Zeron 100 FLT stands for Fasteners for Low Temperature service, the supply state engineered for the bolted joints that have to survive simultaneously below freezing and in chloride-bearing environments. Compared with Zeron 100 FG, the FLT supply state holds higher Charpy reserve at sub-zero temperatures while keeping the same super duplex corrosion envelope (PREN ≥ 40, NACE MR0175 hardness ≤ 28 HRC). FLT corresponds to ASTM A320 Grade L7 in mechanical property class, it's the alloy you specify when the buyer wrote "L7-equivalent" but the service envelope is too corrosive for AISI 4140 carbon steel.
Charpy V-notch 80-130 J at -46°C (typical) · 3-spec avg ≥ 45 J + min ≥ 35 J · PREN ≥ 40 · Hardness ≤ 28 HRC
Where FLT goes: LNG export terminal cryogenic flange bolting (down to -163°C tank ullage temperature on the loading side, -46°C ambient on the receiving side); FPSO topside arctic / North Atlantic deck fasteners; subsea manifold and jumper studs at 1500-3000 m depth where seabed temperature sits near 4°C and pressure plus chloride combine; ROV-installable fasteners that need predictable Charpy at any deepwater temperature; LPG / ethylene / propylene cryogenic loading terminal piping.
All steels have a ductile-to-brittle transition temperature (DBTT). Carbon steel DBTT is around -20°C; below that, Charpy V-notch energy collapses from 100+ J to under 20 J and the steel can fail by brittle fracture under shock loads. AISI 4140 (A193 B7) has DBTT around 0°C, unsuitable for arctic service. ASTM A320 Grade L7 was created as the low-temp variant of B7 with DBTT pushed to -101°C through tighter chemistry and quench-temper cycles. But L7 is still alloy steel, no corrosion resistance.
Super duplex stainless steels are face-centered-cubic (austenite) plus body-centered-cubic (ferrite) in roughly 50/50 mix. The austenite phase has no DBTT down to -270°C; the ferrite phase has DBTT around -50°C. By tightening the ferrite acceptance band toward the lower end (35-45% rather than 35-55%) and selecting heat lots with finer grain size, FLT shifts the composite Charpy curve so that 3-spec average at -46°C is comfortably ≥ 45 J with minimum ≥ 35 J, the NORSOK M-630 acceptance criterion for low-temp super duplex.
| Property | FLT (typical) | FLT (minimum) | vs ASTM A320 L7 | Test method |
|---|---|---|---|---|
| 0.2% Yield Strength | 620-720 MPa | 550 MPa | L7 = 720 MPa min | ASTM A370 |
| Ultimate Tensile | 800-870 MPa | 750 MPa | L7 = 860 MPa min | ASTM A370 |
| Elongation | 30-38 % | 25 % | L7 = 16 % min | ASTM A370 |
| Hardness (body) | 22-27 HRC | 22 HRC | n/a | ASTM E18 |
| Hardness (rolled-thread root) | 25-28 HRC | ≤ 28 HRC | NACE MR0175 ceiling | HV converted per E140 |
| Charpy V-notch @ -46°C | 80-130 J | 45 J avg / 35 J min | L7 = 18 J at -101°C | ASTM E23 |
| Charpy V-notch @ -101°C | 40-70 J | 20 J avg | L7 = 18 J min | ASTM E23 |
| Ferrite content | 38-48 % | 35 % | NORSOK M-630 = 35-55 % | ASTM E562 |
| CPT (G48 Method E) | ≥ 50°C | 50°C | n/a | ASTM G48-E |
| Sigma-phase (A923 Method C) | Pass | 60 J at RT | n/a | ASTM A923 Method C |
| Standard | Coverage of FLT |
|---|---|
| ASTM A320 L7-equivalent | Mechanical envelope match for low-temp bolting; FLT can substitute L7 where corrosion is in scope |
| ASTM A479 / A276 / A1082 | Source bar + finished fastener specs |
| NORSOK M-630 MDS 055 | Super duplex acceptance, FLT meets the low-temp Charpy requirement out of the box |
| NORSOK M-650 | QTR'd manufacturer for North Sea low-temp super duplex |
| NACE MR0175 / ISO 15156-3 | Sour-service hardness ≤ 28 HRC at rolled-thread root |
| API 6A PSL 3 / 3G | Wellhead and Christmas tree, low-temp service envelope |
| API 17D | Subsea wellhead and tree equipment |
| API 6DSS | Subsea pipeline valves, bolting at deepwater temperatures |
| EN 10269 | Steels and Ni alloys for fasteners with low-temp impact requirement |
| EN 13445-2 Annex B | European pressure equipment cryogenic material classification |
| Service envelope | Why FLT |
|---|---|
| LNG export terminal flange bolting (loading arms, manifolds) | Cryogenic Charpy reserve + chloride-bearing seawater spray on outer surface |
