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Ideal Critical Diameter (DI) Calculator

Enter the heat chemistry and the actual austenite grain size at quenching temperature to calculate the ideal critical diameter of non-boron steel. Strictly, DI is the critical diameter at which the center of a round bar reaches 50% martensite under ideal infinite quenching.

DI = DIc × FMn × FSi × FCr × FNi × FMo × FCu × FV
i
This is a composition-based engineering calculation, not a measured hardenability report The page does not clamp or extrapolate out-of-range compositions. For high-carbon high-chromium steels, boron-containing steels, cast irons, or cases where the actual austenite grain size is unknown, use heat-specific Jominy end-quench data or production trials instead.
Geometry
Ideal infinitely long round bar
Microstructural criterion
50% martensite at center
Method status
Empirical multiplying model with validity limits
Input Parameters Composition (wt%)
Example composition:
Presets only demonstrate data entry; engineering calculations should use the material certificate or measured heat chemistry.
ASTM grain size:
C 0.10 – 0.70
%
Mn 0.50 – 1.65
%
Si 0.15 – 0.60
%
Cr 0 – 1.35
%
Ni 0 – 1.50
%
Mo 0 – 0.55
%
Cu 0 – 0.35
%
V 0 – 0.20
%
Calculation Results
Inputs are within the published validity ranges. The result uses the empirical multiplying model for non-boron steel.
Ideal Critical Diameter DI
--
mm
C × grain-size base value = -- in (-- mm)
Alloy Multiplying Factors
Elementwt%Multiplying factor FXLeave-one-out sensitivity
Leave-one-out sensitivity (mm, non-additive)
Use this DI in the round-bar calculator
Empirical calculation model · not measured data
Formula: GB/T 5216-2014 Annex A relationship for ideal critical diameter of non-boron steel.
Input limits: Uses the officially published ASTM A255-20a composition ranges; no calculation is made outside them.
Criterion:50% martensite at the center of an ideal round bar; DI is not the hardenable diameter of a production workpiece in an actual quenchant.
Related Heat Treatment Resources
Maximum Round-Bar Critical Diameter
Combine DI and H to check the 50% martensite center criterion for a target round bar
Grossmann H-Value Guide
Distinguish known or calibrated H values from historical reference ranges
Quenchant H-Range Screening
Screen model feasibility by target size and candidate H range
Measured Jominy Curve Analysis
Enter end-quench distance–hardness data and analyze the 50% martensite crossing
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Data classification

Three data types must remain separate

Every curve and conclusion on this site is identified by how the data was obtained.

01 / MEASURED

Standard-probe measured curve

Describes the medium response for the stated probe and test conditions. It is not the surface or core curve of a production workpiece.

02 / PREDICTED

Software-predicted curve

Calculated from a stated model and inputs. It is a prediction, not measured production data.

03 / WORKPIECE

Real-workpiece internal measured curve

Comes from internal points in the actual workpiece and is used to assess the real cooling process against post-quench hardness, microstructure and properties.

Unless explicitly identified and validated, one data type cannot be converted into or substituted for another.Read the technical method

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