Jominy End-Quench Test: Procedure, Hardenability Curves and Data Interpretation
The Jominy end-quench test is the standard, almost universal method for measuring steel hardenability, the depth to which a steel will harden on quenching, as distinct from the maximum hardness it can reach. This article covers the ISO 642 / ASTM A255 test procedure in detail, explains how to read and interpret a measured hardenability curve, and provides an interactive tool for analysing your own Jominy data against a target hardness or an H-band specification.
Key Takeaways
- Hardenability is not hardness: maximum as-quenched hardness is controlled almost entirely by carbon content, while hardenability, the depth of hardening, is controlled by alloy content, grain size, and quench severity.
- The standard Jominy specimen is a 25 mm (1.0 in) diameter, 100 mm (4 in) long bar, normalised, austenitised, and end-quenched with a controlled water jet on one face.
- Hardness, almost always Rockwell C, is measured at standard distances from the quenched end (commonly J1.5, J3, J5, J7, J10, J13, J15, J20, J25, J32, J40 mm) after grinding two flats to remove decarburised material.
- A steep hardness drop indicates low hardenability; a flat curve indicates high hardenability and deep through-hardening capability.
- Measured curves are compared against H-band specifications (ASTM A304 / ISO 683-2), which define guaranteed minimum and maximum hardness at each standard distance for a given grade.
- Test reproducibility depends on tight control of austenitising temperature, transfer time, and water jet temperature and velocity; ASTM A255 requires the two ground flats to agree within 4 HRC points at every position.
Jominy Curve Interpreter
Enter your own measured Rockwell C hardness at the standard Jominy distances to find the critical Jominy distance for a target hardness, and check a single position against a specification band. This tool interpolates your measured data; it does not predict a curve from composition.
Hardenability vs Hardness
Hardness and hardenability are frequently confused but describe different things. Hardness is the resistance of a surface to localised plastic deformation in its current state; hardenability is the capacity of a steel to develop martensite, or other hard transformation products, to a useful depth when quenched from the austenitising temperature. Two steels of identical carbon content can reach the same maximum surface hardness after quenching, yet behave completely differently a few millimetres beneath the surface: a plain carbon steel such as 1040 will revert to soft pearlite just below the case-hardened skin, while a chromium-molybdenum alloy steel such as 4140 retains a hard, predominantly martensitic structure far deeper into the section because its alloy content suppresses the diffusional pearlite and bainite reactions. The Jominy end-quench test isolates and quantifies this depth-of-hardening behaviour in a single, standardised specimen.
Jominy Test Standard and Specimen
The test is standardised internationally as ISO 642 and in the United States as ASTM A255, with closely equivalent national standards including SAE J406, DIN 50191, and JIS G 0561. The standard specimen is a cylindrical bar 25 mm (1.0 in) in diameter and 100 mm (4 in) long, machined with a small flange or collar at the top end that seats in the support fixture. Where incoming stock is too small to machine a standard specimen, ASTM A255 Annex X1 permits subsize specimens of 19 mm, 12.7 mm, or 6.4 mm (0.75, 0.50, or 0.25 in) diameter, tested with a correspondingly scaled fixture and orifice and correlated back to standard results.
Before austenitising, the specimen is normalised to remove microstructural variation introduced by prior forging, rolling, or heat treatment, so that the resulting hardenability curve reflects only composition, prior austenite grain size, and the austenitising condition used in the test, not an accident of upstream processing history.
Test Procedure Step by Step
1. Austenitising
The normalised specimen is heated to an austenitising temperature specified as approximately 50°C above the Ac3 temperature of the steel, typically in the range 830-870°C for most carbon and low-alloy engineering grades, and held at temperature for 30 minutes once the specimen reaches it, with furnace temperature uniformity controlled within ±5°C. This step must fully homogenise the austenite and establish a known, controlled prior austenite grain size, since coarser grain measurably increases hardenability by reducing the grain-boundary area available for pearlite and bainite nucleation.
2. Transfer to the Quench Fixture
The specimen is removed from the furnace and transferred to the vertical support fixture in no more than 5 seconds. This tight limit prevents significant uncontrolled air-cooling, and the transformation it could trigger, before the standardised end-quench begins.
3. End-Quenching
The fixture holds the specimen vertically with its lower face 12.7 mm (0.5 in) above a 12.5 mm diameter orifice. A water jet at 24 ± 3°C, with a free-jet velocity of approximately 2.5 m/s, is directed onto the bottom face only; the rest of the bar cools by a combination of conduction up the bar and limited contact with ambient air, producing a continuous, monotonically decreasing cooling rate with distance from the quenched face. The quench is maintained for at least 10 minutes before the specimen is removed and air-cooled to room temperature.
