Jominy Hardenability Simulator — Carbon Equivalent and Heat Treatment
Compute the IIW carbon equivalent CE, the max quench hardness HRC_max, the ideal critical diameter D_I and the weldability regime (no preheat / mild preheat / heavy preheat) in real time from the C, Mn, Cr and Ni composition. Visualise the Jominy specimen hardness profile along its length and the required preheat temperature for the current CE.
Parameters
Carbon C
%
Manganese Mn
%
Chromium Cr
%
Nickel Ni
%
Defaults: C = 0.40 percent, Mn = 0.80 percent, Cr = 0.50 percent, Ni = 0.30 percent (Mo = V = Cu = 0). The tool uses the simplified relations CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, HRC_max = 30 + 50 C and D_I = 50 CE (mm). The Jominy hardness profile is shown as HRC(d) = HRC_max exp(-d / (D_I / 10)).
Results
—
Carbon equivalent CE
—
Max quench hardness
—
Ideal critical diameter D_I
—
Weldability regime
Jominy specimen and hardness profile
Top: cylindrical Jominy specimen (left end water-quenched). Bottom: hardness HRC(d) = HRC_max exp(-d / (D_I/10)) along the length. Yellow: hardness marker at d = D_I. Red dashed: HRC_max level.
CE and required preheat temperature
x-axis: carbon equivalent CE in [0, 1.5]. y-axis: required preheat temperature (degC). Blue: empirical curve (CE<0.4: 0 degC, 0.4-0.6: 100 (CE-0.4)/0.2 degC, CE>0.6: 200 degC). Yellow: current CE marker.
Theory & Key Formulas
The IIW (International Institute of Welding) carbon equivalent CE is the standard composite index for the weldability and hardenability of a steel:
C, Mn, Cr and Ni are mass percentages, CE is dimensionless, HRC is the Rockwell C hardness and D_I and d are lengths in mm. This tool uses the simplified expressions for trend-level study.
What is the Jominy hardenability simulator?
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"Hardenability" — isn't that just whether the steel can be quenched hard? And what is a "Jominy test"? I have not heard the term before.
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A great question — "max hardness" and "hardenability" are not the same thing. Max hardness is set almost entirely by carbon (this tool's HRC_max = 30 + 50 C). Hardenability is whether you can keep that hardness all the way through the section — basically, "what diameter can you harden to the centre?" The Jominy end-quench test water-quenches one end of a cylindrical bar and measures the hardness profile along its length to quantify this. With the defaults (C = 0.40, Mn = 0.80, Cr = 0.50, Ni = 0.30) the tool reports CE about 0.653, HRC_max about 50.0, D_I about 32.7 mm and the "Heavy preheating" regime.
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CE shows up everywhere in welding. Is it the same thing as hardenability?
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Two sides of the same coin. The IIW carbon equivalent CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 converts alloying elements into a carbon-equivalent number. Higher CE means higher hardenability — great for the heat-treater because the steel quenches deep, but bad for the welder because the heat-affected zone risks brittle martensite. Sweep Mn from 0 to 2 percent in this tool and watch the regime label move from "no preheat" to "mild preheat" to "heavy preheating".
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D_I is showing 32.7 mm. Does that mean a bar up to 32.7 mm in diameter will harden through to the centre?
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That is the right reading. D_I is defined as the diameter at which a perfectly quenched round bar reaches 50 percent martensite at the centre, a concept introduced by Grossmann in 1942. This tool uses the rough relation D_I = 50 CE. For an automotive crankshaft (60 to 80 mm in diameter) hardened through the section, you would need D_I above 80 mm — roughly CE above 1.6, a heavily alloyed steel. That is why JIS SCM440 (CE about 0.85) or SNCM439 (CE about 1.2) are picked for those parts.
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The preheat-temperature curve has a step at CE = 0.40 and 0.60. Why those numbers?
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Empirical, but rooted in the AWS and JIS welding codes. Below CE = 0.40 the HAZ produces little martensite and a low-hydrogen electrode is enough — no preheat required. Between 0.40 and 0.60 you have a transitional regime where 80 to 150 C of preheat is recommended depending on plate thickness and restraint. Above CE = 0.60 the HAZ will inevitably contain hard, brittle martensite, so 150 to 250 C of preheat plus PWHT (post-weld heat treatment) becomes mandatory. This tool uses a linear interpolation as a simplified visualisation. For example, nuclear piping (SCMV44, CE about 0.65) requires preheating above 150 C and PWHT above 600 C in the JIS code.
FAQ
The Jominy end-quench test (ASTM A255, JIS G 0561) measures the hardness profile along a cylindrical specimen with one end water-quenched, and uses it to characterise hardenability — the ease with which martensite forms — which depends on alloying elements, carbon content and grain size. With the defaults (C = 0.40, Mn = 0.80, Cr = 0.50, Ni = 0.30) the tool reports CE about 0.653, HRC_max about 50.0 HRC, D_I about 32.7 mm and the "Heavy preheating" regime.
