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    2. ASTM A193/A193M Grade B8C Class 2

    ASTM A193/A193M Grade B8C Class 2 Stud Bolts – UNS S34700

    Strain hardened niobium-stabilized 347 stainless steel stud bolts meeting Class 2 diameter-graded mechanical requirements for high-temperature high-pressure pressure bolting.

    ASTM A193/A193M Grade B8C Class 2 Stud Bolts are produced from UNS S34700 niobium-stabilized Type 347 austenitic stainless steel and are strain hardened to achieve the elevated diameter-graded mechanical property requirements of the Class 2 designation under ASTM A193. The niobium stabilization characteristic of this grade ensures that the alloy resists sensitization — chromium carbide precipitation at grain boundaries — during high-temperature service, welding, or thermal cycling, preserving corrosion resistance in the as-used condition even at sustained elevated temperatures. The strain hardening process significantly increases tensile and yield strength compared to the solution annealed Class 1 condition, with Class 2 mechanical requirements varying by diameter in recognition of the practicalities of cold working larger cross sections. Grade B8C Class 2 stud bolts are specified for high-temperature, high-pressure flanged joints in power plants, petroleum refineries, and petrochemical facilities where sustained thermal exposure combined with high pressure loading demands both the stabilized alloy chemistry of 347 and the elevated strength achievable through cold working.

    ASTM A193/A193M Grade B8C Class 2 Chemical Composition - Austenitic Steels

    Grade Description and UNS Designatio Classes C Mn P S Si Cr Ni CbF
    ASTM A193 Grade B8C 347, S34700 1, 1A, 1D, 2 0.08 2.00 0.045 0.030 1.00 17.0-19.0 9.0-12.0 10 × C to 1.10
    Product Analysis VariationB 0.01 0.04 0.010 0.005 0.05 0.20 0.15 0.05 under
    • (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 B8C 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 B8C Class 2 3/4 and under carbide solution treated and strain hardened 125 100 12 35 321 HBW or 35 HRC
    over 3/4 to 1, incl 115 80 15 35 321 HBW or 35 HRC
    over 1 to 11/4 ,incl 105 65 20 35 321 HBW or 35 HRC
    over 11/4 to 11/2 ,incl 100 50 28 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 B8C 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 B8C Class 2 M20 and under carbide solution treated and strain hardened 860 690 12 35 321 HBW or 35 HRC
    over M20 to M24, incl 795 550 15 35 321 HBW or 35 HRC
    over M24 to M30, incl 725 450 20 35 321 HBW or 35 HRC
    over M30 to M36, incl 690 345 28 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 B8C Class 2 Bolting Components Marking - Austenitic Steels

    Grade Class Grade Symbol
    ASTM A193 Grade B8C Class 2 TB B8CSH

    Related ASTM A193 Grades & ASME Companion Dimensions

    Frequently Asked Questions

    What is ASTM A193 Grade B8C Class 2?
    ASTM A193 Grade B8C Class 2 is a bolting material covering 347 SS Nb-stabilized for high-temp (UNS S34700). 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 B8C Class 2?
    The UNS designation for ASTM A193 Grade B8C Class 2 is UNS S34700. This corresponds to 347 SS Nb-stabilized for high-temp. 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 high-temperature advantage of Grade B8C (347 SS) (Class 2)?
    Grade B8C is 347 SS, niobium-stabilized austenitic. The Nb addition prevents chromium-carbide precipitation at the grain boundaries during prolonged exposure between 425–815 °C (sensitization), maintaining inter-granular corrosion resistance for high-temperature service. Specific to ASTM A193 Grade B8C Class 2: Class 2 — solution treated and strain hardened.
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