4. Specimen Preparation and Hardness Measurement
Two flats are ground along the length of the bar, 180 degrees apart, to a depth of 0.38 mm (0.015 in), removing any decarburised or oxidised surface layer that would otherwise bias the hardness reading low. Rockwell C hardness (or Vickers, for very low-hardenability or thin specimens) is then measured along each flat at the standard distances from the quenched end, typically J1.5, J3, J5, J7, J10, J13, J15, J20, J25, J32, and J40 mm. If the two flats disagree by more than 4 HRC points at any position, ASTM A255 requires re-testing on a fresh pair of flats ground 90 degrees from the first pair, and rejection of the specimen if the discrepancy persists. Results are conventionally plotted on the standardised ASTM hardenability chart form to keep results from different laboratories visually comparable.
Predicting a Curve from Composition
This article focuses on running the test and interpreting measured results. To predict an expected Jominy curve and ideal critical diameter from a steel’s chemical composition using the Grossmann multiplying factor method, see MetallurgyZone’s companion Jominy Hardenability Calculator, which also tabulates Jominy position against equivalent cooling rate and bar section size for oil and water quenching.
Reading and Interpreting the Hardenability Curve
Once plotted, the hardenability curve is read as hardness against distance from the quenched end. Every curve starts at its highest value at J1.5, where the cooling rate is most severe, and decreases monotonically as distance increases and the local cooling rate falls. What differs between steels is how quickly that decrease happens, and that rate of decrease is precisely what “hardenability” means in graphical form.
Steep Curve
Hardness falls sharply within the first few millimetres. Typical of plain carbon steels such as 1040. Only a thin martensitic case forms near the quenched face; the bulk of any real part transforms to softer pearlite or bainite unless the section is thin or the quench severe.
Flat Curve
Hardness remains close to its maximum over tens of millimetres. Typical of alloy steels such as 4140 and 4340, where chromium, molybdenum, and nickel suppress the diffusional transformations and allow martensite, or a hard bainite, to form much deeper into a real cross-section.
The single most useful summary number extracted from a curve is the critical Jominy distance: the distance at which the measured hardness crosses a chosen reference value, most often the hardness corresponding to 50 percent martensite for that steel’s carbon content. A flatter curve crosses that reference further from the quenched end, directly indicating deeper hardenability. ASTM A255 publishes correlation tables relating this critical distance to the Grossmann ideal critical diameter DI, the same parameter used in the composition-based prediction method, allowing a measured curve to be converted into an equivalent through-hardening capacity without needing the full composition.
Comparing Curves Against H-Band Specifications
Many engineering steels intended for through-hardened or induction-hardened components are purchased not by chemical composition alone but by a guaranteed hardenability band, or H-band, covered under ASTM A304 and ISO 683-2. An H-band specification, such as that for 4140H or 8620H, defines a minimum and maximum Rockwell C hardness at each standard Jominy distance, established by testing heats manufactured at the extreme composition limits permitted for that grade. Specifying by H-band rather than composition alone is the more reliable basis for heat treatment process design because it directly guarantees the property that determines through-hardening response, rather than an indirect proxy for it.
Checking a measured curve against an H-band is a straightforward point-by-point comparison: at each specified Jominy distance, the measured hardness must fall at or between the published minimum and maximum. A reading above the maximum indicates higher hardenability than guaranteed, which is occasionally acceptable but should be confirmed against the purchase specification; a reading below the minimum indicates the heat does not meet the guaranteed hardenability and is normally cause for rejection. The calculator on this page performs exactly this check at a single specified position; production quality systems typically automate the same comparison across the full set of specified distances.
Relating Jominy Data to Real Part Section Size
A Jominy curve by itself describes the steel, not a finished part. Converting it into a prediction of hardness at a specific position in a real round bar or forging requires combining the Jominy data with the quench severity, characterised by the Grossmann H-factor, of the quenchant and agitation actually used in production. This conversion, originally developed by Lamont in the 1940s and systematised within the Grossmann framework, correlates each Jominy distance to an equivalent cooling rate, and that cooling rate to the surface or centre position of bars of various diameters quenched in oil or water of known severity. MetallurgyZone’s companion Jominy Hardenability Calculator tabulates this Jominy-position-to-section-size correlation directly and is the appropriate next step once a hardenability curve, measured or predicted, is in hand.
