Jominy End-Quench Test for Steel Hardenability

What the test is and where it's used

The Jominy end-quench test measures hardenability — how deeply a steel hardens on quenching — using a single machined bar and one controlled quench. It was developed in 1938 by Walter E. Jominy and A. L. Boegehold at General Motors Research, and remains codified in ASTM A255, ISO 642, and SAE J406.

It is the workhorse test wherever quench-and-temper steels must be specified by cross-section: automotive gears, crankshafts, axle shafts, drive-line components, bearings, springs, fasteners, and forgings. Steelmakers publish Jominy curves (often as a scatter H-band) for guaranteed-hardenability "H" grades such as 4140H and 8620H.

• Design use: confirm a part section will reach a target core hardness after quench.
• Purchasing use: accept or reject heats against an H-band.
• Process use: pick a grade and quench medium together so the core makes >50 % martensite.

Because every steel has nearly the same thermal conductivity, distance along the bar maps one-to-one onto cooling rate — so one bar characterizes an entire alloy.

How the test works, step by step

The mechanism is a deliberately engineered cooling-rate gradient along one specimen:

• Machine: a 25.4 mm diameter × 100 mm long cylinder with a small support flange at the top.
• Austenitize: heat to the grade's austenitizing temperature (e.g. 845–870 °C for 4140), hold ~30 min to fully dissolve carbon into FCC austenite.
• End-quench: drop the bar into a fixture so its lower face sits 12.7 mm above a 12.7 mm orifice; a free-standing column of 24 °C ± 2 °C water strikes only the bottom face. Transfer must take under 5 s to avoid pre-cooling.
• Cool: the quenched end cools at ~270 °C/s and forms martensite; heat conducts axially away, so each position up the bar cools progressively slower.

Grind two flat, parallel lands 0.4 mm deep along the bar (removing any decarburized skin), then read Rockwell C hardness starting 1.6 mm from the quenched end and continuing in 1/16-inch (1.6 mm) steps. Plotting HRC versus distance yields the Jominy curve: high, flat hardness near the end where martensite dominates, falling where slower cooling lets pearlite, ferrite, and bainite form.

Key quantities and a worked example

Hardness at the quenched end is set by carbon content (≈ 60 HRC at 0.6 %C fully martensitic); the rate of falloff is set by alloying. The core relationship links Jominy distance to cooling rate at 700 °C — e.g. J1.6 ≈ 270 °C/s, J8 ≈ 28 °C/s, J16 ≈ 10 °C/s, J40 ≈ 3 °C/s.

Grossmann's ideal critical diameter D_I converts this into a single number: the largest bar diameter that would harden to 50 % martensite at its center in an ideal quench (severity H = ∞). It is built multiplicatively from composition:

D_I = D_I,C × f_Mn × f_Si × f_Ni × f_Cr × f_Mo

• D_I,C = base ideal diameter from carbon + ASTM grain size (e.g. ~0.22 in at 0.4 %C, grain size 7).
• f_x = multiplying factor for each element, roughly f = 1 + k·(wt%). Chromium (k ≈ 2.16) and molybdenum (k ≈ 3.0) are the strongest per-percent; nickel is weak.

Worked example (4140, 0.40 C / 0.9 Mn / 0.95 Cr / 0.2 Mo): D_I,C ≈ 0.21 in; f_Mn ≈ 3.0, f_Cr ≈ 3.05, f_Mo ≈ 1.6, f_Si ≈ 1.2 → D_I ≈ 0.21 × 3.0 × 3.05 × 1.6 × 1.2 ≈ 3.7 in (≈ 94 mm), the hallmark of a deep-hardening alloy steel.

Using Jominy data in real design and selection

The engineering payoff is the Lamont / equivalent-cooling-rate method, which ties a Jominy position to a real bar. Each location in a round bar quenched at severity H (Grossmann's quench severity: still oil ≈ 0.25–0.30, agitated oil ≈ 0.5, still water ≈ 1.0, agitated brine ≈ 2–5) cools at a rate equal to some Jominy distance. Charts give, for example: the center of a 50 mm round in still oil cools like J12–J14.

Design procedure:

• Fix the required core hardness (say 40 HRC for a fatigue-critical shaft).
• Choose the quench medium and part diameter, then read the equivalent Jominy distance for the critical location (surface, 3/4-radius, or center).
• Pick a grade whose published Jominy curve still exceeds the target hardness at that distance.

This lets you trade alloy cost against quench severity: a leaner steel plus a faster quench, or a richer alloy plus a gentle oil quench that limits distortion and quench cracking. Guaranteed-hardenability H-grades ship with a min/max Jominy band so heat treaters can set furnace and quench parameters with confidence.

How it compares to related tests and concepts

Versus a hardness test: a Rockwell or Brinell reading gives hardness at one point; Jominy gives hardness as a function of cooling rate, i.e. hardenability. High hardness ≠ high hardenability — a 1080 plain-carbon steel gets very hard at the surface but shallow-hardens.

• Grossmann bar test: the older method quenches many bars of different diameters and sections each to find the one that is 50 % martensitic at center. It directly yields D_crit and D_I but is slow and wastes material; Jominy replaced it with one bar.
• Calculated D_I (multiplying factors / SAE J406): predicts hardenability from chemistry alone — cheap for melt control, but empirical and less accurate for high-alloy or boron steels.
• CCT / TTT diagrams: give the full microstructure-vs-cooling-rate map; Jominy is essentially a fast physical sampling of the CCT behavior at fixed austenitizing conditions.
• End-quench for aluminum/Ni: the same fixture is adapted to study quench sensitivity in age-hardening alloys, though "hardenability" there means retained solute, not martensite.

Failure modes, trade-offs, and significance

The test's precision depends on rigid control of its variables — most scatter comes from procedural drift, not the steel:

• Water temperature and jet height: the 24 °C ± 2 °C water and 65 mm free column height directly set the end cooling rate; a hot lab or wandering nozzle shifts the whole curve.
• Delay in transfer: >5 s to the fixture lets the end pre-cool, softening J1.6.
• Decarburized or non-parallel ground flats: grinding too shallow leaves a soft skin; grinding-induced heat can temper the surface — both bias HRC low. Always grind wet.
• Grain size and austenitizing: coarser prior-austenite grains and higher soak temperatures both increase measured hardenability, so austenitizing must match the grade spec.

Trade-offs: high hardenability eases through-hardening of thick sections but raises quench-crack and distortion risk and worsens weldability (higher carbon equivalent). Significance: nearly 90 years on, the Jominy curve is still how the steel supply chain communicates hardenability — the bridge between a mill certificate's chemistry and whether a finished shaft will survive its fatigue loads.

Representative Jominy hardness (HRC) at selected distances from the quenched end for common medium-carbon grades (approximate, ~0.40 %C).

Distance from quenched end	Approx. cooling rate	AISI 1045 (plain C)	AISI 4140 (Cr-Mo)	AISI 4340 (Ni-Cr-Mo)
J1.6 mm (1/16 in)	~270 °C/s	55–58 HRC	57–60 HRC	57–60 HRC
J8 mm (5/16 in)	~28 °C/s	30–35 HRC	52–56 HRC	55–58 HRC
J16 mm (10/16 in)	~10 °C/s	24–28 HRC	42–48 HRC	52–56 HRC
J25 mm (16/16 in)	~6 °C/s	22–26 HRC	35–40 HRC	48–53 HRC
J40 mm (25/16 in)	~3 °C/s	20–24 HRC	30–35 HRC	42–48 HRC