The IIW carbon equivalent CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 captures the weldability of a steel in one number. Higher CE means higher hardenability and a greater risk of brittle martensite in the HAZ. CE below 0.40 needs no preheat, 0.40 to 0.60 calls for mild preheating (80 to 150 C), and CE above 0.60 needs heavy preheating (150 to 250 C) plus low-hydrogen electrodes. Sweep Mn from 0.0 to 2.0 percent here and watch CE cross the regime boundaries.
D_I is the round-bar diameter at which an idealised quench reaches 50 percent martensite at the centre, proposed by Grossmann. Higher D_I means higher hardenability and lets larger cross-sections harden through. This tool uses D_I = 50 x CE (mm). It drives material selection for shafts, gears and bolts; bars smaller than D_I harden through to martensite, larger ones contain ferrite and pearlite at the centre.
Under fast quenching the max hardness is set almost entirely by carbon content; alloying elements have only a small effect. This tool uses HRC_max = 30 + 50 C, returning typical 35 to 60 HRC for C = 0.10 to 0.60 percent. This reflects the metallurgical principle that martensite hardness is governed by carbon. Hardenability — the depth to which HRC stays high — is dominated by alloying, so HRC_max, D_I and CE are best read together. Raising C from 0.40 to 0.60 percent here moves HRC_max from 50 to 60 HRC.
Real-world applications
Material selection for automotive driveline parts: crankshafts, camshafts, gears and drive shafts need full-section through-hardening over large diameters, so alloy steels with D_I at least 1.5 times the section diameter (SCM440, SNCM439, SCr420 and similar) are selected. Setting Cr = 1.0 percent, Mo = 0 (fixed for simplicity here) and Ni = 0.3 percent gives CE about 0.75 and D_I about 38 mm. Real SCM440 adds Mo to push D_I to about 60 mm. Gears typically combine carburising (0.8 percent C surface, 0.2 percent C core) to obtain 60 HRC at the surface and 35 HRC at the core in a graded structure.
Welding control for bridge and building structural steels: high-strength steels (SM490B, SM570, SBHS500 with CE = 0.40 to 0.55) need preheat, restraint and hydrogen control governed by JIS Z 3158 and WES 1108. Setting Mn = 1.5 percent and Cr = Ni = 0 here gives CE about 0.65 and the "Heavy preheating" regime — matching real codes that demand 150 to 200 C of preheat and low-hydrogen electrodes (H-5 or better). Lowering CE through TMCP (thermomechanically controlled processing) with small Nb, Ti and V additions for precipitation hardening is the modern strategy.
Safety management of pressure vessels and piping: for thermal-power, nuclear-power and petrochemical pressure vessels (A516, SQV2A) and piping (STPA24, SCMV44), HAZ hardness control is the central tool against rupture and explosion. Materials with CE above 0.60 require PWHT (600 to 750 C tempering) under the ASME B&PV Code or JIS B 8265. Setting Cr = 2.5 percent and Mn = 0.5 percent in this tool gives CE about 0.95 — solidly in the PWHT-required zone. Inspection combines ultrasonic and magnetic-particle testing for both hardness and crack monitoring.
Quality assurance of large forged shafts: power-turbine shafts, marine propulsion shafts and rolling-mill rolls (500 mm class diameter) demand uniform tempered-martensite microstructure all the way to the centre. To meet D_I about 1.0 to 1.5 times the section diameter, low-alloy steels with 2.0 to 4.0 percent Ni (such as SFNCM grades, JIS G 3201) are chosen. Setting Ni = 3.0 percent here gives CE about 0.7 and D_I about 36 mm. Real materials add Mo and V to push D_I to 500 mm class, then oil-quench and high-temperature temper at 600 C for toughness.
Common misconceptions and pitfalls
The most common mistake is the belief that "more carbon means more hardenability". Carbon does set max hardness (HRC_max = 30 + 50 C), but hardenability — the depth to which the section hardens — is largely controlled by Mn, Cr, Mo and Ni. Pushing C above 0.5 percent makes the martensite brittle, lowers Ms and increases retained austenite, so toughness and dimensional stability suffer. Raising C from 0.40 to 0.80 percent here moves HRC_max from 50 to 70 HRC, but real designs cap C around 0.40 to 0.50 percent and rely on alloying for D_I to avoid quench cracking and residual stress.
The second pitfall is "the IIW carbon equivalent alone determines weldability". The IIW formula is calibrated for thick-plate low-alloy steels with CE around 0.4 to 0.6. For low-carbon steel (C below 0.18 percent) the Pcm (Ito-Bessyo) formula Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B is the better choice. In practice plate thickness, restraint, hydrogen, joint geometry (butt vs fillet) and post-weld treatment all matter. Treat the CE here as a first-order index; for critical components consult the JIS Z 3158, WES 1108 or AWS D1.1 thickness-vs-preheat tables.
The last pitfall is "Jominy results map directly onto real components". The Jominy test measures the ideal quench of a standard specimen (25 mm diameter, 100 mm long); real components have different sections, geometries, quench media (water, oil, air) and stirring intensity, all of which change the actual cooling rate. Conversion uses Grossmann's H-value (severity of quench, about 2.0 for stirred water and 0.3 for static oil) and an effective diameter D_e = D x (Q/H). The D_I in this tool assumes ideal quench and needs H-value correction, CCT-diagram cross-checking and prototype destructive testing before it can guarantee the centre hardness of a real part.