Ideal Diameter of Hardenability (ASTM A255)
Computes the Grossmann ideal diameter D_I using the calculated hardenability method of ASTM A255: it starts from the carbon base diameter, D_I = 0.54·√(%C) inches for ASTM grain size No. 7, and multiplies it by the alloy factors (1 + 3.3333·Mn)·(1 + 0.7·Si)·(1 + 2.16·Cr)·(1 + 0.363·Ni)·(1 + 3.0·Mo). The result, already converted to millimetres, is the bar diameter that would still quench to 50 % martensite at its centre in an ideal cooling medium — that is, the index ranking steels by hardening DEPTH, not by peak hardness, which depends almost only on carbon. Because the factors are multiplicative rather than additive, 1 % manganese alone multiplies hardenability by 4.33 while the same 1 % nickel raises it by just 36 % — the order of potency is manganese, molybdenum, chromium, silicon and nickel, and it is why nickel earns its place in engineering steels through toughness rather than hardenability. One reading caveat: D_I is the diameter that would through-harden in an ideal quench of infinite severity — in oil the real critical diameter lands between a third and a half of it. The manganese factor holds up to 1.2 %, beyond which the standard switches expression, and the page rejects it from there on. Enter the carbon, manganese, silicon, chromium, nickel and molybdenum contents.
Result
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Ideal Diameter of Hardenability from ASTM A255
The peak hardness a steel can reach depends almost entirely on carbon. The depth at which that hardness appears depends on the alloy, and that is a different story — it is why 1045 and 4140, at similar carbon levels, behave in opposite ways on an 80 mm shaft. The ideal diameter D_I ranks steels by that depth in a single number in millimetres. It serves to pick between two steel offers, to judge whether a heat that came in at the low end of the composition band still hardens the part, and to justify swapping one grade for another.
The calculated hardenability method of ASTM A255 starts from carbon and multiplies: D_I = 0.54·√(%C) inches, valid for ASTM grain size No. 7, times (1 + 3.3333·Mn)·(1 + 0.7·Si)·(1 + 2.16·Cr)·(1 + 0.363·Ni)·(1 + 3.0·Mo). The page outputs millimetres. With the defaults, a 4140 composition (0.40 C / 0.90 Mn / 0.25 Si / 0.95 Cr / 0.20 Mo), the base is 0.3415 in and the factors 4.000, 1.175, 3.052 and 1.600, giving 7.84 in, or 199.1 mm. The cross-check comes from the standard itself, which tabulates 4.333 for 1.00 % Mn: enter C = 1.00 and Mn = 1.00 with the rest at zero and the page prints 59.4 mm, which is 0.54 × 4.3333 = 2.340 in. A 1045 (0.45 C / 0.75 Mn / 0.25 Si) returns 37.8 mm, 1.49 in.
D_I is the diameter that would harden in a quenchant of infinite severity, which nothing matches. Reaching the real critical diameter means carrying D_I into the Grossmann chart together with the severity H of the medium — near 1.0 for agitated water, 0.35 for oil, 0.02 for air — and the figure drops sharply. The criterion is not full hardness either: it is 50 % martensite at the centre. The 0.54·√(%C) base assumes ASTM grain size 7, and this page has no grain size field. The linear manganese factor holds only to 1.2 %; above that the page still accepts entries, up to 2 %, and overestimates. Boron, worth a 1.5 to 3 times multiplier, has no field, and none of this covers undissolved alloy.
Frequently asked questions
Is the ideal diameter the largest bar I can harden?
Why does manganese count for so much more than nickel?
Can I use the tool for boron-treated steel?
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