ASTM A307 Grade B Heavy Hex Bolts | Low-Carbon Steel, 3/8″ to 2″

Product Introduction
Heavy hex bolts manufactured to ASTM A307/A307M for low-carbon steel bolting. Grade B is distinguished from Grade A by a controlled tensile strength range of 60–100 ksi and is specifically intended for flanged joints in piping systems with cast iron flanges. Diameters 3/8″ through 2″. Heavy hex configuration per ASME B18.2.1; UNC coarse threads per ASME B1.1. Marked “307B” on the head. Typically paired with ASTM A563 Grade A heavy hex nuts.

Material & Mechanical Properties
Low or medium carbon steel (typically 1018–1020 or equivalent). Not quenched and tempered.

• Tensile strength: 60,000 psi minimum; 100,000 psi maximum

• No specified minimum yield strength

• Core hardness: Rockwell B69 minimum to B95 maximum
  Because Grade B has a maximum tensile limit, it is not interchangeable with higher-strength structural grades such as A325 or A449. Mechanical properties are certified by lot; mill test reports are available.

Geometry & Thread
Dimensions conform to ASME B18.2.1 heavy hex. Width across flats: 3/8″ = 0.544″–0.562″; 1/2″ = 0.725″–0.750″; 1″ = 1.450″–1.500″; 2″ = 2.900″–3.000″. Head height: 3/8″ = 0.226″–0.268″; 1/2″ = 0.302″–0.364″; 1″ = 0.591″–0.700″; 2″ = 1.175″–1.388″. UNC coarse pitch per ASME B1.1, Class 2A fit. The heavy hex head provides a larger bearing surface and wider wrenching flats than standard hex bolts, improving load distribution in flange assemblies.

Surface Finish
Plain (uncoated), zinc plated with clear or Cr+3 passivation, or hot-dip galvanized. When hot-dip galvanized, threads are tapped oversize after coating to maintain assembly fit. Finish selection is based on the service environment and the flange material.

Marking & Certification
Each bolt head is permanently marked with “307B” and the manufacturer’s identification mark. Grade A bolts are marked “307A”; unmarked low-carbon bolts are also considered Grade A per A307 convention. Full traceability is maintained through lot control. Inspection certificates and material test reports are available on request.

Assembly
Per ASTM A307, Grade B bolts in sizes 1/4″ through 4″ are matched with ASTM A563 Grade A heavy hex nuts. Higher proof-load grades (C, D, DH) may be used if specified. No washer is mandatory under A307 for Grade B, but hardened washers to ASTM F436 may be specified for load distribution on non-ferrous or brittle flange materials.

Application Suitability
Flanged joints in piping systems with cast iron flanges. The controlled tensile range of Grade B provides sufficient strength for bolting cast iron flanges without exceeding the compressive limits of the flange material. The heavy hex configuration ensures adequate bearing area on the flange face. Also used in general industrial applications where a low-carbon, heavy hex bolt with moderate strength is specified. Not intended for preloaded structural joints or applications requiring high-strength bolting.