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Hardenability calculator

Grossmann ideal critical diameter DI from composition, by the ASTM A255 multiplying-factor method — then your quench and your section size, to answer the question that actually matters: will the core harden?

Composition

Weight percent. Carbon must be between 0.05 and 0.90 for the A255 method.

Load a typical composition
Your part

DI

Ideal critical diameter — the steel alone, perfect quench

—
—
Quench severity H—
Effective hardening dia.—
Core martensite at your section—
Through-hardens up to—

Take this composition further. The same chemistry, with your route and heat treatment, gives yield, tensile, hardness, elongation and fatigue life — and METALLAI applies this very hardenability correction to the core of a thick section rather than reporting the coupon value. Open it in METALLAI with this composition →

How it is calculated

The ASTM A255 multiplying-factor method at grain size 7. A base diameter is set by carbon, then multiplied once per alloying element. These are the expressions METALLAI uses internally.

base(C) piecewise, ASTM grain size 7: C ≤ 0.39 0.54·C C ≤ 0.55 0.171 + 0.001·C + 0.265·C² C ≤ 0.65 0.115 + 0.268·C − 0.038·C² C ≤ 0.75 0.143 + 0.200·C C ≤ 0.90 0.062 + 0.409·C − 0.135·C² f(Mn) = 3.3333·Mn + 1.0 (Mn ≤ 1.20) = 5.10·Mn − 1.12 (Mn > 1.20) DI = base × f(Mn) × (1 + 0.70·Si) × (1 + 0.363·Ni) × (1 + 2.16·Cr) × (1 + 3.00·Mo) × (1 + 0.365·Cu) × (1 + 1.73·V)

Every element is capped, not extrapolated

Each multiplying factor is only tabulated over a range. Past the top of its table this calculator holds the element at the cap rather than continuing the straight line — which keeps the estimate on the conservative, smaller-DI side for rich steels instead of inventing hardenability that the tables never measured.

ElementFactorCapped at (wt %)
Manganese3.3333·Mn + 1  /  5.10·Mn − 1.121.95
Silicon1 + 0.70·Si2.00
Nickel1 + 0.363·Ni3.50
Chromium1 + 2.16·Cr1.75
Molybdenum1 + 3.00·Mo0.55
Copper1 + 0.365·Cu0.55
Vanadium1 + 1.73·V0.20

From DI to your actual part

DI assumes a perfect quench. A real medium achieves a fraction q(H) of it, rising toward 1 as severity increases:

MediumHq(H)
Brine2.000.88
Water1.000.78
Polymer0.600.68
Oil0.450.62
Forced air0.100.35
Still air0.020.15
Furnace0.010.10

The centre is 50 % martensite when the section equals the effective diameter DI·q(H), essentially fully hardened below about 0.35 × it, and falls away quickly beyond.

q(H) is the largest uncertainty here, and it is an approximation. It follows the shape of the Grossmann DH/DI relation rather than a measured curve for your furnace. Agitation, part geometry, fixturing and bath temperature all move it. Treat the core fraction as a ranking between options, not as a number to certify against — a Jominy test on your own heat is the measurement.

Why hardenability, not hardness

Hardness tells you what a surface reached. Hardenability tells you how deep the transformation went. A steel can hit its target hardness on a test coupon and still leave a soft, partly-bainitic core in a thick section — which is where fatigue cracks start, and which a tensile test on a small specimen will never warn you about.

Section size is the input most often missing from a property prediction, and the one that most often explains a disappointing result. It is why a 4140 shaft at 25 mm and the same 4140 at 150 mm are, mechanically, two different materials.

Questions

What is the ideal critical diameter DI?
The largest bar diameter that would reach 50 % martensite at its centre in a perfect quench — one removing heat infinitely fast. It is a property of the steel alone, not of your process, which is exactly what makes it useful: it separates what the alloy can do from what the quench does to it.
What is quench severity H?
Grossmann's measure of how fast a medium pulls heat out. Brine 2.0, water 1.0, polymer 0.6, oil 0.45, forced air 0.10, still air 0.02, furnace 0.01. The effective hardening diameter is DI × q(H), so the same steel through-hardens a far larger section in brine than in oil — at the cost of distortion and quench-crack risk.
How do you calculate hardenability from composition?
The ASTM A255 multiplying-factor method: a base diameter from carbon and grain size, multiplied by one factor per alloying element. Chromium and molybdenum are the strongest multipliers per weight percent — 2.16 and 3.00 respectively, against 0.363 for nickel. Defined for carbon from 0.05 to 0.90 %.
Will my part through-harden?
As a working rule, essentially fully martensitic below about 0.35 × the effective hardening diameter, 50 % at the diameter itself, falling away quickly beyond. This calculator reports the estimated core fraction for the section and quench you enter. It is an estimate from composition; a Jominy test on your heat is the measurement.
Why does the grain size matter?
Coarser prior-austenite grains raise hardenability, because there is less grain-boundary area to nucleate ferrite and pearlite on. The A255 base curve is tabulated per grain size; this calculator uses ASTM grain size 7, the usual assumption for a fine-grained steel. A coarser-grained heat will out-perform this estimate — usually at the cost of toughness.

DI is one term of the answer

METALLAI takes the same composition plus your route, heat treatment and section thickness and predicts yield, tensile, hardness, elongation and fatigue life — with the core-versus-surface correction applied, not assumed away.

Run the full prediction — free Ac1 / Ac3 calculator