By Ma Yajiada, Chief Engineer · Reviewed by Ma Yajiada, PE · Citations: ASME SA-193-2023, ASTM A194-23, ASME B16.5-2020 §7, NACE MR0175 / ISO 15156-3:2020, ASTM A320-23

Why the bolt grade matters more than the flange grade

A flange joint is only as strong as the studs that hold it together. The flange body sees hoop stress — predictable, calculable, well covered by ASME B16.5 and ASME Section VIII. The studs see axial load — and that load is governed by SA-193 tensile class, yield class, hardness limit, and temperature derating. Pick the wrong stud grade and one of three things happens: (1) the bolt stretches under load and the joint leaks; (2) the bolt hardens past 22 HRC and fails NACE MR0175 in a sour service audit; (3) the bolt corrodes at the dissimilar-metal junction with a stainless flange and snaps within 18 months. The flange itself can be perfect — A105, A350 LF2, A182 F316 — but if the studs are wrong, the joint fails.

There are seven SA-193 grades that show up on flange RFQs in our order book. Four are interchangeable in narrow service windows. Three are restricted by temperature, environment, or hardness limits that the buyer often does not realise are coded into the standard. Below is the SA-193 grade family, the Table 2 mechanical property data, and the four-service rule that decides which grade goes into the RFQ.

SA-193 grade family at a glance

ASME SA-193-2023 "Specification for Alloy-Steel and Stainless Steel Bolting Materials for High-Temperature or High-Pressure Service" lists 7 grades that show up on flange bolting RFQs. The grade letter (B / L) tells you the service class — B for high-temperature / high-pressure, L for low-temperature (cryogenic). The grade number (7 / 7M / 8 / 8M / 16) tells you the alloy system.

Tensile, yield, hardness — the SA-193 Table 2 data

Mechanical properties per ASME SA-193-2023 Table 2 (Class 1 for stainless grades, Class 1 for alloy grades in the size range 1/4" to 4"). All values are minimum unless noted otherwise.

Two numbers matter most when you are reading a mill certificate. (1) Hardness: B7M and L7M are capped at 22 HRC per NACE MR0175 — this is the most common cause of stud rejection at the MTC review stage. (2) Yield: B7M has a lower yield floor (550 MPa vs 720 MPa for B7), which means the allowable bolt stress in ASME PCC-1 is reduced proportionally. You cannot simply swap B7 for B7M and expect the same torque capacity — the bolt will stretch more under the same load.

NACE MR0175 — why B7M is not just "softer B7"

NACE MR0175 / ISO 15156-3:2020 limits the maximum hardness of carbon and low-alloy steel bolting to 22 HRC in H₂S-containing environments (sour service). The reason is sulfide stress cracking (SSC) — atomic hydrogen generated by H₂S corrosion diffuses into high-strength steel, and at hardness above ~22 HRC the steel becomes susceptible to cracking. B7 in the as-quenched-and-tempered condition lands at 28–35 HRC, well above the cap. B7M is the same alloy chemistry (AISI 4140 / 4142 / 4145) but receives a controlled final temper to bring hardness into the 18–22 HRC window — this also drops tensile and yield, which is why B7M is not a drop-in replacement for B7 in the torque calculator.

When the RFQ line item says "NACE MR0175", every stud and every nut on that flange must meet the 22 HRC limit. This applies to the bolt (B7M, L7M, B8M, or B8M Class 2), the nut (A194 Grade 7M or 7), and any washers. A common audit failure we see: the buyer specifies B7M studs but accepts standard 2H nuts — 2H is a different hardness class and is not NACE-compliant. The right pairing is B7M studs + A194 Grade 7M nuts.

4-service decision tree

Run this top-down. The first matching branch decides the bolt grade.

Rule 1 — Pressure class drives the alloy system

ASME B16.5-2020 §7 groups stud bolt grades by class. Class 150 through 600 defaults to B7 (or L7 for –46 °C and below). Class 900 and above — including Class 1500 and 2500 — defaults to B16 because of the higher design temperature and creep exposure. If your RFQ says "Class 900" without specifying a bolt grade, the standard expects B16.

Rule 2 — NACE sour service caps hardness

If the process stream contains H₂S above the NACE partial-pressure threshold (0.0003 MPa / 0.05 psi for carbon steel, or per the environmental severity table in MR0175 §6), swap B7 for B7M, or B16 for a hard-equivalent that meets 22 HRC. For stainless bolting in sour service, B8M replaces B8.

Rule 3 — Cryogenic service requires impact testing

For design temperatures at or below –46 °C (–50 °F), ASME B31.3 requires impact-tested bolting per ASTM A320. The standard grade is L7 (carbon alloy, same strength as B7 but Charpy V-notch tested at –46 °C). For combined cryogenic + sour, L7M is the answer.

Rule 4 — Dissimilar metal flange joint

If the flange is stainless (A182 F304, F316, F51 Duplex) and the bolts are carbon steel (B7), the joint is a guaranteed galvanic couple — see our companion article on galvanic corrosion in dissimilar metal joints. Two paths: (a) upgrade the bolting to match the flange (B8 / B8M for stainless-on-stainless), or (b) keep B7 but add dielectric bolt sleeves and isolation gaskets. For offshore and subsea, the standard answer is (a) — match the alloy system.

Nut pairing: ASTM A194 Grade 2H / 7 / 8

The stud bolt is only half the joint. ASTM A194 covers the matching nut grades, and the pairing matters because nut hardness must equal or exceed bolt hardness — otherwise the nut threads strip before the bolt reaches yield.

The most common procurement error: ordering "B7 stud with 2H nut" for a sour-service project. 2H nuts are not NACE-rated — the NACE-rated equivalent is 2HM. The mill certificate will show 2H chemistry and hardness, which fails the NACE audit on the nut side even when the B7M studs pass.

4 procurement pitfalls we see every month

Pitfall 1 — Specifying "B7 equivalent" without NACE wording

RFQ says "ASTM A193 B7 stud bolts, NACE". Some mills interpret this as B7 with NACE paperwork only — hardness is not verified, and the bolt is actually B7 (28–35 HRC), not B7M (≤ 22 HRC). The fix: write "SA-193 Grade B7M, hardness verified ≤ 22 HRC per NACE MR0175" on the RFQ line. Always.

Pitfall 2 — Class 900 RFQ with no bolt grade specified

Buyer asks for "ASME B16.5 Class 900 weld neck flanges, B7 stud bolts". B7 is not the default for Class 900 — the standard expects B16 because of the higher temperature envelope. The flange supplier will either substitute B16 (price delta ~15%) or push back. Write "Class 900, B16 stud bolts" in the original RFQ.

Pitfall 3 — Stainless flange + carbon steel bolt (galvanic couple)

A182 F316L weld neck + SA-193 B7 studs. The flange is 316 stainless, the studs are carbon steel — in any wet environment, the galvanic series puts the B7 stud as the anode and the F316L flange face as the cathode. The stud corrodes at the thread root within 18–36 months. Fix: B8M studs + A194 Grade 8M nuts, or add dielectric sleeves. See our galvanic corrosion article for the full mitigation rule.

Pitfall 4 — No nut grade on the RFQ

Buyer orders "B7 studs" and assumes the supplier picks the right nut. Some suppliers quote A194 Grade 2 (light hex), not 2H (heavy hex). Grade 2 has lower load-bearing capacity and will fail before the B7 stud reaches yield. Specify "ASTM A194 Grade 2H heavy hex nut" on every B7 stud RFQ.

References & standards cited