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    2. ASTM A193/A193M Grade B8ML4CuN Class 1D

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

    High-molybdenum, copper and nitrogen-bearing austenitic stainless stud bolts in Class 1D condition for extreme corrosion resistance and elevated-temperature creep performance.

    ASTM A193/A193M Grade B8ML4CuN Class 1D Stud Bolts are manufactured from UNS S31730 high-alloy austenitic stainless steel incorporating approximately 4% molybdenum, copper, low carbon, and controlled nitrogen, and are supplied in the solution annealed Class 1D condition per ASTM A193, which adds grain size control to optimize long-term creep and stress rupture properties at elevated service temperatures. The elevated molybdenum level combined with copper provides extraordinary corrosion resistance against pitting, crevice corrosion, and attack from reducing acids in concentrations that overwhelm standard 316-type grades, while nitrogen strengthening compensates for the low carbon content and the Class 1D grain size requirement enhances thermal stability under sustained high-temperature loading. Grade B8ML4CuN Class 1D represents the most technically comprehensive specification available in this alloy family, combining maximum corrosion capability with microstructure-optimized elevated-temperature mechanical performance for critical bolting in specialty chemical reactors, high-temperature corrosive process systems, and advanced industrial applications where every aspect of fastener metallurgy must be precisely controlled.

    ASTM A193/A193M Grade B8ML4CuN Class 1D 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 1D 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 1D 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 1D 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 1D 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 1D Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8ML4CUN Class 1D TB B104

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8ML4CUN Class 1D?
    ASTM A193 Grade B8ML4CUN Class 1D is a bolting material covering AL-6XN super-austenitic 6-Mo+N (UNS S31730). Class 1D — Class 1 base properties for lower-temperature service. 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 1D?
    The UNS designation for ASTM A193 Grade B8ML4CUN Class 1D 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 1D' designator denotes the heat treatment condition under ASTM A193 Table 3.
    How does Grade B8ML4CuN compare to standard 316 in seawater service (Class 1D)?
    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 1D: Class 1D — Class 1 base properties for lower-temperature service.
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