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    2. ASTM A193/A193M Grade B8MLCuN Class 2

    ASTM A193/A193M Grade B8MLCuN Class 2 Stud Bolts – UNS S31254

    Strain hardened super austenitic 254 SMO stainless steel stud bolts in Class 2 with diameter-graded strength for extreme corrosion resistance in high-pressure service.

    ASTM A193/A193M Grade B8MLCuN Class 2 Stud Bolts are produced from UNS S31254, the super austenitic 254 SMO stainless steel with approximately 6% molybdenum, 20% chromium, 18% nickel, high nitrogen, and copper, and are strain hardened to achieve the diameter-graded elevated mechanical requirements of the Class 2 designation under ASTM A193. This grade represents the most corrosion-resistant austenitic stainless steel available under the A193 Class 2 framework, with a PREN value typically exceeding 42, providing resistance to pitting, crevice corrosion, and stress corrosion cracking that far surpasses standard 316-type grades even in the most aggressive seawater, hot chloride brine, and acidic chemical environments. Cold working to Class 2 levels further raises the already substantial strength of this super austenitic alloy. Grade B8MLCuN Class 2 stud bolts are specified for the most critical high-pressure, high-corrosion bolting applications in offshore subsea systems, desalination high-pressure membrane housings, seawater cooling flanges, and chemical process equipment handling concentrated acids or hot chloride solutions.

    ASTM A193/A193M Grade B8MLCuN Class 2 Chemical Composition - Austenitic Steels

    Grade Description and UNS Designatio Classes C Mn P S Si Cr Ni Mo Cu N
    ASTM A193 Grade B8MLCUN S31254 1A, 1B, 1D, 2 0.020 1.00 0.030 0.010 0.80 19.5-20.5 17.5-18.5 6.0-6.5 0.50-1.00 0.18-0.25
    Product Analysis VariationB 0.005 0.03 0.005 0.002 0.05 0.20 0.15 0.10 . . . 0.02
    • (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 B8MLCuN 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 B8MLCUN 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 B8MLCuN 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 B8MLCUN 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 B8MLCuN Class 2 Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8MLCUN Class 2 TB B8JSH

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8MLCUN Class 2?
    ASTM A193 Grade B8MLCUN Class 2 is a bolting material covering 6-Mo 254 SMO super-austenitic (UNS S31254). 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 B8MLCUN Class 2?
    The UNS designation for ASTM A193 Grade B8MLCUN Class 2 is UNS S31254. This corresponds to 6-Mo 254 SMO super-austenitic. 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.
    What is the chloride-pitting envelope for Grade B8MLCuN (Class 2)?
    B8MLCuN is 254 SMO (UNS S31254) — a 6-Mo super-austenitic with PREN ≈ 43, rated for service in seawater, ferric-chloride, and severe sour-service environments up to 320 °C. Far exceeds 316 (PREN 24) in chloride pitting resistance. Specific to ASTM A193 Grade B8MLCUN Class 2: Class 2 — solution treated and strain hardened.
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