ASTM A193 Grade B7 Black Heavy Hex Bolts | Chromium-Molybdenum Alloy Steel, 1/2″ to 2″

Product Introduction
Heavy hexagon head bolts manufactured to ASTM A193/A193M Grade B7, the standard specification for chromium-molybdenum alloy steel bolting in high-temperature and high-pressure service. Produced from AISI 4140/4142 alloy steel, oil-quenched and tempered at a minimum of 593 °C (1,100 °F). Diameters 1/2″ through 2″ inclusive. Heavy hex dimensions per ASME B18.2.6; UNC coarse threads per ASME B1.1, Class 2A fit. The black finish is typically black oxide or phosphate, providing a non-reflective surface and controlled friction characteristics for consistent preload during assembly. Used for flanged joints in ASME pressure classes 150 through 2500, in refineries, petrochemical plants, and power generation.

Material & Mechanical Properties
Chromium-molybdenum ferritic alloy steel with controlled chemistry: carbon 0.38–0.48%, chromium 0.80–1.10%, molybdenum 0.15–0.25%. The Cr-Mo composition provides through-thickness hardenability in heavy sections and retains usable strength up to 450 °C (840 °F).

• Minimum tensile strength: 125 ksi (862 MPa) for diameters up to 2-1/2″

• Minimum yield strength: 105 ksi (724 MPa)

• Minimum elongation: 16%

• Minimum reduction of area: 50%

• Hardness: 235–331 HB (maximum 35 HRC)
  Above 450 °C, B7 loses yield strength rapidly and ASTM A193 B16 (Cr-Mo-V) should be specified. For service below –29 °C, impact-tested ASTM A320 L7 is required. Mechanical and chemical properties are certified by heat lot; Mill Test Reports are available.

Geometry & Thread
Dimensional conformity to ASME B18.2.6 for heavy hex bolts. For 1/2″: width across flats 7/8″, head height approximately 21/64″, thread length 1.00″ for lengths up to 6″. For 1″: width across flats 1-5/8″. Threads are UNC coarse for 1″ and smaller (1/2″-13, 5/8″-11, 3/4″-10, 7/8″-9, 1″-8) and 8-thread UN series for 1-1/8″ and larger (1-1/8″-8UN, 1-1/4″-8UN, 1-1/2″-8UN, 2″-8UN), all Class 2A fit. Heavy hex geometry provides a larger bearing surface and wider wrenching flats than a standard hex head, distributing clamping load more effectively across flange faces.

Surface Finish
Black oxide or phosphate coating, applied for a non-reflective protective surface without the dimensional buildup of plating. For coated assemblies, both the bolt and the mating 2H nut should be finished by the same process to maintain consistent friction characteristics. Alternative finishes, including PTFE, zinc-nickel, and hot-dip galvanizing, are available where service conditions require them, subject to temperature and hydrogen embrittlement limits.

Marking & Certification
Each bolt 3/8″ diameter and larger is permanently marked with the “B7” grade symbol and the manufacturer’s identification mark. Heat code traceability links each finished fastener to its Mill Test Report containing chemical and mechanical test results. Grade B7 bolts are paired with ASTM A194 Grade 2H heavy hex nuts, quenched and tempered carbon steel with hardness 24–35 HRC (248–352 HB) and minimum proof load stress of 95 ksi for sizes up to 1-1/2″. EN 10204 Type 3.1 inspection certificates are available on request.

Application Suitability
Flanged joints in ASME pressure classes 150 through 2500, including refineries, petrochemical plants, and power generation facilities. The black finish is suited to industrial bolting where a protective, non-reflective surface is preferred without plating buildup. For continuous service above 450 °C or impact requirements below –29 °C, alternative grades (B16 or L7) should be specified.