| FPSO topside arctic / North Atlantic deck fasteners | Sub-zero deck temperatures + saltwater spray + topside vibration loading |
| Subsea manifold & jumper studs (deepwater) | 4°C seabed temperature + 1500-3000 m hydrostatic + chloride; FLT covers full range |
| ROV-installable subsea fasteners | Charpy at any deepwater temperature; predictable installation torque |
| LPG / ethylene / propylene cryogenic loading piping | Sub-zero process temperature + salt-air ambient; A320 L7 envelope without alloy steel |
| Arctic offshore platform topside flange bolting | Year-round sub-zero service; corrosion + temperature combine |
| Polar research vessel piping fasteners | Continuous low-temp + saltwater + cyclic loading |
| Cryogenic gas separation plants (air separation, ASU) | -150°C+ service on cold-side flanges; chloride from cooling water on outside |
| Service temperature | FG suitable? | FLT suitable? | Recommendation |
|---|---|---|---|
| Above 0°C continuous | Yes | Yes | FG, cheaper, same yield envelope |
| 0°C to -29°C | Yes (FG meets 50-90 J at -50°C) | Yes | FG acceptable; FLT adds margin |
| -29°C to -46°C (NORSOK envelope) | Borderline | Yes, designed for it | FLT (specify on PO) |
| -46°C to -60°C | No | Yes | FLT (mandatory) |
| -60°C to -101°C (LNG envelope) | No | Yes (Charpy 40-70 J at -101°C) | FLT with project-specific Charpy at LNG temp |
| Below -101°C continuous | No | Borderline | Consider 304L or Inconel 625; super duplex DBTT at sub-LNG temps is project-by-project |
In mechanical-property class, yes, FLT meets or exceeds L7's yield, tensile, elongation, and impact toughness at -46°C. In metallurgical family, no, L7 is AISI 4140 alloy steel quenched-and-tempered to L7 envelope. FLT is super duplex stainless. The two are equivalent only in the load-bearing envelope they cover; corrosion resistance of FLT is dramatically better. The right way to spec FLT in buyer documents is "ASTM A1082 Zeron 100 (S32760), low-temperature variant per buyer impact-test requirement at -46°C, NACE MR0175 / ISO 15156-3 compliant".
Yes, using ER2594 (overmatching) or ER2553 (matching) super duplex filler. Heat-affected zone of any super duplex weld reverts to SA-equivalent ferrite balance, so post-weld solution-anneal is recommended for restoring uniform properties when the weld is in the load path. For sub-zero impact-tested joints, the weld qualification record (PQR) must include Charpy V-notch at the design minimum metal temperature.
The ferrite phase is what limits low-temp Charpy in super duplex. Reducing ferrite from 55% to 45% leaves more austenite (which has no DBTT to -270°C) and shifts the composite Charpy curve to lower temperatures. The narrower 38-48% band is achievable through tighter melt chemistry control + finer grain-size lots; we cherry-pick heats that hit this window for our FLT inventory rather than blending mixed-ferrite heats.
FLT holds 40-70 J Charpy at -101°C and is project-acceptable for LNG flange bolting where the design minimum metal temperature is set at the loading-arm interface (~-46°C ambient envelope, even though the LNG is colder). For the inboard bolted joints actually in contact with -163°C LNG, super duplex is the wrong alloy, the standard call there is austenitic 304L or Inconel 625 (no DBTT to LNG temp). The FLT envelope is "outboard cryogenic" not "deep-cryogenic".
Heavy hex nuts ship in SA condition (hot-forged, then SA, then tapped). Specifying "FLT heavy hex nut" effectively means SA bar + heat selected for low-temp Charpy, we cherry-pick the heat lot to meet -46°C requirement on the same Charpy criterion as FLT bar. The cert reflects this. Stud bolts and hex bolts in FLT are CNC-turned from FLT-controlled bar.
Send the spec (form, size, length, design minimum metal temperature, standards / MDS to comply with, quantity, destination port) to info@torqbolt.com or +91-22-66157017 / WhatsApp. We reply with stock availability, mill route, lead time, MTC tier (incl. Charpy temperature matrix), and price within one business day (Mumbai IST).
