AISI 304 austenitic stainless steel · Solution-treated (Class 1) or strain-hardened (Class 2)
ASTM A320 Grade B8 is an austenitic stainless steel bolting grade specified per ASTM A320/A320M-21 for low-temperature service. Assigned UNS number S30400. Manufactured from AISI 304 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 B8 is the baseline 304 stainless A320 grade for general corrosion resistance at low temperatures. Grade B8 is intended for stud bolts and hex bolts in pressure vessels, valves, flanges, and fittings for low-temperature service. Grade B8 is equivalent to ASME SA-320 Grade B8 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 | |
|---|---|---|---|---|---|---|---|---|---|
| ASTM A320 B8 | S30400(304) | Range,% | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 8.0-11.0 | 18.0 - 20.0 |
| 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 B8 Class 1 | All diameters | carbide solution treated | 75 [515] | 30 [205] | 30 | 50 | 223 HBWDor 96 HRB |
| ASTM A320 B8 Class 2 | 3/4 [20] and under | carbide solution treated and strain hardened | 125 [860] | 100 [690] | 12 | 35 | 321 HBW or 35 HRC |
| over 3/4 to 1 [20 to 25], incl | 115 [795] | 80 [550] | 15 | 30 | 321 HBW or 35 HRC | ||
| over 1 to 11/4 [25 to 32], incl | 105 [725] | 65 [450] | 20 | 35 | 321 HBW or 35 HRC | ||
| over 11/4 to 11/2 [32 to 40], inclA | 100 [690] | 50 [345] | 28 | 45 | 321 HBW or 35 HRC |
Section 10.2 of ASTM A320 requires Grade B8 bolts, studs, and stud bolts to be equipped with austenitic alloy nuts to ASTM A194/A194M Grade 8; impact tests are not required for Grade 8 nuts above -425°F [-255°C]. Washers for austenitic bolting are made of austenitic steel as agreed between manufacturer and purchaser (10.5); ASTM F436 hardened steel washers are specified for ferritic bolting only (10.4). TorqBolt supplies kit-matched A320 B8 + A194 Grade 8 + austenitic washer sets with a single EN 10204 3.1 or 3.2 certificate covering all three components.
TorqBolt’s ASTM A320 Grade B8 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 (manufacturer’s test report); 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 B8 equivalence, NACE MR0175 / ISO 15156-3 sour-service compliance (where applicable), and PED 2014/68/EU for European pressure-equipment buyers.
