Tool Steel Hardenability Chart

The section shows how deep each tool steel will harden, the hardness it reaches at the core, and the corresponding hardening temperature. Water-hardening grades harden a thin skin, oil-hardening grades go deeper, and air-hardening grades such as D2, A2, and S7 harden through the largest blocks, which determines whether a given grade can make a thick die section at all.

Section size each grade will harden through

Quench medium and the diameter the grade hardens through at that quench, with the hardness reached straight out of the quench. Hardening and tempering temperatures for the same grades are in the hardening and tempering chart.

Grade	Hardens through	Quench	As-quenched HRC ^(a)
W2	Chill depth 1/8–3/16 in. (3.2–4.8 mm)	Water	68 HRC
O1	Through 1-3/4 in. (44.5 mm) diameter	Oil	64 to 65
S5	Through 3 in. (76.2 mm) diameter	Oil	62–63
A2	Through 4-1/2 in. (114.3 mm) diameter	Air	62–63
S7	Through 2-1/2 in. (63.5 mm) diameter	Air	59–60
D2	Through 5 in. (127 mm) diameter	Air	61–63
M2	Through 1-1/2 in. (38.1 mm) diameter	Air	65–66
H13	Through 2-1/2 in. (63.5 mm) diameter	Air	52–54

Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-6, from Bethlehem Steel grade data sheets.

Hardening practice for die steels, round section

The section the grade fully hardens through, the hardness it is normally put to work at and the austenitizing temperature that produces it. Wear and toughness ratings for the same grades are in the general properties chart.

Grade	Hardens through	Working HRC	Hardening temperature	Quench
W2	Chill depth approximately 1/8 in. (3.2 mm) per side	60–62	1,400–1,450 °F (760–788 °C)	Water quench
S7	Through 2-1/2 in. (63.5 mm) round	56–58	1,725 °F (941 °C)	Air quench
A2	Through 4-1/2 in. (114.3 mm) round	58–60	1,775 °F (968 °C)	Air quench
D2	Through 5 in. (127 mm) round	58–60	1,850 °F (1,010 °C)	Air quench

Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-8, quoting Bethlehem Steel data sheets for W2, S7, A2 and D2.

Hardening practice for mold steels, square section

Mold steels are quoted as square sections rather than round, because that is the shape of a mold block. A2 and S7 appear in both tables for that reason.

Grade	Hardens through	Working HRC	Hardening temperature	Quench
A2	Through 4-1/2 in. (114.3 mm) square	58–60	1,775 °F (968 °C)	Air quench
S7	Through 2-1/2 in. (63.5 mm) square	56–58	1,725 °F (941 °C)	Air quench
420 SS	Through 1-1/2 in. (38.1 mm) square	48–50	1,750–1,850 °F (954–1,010 °C)	Air quench
H13	Through 1-1/2 in. (38.1 mm) square	46–48	1,850 °F (1,010 °C)	Air quench
P-20	Prehardened, no hardening step	28–32	Prehardened on delivery	None

Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-10, from Bethlehem Steel data sheets.

How hard the other tooling materials are

Hardness of the coatings and non-steel materials that share a die with tool steel, in Knoop microhardness and the Rockwell C value usually quoted for them. Surface treatments that produce these layers are described on the surface treatment page.

Material or coating	Knoop (HK)	Rockwell C (HRC) ^(b)
Diamond	9,000	99
Cubic boron nitride	5,000	94
Titanium nitride	2,800	87
Vanadium carbide	2,500	85
Silicon carbide	2,400	84
Aluminum oxide	2,100	83
Tungsten carbide	1,900	82
Zirconium oxide	1,500	77
Gas nitrided case	850–1,000	65–70
Chrome plating	900–1,000	68–70
Hardened high speed steels	800–850	63–65
Hardened tool steels	460–690	45–60
Annealed steels	185	20

Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 5-11, from Bry Coat hardness conversion estimates.

Knoop and Rockwell C are not on the same scale. The values above 68 HRC are approximations, not conversions from hardness data, because the relationship between the two scales loses accuracy as Knoop values climb.
The diameters above describe a round section cooled on all sides. Actual depth varies with part design, part mass, hardening temperature, and quench speed, and the core of a heavy rectangular block cools more slowly than the same weight of round bar.

Reading the table

Steels with good hardenability harden deeper and more safely. A grade that transforms at a slow cooling rate can be quenched in air, and an air quench puts far less stress into the part than water or oil, so the section is less likely to crack or to move out of shape. That is why the air-hardening grades carry the heaviest die sections, and why quench cracking is usually a story about a low-hardenability grade in a heavy section.

