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Resources/Materials & grades/A194 2H

Materials & grades

ASTM A194 Grade 2H heavy hex nut steel (2HM, 4, 7; matches A193 B7)

ASTM A194 Grade 2H is a medium-carbon steel nut, quenched and tempered to 24–35 HRC, made to pair with ASTM A193 B7 studs. Grade 2HM is the lower-hardness version for B7M, and Grade 7 is the alloy nut used with B16 and A320 L7.

Names by standard

What it is called in each region

RegionSystemDesignationNormaMatch
USAASTM A194/A194MGrade 2HHeavy hex nut, medium-carbon steel, quenched and tempered. Pair with A193 B7.ASTM A194/A194MSame material
ASTM A194/A194MGrade 2HMSame chemistry as 2H, tempered to 159–235 HBW (99 HRB maximum) for stress-corrosion service. Pair with A193 B7M.ASTM A194/A194MClosest equivalent
ASTM A194/A194MGrade 7Alloy-steel (4140 type) nut, quenched and tempered to the same 2H hardness range. Pair with B16 and A320 L7. Grade 4 was the older companion grade; the current table read does not list it.ASTM A194/A194MClosest equivalent
UKBS EN ISO 898-2Property class 10UK adopts ISO 898-2. Class 10 is a nut property class for room-temperature use with bolts up to class 10.9; it carries no elevated-temperature rules and no 2H tempering minimum.BS EN ISO 898-2Closest equivalent
EUEN ISO 898-2Property class 10Quenched and tempered carbon-steel nut, 272–353 HV, for bolts up to class 10.9. Not the same as A194 2H.EN ISO 898-2Closest equivalent
EN 10269 nameC35E (1.1181)Carbon steel in EN 10269 for fasteners, but with 0.32–0.39% carbon against 0.40% minimum for 2H.EN 10269Closest equivalent
ÍndiaIS 1367 (Part 6)Property class 10Identical with ISO 898-2:2022. The Indian standard has no high-temperature 2H equivalent that we could verify.IS 1367 (Part 6):2025Closest equivalent
JapanJIS B 1052-2Property class 10Japanese nut property-class standard listed against ISO 898-2. No JIS nut with A194's tempering rule was found.JIS B 1052-2Closest equivalent

On equivalence

  • A194 2H is an ordering specification for a heat-treated nut with set hardness and proof stress. It is not the same as an ISO 898-2 property class. Class 10 nuts are quenched and tempered carbon steel at 272–353 HV, rated for bolts up to 10.9. They do not carry the elevated-temperature tempering rule or the heavy-hex dimensions of 2H.
  • ISO 898-2 and IS 1367 (Part 6) both use property classes. IS 1367 (Part 6):2025 is identical with ISO 898-2:2022.
  • EN 10269 C35E has 0.32–0.39% carbon and cannot meet the 0.40% carbon minimum of A194 2H. It is the nearest EN 10269 carbon steel, not an A194 equivalent.
  • Japan: JIS B 1052-2 is a nut property-class standard with no 2H-type tempering rule found. Use the ASTM name on drawings.
  • Some supplier pages quote 2H as 248–352 HB. The A194 table gives 248–327 HBW and 24–35 HRC for nuts up to 1½ in, which we used; 352 HB is the upper limit for Grade 2.

Chemical composition

Composition limits, mass %

Element2HUSA · ASTM A194/A194M, Grade 2H (heat analysis)7USA · ASTM A194/A194M, Grade 7 (typical steels 4140, 4142, 4145)C35E (1.1181)EU · EN 10269 (cast analysis)
C≥0.400.38–0.480.32–0.39
Si≤0.400.15–0.35≤0.40
Mn≤1.000.75–1.000.50–0.80
P≤0.040≤0.035≤0.030
S≤0.050≤0.040≤0.015
Cr—0.80–1.10≤0.40
Ni——≤0.40
Mo—0.15–0.25≤0.10

