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    2. ASTM A193/A193M Grade B8MN Class 1B

    ASTM A193/A193M Grade B8MN Class 1B Stud Bolts – UNS S31651

    Molybdenum and nitrogen-bearing 316N stainless steel stud bolts in the Class 1B condition for enhanced strength and superior corrosion resistance.

    ASTM A193/A193M Grade B8MN Class 1B Stud Bolts are produced from UNS S31651 austenitic stainless steel, which is the nitrogen-enhanced version of Type 316 molybdenum-bearing stainless steel, commonly designated 316N, and are supplied in the Class 1B condition involving solution annealing followed by light cold drawing to achieve closer dimensional tolerances and modest improvement in mechanical properties. The molybdenum content of 2–3% provides significantly enhanced resistance to pitting and crevice corrosion in chloride-containing media beyond what standard 304-type grades can offer, while the controlled nitrogen addition of 0.10–0.16% improves yield and tensile strength through solid solution strengthening. Together, these alloying strategies make Grade B8MN Class 1B stud bolts well suited for flanged connections, pressure vessels, and process piping systems operating in chloride-rich, marine, offshore, and aggressive chemical environments where both mechanical performance and corrosion resistance must be maintained simultaneously over the service life of the fastener.

    ASTM A193/A193M Grade B8MN Class 1B 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 1B 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 1B All diameters carbide solution treated 80 35 30 40 223 HBW or 96 HRBC
    • (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 1B 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 1B All diameters carbide solution treated 550 240 30 40 223 HBW or 96 HRBC
    • (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 1B Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8MN Class 1B TB B8MN or TB B9J

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8MN Class 1B?
    ASTM A193 Grade B8MN Class 1B is a bolting material covering 316N nitrogen-enhanced 316 (UNS S31651). Class 1B — solution treated higher-strength. 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 1B?
    The UNS designation for ASTM A193 Grade B8MN Class 1B 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 1B' designator denotes the heat treatment condition under ASTM A193 Table 3.
    Why specify Grade B8MN (316N) instead of B8M (316) (Class 1B)?
    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 1B: Class 1B — solution treated higher-strength.
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