hardenability

name	hardenability
description	Steel hardenability — Jominy end-quench test, Grossmann H-factor, ideal critical diameter (DI), hardness prediction, alloy contribution, boron effect, case vs. core hardness, ASTM A255, hardenability bands, heat treatment selection.
metadata	{"priority":7,"promptSignals":{"phrases":["steel hardenability","Jominy test","ideal critical diameter","quench hardness","hardenability band","Grossmann factor"],"minScore":3}}

Steel Hardenability — Complete Skill

Hardenability Fundamentals

Hardenability: ability of steel to be hardened to depth by quenching; measured by how far from quenched surface martensite forms Not the same as hardness: hardness = maximum attainable hardness at surface (function of carbon content only); hardenability = depth penetration of hardening

Carbon content and maximum hardness: HRC_max ≈ 30 + 50 × %C [approximate; for %C = 0.1–0.6%] Or: HRC_max = 17.36 + 92.8 × (%C)^0.67 (more accurate) For 0.40% C steel: HRC_max ≈ 57–60 (martensitic)

Martensite start temperature (Ms): Ms [°C] = 561 - 474 × %C - 33 × %Mn - 17 × %Ni - 17 × %Cr - 21 × %Mo Below Ms: martensite forms on cooling; quench rate must exceed critical to reach Ms without decomposition

Jominy End-Quench Test (ASTM A255)

Test procedure:

• Austenitize standard bar (25.4 mm dia × 100 mm) at specified temperature
• Quench one end with 24°C water jet; bar cools at varying rates along its length
• Measure Rockwell hardness at 1.6 mm intervals from quenched end

Jominy distance (J) and cooling rate: J = 0 mm (quenched end): cooling rate ≈ 200°C/s (very fast; similar to water quench on small part) J = 25 mm: cooling rate ≈ 15°C/s (similar to oil quench) J = 50 mm: cooling rate ≈ 5°C/s (similar to air quench on thin section)

Jominy curve interpretation: High hardenability: hardness stays high (near HRC_max) to J = 50 mm and beyond Low hardenability: hardness drops quickly from J = 5–15 mm; soft core forms in thick sections

Example (SAE 1040 vs. 4140): SAE 1040 (plain carbon): J=0: HRC 60; J=5: HRC 45; J=10: HRC 25 (low hardenability) SAE 4140 (alloyed): J=0: HRC 58; J=5: HRC 55; J=25: HRC 45 (high hardenability)

Grossmann Ideal Critical Diameter (DI)

DI: diameter of round bar that will have 50% martensite at center when quenched ideally (H = ∞) Actual critical diameter (D): D = DI × f(H) [H = Grossmann quench severity factor]

Quench severity H:

Medium	H [inch⁻¹]
Still air	0.02
Oil (still)	0.25–0.30
Oil (agitated)	0.40–0.60
Water (still)	0.90–1.00
Water (agitated)	1.0–1.5
Brine (agitated)	2.0
Ideal quench	∞

DI calculation (Grossmann): DI = C_base × f_C × f_Mn × f_Ni × f_Cr × f_Mo × f_Si × f_B C_base = base for grain size (ASTM 7: 0.162; ASTM 8: 0.171); from table f_alloy = multiplying factors per alloying element (from ASTM A255 table)

Example (SAE 4340, 0.40%C, 0.70%Mn, 1.70%Ni, 0.77%Cr, 0.23%Mo, ASTM 8 grain): C_base = 0.171 f_C(0.40%) = 4.89; f_Mn(0.70%) = 3.77; f_Ni(1.70%) = 1.46; f_Cr(0.77%) = 2.22; f_Mo(0.23%) = 1.92 DI = 0.171 × 4.89 × 3.77 × 1.46 × 2.22 × 1.92 = 16.2 inches (411 mm) → With oil quench (H=0.4): D = DI × f(H) ≈ 4–5 in (100–125 mm) → center hardness achievable at this diameter

