Black Finish Chromium-Molybdenum Alloy Steel Heavy Hex Bolt B7 Stud Bolt in 1/2

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

This is a heavy hexagon head bolt manufactured to ASTM A193/A193M Grade B7, the standard specification for chromium-molybdenum alloy steel bolting in high-temperature and high-pressure service. The bolt is produced from AISI 4140/4142 alloy steel, oil-quenched and tempered at a minimum of 593°C (1,100°F) to develop the required strength and toughness combination. Available in diameters 1/2″ through 2″ inclusive, with heavy hex dimensions per ASME B18.2.6 and UNC coarse threads per ASME B1.1, Class 2A fit. The black finish typically denotes a black oxide or phosphate coating, applied to provide a non-reflective surface and controlled friction characteristics for consistent preload during assembly. The B7 heavy hex bolt is the workhorse fastening element for flanged joints in ASME pressure classes 150 through 2500, widely used in refineries, petrochemical plants, and power generation facilities.

What are the mechanical property and material composition requirements for Grade B7 bolts in the 1/2″–2″ range?

For diameters up to 2-1/2″ (which includes the entire 1/2″–2″ range), Grade B7 requires minimum tensile strength of 125 ksi (862 MPa), minimum yield strength of 105 ksi (724 MPa), minimum elongation of 16%, minimum reduction of area of 50%, and hardness range of 235–331 HB. The maximum hardness is capped at 35 HRC to ensure adequate toughness. The material is a chromium-molybdenum ferritic alloy steel with controlled chemistry: carbon 0.38–0.48%, chromium 0.80–1.10%, and molybdenum 0.15–0.25%. This Cr-Mo composition provides the hardenability necessary for through-thickness strength in heavy sections and retains usable strength up to a maximum continuous service temperature of 450°C (840°F); above this limit, B7 loses yield strength rapidly, and ASTM A193 B16 (Cr-Mo-V) should be specified instead.

What are the dimensional specifications and thread configurations for 1/2″–2″ heavy hex B7 bolts?

The bolt conforms dimensionally to ASME B18.2.6 for heavy hex structural bolts. For the 1/2″ size, the width across flats is 7/8″ (0.875″), head height is approximately 21/64″ (0.328″), and the thread length for bolts up to 6″ is 1.00″. For 1″ nominal diameter, the width across flats increases to 1-5/8″, with head height proportionally larger per the ASME B18.2.6 tables. Threads are UNC coarse series for 1″ and smaller (e.g., 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 (e.g., 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.

What are the matching nut, surface finish, and traceability requirements for a black B7 heavy hex bolt assembly?

Grade B7 bolts must be paired with ASTM A194 Grade 2H heavy hex nuts, which are quenched and tempered carbon steel nuts with a hardness range of 24–35 HRC (248–352 HB) and a minimum proof load stress of 95 ksi for sizes up to 1-1/2″. The black oxide or phosphate finish on the bolt is commonly used in industrial bolting where a protective, non-reflective surface is desired without the dimensional buildup of plating. For coated assemblies, both the bolt and the 2H nut should be finished by the same process to maintain consistent friction characteristics. Each bolt 3/8″ diameter and larger must be permanently marked with the “B7" grade symbol and the manufacturer’s identification mark on one end; heat code traceability links the finished fastener to its Mill Test Report (MTR) containing chemical and mechanical test results. Assemblies can be supplied with EN 10204 Type 3.1 inspection certificates upon request, providing full heat traceability and mechanical test records.