Mechanical properties

Strength and hardness

ConditionTensile strengthYield strengthElongationHardness
Grade 2H, quenched and tempered, minimum tempering temperature 850 °F (455 °C)Specified, finished nut up to 1½ in (M36). Proof stress 175 ksi (1205 MPa) for heavy hex.ASTM A194/A194M Grade 2H, via Torqbolt———248–327 HBW / 24–35 HRC
Grade 2H, quenched and tempered, minimum tempering temperature 850 °F (455 °C)Specified, finished nut over 1½ in (over M36). Proof stress 175 ksi (1205 MPa) for heavy hex.ASTM A194/A194M Grade 2H, via Torqbolt———212–327 HBW / ≤35 HRC
Grade 2HM and 7M, quenched and tempered, minimum tempering temperature 1150 °F (620 °C)Specified, finished nut. Proof stress 150 ksi (1035 MPa) for heavy hex. Every nut hardness tested.ASTM A194/A194M Grades 2HM and 7M, via Torqbolt———159–235 HBW / 84–99 HRB
Grade 7, quenched and tempered, minimum tempering temperature 1100 °F (595 °C)Specified, finished nut. Proof stress 175 ksi (1205 MPa) for heavy hex.ASTM A194/A194M Grade 7, via Torqbolt———248–327 HBW / 24–35 HRC
ISO 898-2 property class 10 nut, quenched and temperedSpecified, thread M5 to M16, coarse thread. For bolts up to class 10.9. Comparison only.ISO 898-2, via Torqbolt———272–353 HV
EN 10269 C35E bar, quenched and tempered (+QT)Specified, diameter up to 60 mm; yield and elongation are minimums. Reduction of area at least 45%. Comparison only; stock for fasteners, not nuts to A194.EN 10269, C35E, via Torqbolt500–650 MPa73–94 ksi≥300 MPa≥44 ksi≥22 %—

In practice

Behaviour, heat treatment and uses

Characteristics

  • 2H is a heavy hex nut in medium-carbon steel with at least 0.40% carbon, quenched and tempered at 850 °F (455 °C) minimum. The target hardness is 248–327 HBW (24–35 HRC) for nuts up to 1½ in.
  • The nut is made stronger than the stud. Proof stress is 175 ksi for a 2H heavy hex nut, above the 125 ksi minimum tensile strength of a B7 stud up to 2½ in, so an overload stretches the stud, not the thread.
  • 2HM and 7M are tempered at 1150 °F (620 °C) or higher to 159–235 HBW so that nuts for B7M and L7M service stay at or below 99 HRB. Every 2HM and 7M nut is hardness tested; 2H is sampled.
  • Grade 7 is an alloy nut (4140-type) for B16 and A320 L7 studs. A 2H nut relaxes faster than a B16 stud at high temperature and becomes the weak link.
  • Grade 4 appears in older A194 and A320 practice for low-temperature service. A fastener guide notes it was removed from A194 in 2017, and a table of the current grades read here does not list it.
  • A194 specifies no coating. Coatings such as zinc plating or galvanizing are addressed by other specifications.
  • A194 covers nut sizes ¼ to 4 in (M6 to M100 in the metric version) for carbon, alloy and martensitic stainless steels, and from ¼ in for austenitic nuts.

Heat treatment

  • Nuts are made from hot-wrought bar, quenched and tempered. A194 sets a minimum tempering temperature of 850 °F (455 °C) for 2H, 1100 °F (595 °C) for grade 7 and 1150 °F (620 °C) for 2HM and 7M.
  • The letters H and M in the grade mean the nut is heat treated.
  • Hardness is checked on finished nuts (sample for 2H, 100% for 2HM and 7M).

Usos típicos

  • Heavy hex nuts for A193 B7 studs on pipe flanges, valves and pressure vessels
  • 2HM nuts for A193 B7M studs in sour or stress-corrosion service
  • Grade 7 nuts for A193 B16 and A320 L7 studs

Bolting steels

Similar grades compared

GrauC %Mn %Cr %Ni %Tensile, first listed condition
A194 2H≥0.40≤1.000.80–1.10≤0.40500–650 MPa
A193 B70.37–0.490.65–1.100.75–1.20—≥860 MPa
A193 B160.36–0.470.45–0.700.80–1.15—≥860 MPa
A320 L70.38–0.480.75–1.000.80–1.10—≥860 MPa
A193 B8 / B8M≤0.08≤2.0018.0–20.08.0–11.0≥515 MPa

Composition from the US standard where one exists, otherwise the first standard listed on each grade's page. Strength conditions differ between grades; open a grade for its full table.

Perguntas respondidas

Common questions

What nut goes with A193 B7?

ASTM A194 Grade 2H heavy hex nuts. For B7M, use 2HM, which has a lower hardness (159–235 HBW). For B16 and A320 L7, use Grade 7 alloy nuts.

What is the difference between A194 2H and 2HM?