Alloy Element Effects on Hardenability

Manganese (Mn): most cost-effective hardenability element; f_Mn = 1 + 3.24 × %Mn (Jominy shift) Chromium (Cr): strong carbide former; increases hardenability; f_Cr = 1 + 2.16 × %Cr Molybdenum (Mo): strong effect; also reduces temper brittleness; f_Mo = 1 + 3.0 × %Mo Nickel (Ni): moderate effect; excellent toughness contribution; f_Ni = 1 + 0.363 × %Ni Silicon (Si): minor direct effect; strengthens ferrite; f_Si = 1 + 0.70 × %Si Vanadium (V): strong grain refiner; raises Jominy hardness at J=25+ mm Boron (B): extremely potent at trace levels (0.001–0.005%); multiply DI by 1.5–2.0×

Boron Effect

Boron hardenability mechanism: B segregates to austenite grain boundaries → delays ferrite nucleation → dramatic hardenability increase at very low concentrations Effective boron: must have free B (not tied up as BN); requires titanium or aluminum to bind N Typical composition: 0.0005–0.003% B (add to SAE 1020 or 1040 → hardenability similar to 4140) Limitation: boron less effective in high-carbon steels (>0.60%C); also less effective in coarse grain size

Hardenability Bands (H-Steels)

H-steel specification (ASTM A304): H suffix (e.g., 4140H): guaranteed hardenability band; min and max Jominy curve Composition slightly modified (typically ±0.05% C range vs. ±0.03% for standard) Used for: production consistency; interchangeable heat lots; automotive/industrial

Key hardenability bands:

Grade	DI Range [in]	Typical Application
1040H	0.6–1.5	Lightly stressed shafts
4140H	3.0–7.0	Gears, shafts, structural
8620H	2.0–4.5	Carburized gears (case+core)
4340H	7.0–13.0	Heavy-section high-strength
300M	> 15	Aircraft undercarriage

Prediction of Core Hardness in Production

From Jominy curve to section diameter:

1. Determine quench medium and H-factor
2. For shaft of diameter D: equivalent Jominy distance J_equiv = D / (4H) [inch; simplified] Or use published charts (ASM Handbook Vol. 4)
3. Read Jominy hardness at J_equiv → predicted core HRC

More accurate: J_surface = 0 mm (always); J_center = function of D and H (from chart) For D = 50 mm, oil quench (H=0.4): J_center ≈ 15–20 mm → read J15–20 from Jominy curve

Minimum core hardness for strength: Shaft: HRC 28–35 at center (for adequate fatigue strength) Gear: HRC 28–38 core (for tooth impact strength per AGMA 923) If predicted core HRC < minimum: change quench medium, alloy grade, or reduce section size

Heat Treatment Selection

Quench medium selection: Oil: prefer for distortion-sensitive parts; hardened steels with DI > 3 in; less cracking risk Water: lower-alloy steels (DI < 2 in) to achieve core hardness; higher cracking risk on corners Polymer (aqueous): adjustable quench severity (5–15%); between oil and water Marquench (martempering): oil at 120–200°C; slow thermal shock; reduced distortion for complex parts

Process sequence (typical for through-hardened steel): Austenitize at T_austenitize [°C] → quench → temper 1 hour at T_temper T_austenitize: 800–870°C (medium carbon steel); 790–850°C (alloy steel) T_temper: 200°C → HRC 55–60; 400°C → HRC 45–50; 600°C → HRC 28–35

Standards

Standard	Scope
ASTM A255	Jominy hardenability test
ASTM A304	H-steel hardenability band specification
SAE J406	Methods for determination of hardenability
ASM Handbook Vol. 4	Heat treating — hardenability complete reference
AMS 2759	Heat treatment of steel parts

Output

Provide: steel grade and composition (key elements: C, Mn, Ni, Cr, Mo, B [wt%]), calculated DI [in or mm], Jominy curve (HRC at J=0, 5, 10, 15, 20, 25, 32, 40, 50 mm), quench medium and H-factor, section diameter D [mm], equivalent Jominy distance at center J_center [mm], predicted center hardness HRC, minimum required core hardness HRC (by application), adequacy (yes/no), if inadequate: recommended higher-hardenability grade, and applicable standard (ASTM A255, ASTM A304, AMS 2759).