A194 Grade 2H and A563 Grade DH: The Nut Comparison That's Less Complicated Than It Looks
If you've ever ordered high-strength nuts and wondered whether 2H and DH are interchangeable, you're asking the right question. The short answer is: often yes, with specific exceptions, and F3125 spells out exactly when.
The longer answer requires understanding that A194 and A563 are different specifications serving different primary purposes — and that 2H and DH are just two grades within each, not the whole story. Here's how both specs work and when the substitution is and isn't allowed.
What A194 covers
ASTM A194 is the nut specification for high-pressure and high-temperature service — the companion to A193 and A320 bolting. It covers a range of grades matched to specific stud grades: Grade 2H with A193 B7; Grade 2HM with B7M in sour service; Grade 7 or 16 with B16; Grade 8 with B8; Grade 8M with B8M.
The full A194 grade list is built around process piping and pressure equipment — refineries, power plants, chemical facilities. Every A193 stud grade has a corresponding A194 nut grade calibrated to match it in material and mechanical performance.
What A563 covers
ASTM A563 is the structural nut specification — the companion to F3125 structural bolts, F1554 anchor bolts, A307, and A449. Its grade list is built around construction and general mechanical assembly: Grade A for light-duty anchor bolt work, Grade DH for high-strength structural connections, Grade DH3 for weathering-steel structural work.
Where A194 is organized around temperature, pressure, and corrosion service, A563 is organized around structural connection strength and weathering compatibility.
Grade 2H and Grade DH — what the specs say
This is where the two look nearly identical on paper, and the reason the substitution question comes up constantly.
A194 Grade 2H is medium carbon steel, quenched and tempered, heavy hex. Proof load stress of 175 ksi, hardness between 24 HRC minimum and 38 HRC maximum.
A563 Grade DH is carbon steel, quenched and tempered, heavy hex. Proof load stress of 175 ksi, hardness between 24 HRC minimum and 38 HRC maximum.
Same proof load, same hardness range, same profile. The mechanical properties are effectively identical, which is exactly why F3125 permits the substitution.
The substitution F3125 allows
ASTM F3125 explicitly permits A194 Grade 2H heavy hex nuts as a substitute for A563 Grade DH on Type 1 structural bolts — A325, A490, F1852, and F2280. All the A563 requirements still apply: coating grade, overtap amounts, lubrication, proof-load testing. The substitution just means a shop stocking 2H for their A193 process bolting work can cover structural bolt orders from the same bin.
That's a practical supply advantage. Many distributors and contractors carry 2H as their standard heavy hex nut because it covers both A193 process bolting and F3125 structural bolting in one grade.
Where the substitution doesn't work
2H cannot substitute for A563 Grade DH3. DH3 is weathering steel — it's specified for F3125 Type 3 structural bolts used with weathering-steel structural members on bridges and exposed construction where the assembly is designed to weather uniformly without paint. 2H is standard carbon steel and doesn't match the weathering alloy chemistry. If the bolt is Type 3, the nut must be DH3.
Similarly, DH doesn't cross over into process piping service. A563 DH is a structural spec nut. It doesn't carry the A194 pedigree, documentation requirements, or temperature-service testing that process bolting demands. For A193 B7 studs on a refinery flange, the call is A194 2H — not A563 DH, even though the mechanical properties are essentially the same.
The grades beyond 2H and DH
The reason 2H and DH get compared is that they're the two most commonly specified heavy hex nuts in industrial work. But neither A194 nor A563 is a two-grade spec.
A194 covers fourteen grades including 2HM for sour service, Grade 4 and 7 for alloy-steel high-temperature applications, Grade 16 for the hottest B16 service, and Grades 8 and 8M for stainless B8 and B8M studs. Each of those grades exists because a different A193 or A320 stud grade demands a calibrated nut match.
A563 covers eight grades including Grade A for anchor bolt service with F1554 Grades 36 and 55, Grade C for F1554 Grade 105 anchor bolts, and DH3 for weathering structural connections. The full grade list exists because different structural applications — from light anchor work to high-strength weathering-steel bridges — have different nut requirements.
Specifying only "2H" or "DH" without the spec context works in most cases because those are the default grades for their respective worlds. But if the application is outside the standard pairing — sour service, high temperature, weathering steel, stainless — the grade needs to match the service, not just the bolt strength.
Marking
Both nuts mark the grade on one face. A563 DH nuts show "DH" plus the manufacturer's identification. A194 2H nuts show "2H" plus the manufacturer's identification. Type 3 and sour-service grades add supplementary markings. If you're looking at an unmarked heavy hex nut and need to confirm the grade, the cert is the only way — head marking tells you the grade, not the spec family.
What California Fastener supplies
We stock A194 Grade 2H as our standard heavy hex nut for both A193 process bolting and F3125 structural applications — it covers both worlds from one inventory position. A563 DH3 is stocked for weathering-steel structural projects. The full A194 grade range — 2HM, 4, 7, 7M, 8, 8M — is available with mill certs for process bolting orders.
Contact California Fastener to discuss nut grades, quantities, and lead times.
