ASTM A193 and A194 Bolting and Nuts for High-Temperature and High-Pressure Service

What ASTM A193 and A194 Cover

ASTM A193 is the specification for alloy and stainless steel bolting material, studs, stud bolts, screws and headed bolts, for pressure vessels, valves, flanges and fittings operating at high temperature, high pressure, or both. ASTM A194 is its partner, covering carbon, alloy and stainless steel nuts intended to be assembled with that bolting under the same conditions. One standard sets the bolt, the other sets the nut, and the two are written to be used as a pair.

Each standard defines the chemistry, heat-treat condition, mechanical properties and testing for every grade, but not the thread form or dimensions, which come from ASME B1.1 and B18.2. That separation lets a stud and nut carry a single traceable material pedigree from melt to the installed flange joint, while the thread geometry is governed independently.

A193 Bolting Grades

The A193 grade marking tells you both the alloy family and the strength class. The alloy-steel B7 grade is the workhorse for general high-temperature and high-pressure flanges, while the B8 and B8M austenitic stainless grades are chosen when corrosion resistance matters. Within the stainless grades, Class 1 is solution annealed for maximum corrosion resistance, and Class 2 is strain-hardened to raise strength at some cost to ductility.

Choosing Between B7, B8 and B8M

B7 gives the highest strength per rupee and suits dry, non-corrosive high-temperature flanges, but it has no inherent corrosion resistance and needs coating in wet or chloride-laden environments. Step up to B8 when the joint sees corrosion or sustained elevated temperature, and to B8M when chlorides are present. The deciding element is molybdenum: 316 carries roughly 2 to 3 percent molybdenum that 304 lacks, lifting its pitting resistance equivalent number from about 18 to about 24. For coastal, marine and chloride-process bolting, that difference is why B8M earns its place over B8.

A common procurement error is choosing Class 2 strain-hardened studs purely for the higher strength figure without accounting for the reduced ductility and the larger diameters where the Class 2 strength bonus falls away. Confirm the required proof load against the actual class before specifying, since over-specifying strength on a corrosion-driven joint rarely pays off.

A194 Nut Grades and Matching

A194 nuts are selected to match the bolting grade so the assembled joint has balanced strength and compatible corrosion behavior. Grade 2H is the hardened carbon-steel nut that pairs with B7 studs, while grades 8 and 8M are the austenitic stainless nuts that match B8 and B8M respectively. Mixing an incompatible nut and bolt, such as a plain carbon nut on a stainless stud in a chloride environment, can set up galvanic attack, so the pairing matters as much as the individual grade.

Grade and Property Reference

The corrosion behavior of the stainless bolting grades follows directly from their chemistry. The table below sets out the two austenitic grades behind B8 and B8M, drawn from the same metallurgy that governs the plate and bar forms.

Both B8 and B8M are austenitic, so neither is immune to chloride stress-corrosion cracking at elevated temperature. Where the service is sour, hot concentrated chloride, or strongly acidic beyond the reach of 316, the bolting is usually specified in nickel alloys such as Inconel or Hastelloy rather than standard A193 stainless, and the hardness limits of NACE MR0175 / ISO 15156 are called out on the order.

Where A193 and A194 Fit in a Bolted Joint

A193 studs and A194 nuts are the fastening members of a flanged piping system, and they are specified alongside the flange, the pipe and the forged fittings so the whole joint shares one engineered pedigree. The material standards set the metal, and the ASME B16 and B18 standards set the dimensions and thread form.

Matching the bolting grade to the flange grade keeps the joint metallurgically consistent: a 316 flange on a chloride service line deserves B8M bolting and grade 8M nuts, not plain B7. For low-carbon welded assemblies held in the 425 to 815 degree C range, the same sensitization discipline that governs plate applies to any welded fastener attachments.

ASTM A193 and A194 Fasteners at Nandini Steel

Nandini Steel is an ISO 9001:2015-certified stockist and supplier of stainless, nickel-alloy and alloy-steel products, established in 2003 and based in Mumbai. We hold A193 studs, stud bolts and screws and A194 nuts in the common B7, B8, B8M, grade 2H, grade 8 and grade 8M designations as part of our stainless steel fastener and alloy steel fastener ranges, with Inconel and Hastelloy bolting for the most aggressive service.

Everything is ex-stock and available for immediate dispatch, and every order ships with an EN 10204 3.1 or 3.2 mill test certificate documenting the heat number, chemistry, mechanicals and heat-treat condition for full traceability. As a certified specialist in 904L and the high-nickel alloys, we can supply matched bolting for demanding chemical and sour-service joints where standard stainless falls short. These fasteners serve oil and gas, petrochemical and pressure vessel and boiler plants across more than 36 export countries.

Frequently Asked Questions

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For live pricing and stock availability on ASTM A193 and A194 bolting and nuts, contact Nandini Steel at reachus@nandinisteel.com, call +91 22 43404600, or message WhatsApp +91 99202 09184 with your grade, size and quantity.