Both use the same chemistry, but 2HM is tempered at 1150 °F (620 °C) or higher to 159–235 HBW so that it stays at or below 99 HRB for stress-corrosion cracking service. 2H is 248–327 HBW and is for B7. Proof stress is 175 ksi for 2H and 150 ksi for 2HM.

Is a 2H nut the same as a class 10 nut?

No. A class 10 nut is an ISO 898-2 room-temperature property class (272–353 HV) with no heavy-hex or elevated-temperature tempering rules. 2H is a heavy hex A194 nut, quenched and tempered to 24–35 HRC. A class 10 nut is not a stand-in for 2H on a B7 stud.

What hardness is an A194 2H nut?

Finished 2H nuts up to 1½ in are 248–327 HBW, which is 24–35 HRC. Larger nuts are 212–327 HBW with 35 HRC maximum. Some supplier pages quote 248–352 HB, but the A194 table's upper limit for 2H is 327 HBW.

Why not use a 2H nut on B16 studs?

At high temperature 2H relaxes more than a B16 stud, so the nut becomes the weak point and bolt load is lost. Grade 7 nuts are the pairing for B16.

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Keep reading

A193 B7ASTM A193 Grade B7 is a quenched and tempered chromium-molybdenum steel (AISI 4140 or 4142 type) for studs and bolts in high-pressure and high-temperature service, with a minimum tensile strength of 125 ksi up to 2½ in.A193 B16ASTM A193 Grade B16 is a chromium-molybdenum-vanadium bolting steel for higher temperatures than B7.A320 L7ASTM A320 Grade L7 is a quenched and tempered chromium-molybdenum bolting steel, chemically like A193 B7, that must pass a Charpy impact test at −150 °F (−101 °C).A193 B8 / B8MASTM A193 Grades B8 and B8M are austenitic stainless steel bolting made from AISI 304 (B8) and AISI 316 (B8M).Bolt property classesMetric bolts in the UK, EU, India and Japan all use the ISO 898-1 property classes (4.6, 8.8, 10.9, 12.9), while US inch bolts use SAE J429 grades and ASTM specifications.Quenching and temperingQuenching and tempering hardens steel by cooling it rapidly from the austenite range to form martensite, then reheats it below Ac1 to trade some hardness for toughness.

References

Sources

  1. ASTM International, A194/A194M scope and abstract (nuts for high-pressure or high-temperature service) — Scope and size range; the full tables are paywalled.
  2. Torqbolt, ASTM A194/A194M nuts — A194 composition, hardness, proof stress and tempering temperatures for 2H, 2HM and 7.
  3. Baco, ASTM A194 grades — Cross-check of the 2H and 2HM composition and tempering temperatures.
  4. Projectmaterials, What is ASTM A194 — Nut-to-stud pairings for 2H, 2HM and 7.
  5. Hex Technology, The Stud Guide: B7s, B16s and other common bolt materials — Typical service ranges, nut pairings and grade-selection notes from a fastener maker.
  6. Torqbolt, ISO 898-2 nuts — Class 10 nut hardness, steel type and mating bolt class.
  7. Torqbolt, EN 10269 bolting steels overview — Grades covered by EN 10269 and the compositions and properties of X5CrNi18-10, X5CrNiMo17-12-2, C35E and 21CrMoV5-7.
  8. Standards.ie (NSAI shop), EN 10269:2013 Steels and nickel alloys for fasteners with specified elevated and/or low temperature properties — Title, scope and status of the CEN standard.
  9. Bureau of Indian Standards, IS 1367 (Part 6):2025, identical with ISO 898-2:2022 — Indian property classes for nuts follow ISO 898-2.
  10. Wermac, A193 B7 and A193 B7M studs — Inch-series B7 and B7M strength and hardness rows; B7M testing and nut pairing.
  11. ASTM A194/A194M-16, hosted copy of the full standard (Tables 1, 2, 3, 7, section 6) — 2H, 2HM, 7 and 7M composition, hardness, minimum tempering temperatures, proof stresses, size scope (¼ to 4 in, M6 to M100). 2016 edition, read by the independent checker.
  12. Wermac, A194 Grade 7 and Grade 4 nuts — Grade 4 phased out of A194 in 2017 with Grade 7 as the replacement.
  13. Fastenright, ASTM A194 nuts — Size ranges: ¼ to 4 in and M6 to M100 for carbon, alloy and martensitic nuts.

Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.