AISI 316 molybdenum-bearing austenitic stainless steel · Solution-treated (Class 1) or strain-hardened (Class 2)
ASTM A320 Grade B8M is an austenitic stainless steel bolting grade specified per ASTM A320/A320M-21 for low-temperature service. Assigned UNS number S31600. Manufactured from AISI 316 molybdenum-bearing austenitic stainless steel. The grade is supplied in Class 1 (solution-treated) or Class 2 (strain-hardened) condition, with Class 2 offering higher tensile strength through controlled cold work. Grade B8M offers improved chloride-pitting and crevice-corrosion resistance versus B8. For sour service, NACE MR0175/ISO 15156-3 accepts austenitic stainless steel only when solution annealed, free of cold work intended to enhance mechanical properties, at 22 HRC maximum and within the Table A.2 environmental limits, so strain-hardened Class 2 does not qualify. Grade B8M is intended for stud bolts and hex bolts in pressure vessels, valves, flanges, and fittings exposed to corrosive or low-temperature environments. Grade B8M is equivalent to ASME SA-320 Grade B8M in Section II of the ASME Boiler and Pressure Vessel Code.
| Grade Symbol | UNS Designation | C, max | Mn, max | P, max | S, max | Si, max | Ni | Cr | Mo | |
|---|---|---|---|---|---|---|---|---|---|---|
| ASTM A320 B8M | S31600(316) | Range,% | 0.08 | 2.00 | 0.045 | 0.030, max | 1.00 | 10.0 - 14.0 | 16.0 - 18.0 | 2.00 - 3.00 |
| Product Variation, % Over or Under | 0.01 over | 0.04 over | 0.010 over | 0.005 over | 0.05 over | 0.15 | 0.20 | 0.10 | ||
| Grade and Class | Diameter, in. [mm] | Heat Treatment | Tensile Strength, min, ksi [MPa] | Yield Strength, min, ksi [MPa] (0.2 % offset) | Elongation in 2 in. or 50 mm, min, % | Reduction of Area, min, % | Hardness, max |
|---|---|---|---|---|---|---|---|
| ASTM A320 B8M Class 1 | All diameters | carbide solution treated | 75 [515] | 30 [205] | 30 | 50 | 223 HBWDor 96 HRB |
| ASTM A320 B8M Class 2 | 3/4 [20] and under | carbide solution treated and strain hardened | 110 [760] | 95 [655] | 15 | 45 | 321 HBW or 35 HRC |
| over 3/4 to 1 [20 to 25], incl | 100 [690] | 80 [550] | 20 | 45 | 321 HBW or 35 HRC | ||
| over 1 to 11/4 [25 to 32], incl | 95 [655] | 65 [450] | 25 | 45 | 321 HBW or 35 HRC | ||
| over 11/4 to 11/2 [32 to 40], inclA | 90 [620] | 50 [345] | 30 | 45 | 321 HBW or 35 HRC |
For low-temperature service consistency, ASTM A320 Grade B8M stud bolts are paired with ASTM A194 Grade 8 series austenitic heavy hex nuts (A320 10.2 lists Grades 8, 8C, 8T, 8F, 8M, 8LN, and 8MLN) and austenitic steel washers as agreed between manufacturer and purchaser (A320 10.5). ASTM F436 hardened steel washers are for ferritic bolting (A320 10.4). TorqBolt supplies kit-matched A320 B8M + A194 + austenitic washer sets with a single EN 10204 3.1 or 3.2 certificate covering all three components.
TorqBolt’s ASTM A320 Grade B8M stud bolt production route is qualified to NORSOK M-650 for North Sea oil & gas offshore service. Each manufacturing record links heat number, melt analysis, heat-treatment batch, mechanical test, dimensional inspection, surface treatment, and dispatch. Standard certification is EN 10204 Type 3.1 (inspection certificate with test results, validated by the manufacturer’s authorized inspection representative); Type 3.2 third-party witnessed by Lloyd’s Register, DNV, BV, ABS, or TUV is available on request. The qualification packet covers ASME SA-320 B8M equivalence, NACE MR0175 / ISO 15156-3 sour-service compliance (where applicable), and PED 2014/68/EU for European pressure-equipment buyers.