The quench medium matters as much as the grade. O1 hardens through 1-3/4 in. only when the oil is agitated and kept cool, and a part quenched too slowly will read low in the core while reading full hardness at the corner. Where a part must be quenched in water to harden, the section is limited by the risk of cracking rather than by the hardness target.

Air hardening also means less movement. D2 grows by plus or minus 0.0005 in. per in. of dimension through the whole cycle, versus 0.0015 in. per in. for an oil-hardening grade, the difference between grinding to size after hardening and hoping the block comes out close. The figures for every family are in the size change chart, and the stock to remove from as-supplied hot-rolled bar is in the machining allowance chart.

High hardenability is not the same as high working hardness. H13 hardens through 2-1/2 in. yet works at 46 to 48 HRC, because a hot work die trades hardness for toughness at temperature. The hot hardness these grades hold is shown in the hot hardness chart.

A shallow-hardening grade is the right answer for some tools. W2 gives a hard skin over a tough core in one quench, which suits a punch that needs a wear face and a body that will not shatter. P20 arrives prehardened at 28 to 32 HRC and is never hardened by the toolmaker, which is why the large mold blocks are made from it. Both routes are covered in the selection by application pages.

Unless you check the core, a low surface reading says nothing about hardenability. Decarburization and a slow quench both soften the skin of a part that hardened correctly inside. Cut or grind a few thousandths off before testing, and use the hardness-testing methods when a file isn’t enough. Values read on other scales convert in the hardness conversion calculator.

Hardenability comes from carbon and from the alloying elements that hold it in solution: chromium, molybdenum, manganese, and tungsten. Each grade’s composition is in the composition chart.

The numbers in the table come from the end-quench test, which measures hardenability. A 25 mm round bar is austenitized, fixed in its fixture, and quenched from one end only, so a single specimen covers a full range of cooling rates along its length.

Jominy end-quench hardenability test: the 25 mm round specimen in its fixture with the water jet spraying the lower end, with specimen dimensions and quench conditions labelled — Fig. 5.20. The Jominy end-quench specimen and its fixture. The 25 mm diameter x 100 mm bar is quenched from the lower end by an unimpeded jet of water at 24 ± 3 °C, and hardness is then read every 1/16 in. from the quenched end. One test at each cooling rate makes the hardness-against-distance figures in the tables above comparable across grades. Source: B. Liščić, Hardenability of Steel, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 5.20.

The tables above give the numbers. The diagram below shows why they come out the way they do: a calculated continuous cooling transformation diagram, with the cooling rates a real quench produces drawn across it. A fast curve misses the ferrite and pearlite start lines and lands in martensite; a slow one crosses them first and finishes as ferrite and pearlite, which is where the soft patches and the hardness below the drawing come from.

Calculated continuous cooling transformation diagram for a 0.37C 1.45Mn 0.25Si steel with cooling curves for 100, 10, 1 and 0.1 C per second — Fig. 2.20b. A calculated continuous cooling transformation diagram for a 0.37% C, 1.45% Mn, 0.25% Si steel, with the cooling curves for 100, 10, 1 and 0.1 °C/s drawn across it. The marked curves are the start (0.1%) and near completion (99.9%) of the ferrite, pearlite and bainite reactions. Read across the four cooling rates and the point of the hardenability tables above is visible: the medium and the section size decide whether the quench clears the transformation noses, and the grade name only shifts where they sit. Panel (b) of Fig. 2.20; diagram calculated with JMatPro, reproduced from S.S. Babu, Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006.

Match the quench to the section

Water quench for a thin section in a shallow-hardening grade, oil for the middle range, air for anything heavy or awkward. A quench faster than the section needs buys nothing and costs stress. Aobo Steel lists the round bar and plate sizes they carry in the tool steel catalog.

Shallow-hardening grades are the awkward case for that test because their hardenability is spent in the first few millimeters of the bar. Two special L specimens are used for them instead.

For everything that is not shallow hardening, the bar diameter that hardens through in a given medium is read off a diagram like this one.

Lamont diagram for the centre of round bars: bar diameter against the distance from the quenched end of a Jominy probe, for several quench severities H — Fig. 9.56. Lamont diagram for the centre of round bars: bar diameter against the distance from the quenched end of a Jominy probe, for a family of quench severities H. The diagram converts an end-quench curve into the diameter that will harden through at a given quench severity, which is the calculation behind matching the quench to the section: a shallow-hardening grade needs a high H and a small diameter, and the same grade at a larger diameter needs a faster medium. Read it with the H values of the common media, which run from about H 0.2 for a still oil to about H 2.8 for agitated water. Source: H.M. Tensi, A. Stich and G.E. Totten, Quenching and Quenching Technology, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 9.56.

