H13 Austenitizing Temperature Chart
Austenitizing temperature sets three things at once in H13. It decides how much of the carbide goes back into solution, how much the austenite grain grows, and how hard the steel is when it leaves the quench. The three tables below are measured values from one published study on H13 die steel, at austenitizing temperatures of 1024, 1066 and 1107 °C held for 25, 50 and 80 minutes. This is a reference summary of published practice and not a specification of ours.
How the three measurements were taken
All nine rows describe the same nine austenitizing conditions, so a row in one table can be read against the same row in the others. The 25 minute specimens were austenitized in a neutral salt bath and air cooled. The 50 minute and 80 minute specimens were cooled at rates that simulate the core of a 152 mm and a 305 mm round bar, because a heavy die block does not cool at the rate of a small sample. Hardness figures are the average of five tests rounded to the nearest 0.5 HRC.
The study ran three austenitizing temperatures rather than the working range alone. Normal practice for H13 sits at 1010 to 1024 °C and 1107 °C is above it, so the nine rows trace the direction each property moves as the temperature is raised and show where the structure starts to become uneven.
Grain size after austenitizing
ASTM grain size against temperature and soak time
| Temperature (°C) | Soak time (min) | Average ASTM grain size | Standard deviation | 95% confidence interval |
|---|---|---|---|---|
| 1024 | 25 | 10.1 | 0.31 | 9.9 to 10.3 |
| 1024 | 50 | 9.7 | 0.21 | 9.5 to 9.9 |
| 1024 | 80 | 9.8 | 0.47 | 9.5 to 10.1 |
| 1066 | 25 | 8.8 | 0.37 | 8.5 to 9.1 |
| 1066 | 50 | 8.8 | 0.27 | 8.6 to 9.0 |
| 1066 | 80 | 8.2 | 0.64 | 7.7 to 8.7 |
| 1107 | 25 | 7.1 | 1.24 | 6.2 to 7.9 |
| 1107 | 50 | 6.8 | 1.31 | 6.1 to 7.5 |
| 1107 | 80 | 5.9 | 2.15 | 4.4 to 7.4 |
Source, Table V, Results of the Grain Size Ratings, printed page 129 of Tool Materials for Molds and Dies, Application and Performance, edited by G. Krauss and H. Nordberg, PDF page 136 of the copy consulted. The figures were read from the page image rather than from the scanned text layer.
Specimens were austenitized in neutral salt, step-quenched to 760 °C in salt for 10 minutes and air cooled. The grain size numbers were converted from the Snyder-Graft intercept grain size and the interval is calculated with the Student t test statistic. A higher ASTM number means a finer grain.
Carbide taken into solution
Volume per cent primary carbide after austenitizing
| Temperature (°C) | Soak time (min) | Cooling | Average volume % | Standard deviation | 95% confidence interval |
|---|---|---|---|---|---|
| 1024 | 25 | Air cooled | 2.24 | 0.65 | 2.06 to 2.42 |
| 1024 | 50 | 152 mm round, simulated | 1.97 | 0.63 | 1.80 to 2.14 |
| 1024 | 80 | 305 mm round, simulated | 2.00 | 0.65 | 1.83 to 2.17 |
| 1066 | 25 | Air cooled | 1.60 | 0.29 | 1.47 to 1.73 |
| 1066 | 50 | 152 mm round, simulated | 1.42 | 0.34 | 1.27 to 1.57 |
| 1066 | 80 | 305 mm round, simulated | 1.35 | 0.44 | 1.14 to 1.55 |
| 1107 | 25 | Air cooled | 1.30 | 0.26 | 1.18 to 1.42 |
| 1107 | 50 | 152 mm round, simulated | 0.88 | 0.28 | 0.74 to 1.02 |
| 1107 | 80 | 305 mm round, simulated | 0.92 | 0.35 | 0.80 to 1.00 |
Source, Table VI, Results of the Volume % Primary Carbide Determination, printed page 130 of the same book, PDF page 137 of the copy consulted. Read from the page image rather than from the scanned text layer.
