Understanding Bolting Specifications: ASTM, SAE and Metric Bolting Materials

• Reading a callout
• Who writes the specs
• ASTM A193 and A320
• A194 nuts
• ASTM general and structural
• SAE J429
• Metric classes
• Stainless and nickel
• Comparing grades
• Markings
• Ordering checklist

How to read a bolting callout

A complete bolting description has several parts, and each one matters. The specification names the document that sets the requirements. The grade sets the chemistry and heat treatment. For some materials, such as strain-hardened stainless, a class sets the strength level. Size, thread and length come from separate dimensional standards, and the nut is a separate specification with its own grade.

Material specifications are separate from dimensional standards. ASTM A193 says what a B7 stud is made of and how it is tested. ASME B1.1 sets the thread, ASME B18.2.1 and B18.2.2 set bolt head and nut dimensions, and ASME B16.5 sets which stud size and length a flange needs. Use our stud bolt length calculator for the last one.

Who writes bolting specifications

Some ASTM specifications cover inch and metric sizes in one document, such as F3125, which has Grades A325 (120 ksi) and A325M (830 MPa). Others come as pairs, such as A193 and A193M. The M version uses the same grade names with SI units and metric threads, so an A193M B7 stud is still a B7 stud.

ASTM pressure and temperature bolting

These are the specifications called out by piping and pressure vessel codes for flanged joints in refineries, chemical plants and power plants. Strength minimums for quenched and tempered grades fall with diameter because large bars cannot be hardened as deeply. Markings for the most common grades are on our Bolting Grades and Markings sheet.

ASTM A193 ferritic (alloy and martensitic) grades

ASTM A193 austenitic (stainless) grades

A193 lists fourteen austenitic grade families. The letters after B8 identify the alloy: C for 347, T for 321, M for 316, P for 305, N for nitrogen-strengthened, L for low carbon, and a trailing A for material solution treated after machining. Every stainless grade must be ordered with a class.

What the stainless classes mean

Strain-hardened Class 2, 2B and 2C stainless may not have uniform properties through the section, especially over 3/4 in. diameter. At 1-1/2 in. and larger, core properties may be lower than test results taken at half radius.

ASTM A320 and other pressure bolting

ASTM A194 nut grades

ASTM A194 covers the heavy hex nuts used with A193, A320 and A453 bolting. The nut grade number usually mirrors the stud grade: 2H or 7 for B7, 2HM or 7M for B7M, 8M for B8M. Proof stress is the load the nut must carry without stripping or permanent set, divided by the stress area of the mating bolt.

Every A194 nut carries the grade and the manufacturer’s mark on one face. A “B” is added when the nut is machined from bar, an “A” marks stainless nuts solution treated after finishing, and an “L” suffix (such as 7L) marks nuts that are impact tested for low-temperature service. For sour service, the 2HM and 7M hardness limits apply. See the Sour Service Bolting Quick Reference. Pairings shown are common practice; the governing code and the purchaser’s specification decide the required nut grade.

ASTM general, structural and anchor bolting

These specifications cover machinery, structural steel and foundation bolting. They are paired with ASTM A563 nuts and, for structural work, ASTM F436 hardened washers.

F3125 replaced six older specifications in 2015, including A325 and A490, which are now grades within it. A325 is 120 ksi for all diameters to 1-1/2 in. The current edition also adds a 144 ksi class.

SAE J429 inch grades

SAE J429 is the familiar Grade 2, 5 and 8 system for inch hex cap screws and bolts in machinery and equipment. It stops at 1-1/2 in. Above that size, ASTM A449 is the usual Grade 5 equivalent and A354 BD the usual Grade 8 equivalent. Matching nuts are SAE J995 of the same grade.

Metric property classes (ISO 898-1 and ISO 3506)

Metric bolts do not use grades. They use a property class that encodes the strength directly in the number.

Metric nuts are classed by a single number that matches the first number of the bolt class: a class 10 nut for a class 10.9 bolt. Metric pressure bolting is usually ordered to ASTM A193M, A194M and A320M, or to European material standards. In those standards, 42CrMo4 is the chemistry closest to the 4140 steel used for B7.

Stainless and nickel alloy bolting

Stainless and nickel alloy bolting is ordered either to a bolting specification or to the bar specification it is machined from. The two routes give different testing and certification, so the drawing or purchase order should say which one applies.

For corrosion and temperature screening across these alloys, use our Metallurgy Selector or the Fastener Materials Matrix.

Comparing grades across systems

Grades of similar strength are often treated as rough equivalents. The table below is a starting point for cross-referencing only. Each specification has its own testing, size range, marking and nut pairing, so a substitution should be approved by the engineer or purchaser.

Strength is only half the choice. Temperature range decides where each material can be used:

Head and end markings

Most specifications require a grade mark and the manufacturer’s identification mark. Bolts and cap screws are marked on the head, studs on one end, and nuts on one face.

A marking shows what the part claims to be. The mill test report, traced to the heat number, is the proof. Patriot Bolt also verifies alloy chemistry in house with XRF analysis.

Ordering checklist

1. Specification, grade and class, for example ASTM A193 Grade B8M Class 2, or SAE J429 Grade 8, or ISO 898-1 class 10.9.
2. Size, thread and length, for example 1-8UNC × 7-1/2 in., or M24 × 3 × 180 mm.
3. Nut specification and grade, for example ASTM A194 Grade 2H heavy hex.
4. Supplementary requirements, such as impact testing, NACE MR0175 / ISO 15156 hardness limits or 100% hardness testing.
5. Coating or finish, such as PTFE, zinc-nickel or hot-dip galvanizing. See Fastener Coatings.
6. Certification: mill test reports with heat numbers and a certificate of conformance.

Common questions

Need bolting to a specific spec? Patriot Bolt machines stud bolts, bolts and nuts to ASTM and SAE specifications from B7 and B7M to stainless and nickel alloys, with mill test reports on every order. Send us the specification and grade from your drawing or bolting list. Request a quote or call (281) 581-0176.