Two L specimens used for Jominy hardenability testing of shallow-hardening steels, dimensions in millimetres — Fig. 5.24. The two L specimens used for end-quench testing of shallow-hardening steels, all dimensions in millimetres. The nose of the L sits nearer the water jet than it would on a standard bar, which stretches out the cooling rate over the critical first millimetres so the hardness drop can be resolved. This is the specimen used for the water-hardening carbon grades such as W1 and W2, where the hardened case is only a few millimetres deep. Source: B. Liščić, Hardenability of Steel, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 5.24.

Prehardened is a different purchase

P20 and the prehardened mold grades arrive at working hardness and are machined straight into the mold. The hardenability tables don’t apply unless the block is later hardened, which most mold shops avoid. Mold grades are grouped under plastic mold steels.

Keep the cycle with the steel

Hardenability sets what is possible; the furnace cycle decides what you get. The heat treatment guide lists the stages a tool steel cycle must include, and the mill certificate proves the analysis used to calculate hardenability.

Section size beats grade name

Two dies in the same grade behave differently when one weighs four times as much as the other. Where the section is the problem, the answer is usually a different grade rather than a longer soak time, and the grade selection guide works through that trade in the order a die shop faces it.

The same curve answers the section’s question: what is the largest bar that will still hold 50% martensite at the center in a given quenchant. The chart below reads that off for two grades at once.

The two ways of describing the same quench, Jominy distance and H value, are tied together by one calibration curve.

Quench severity in terms of Grossman H value against Jominy distance, from 6 to 24 sixteenths of an inch — Fig. 10.17. Quench severity (Grossman H value) versus Jominy distance, from 6 to 24 sixteenths of an inch. The curve calibrates the two descriptions of the same quench, so a shop with an end-quench curve but no quenchometer can still assign a severity number to its medium, and a shop with the severity number can predict where the end-quench curve should sit. The figure is credited in the source to R. Kern, Heat Treating, 1985. Source: M. Narazaki and G.E. Totten, Distortion of Heat-Treated Components, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 10.17.

Determining the critical diameter of round bars from the Jominy hardenability curves of two steels, with hardness and bar diameter against distance from the quenched end — Fig. 5.48. Determining the critical diameter of a round bar from its end-quench curve. The upper chart carries the Jominy hardness of a 0.45% carbon steel (Ck45) and a 0.44% C chromium-molybdenum steel (50CrMo4) against distance from the quenched end, with the 48 and 45 HRC level that corresponds to 50% martensite marked across it. The lower chart converts that distance into a bar diameter for two quenching intensities, H = 5.0 (vigorous brine) and H = 0.4 (still oil). The shallow-hardening grade loses the 50% martensite at a much smaller diameter, which is the trade against section size described above. Source: B. Liščić, Hardenability of Steel, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 5.48.

A hardenability band puts the whole argument on one chart. It gives the hardness range a grade can be expected to hold at a given distance from the quenched end, and the band width is the scatter the steelmaker is allowed for that grade.

Steel hardenability band, HRC range against distance from the cooling surface in mm — Fig. 3.21. A steel hardenability band: the HRC range (shaded) a grade meets at each distance from the cooling surface. The upper curve is the high-hardenability heat and the lower curve the low one. A heavy section reads at the right-hand end of the chart, where the band is both narrow and low, which is the section-size effect the tables above are built on. Source: A.V. Sverdlin and A.R. Ness, Fundamental Concepts in Steel Heat Treatment, in Steel Heat Treatment: Metallurgy and Technologies (G.E. Totten, ed.), CRC Press, 2006, Fig. 3.21.

Tool steel heat treatment troubleshooting chart · Steel heat color chart · Tool steel tempering chart · Tool steel annealing and normalizing chart · D2 austenitizing temperature · Tool steel forging temperature guide · H steel composition chart

Sources: Tool and Die Making Troubleshooter (R. M. Leed, Hanser Gardner), Tables 5-6, 5-8, 5-10 and 5-11, themselves from Bethlehem Steel Corporation grade data sheets (1976 to 1981) and Bry Coat hardness conversion estimates. Reference data for comparison only. Confirm the quench and the depth on your own furnace with a test piece before production. Aobo Steel supplies tool steel in the annealed condition, and the customer performs hardening.