The short answer: ASTM A193 Grade B7 is quenched-and-tempered chromium-molybdenum alloy-steel bolting for high-temperature, high-pressure and other special-purpose joints. Its minimum tensile strength is 125 ksi through 2 1/2 in diameter, then decreases for larger diameters. B7 is strong, but it is not stainless steel: coating, service temperature, nut grade and traceability must be specified separately.[1] [2]
If you are deciding between alloy steel and stainless steel, start with ASTM A193 B7 vs B8. This guide stays focused on specifying B7 correctly.
What material is ASTM A193 Grade B7?
B7 is a chromium-molybdenum alloy-steel grade within ASTM A193/A193M. The material is quenched and tempered to develop its required strength. It is commonly produced from 4140- or 4142-type steel, but a generic 4140 certificate is not proof of B7 compliance: the finished bolting must meet the applicable A193 chemistry, heat treatment, mechanical testing, dimensions, marking and certification requirements.[1] [2]
ASTM A193/A193M covers bolting material and components for pressure vessels, valves, flanges and fittings. B7 is therefore a material grade, not a complete joint specification and not the name of one fixed product shape.[1]
| Item | What B7 tells you | What must still be stated |
|---|---|---|
| Material route | Chromium-molybdenum alloy steel | Required ASTM edition and any project restrictions |
| Heat treatment | Quenched and tempered | Test reports and any supplementary requirements |
| Strength | Diameter-dependent A193 B7 minimums | Nominal diameter and governing design load |
| Corrosion protection | Nothing by itself | Plain, coated or plated condition |
| Complete joint | Bolting grade only | Nut grade, dimensions, thread and assembly requirements |
ASTM A193 B7 strength by diameter
The inch-series minimum mechanical properties change at exact nominal-diameter boundaries.[1] [2]
| Nominal diameter | Minimum tensile strength | Minimum yield strength |
|---|---|---|
| Up to and including 2 1/2 in | 125 ksi | 105 ksi |
| Over 2 1/2 through 4 in | 115 ksi | 95 ksi |
| Over 4 through 7 in | 100 ksi | 75 ksi |
Do not apply the 125 ksi row to every B7 diameter. For example, a 3 in B7 stud falls in the 115 ksi tensile and 95 ksi yield row. These are material-specification minimums, not allowable design stresses; the governing pressure, piping or equipment code determines the allowable stress at design temperature.
Product form, threads and marking
B7 is widely supplied as stud bolts, threaded rod and headed bolts. ASTM A193’s default inch-series thread requirements are ASME B1.1 Class 2A, with coarse threads through 1 in and 8-thread series above 1 in unless the purchaser specifies otherwise. Unless the purchase order specifies another head style, headed bolts are heavy hex with dimensions under ASME B18.2.1.[3]
The grade symbol and manufacturer identification are required on the bolt head or on one end of a stud, subject to the standard’s provisions for small products and alternate marking locations. Marking is useful for receiving inspection, but it does not replace a material test report linked to the heat or lot. An RFQ should identify whether the required item is a full-thread stud, tap-end stud, threaded rod or headed bolt; “B7 bolt” alone leaves the geometry unresolved.
B7 is not corrosion-resistant stainless steel
Bare B7 can corrode in wet or chemically aggressive service. Specify a project-approved finish separately, checking its temperature range, thread allowance and friction behavior. The bolt and nut must have compatible finishes.
Which nut goes with B7?
ASTM A194 Grade 2H is the common heavy hex nut pairing for B7. Specify the nut independently from the bolt.[4] The compatibility guide covers thread, finish and assembly selection.
B7 and B7M are different grades
ASTM A193 Grade B7M is not a casual substitute for B7. Its minimum strength is lower than B7 through 4 in, and it has a controlled hardness range with 100% hardness inspection. If a drawing calls for B7M, quote and certify B7M; do not supply B7 with an assumed equivalence.[2] [5]
B7 details to confirm before ordering
Confirm the diameter row, product form, finish and B7 marking. A 3 in stud, for example, belongs to the 115 ksi tensile / 95 ksi yield row. Use the assembly ordering examples to add the nut, matching thread, dimensions, service conditions and quantity.
Requesting B7 bolting? Send the diameter, thread, length, product form, finish, nut grade, quantity and service temperature. HexFastener can review the callout and required documentation before quoting.
References
- ASTM International. ASTM A193/A193M-26 — Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications. Current scope and purchaser requirements.
- Portland Bolt. ASTM A193 Grades. B7 material, heat treatment and mechanical-property summary.
- Portland Bolt. ASTM A193. Product configurations, default inch-series threads and identification marking.
- ASTM International. ASTM A194/A194M-26 — Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both. Nut specification scope and requirements.
- Portland Bolt. ASTM A193 B7 vs B7M. Practical differences in mechanical requirements and hardness testing.