STP756: Weld Procedure, Filler Metal, and Post-Weld Heat Treatment—Their Effect on the Hardness and Quality of Welds in CA6NM Alloy

    Loveless, RW
    Director of quality assurance and engineering product manager, Worthington Group, McGraw-Edison Co., Harrison, N.J.

    Smith, WC
    Director of quality assurance and engineering product manager, Worthington Group, McGraw-Edison Co., Harrison, N.J.

    Templeton, HC
    Director of quality assurance and engineering product manager, Worthington Group, McGraw-Edison Co., Harrison, N.J.

    Pages: 9    Published: Jan 1982


    Abstract

    The hardness of repair welds in CA6NM alloy has consistently been higher than the hardness of the base metal, regardless of the post-weld heat treatment used. The purpose of this investigation was to establish the optimum post-weld heat treatment to use in order to obtain the lowest hardness and to establish expected hardness limits for repair welds.

    Simulated repair welds 13 mm deep, 25 mm wide, and 51 mm long (½ by 1 by 2 in.) made in cast CA6NM plates 51 by 178 by 305 mm (2 by 7 by 12 in.) by the shielded metal arc weld (SMAW) and gas tungsten arc weld (GTAW) processes using 12Cr-4Ni filler metal were evaluated as to hardness in the as-welded condition and after various post-weld heat treatments. The plates were normalized at 1050°C (1925°F) and double tempered at 675 + 607°C (1250 + 1125°F) prior to welding and conformed to the tensile requirements of ASTM A487, Grade CA6NM. Plate hardness was 255 to 269HB.

    As-welded SMAW welds were 387HB to 418HB; the heat-affected zone (HAZ) hardness was 340HB. Single-tempered SMAW welds [temper 607°C (1125°F)] were 302HB. Double-tempered SMAW welds [temper 675 + 607°C (1250 + 1125°F)] were 293HB.

    SMAW welds which were normalized at 1050°C (1925°F) followed by tempering were harder than welds tempered only. Single tempering at 607°C (1125°F) resulted in weld hardness of 321HB; double tempering at 675 + 607°C (1250 + 1125°F) produced a weld hardness of 302HB.

    GTAW welds (no molybdenum in filler metal) resulted in lower hardness of welds, 241HB to 286HB after double tempering at 675 + 607°C (1250 + 1125°F). Normalizing of GTAW welds followed by the double-tempering treatment produced a weld hardness of 235HB.

    A smaller simulated repair 6 mm deep, 13 mm wide, 25 mm long (¼ by ½ by 1 in.) was made in one plate by the SMAW process using nickel-chromium-iron filler metal (Inconel 600). Hardness was determined in the as-welded condition only—weld hardness 170HB to 187HB, HAZ hardness 302 HB.

    All plates and welds were nondestructively examined by radiographic, ultrasonic, and penetrant techniques, and met the requirement of Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, as per Table 1.

    All welding was done in the flat position without preheat, using procedures qualified in conformance with Section IX of the ASME Code. Root passes of SMAW welds were made with 3.175-mm (⅛ in.) electrodes; 3.968-mm (5/32 in.) electrodes were used for final passes. GTAW welds were made with 3.175-mm (⅛ in.) filler wire throughout.

    The lowest hardness of welds made with filler metal of the same composition as the base metal is obtained by a double-temper post-weld heat treatment of 675 + 607°C (1250 + 1125°F). Expected weld hardness will be about 293HB. Minor repair welds can be made with nickel-chromium-iron filler metal with weld hardness and strength compatible with the cast alloy without post-weld heat treatment.

    Keywords:

    cast alloy, filler metal, hardness, martensitic stainless steel, molybdenum, nondestructive examination, normalizing, nitrogen, post-weld heat treatment, tempering, weld procedure, weld repair, carbon content


    Paper ID: STP28453S

    Committee/Subcommittee: A01.22

    DOI: 10.1520/STP28453S


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