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    2. ASTM A193/A193M Grade B8MN Class 2

    ASTM A193/A193M Grade B8MN Class 2 Stud Bolts – UNS S31651

    Strain hardened molybdenum and nitrogen-bearing 316N stainless stud bolts in Class 2 for maximum corrosion resistance and high-strength pressure bolting service.

    ASTM A193/A193M Grade B8MN Class 2 Stud Bolts are manufactured from UNS S31651 Type 316N molybdenum and nitrogen-bearing austenitic stainless steel and strain hardened to achieve the diameter-dependent elevated mechanical property requirements of the Class 2 designation under ASTM A193. The combination of nitrogen strengthening and cold working in this grade produces a particularly efficient path to high mechanical properties — the nitrogen adds strength in the pre-cold-worked condition, and the subsequent strain hardening further amplifies both tensile and yield strength to Class 2 levels, often requiring less total cold work than non-nitrogen-bearing grades to achieve the same mechanical performance. The 2–3% molybdenum content provides superior pitting corrosion resistance compared to standard 304 types, making Grade B8MN Class 2 stud bolts an attractive premium specification for high-pressure offshore, marine, and chemical service flanges where both elevated mechanical performance and molybdenum-enhanced corrosion resistance are simultaneously specified by design codes and service environment requirements.

    ASTM A193/A193M Grade B8MN Class 2 Chemical Composition - Austenitic Steels

    Grade Description and UNS Designatio Classes C Mn P S Si Cr Ni Mo N
    ASTM A193 Grade B8MN 316N, S31651 1A, 1B, 1D, 2 0.08 2.00 0.045 0.030 1.00 16.0-18.0 10.0-13.0 2.00-3.00 0.10-0.16
    Product Analysis VariationB 0.01 0.04 0.010 0.005 0.05 0.20 0.15 0.10 0.01
    • (A) Values are maximums unless a range or a minimum is indicated. Where ellipses appear in this table, there is no requirement and the element need not be determined or reported. The intentional addition of Bi, Se, Te, and Pb is not permitted.
    • (B) Product Analysis-Individual determinations sometimes vary from the specified limits as shown in the tables. The several determinations of any individual element in a heat may not vary both above and below the specified range. Product variation limits are over for maximums, over or under for ranges, and under for minimums, unless otherwise indicated.
    • (C) Typical steel compositions used for this grade include 4140, 4142, 4145, 4140H, 4142H, and 4145H.
    • (D) For bar sizes over 31/2 in. [90 mm], inclusive, the carbon content may be 0.50% max. For the B7M grade, a minimum carbon of 0.28% is permitted, provided that the required tensile properties are met in the section sizes involved, the use of AISI 4130 or 4130H is allowed.
    • (E) Total of soluble and insoluble.
    • (F) Columbium and Niobium are alternate names for element 41 in the Periodic Table of the Elements.

    ASTM A193/A193M Grade B8MN Class 2 Mechanical Requirements - Austenitic Steels (Inch)

    Grade Diameter,
    in.     		Heat TreatmentB Tensile Strength,
    min, ksi     		Yield Strength, min,
    0.2 % offset, ks     		Elongation in 4 D,min Reduction of Area, min Hardness,
    max
    ASTM A193 Grade B8MN Class 2 3/4 and under carbide solution treated and strain hardened 110 95 15 45 321 HBW or 35 HRC
    over 3/4 to 1 incl 100 80 20 45 321 HBW or 35 HRC
    Over 1 to 11/4 ,incl 95 65 25 45 321 HBW or 35 HRC
    over 11/4 to 11/2 ,incl 90 50 30 45 321 HBW or 35 HRC
    • (A) To meet the tensile requirements, the Brinell hardness shall be over 200 HBW (93 HRB).
    • (B) Class 1 is solution treated. Class 1A is solution treated in the finished condition for corrosion resistance; heat treatment is critical due to physical property requirement. Class 2 is solution treated and strain hardened. Austenitic steels in the strain-hardened condition may not show uniform properties throughout the section particularly in sizes over 3/4 in. in diameter.
    • (C) For sizes3/4 in. in diameter and smaller, a maximum hardness of 241 HBW (100 HRB) is permitted.
    • (D) For diameters 11/2 and over, center (core) properties may be lower than indicated by test reports which are based on values determined at1/2 radius.

    ASTM A193/A193M Grade B8MN Class 2 Mechanical Requirements - Austenitic Steels (Metric)

    Grade Class ,
    mm     		Diameter Heat TreatmentB Tensile Strength,
    min,MPa     		Yield Strength, min,
    0.2 % offset, MPa     		Elongation in 4 D,min % Reduction of Area, min % Hardness,
    max
    ASTM A193 Grade B8MN Class 2 M20 and under carbide solution treated and strain hardened 760 655 15 45 321 HBW or 35 HRC
    over M20 to M24, incl 690 550 20 45 321 HBW or 35 HRC
    over M24 to M30, incl 655 450 25 45 321 HBW or 35 HRC
    over M30 to M36, incl 620 345 30 45 321 HBW or 35 HRC
    • (A) To meet the tensile requirements, the Brinell hardness shall be over 200 HBW (93 HRB).
    • (B) Class 1 is solution treated. Class 1A is solution treated in the finished condition for corrosion resistance; heat treatment is critical due to physical property requirement. Class 2 is solution treated and strain hardened. Austenitic steels in the strain-hardened condition may not show uniform properties throughout the section particularly in sizes over M20 mm in diameter.
    • (C) For sizes M20 mm in diameter and smaller, a maximum hardness of 241 HBW (100 HRB) is permitted.
    • (D) For diameters M38 and over, center (core) properties may be lower than indicated by test reports which are based on values determined at1/2 radius.

    ASTM A193/A193M Grade B8MN Class 2 Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8MN Class 2 TB B8YSH

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8MN Class 2?
    ASTM A193 Grade B8MN Class 2 is a bolting material covering 316N nitrogen-enhanced 316 (UNS S31651). Class 2 — solution treated and strain hardened. Used in high-temperature, high-pressure flange, pressure-vessel and heat-exchanger bolting service per ASTM A193/A193M.
    What is the UNS designation for ASTM A193 Grade B8MN Class 2?
    The UNS designation for ASTM A193 Grade B8MN Class 2 is UNS S31651. This corresponds to 316N nitrogen-enhanced 316. The UNS code is universal across ASTM, ASME (SA-193), PED 2014/68/EU and NORSOK material specifications. The 'Class 2' designator denotes the heat treatment condition under ASTM A193 Table 3.
    Why specify Grade B8MN (316N) instead of B8M (316) (Class 2)?
    B8MN is 316N — nitrogen-enhanced 316. The 0.10–0.16% N addition increases yield strength to 50 ksi minimum (vs 30 ksi for standard B8M Class 1) without sacrificing the corrosion resistance of 316. Used where 316 chemistry is needed with higher proof load. Specific to ASTM A193 Grade B8MN Class 2: Class 2 — solution treated and strain hardened.
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