Common Errors and Sources of Test Variability
Transfer Delay
Any transfer time meaningfully longer than the 5 second limit allows uncontrolled air cooling and possible partial transformation before the controlled quench begins, shifting the entire curve in an unpredictable, non-reproducible way.
Water Temperature, Flow, or Orifice Wear
Because the whole curve is anchored to a single, tightly specified cooling condition, water outside the 24 ± 3°C range, jet velocity drift, or a worn or partially blocked orifice changes the cooling rate at every position and biases the entire curve up or down rather than at one point only.
Incomplete Removal of Decarburised Material
Grinding the flats to less than the specified 0.38 mm depth can leave residual surface decarburisation, which reads artificially soft and is most misleading near the quenched end, where the true hardness should be highest.
Specimen Diameter and Straightness
A specimen machined outside the diameter tolerance, or one that is not straight and does not sit centred and vertical in the fixture, alters the effective heat-transfer path from the quenched face and can also explain a two-flat discrepancy exceeding the 4 HRC acceptance limit.
Insufficient Austenitising Time or Temperature Control
Under-temperature or under-time austenitising leaves undissolved carbides or an inhomogeneous austenite, lowering apparent hardenability, while excessive temperature or time coarsens the prior austenite grain size, which the grain boundary nucleation argument shows will artificially raise it.
Industrial Applications and Significance
The Jominy test underlies the selection and quality control of nearly every through-hardened or case-hardened steel component in mechanical engineering. Automotive and machinery manufacturers specify shafts, gears, and fasteners by H-band to guarantee consistent quenching and tempering response across heats and suppliers; forging producers use Jominy data to confirm that heavy-section parts will achieve adequate core hardness rather than a soft, coarse pearlitic centre; and case-hardening grades such as 8620 are screened on their core composition to ensure adequate core strength beneath a carburised case. Because the curve reflects the underlying transformation behaviour captured in the TTT diagram and CCT diagram for that steel, Jominy data is also routinely used alongside those diagrams when designing or troubleshooting an industrial heat treatment cycle.
Frequently Asked Questions
How do you read a Jominy hardenability curve?
A Jominy curve plots hardness, almost always Rockwell C, on the vertical axis against distance from the quenched end on the horizontal axis. The curve always starts at its highest hardness at the quenched end, where the cooling rate is fastest, and either stays high over a long distance, indicating deep hardenability, or falls away quickly, indicating shallow hardenability. The distance at which the curve crosses a chosen reference hardness, commonly the hardness corresponding to 50 percent martensite for that steel, is the key single-number summary of the curve and is often called the critical Jominy distance.
What does a steep versus a flat Jominy curve indicate about a steel?
A steep curve, where hardness drops sharply within the first few millimetres of the quenched end, indicates low hardenability: only a thin martensitic case forms near the quenched face and the interior of any real part transforms to softer pearlite or bainite unless the section is very thin or the quench is very severe. A flat curve, where hardness remains high over tens of millimetres, indicates high hardenability and allows martensite, or a hard bainitic structure, to form much deeper into a real cross-section under the same quenching conditions.
Why is the Jominy specimen normalized before the test, and why is it austenitized rather than tested as-received?
Normalizing removes the prior thermal and mechanical history of the bar, such as variation introduced by forging, rolling, or prior heat treatment, so that the measured hardenability reflects only the steel’s composition and the austenitizing condition used in the test, not an accident of how the stock was previously processed. Austenitizing is required because hardenability describes how the austenite formed on heating transforms during cooling; the specimen must start from a fully austenitic, compositionally homogeneous condition with a controlled grain size for the resulting curve to be meaningful and reproducible between laboratories.
What hardness scale is used for Jominy testing, and why does it sometimes change along the bar?
Rockwell C is the standard scale because most steels evaluated by the Jominy test are hardenable grades that read above the practical 20 HRC floor of the diamond cone scale over at least the near-quenched-end region. On steels with low hardenability, however, hardness toward the slow-cooled far end of the bar can fall below roughly 20 HRC, where HRC becomes unreliable; in that situation Rockwell B or Brinell is substituted for the far-end readings, exactly the scale-selection logic covered in detail in MetallurgyZone’s guide to choosing between Vickers, Brinell, and Rockwell hardness tests.
Why are two flats ground on opposite sides of the Jominy bar rather than just one?
Measuring both flats provides a built-in repeatability check: if the two hardness traverses at any given position differ by more than 4 HRC points, ASTM A255 calls for the test to be repeated on a fresh pair of flats ground 90 degrees from the first pair, and for the specimen to be rejected and retested entirely if the second attempt also exceeds that spread. This check catches non-uniform quenching, off-axis specimen mounting, and localized decarburization or grinding damage that a single traverse would not reveal.
