A194 Gr. 8S Nut

QS Fastener: A194 Gr. 8S Nut

Name: A194 Gr. 8S Nut
Standard: ASME B18.2.4.1M
Material: A194 Gr. 8S
Size: M12
Surface: Natural color

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A194 Gr. 8S Nut, conforming to ASME B18.2.4.1M, M12 size, is fully interchangeable with high-pressure flanges and valves. The grade 8S material delivers low-temperature toughness down to –196 °C and 1000 MPa-class tensile strength. Its natural, coating-free surface eliminates hydrogen-embrittlement risk, making A194 Gr. 8S Nut the secure choice for locking connections in LNG, petrochemical and ultra-low-temperature services.

A194 Gr. 8S Nut belongs to the ASTM A194 system and is an alloy-steel grade purpose-built for “ultra-low temperature + high pressure” service. Compared with conventional grades 2H, 8, 8M and 8C, the design philosophy, metallurgical control and performance validation of A194 Gr. 8S Nut all revolve around the extreme –196 °C operating window. The key differences are detailed below:

1. Low-temperature toughness
A194 Gr. 8S Nut achieves Charpy V-notch impact energy ≥ 27 J at –196 °C, with grain size ≥ 7 and a ductile-to-brittle transition temperature (DBTT) below –200 °C. Grades 8/8M/8C, by contrast, only require ≥ 27 J at –46 °C and exhibit a DBTT around –80 °C. During start-stop cycles in LNG cargo tanks or liquid-nitrogen lines, temperature swings can reach 250 °C; the fibrous fracture morphology of A194 Gr. 8S Nut completely suppresses low-temperature brittle failure and prevents catastrophic rupture.

2. Resistance to hydrogen embrittlement & stress-corrosion cracking
A194 Gr. 8S Nut is produced by vacuum degassing + electro-slag remelting, with tight chemistry limits: C 0.08–0.12 %, Ni 0.80–1.00 %, S ≤ 0.020 %, P ≤ 0.025 %, carbon equivalent CE ≤ 0.45. Consequently:
① Martensite and carbide precipitation are minimized, curbing hydrogen-trapping sites;

② Nickel stabilizes austenite, raising the chloride-ion SCC threshold to ≥ 70 MPa·m½;

③ The natural, coating-free surface avoids hydrogen pick-up and blistering associated with electroplating. Grade 2H (C 0.40–0.50 %) is prone to hydrogen-induced cracking under high H₂S partial pressures; grade 8M (Mo) resists pitting but its hydrogen-embrittlement-insensitive zone only extends to –29 °C.

3. Strength & hardness match
After oil quenching at 980 °C followed by double tempering at 650 °C, A194 Gr. 8S Nut reaches 321–352 HBW hardness, yield ≥ 760 MPa and tensile ≥ 965 MPa. When paired with ASTM A320 B8/B8M Class 2 bolts, a fully matched “8S-8S” assembly is created with a potential difference < 50 mV, eliminating galvanic corrosion at flange seal faces. Traditional 2H nuts (248–327 HBW) must mate with B7 bolts (235–302 HBW), where the dissimilar-steel potential can reach 250 mV and galling occurs within 12 months in coastal salt-spray environments.

4. Process traceability
Each lot of A194 Gr. 8S Nut is delivered with an EN 10204 3.2 certificate covering nine full-inspection items: –196 °C impact, hardness, grain size, non-metallic inclusions, etc. Grades 8/8M usually ship with only 3.1 certificates and lot-sampled impact testing, resulting in high data scatter that fails to meet the mandatory inspection requirements of ASME B31.3 cryogenic piping codes.

Conclusion: At –196 °C extreme cold, 40 MPa high pressure, and in the combined presence of chlorides and sulfur, A194 Gr. 8S Nut provides the trinity of “high toughness, hydrogen-embrittlement immunity, and same-material pairing,” delivering zero-failure locking assurance for LNG receiving terminals, liquid-hydrogen storage tanks and cryogenic air-separation columns—a solution that grades 2H, 8, 8M and 8C simply cannot match.

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