The 25 minute specimens were treated in neutral salt and the 50 minute and 80 minute specimens in an electric resistance muffle furnace under argon. The interval uses the normal probability distribution. The figure is the volume fraction of primary carbide, that is the carbide still undissolved in the as-quenched structure.
Hardness as quenched
Rockwell C hardness after quenching
| Temperature (°C) | Soak time (min) | Cooling | Hardness (HRC) |
|---|---|---|---|
| 1024 | 25 | Air cooled | 52.5 |
| 1024 | 50 | 152 mm round, simulated | 48.5 |
| 1024 | 80 | 305 mm round, simulated | 47.5 |
| 1066 | 25 | Air cooled | 53.5 |
| 1066 | 50 | 152 mm round, simulated | 49.0 |
| 1066 | 80 | 305 mm round, simulated | 48.0 |
| 1107 | 25 | Air cooled | 55.0 |
| 1107 | 50 | 152 mm round, simulated | 50.0 |
| 1107 | 80 | 305 mm round, simulated | 48.0 |
Source, Table VIII, As-Quenched Rockwell C Hardness, printed page 133 of the same book, PDF page 140 of the copy consulted. Read from the page image rather than from the scanned text layer.
Treatment conditions are those of Table VI. Each figure is the average of five tests rounded to the nearest 0.5 HRC and is the hardness straight out of the quench, before any tempering.
Reading the three tables together
Grain size falls close to a straight line as the temperature rises, with a correlation coefficient of minus 0.8624 across the 1024 to 1107 °C range. Against normal practice at 1010 to 1024 °C, austenitizing at 1066 °C lowers the average ASTM grain size number by about 13 percent, which the source does not treat as a serious loss of structure. Soak time has no statistically significant effect on grain size. The exception is 1107 °C, where the standard deviation widens to 1.24, 1.31 and 2.15 against 0.21 to 0.47 at the two lower temperatures, so the grain structure is no longer uniform from piece to piece.
Carbide solutioning is close to linear as well, with a correlation coefficient of minus 0.9450. A 50 °C rise in austenitizing temperature cut the volume fraction of primary carbide by about 33 percent in this material, and that is most of the change in wear resistance the treatment can make. Soak time and the cooling rate after austenitizing made no statistically significant difference to the carbide volume, so temperature is the lever rather than time at temperature.
Hardness straight out of the quench rises with the austenitizing temperature, at a correlation coefficient of 0.9926 for the 25 minute air cooled specimens, with 0.9806 and 0.8695 for the two simulated round bar sections. The same table shows the section effect plainly. Air cooled specimens come out between 52.5 and 55.0 HRC, while the 305 mm simulation lands between 47.5 and 48.0 HRC at the same austenitizing temperature, so a heavy block quenches slower and harder is not the result.
The three tables together describe the trade at the heart of H13 heat treatment. A higher austenitizing temperature dissolves more carbon and alloy, which lifts the as-quenched hardness and the secondary hardening response after tempering, while the undissolved carbide that carries wear resistance is reduced and the grain coarsens. A lower temperature leaves more carbide for wear and a finer grain for toughness, at a lower as-quenched hardness. In this data the temperature within the working band does most of the work and the soak time does very little.
One caution on the range covered. The nine rows come from one study on one heat of H13 and the 1107 °C rows sit above normal practice. Read them for the direction each property moves and for the size of the change, then confirm the figures for a particular die against the material test certificate and the heat treatment specification.
Related reference data
The grade these figures belong to is on H13 tool steel. Heat treatment practice is set out in the H13 heat treatment guide, and the hardness the finished die works at is on H13 steel hardness. For the grain size number itself, including what each ASTM number averages in grain diameter, see the ASTM grain size chart for tool steel. Carbide volume per grade across the tool steel families is listed on carbide volume fractions in tool and knife steels, and the same austenitizing comparison for the cold work side is on D2 austenitizing temperature.
Compiled from Tool Materials for Molds and Dies, Application and Performance (G. Krauss and H. Nordberg, eds.), the chapter on H13 die steel heat treatment and properties, Tables V, VI and VIII on printed pages 129, 130 and 133. The tables are reproduced as published and are a reference summary rather than an Aobo Steel specification, so treat them as the behaviour the trade measures rather than as a heat treatment instruction for a particular die.