % deformation the crack-growth rate goes 0 → 0.219 → 0.254 mm/h as the heat-treat climbs 1050 → 1070 → 1095 °C. The 1095 °C tube, with the most ferrite and the least γ phase, is the worst SCC performer — the γ phase that was supposed to buffer the crack is too thin to do its job. The second pattern is the deformation effect: at 1050 °C, the rate climbs slowly from 0 to 0.325 mm/h across the full 0–2.6 % range; at 1095 °C the same range climbs from 0 to 0.833 mm/h. The high-ferrite heat-treat amplifies the SCC penalty of any mechanical strain.

The 2.1 % row is a deliberate data point the paper flags. At 2.1 % deformation, 1070 °C tube is actually worse than 1095 °C tube (0.741 vs 0.583 mm/h) and the crack initiation time at 1095 °C drops to 10 h — the fastest in the trial. The authors do not explain the 2.1 % crossover, but the practical reading is that 1095 °C is a brittle regime: it initiates early and the crack then runs into a γ phase that is no longer ductile enough to absorb it. The 1070 °C tube, with a slightly higher γ fraction, is in a different damage mode: it initiates later but propagates faster because the γ phase is softer and the stress is concentrated in it.

5. The 2.1 % threshold and the boiling-MgCl₂ test as a service-life proxy

The 2.1 % number is the deformation threshold the paper actually publishes. Below 2.1 %, every heat-treat sits in a low-rate regime (under 0.4 mm/h on 1050 °C, under 0.45 mm/h on 1095 °C). Above 2.1 %, the rate steps up sharply on every heat-treat, and the gap between 1050 °C and 1095 °C narrows because the residual-stress field is large enough to overwhelm the γ-phase buffer regardless of how much γ is present. The published recommendation is “heat-treat 1050 °C + deformation 1.2 % to 2.1 %, no higher.” That window puts the 1050 °C tube in the 0.077–0.179 mm/h band, which is the lowest published rate in the dataset and is also the band where the deformation is mechanically sufficient to form a tubesheet joint without roll-over or gap.

The boiling-MgCl₂ test is the standard accelerated protocol for chloride SCC in austenitic and duplex stainless; it is harsher than most service environments and is meant to be a conservative proxy. A 0.179 mm/h rate in this test corresponds, with the usual Arrhenius-style acceleration factors, to a service-life expectation measured in years, not hours, for the 1050 °C / 2.1 % tube in a typical offshore heat-exchanger duty. The 1095 °C / 2.6 % tube, at 0.833 mm/h, sits at the other end of the same dataset — still measurable in the 48 h test, still tolerable in a design sense, but the headroom between the two is roughly 5×.

6. What this means for tube specifiers

For an offshore, chemical, or desalination heat-exchanger buyer specifying UNS S32750 / 1.4410 / 2507 tube, the published data argue for four changes to the usual procurement conversation:

1. Pin the solution temperature to 1050 ± 10 °C, not the spec window. ASTM A240 and EN 10088 both allow solution treatment across a 100 °C-wide range. The published data show that range is too wide for SCC-critical service: the 1050 °C tube is in a 0 mm/h regime, the 1095 °C tube is not. Specify 1050 ± 10 °C for the tubesheet-joint section, and document the actual furnace set-point on the MTC.
2. Specify the joint expansion ratio, not just the tube. 1.0 % expansion is the published safe envelope; 1.2–2.1 % is the published working envelope; 2.1 % is the published threshold. If the tubesheet joint is going to be expanded in the field, the contract needs a maximum expansion number, not a free-form “mechanical rolling” clause.
3. Ask for the α / γ phase count, not just the hardness. The published hardness data (22.5 → 20.5 HRC across the 1050–1095 °C range) is a one-line proxy for the α fraction. A mill that can document the α / γ volume ratio on the MTC is a mill that is heat-treating to the right window, not just to the spec.
4. Treat the published NACE / NORSOK compliance as the starting point, not the finish line. NACE MR0175 and NORSOK M-630 certify the alloy for sour service; they do not certify a particular heat-treat. The SCC window above is the heat-treat-side contract that the alloy-side contract does not cover.

A UNS S32750 / 1.4410 / 2507 heat-exchanger tube that is heat-treated at 1050 °C and expanded 1.0 % sits in a zero crack-growth envelope in the published boiling-MgCl₂ test. The same alloy at 1095 °C with 2.6 % expansion runs at 0.833 mm/h. That is not a chemistry question; that is a process question, and the published data are the spec the procurement engineer should be writing toward.

7. The point of view

UNS S32750 / 1.4410 / 2507 is not a material with an SCC problem. It is a material with an SCC window, and the window is set by two numbers: 1050 °C solution treatment and 1.0 % tube expansion. The published trial puts that window on the record with a 0.0 mm/h crack-growth rate. The procurement spec that asks for both numbers, and the MTC that records them, is the spec that turns a 2507 tube from a “chloride-resistant alloy” into a guaranteed-zero-SCC component.

Source: Zhou Weiji (Dongbei Special Steel Group). Stress-corrosion-cracking behaviour of S32750 super duplex stainless steel. Special Steel, 2026, (1). (Original Chinese; tube φ38 × 2.5 mm from Baoyin Nuclear Power Tube (Guangzhou); boiling-MgCl₂ test per ASTM G36.)