What happens if the water temperature or flow rate is outside the standard specification?
Because the entire Jominy curve is anchored to the cooling rate produced by a tightly controlled water jet, water that is warmer, colder, or flowing faster or slower than the standard condition shifts the cooling rate at every position along the bar and therefore shifts the whole hardness-versus-distance curve up or down. This is precisely why ISO 642 and ASTM A255 specify the orifice diameter, jet velocity, and water temperature within narrow tolerances: without that control, Jominy results from different laboratories, or even different days in the same laboratory, would not be comparable.
Can the Jominy end-quench test be used to evaluate case-hardening (carburizing) steel grades?
Yes. For carburizing grades such as 8620, the Jominy test is run on the core composition, that is, the steel before carburizing, because core hardenability controls the strength and toughness beneath the carburized case and governs resistance to case crushing and spalling under contact fatigue. The case itself, with its much higher local carbon content, is evaluated separately through case depth and case hardness measurement rather than through the core Jominy curve.
How is a measured Jominy curve compared against an H-band specification for pass or fail acceptance?
An H-band specification, covered under ASTM A304 and ISO 683-2, defines a guaranteed minimum and maximum hardness at each standard Jominy distance for a given grade, such as 4140H. The measured curve from an incoming heat is compared point by point against that envelope at the distances called out in the specification; if every measured hardness falls within the published minimum-maximum band, the heat is accepted, and any point falling outside the band, high or low, is cause for rejection or further evaluation.
What is the maximum bar diameter the standard Jominy test can characterize directly, and what about smaller stock?
The standard specimen is a fixed 25 mm (1.0 in) diameter bar regardless of the diameter of the stock it represents; the test characterizes the steel’s hardenability, not a specific part size, and that hardenability is then mapped onto real section sizes using cooling-rate correlation charts. ASTM A255 also permits subsize specimens of 19, 12.7, or 6.4 mm diameter, tested with a correspondingly modified fixture and orifice, for evaluating hardenability directly from bar stock too small to machine a standard specimen from.
Why must the specimen be transferred to the quench fixture within 5 seconds of leaving the furnace?
A hot specimen exposed to air begins cooling immediately, and if that air-cooling period is too long, the surface or even the bulk of the bar can begin to transform before the controlled end-quench ever starts, which would corrupt the resulting hardness-distance relationship. The 5 second transfer limit keeps this uncontrolled pre-quench cooling negligible so that the cooling rate experienced by the specimen is governed almost entirely by the standardized water jet rather than by handling time, which varies between operators and laboratories.
Recommended Reference Materials
ASM Handbook, Volume 4: Heat Treating
Authoritative reference covering the Jominy test, Grossmann hardenability theory, hardenability bands, and quench severity, with full reference tables and worked examples.
View on AmazonSteel Heat Treatment Handbook – Totten & Howes
Comprehensive treatment of hardenability fundamentals, quench media, and quench severity, complementary to interpreting Jominy data for real component sections.
View on AmazonBench-Mount Rockwell Hardness Tester (B/C Scale)
A dedicated bench Rockwell tester is the standard instrument for the HRC traverse along the ground flats of a Jominy specimen.
View on AmazonMetallographic Specimen Grinder/Polisher
Used to grind the two flats along the Jominy bar to the specified depth without introducing grinding heat that could bias near-surface hardness readings.
View on AmazonDisclosure: MetallurgyZone participates in the Amazon Associates programme. If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.
Further Reading
Jominy Hardenability Calculator
Predict a Jominy curve and ideal critical diameter from steel composition using the Grossmann method.
Martensite Formation in Steel
The transformation mechanism behind the high hardness measured near the quenched end of a Jominy bar.
Quenching and Tempering of Steel
How hardenability data feeds directly into industrial heat treatment cycle design.
TTT Diagram Explained
The isothermal transformation kinetics underlying the Jominy curve’s shape.
CCT vs TTT Diagrams
Continuous cooling transformation behaviour, directly relevant to interpreting a continuously varying Jominy cooling rate.
Hardness Testing Methods
Principles of Vickers, Rockwell, Brinell, and Knoop hardness testing used to generate Jominy data.
Vickers vs Brinell vs Rockwell Hardness
Detailed comparison and a conversion calculator for the three hardness scales referenced throughout this article.
Calculators Hub
Browse all interactive metallurgy and heat treatment calculators on MetallurgyZone.
