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    2. ASTM A193/A193M Grade B8ML4CuN Class 1

    ASTM A193/A193M Grade B8ML4CuN Class 1 Stud Bolts – UNS S31730

    High-molybdenum, copper and nitrogen-alloyed austenitic stainless stud bolts engineered for severely corrosive chemical and process industry environments.

    ASTM A193/A193M Grade B8ML4CuN Class 1 Stud Bolts are produced from UNS S31730, a highly alloyed austenitic stainless steel featuring elevated molybdenum content (approximately 4%), copper addition, low carbon content, and controlled nitrogen enhancement. The high molybdenum level dramatically improves resistance to pitting corrosion, crevice corrosion, and stress corrosion cracking in chloride and other halide environments, while the copper provides additional resistance to reducing acids such as sulfuric acid and improves general corrosion performance in certain chemical media. Nitrogen serves as a solid solution strengthener, compensating for the strength reduction from the low carbon content. Supplied in the solution annealed Class 1 condition per ASTM A193, these stud bolts are reserved for the most demanding corrosive service conditions encountered in chemical processing, pulp and paper, marine environments, and desalination facilities, where standard 316-type grades are inadequate and a higher-performance austenitic solution is required.

    ASTM A193/A193M Grade B8ML4CuN Class 1 Chemical Composition - Austenitic Steels

    Grade Description and UNS Designatio Classes C Mn P S Si Cr Ni Mo Cu N
    ASTM A193 Grade B8ML4CUN S31730 1, 1A, 1D 0.030 2.00 0.040 0.010 1.00 17.0-19.0 15.0-16.5 3.0-4.0 4.0-5.0 0.045
    Product Analysis VariationB 0.005 0.04 0.005 0.002 0.05 0.20 0.15 0.10 0.15 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 B8ML4CuN Class 1 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 B8ML4CUN Class 1 All diameters carbide solution treated 70 25 35 50 90 HRB
    • (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 B8ML4CuN Class 1 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 B8ML4CUN Class 1 All diameters carbide solution treated 480 175 35 50 90 HRB
    • (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 B8ML4CuN Class 1 Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8ML4CUN Class 1 TB B8YY or TB B8ML4CuN  

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8ML4CUN Class 1?
    ASTM A193 Grade B8ML4CUN Class 1 is a bolting material covering AL-6XN super-austenitic 6-Mo+N (UNS S31730). Class 1 — solution treated. 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 B8ML4CUN Class 1?
    The UNS designation for ASTM A193 Grade B8ML4CUN Class 1 is UNS S31730. This corresponds to AL-6XN super-austenitic 6-Mo+N. The UNS code is universal across ASTM, ASME (SA-193), PED 2014/68/EU and NORSOK material specifications. The 'Class 1' designator denotes the heat treatment condition under ASTM A193 Table 3.
    How does Grade B8ML4CuN compare to standard 316 in seawater service (Class 1)?
    B8ML4CuN is AL-6XN (UNS S31730/N08367) — a 6%-Mo super-austenitic with PREN ≈ 46. In seawater and chloride-rich service it offers chloride pitting and crevice corrosion resistance roughly an order of magnitude better than 316, with critical crevice temperature ≥ 35 °C vs 316 ≈ -3 °C. Specific to ASTM A193 Grade B8ML4CUN Class 1: Class 1 — solution treated.
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