Heat treatment, or the mill's rod rolling and annealing, can take carbon out of the surface of a steel bolt or put carbon in, and too much of either harms the bolt's safety and performance. ASTM F2328 says the harder and stronger the product, the more likely it is to fail if the limits are exceeded, so the limits tighten with strength: the zone free of decarburization must be at least half the thread height for Class 1, two-thirds for Class 2 and three-quarters for Class 3, which allows no gross decarburization at all. ASTM A962, the common requirements behind A193, A194 and A320, holds carbon and alloy steel bolting to the Class 3 limits whatever the grade, measured after all heat treatment.
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model dataMethod, classes and limits from ASTM F2328 (section through the thread axis, 2 to 4% nital or picral at 100×, N and G from Tables 1 and 2, Vickers 300 gf or Knoop 500 gf under 40 threads per inch, points 1 to 3, the 30-point rule and the four-thread referee); Class 3 for carbon and alloy steel bolting from ASTM A962. Thread drawn to the 1"-8 UNR form, H = 0.077 in. The depth of the light band and the hardness readings are for illustration; each indent is drawn at the size its reading gives. The cut-off machine, polisher, tester and monitor are generic.
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