Cr-Mo-V Hot Work Steel Thermal Constants and Transformation
Thermal conductivity, specific heat, the heat transfer coefficient of the quench surface and the elastic, yield and expansion behaviour of a Cr-Mo-V hot work die steel, printed by the source in the austenite and the bainite condition, together with the start and finish temperature of its transformation. A reference summary of published practice and not an Aobo Steel specification.
What these figures are used for
A hot work die is heated and cooled by numbers. Conductivity and specific heat set how much heat a block holds at temperature and how quickly it gives that heat up during the quench, and the heat transfer coefficient of the surface sets the other half of the same exchange on the medium side. Elastic modulus and yield stress set how far the block moves under load at temperature, and the expansion figures set how much it moves when it heats and cools.
The source describes the steel only as a Cr-Mo-V steel of 7800 kg per cubic metre, and prints every figure twice, once for austenite and once for bainite. Austenite is the structure the steel holds above the transformation range and bainite is the structure it takes when the quench is too slow to make martensite, so the two columns bracket the two ends a block passes through on its way from the furnace to the quench tank.
Thermal constants
The two structures are not printed over the same temperature range. Austenite runs from 200 to 1000 °C and bainite from 20 to 600 °C, which is the range in which the source measured each one, so the dotted cells are ranges where the steel is not in that structure rather than gaps in the measurement. Austenite conducts at 24 to 28 W per metre per kelvin and bainite at 41.9 to 44.2 W, so a block held above the transformation range gives up its heat at a little over half the rate of the same block once it has transformed.
The heat transfer coefficient falls steeply as the temperature rises and then flattens, from 5250 W per square metre per kelvin at 20 °C to 1750 at 100 °C, 1050 at 150 °C and a flat 700 from 200 °C to 1000 °C. It is a property of the surface and the quench medium rather than of the structure, which is why one column serves both states.
Thermal constants of the Cr-Mo-V steel in the austenite and the bainite condition
| Temp., °C | Conductivity, austenite, W per m per K | Conductivity, bainite, W per m per K | Specific heat, austenite, J per kg per K | Specific heat, bainite, J per kg per K | Heat transfer coefficient, W per sq m per K |
|---|---|---|---|---|---|
| 20 | n/a | 44.2 | n/a | 481 | 5250 |
| 100 | n/a | 43.5 | n/a | 480 | 1750 |
| 150 | n/a | 43 | n/a | 494 | 1050 |
| 200 | 24.4 | 42.6 | 615 | 502 | 700 |
| 400 | 24.4 | 41.9 | 624 | 527 | 700 |
| 600 | 24.4 | 41.9 | 632 | 553 | 700 |
| 800 | 26.7 | n/a | 636 | n/a | 700 |
| 1000 | 27.9 | n/a | 645 | n/a | 700 |
Source, thermal constants of the Cr-Mo-V steel, printed page 4-28 of the report (PDF page 89).
Austenite and bainite are the two conditions the source prints. It marks a cell with a dot where it has no value and those cells are marked n/a here, so a missing value is not read as a zero. Density is 7800 kg per cubic metre. A reference table only, it is not an Aobo Steel specification.
Mechanical properties
Elastic modulus is printed for the steel as a whole and falls steadily with temperature, from 205 GN per square metre at 20 °C to 85 at 1000 °C, a fall of 59 per cent across the range. Yield stress separates the two structures sharply. Bainite holds 860 MN per square metre at every temperature up to 600 °C while austenite falls from 105 at 200 °C to 20 at 1000 °C. Thermal expansion runs at 11 millionths per kelvin in bainite at every temperature and between 21 and 25 millionths per kelvin in austenite, which is the usual split between a body centred and a face centred structure and the reason the two expansion columns cannot be used interchangeably across a quench.
Mechanical properties of the Cr-Mo-V steel in the austenite and the bainite condition
| Temp., °C | Young’s modulus, GN per sq m | Yield stress, austenite, MN per sq m | Yield stress, bainite, MN per sq m | Expansion, austenite, millionths per K | Expansion, bainite, millionths per K |
|---|---|---|---|---|---|
| 20 | 205 | n/a | 860 | n/a | 11 |
| 100 | 201 | n/a | 860 | n/a | 11 |
| 150 | 198 | n/a | 860 | n/a | 11 |
| 200 | 194 | 105 | 860 | 21 | 11 |
| 400 | 173 | 95 | 860 | 21 | 11 |
| 600 | 145 | 85 | 860 | 23 | 11 |
| 800 | 115 | 60 | n/a | 25 | n/a |
| 1000 | 85 | 20 | n/a | 25 | n/a |
Source, mechanical properties of the Cr-Mo-V steel, printed page 4-28 of the report (PDF page 89).
The source prints expansion multiplied by 10 to the power minus 5 per kelvin, from 1.1 to 2.5, and the values are written out here in millionths per kelvin, so 2.1 becomes 21. Poisson’s ratio 0.3 and a strain hardening coefficient of 0.05 E belong to the same steel and are stated in the source footnote. A reference table only, it is not an Aobo Steel specification.
Phase transformation
One reaction is described, with the temperature at which it starts, the temperature at which it finishes, the heat it releases and the linear change that comes with it. The transformation releases 753 kJ per kilogram of steel and the block grows 0.38 per cent in length as it runs. The source names neither the transformation nor the structure it produces, and the row is printed here as the source gives it.
Start and finish temperature, latent heat and dilatation of the transformation
| Start temperature, °C | Finish temperature, °C | Latent heat, kJ per kg | Dilatation, per cent |
|---|---|---|---|
| 520 | 315 | 753 | 0.38 |
Source, characteristics of phase transformation of the Cr-Mo-V steel, printed page 4-28 of the report (PDF page 89).
The source prints latent heat as 7.53 multiplied by 10 to the power 5 J per kg, which is 753 kJ per kg, and gives no name for the transformation or for the structure it produces. Dilatation is a linear change in per cent. A reference table only, it is not an Aobo Steel specification.
Reading the tables
Two constants sit in the footnotes rather than in the tables. Poisson’s ratio is 0.3 and the strain hardening coefficient is 0.05 times the elastic modulus, both quoted by the source for the same steel, and density is given as 7800 kg per cubic metre, which sits between the 7.76 and the 7.89 g per cubic centimetre that the density chart on this site lists for the chromium hot work grades.
The dotted cells are worth a second look before the tables are used in a calculation. They are ranges where the steel is not in that structure, and a spread of the values on either side of one is not a trend. The austenite expansion column, for example, is flat at 21 millionths per kelvin from 200 to 400 °C and then climbs to 25 by 800 °C, while the bainite conductivity moves by less than one unit between 200 and 600 °C.
Related reference data
The heat treatment cycle that produces these structures is set out on the H13 heat treatment guide, and specific heat, conductivity, density, resistivity and elastic modulus for four named hot work grades are on the hot work tool steel physical properties page. Conductivity for seven other grades is on the thermal conductivity chart, density and mean expansion over a much longer grade list are on the density and thermal expansion chart, and the chromium hot work grades that share this Cr-Mo-V base are set out on the H13 tool steel page and the H11 tool steel page.
Reference data compiled from Improved Life of Die Casting Dies of H13 Steel by Attaining Improved Mechanical Properties and Distortion Control During Heat Treatment (J. F. Wallace and D. Schwam, US Department of Energy final report DOE/ID/13320-3, October 1998). The three tables are printed together on page 4-28 of that report, which is PDF page 89. Every figure on this page was read from the page image rather than from an automatic text layer.