ASTM A193: How the Grade System Works and Which Grade You Actually Need
ASTM A193 is the bolting specification for anywhere that gets hot, pressurized, or both. Refineries, power plants, chemical processing facilities, boilers, compressor casings — if flanges are being torqued under process conditions, A193 is almost certainly the spec governing the studs.
The reason A193 trips people up isn't complexity. It's that the spec covers two fundamentally different material families under one designation, and buyers sometimes treat it as a single grade when it's really a family of six. Ordering "A193" without a grade is like ordering "stainless" without an alloy — technically meaningful, practically incomplete.
This post explains how the grade system works, what each common grade delivers, and how to make the call.
The two families inside A193
A193 is split into alloy steel grades and austenitic stainless steel grades. They serve different problems and shouldn't be confused.
Alloy steel grades — B5, B7, B7M, B16 — are chromium-molybdenum steels, quenched and tempered to high strength. These are the workhorse grades: heat-resistant, high-strength, and specified across the majority of process piping bolting. B7 alone covers probably 80% of refinery and petrochemical flange work.
Stainless grades — B8, B8M, and their variants — are austenitic 304 and 316 stainless, solution-annealed or strain-hardened. These go where corrosion resistance is the primary design constraint: chloride-bearing fluids, aggressive chemical service, food and pharmaceutical process equipment.
The families don't substitute for each other. B7 at 125 ksi tensile and B8 Class 1 at 75 ksi tensile are both "A193" — they are not interchangeable on a flange.
Grade B7 — the one you'll order most
B7 is chromium-molybdenum steel (AISI 4140 / 4142 family), quenched and tempered. It's the standard stud material for process piping, pressure vessels, heat exchangers, and compressors across the oil and gas, petrochemical, and power generation industries.
Mechanical properties are diameter-dependent. Up to 2-1/2": 125 ksi minimum tensile, 105 ksi minimum yield. From 2-5/8" through 4": 115 ksi tensile, 95 ksi yield. From 4-1/8" through 7": 100 ksi tensile, 75 ksi yield. Practical upper service temperature is around 750°F (400°C) — above that, creep starts to govern, and B16 becomes the better choice.
Pair with A194 Grade 2H nuts. That's the industry default and has been for decades. Grade 7 nuts are the alloy-steel alternative and are interchangeable with 2H in most services; they become the preferred choice above roughly 850°F.
Grade B7M — B7 with a hardness ceiling
B7M uses the same chromium-molybdenum chemistry as B7 but is produced to a controlled maximum hardness of 235 HBW (22 HRC). That hardness limit is what NACE MR0175 / ISO 15156 requires for sour service — environments containing hydrogen sulfide, where high-hardness steel is susceptible to sulfide stress cracking.
If H₂S is in the process stream, specify B7M instead of B7. Same strength class (100 ksi tensile minimum), same diameter range, different hardness control. B7M requires 100% hardness testing of every piece in the lot. Pair with A194 Grade 2HM nuts — same logic, same hardness ceiling.
Grade B16 — when the temperature goes higher
B16 is a chromium-molybdenum-vanadium alloy, heat-treated to similar strength as B7 at common diameters: 125 ksi tensile minimum up to 2-1/2", stepping down to 110 ksi from 2-5/8" through 4" and 100 ksi above that. The vanadium addition and higher tempering temperature give B16 significantly better creep resistance at elevated temperature — usable up to approximately 1100°F (593°C), where B7 would have crept out long before.
Pair with A194 Grade 7 nuts up to about 850°F, Grade 16 above that. B16 is specified for steam turbines, boilers, and the hottest pressure vessel service — applications where B7 simply can't maintain clamping force at temperature.
Grade B8 — 304 stainless for corrosion service
B8 is austenitic 304 stainless. It goes where the process fluid or environment would attack carbon and alloy steels — chemical plants, food and pharmaceutical equipment, general corrosion-resistant piping. Class 1 (solution-annealed) gives you 75 ksi tensile and 30 ksi yield with maximum corrosion resistance. Class 2 (strain-hardened) develops up to 125 ksi tensile at small diameters, dropping significantly as diameter increases.
The detail that matters most on B8 is that Class 2 strength is diameter-dependent in a way most people don't account for. A 3/4" stud and a 1-1/2" stud are both "B8 Class 2" with a 50 ksi yield difference between them. If load is a design input, confirm the diameter-specific number.
B8 is not suitable for chloride-bearing environments. That's where B8M comes in. Pair with A194 Grade 8 nuts.
We cover B8 in depth in its own post — ASTM A193 B8 Stainless Steel — Specs, Uses & Applications.
Grade B8M — 316 stainless when chlorides are present
B8M is 316 stainless. The jump from B8 to B8M isn't more chromium — 316 actually has slightly less chromium than 304. What it adds is 2–3% molybdenum, and that molybdenum is what provides resistance to chloride-induced pitting and crevice corrosion. Marine service, offshore platforms, desalination, brine, seawater piping, chemical process streams with halide chemistry — these are B8M applications.
Same class structure as B8: Class 1 at 75 ksi tensile for maximum corrosion resistance, Class 2 strain-hardened up to 125 ksi at small diameters with the same diameter-dependent drop. Pair with A194 Grade 8M nuts — 316 stainless to match.
We cover B8M in depth in its own post — ASTM A193 B8M Stainless Steel — Specs, Uses & Applications.
A note on low-temperature service
A193 does not cover low-temperature bolting. If your design temperature goes below -20°F (-29°C), you need ASTM A320 — the companion specification that adds Charpy impact requirements to ensure the material stays tough at cryogenic and low-temperature service conditions. A320 covers L7, L7M, L43, and Class 2 austenitic grades. Don't spec A193 for cold service and assume it's equivalent.
How to pick
The grade decision usually follows a short chain. First: what's the maximum service temperature? If it's above 750°F, B16. If it's below -20°F, A320. Everything in between, A193 applies.
Second: is H₂S present? If yes, B7M with 2HM nuts, full stop.
Third: what's the corrosive environment? Chlorides, seawater, marine or chemical process service → B8M. General corrosion-resistant service without chloride exposure → B8. No significant corrosion threat → B7.
If you're not sure which grade your application calls for, we work through this with customers regularly. It's a short conversation that saves a long rework.
What California Fastener supplies
We stock B7, B7M, B8, B8M, and B16 in stud bolt, threaded rod, and hex bolt configurations with matching A194 nuts across a full range of diameters. Mill certs ship with every order — heat number, chemistry, mechanical properties, and heat treatment records. Sour-service orders (B7M, 2HM) include NACE MR0175 compliance documentation on request.
Contact California Fastener to discuss A193 grade selection, quantities, and lead times.
