Hot Work Tool Steel Tensile Strength at Elevated Temperature

Hot work tool steels are chosen for the strength they keep while they are hot, and the figures that answer that question are tensile and yield strength measured at the working temperature rather than at room temperature. Three chromium hot work grades are collected here. H11 and H13 are the two steels most aluminum die casting and hot forging tooling is made from, and H12 is the more highly alloyed grade used where the tool has to survive heavier shock. Every figure belongs to the exact heat treatment and the exact testing temperature named beside it, so read the table as the measured behaviour of that one condition. This is a reference summary of published practice and not a specification of ours.

What the figures measure

Tensile strength is measured on a specimen pulled at the temperature in the first column, so the number describes the steel while it is hot and not after it has cooled. Above about 400 °C every chromium hot work steel loses strength steadily, and the rate at which it loses it is what separates one grade from another. Elongation and reduction of area are measured on the same specimen and rise as the strength falls, which is why the toughest condition of a hot work steel is never its strongest one.

The three grades were tested after different heat treatments and each table is preceded by the treatment it belongs to. Two of the H12 conditions were tested after oil quenching from 1050 °C and a third after air cooling from 980 °C, so the H12 figures are not one set. The source does not report a yield strength for every condition, and does not report the hardness after testing for every condition; those cells are marked Not reported rather than carried across from another condition.

Tensile properties of H11 tool steel at temperature

Air cooled from 1010 °C and double tempered 2.5 h at 570 °C

Source, Table 13-4, Tensile properties of chromium-molybdenum hot-work tool steels at elevated temperatures, H11 of Tool Steels (G. A. Roberts and G. Krauss). Composition 0.40% C, 0.92% Si, 5.09% Cr, 1.34% Mo, 0.52% V; austenitized at 1010 °C (1850 °F) and air cooled, then double tempered 2.5 h at 570 °C (1060 °F). The room temperature row is printed as Room in the source and carries no separate degree figure.

H11 holds 1482 MPa yield strength at room temperature and still holds 1289 MPa at 425 °C, so the tool has lost only about a tenth of its yield strength by the temperature at which aluminum die casting runs. Past 480 °C the fall steepens, and above 540 °C it is fast, with yield strength down to 965 MPa at 540 °C and to 434 MPa at 650 °C. Elongation and reduction of area climb as the strength drops, from 10.0 per cent elongation at room temperature to 18.9 per cent at 650 °C, which is the ductility that lets a hot tool bend rather than crack when it is overloaded. The hardness after testing shows how far the specimen softened while it was being pulled, holding 50 HRC up to 540 °C but falling to 47 HRC at 595 °C and 41 HRC at 650 °C.

Tensile properties of H13 tool steel at temperature

Oil quenched from 1010 °C and tempered 2 h at 565 °C

Source, Table 13-4, H13 of Tool Steels (G. A. Roberts and G. Krauss). Composition 0.40% C, 1.00% Si, 5.25% Cr, 1.15% Mo, 1.00% V; austenitized at 1010 °C (1850 °F) and oil quenched, then tempered 2 h at 565 °C (1050 °F). The source reports no yield strength and no hardness after testing for this condition.

The H13 condition reported here was tested only from 480 °C upwards, which is the range that matters inside a die. Tensile strength falls from 1531 MPa at 480 °C to 814 MPa at 650 °C, and the elongation measured on the same specimens rises from 9 to 22 per cent over that span. H13 carries 52 HRC before testing at every temperature in the table, so the loss of strength with temperature happens without a matching loss in the as-tempered hardness. The source does not report yield strength or the hardness after testing for this condition.

Tensile properties of H12 tool steel at temperature

Oil quenched from 1050 °C and tempered 2 h at 565 °C

Source, Table 13-4, H12 oil quenched from 1050 °C of Tool Steels (G. A. Roberts and G. Krauss). Composition 0.33% C, 0.85% Si, 5.00% Cr, 1.25% W, 1.45% Mo, 0.23% V; austenitized at 1050 °C (1925 °F) and oil quenched, then tempered 2 h at 565 °C (1050 °F). The source reports no yield strength and no hardness after testing for this condition.

Air cooled from 980 °C and tempered 1.5 h at 565 °C

Source, Table 13-4, H12 air cooled from 980 °C of Tool Steels (G. A. Roberts and G. Krauss). The steel was austenitized at 980 °C (1800 °F), air cooled, then tempered 1.5 h at 565 °C (1050 °F). The source reports no hardness after testing for this condition.

H12 is the tungsten-bearing member of the chromium hot work group, and the two conditions above show how much the starting heat treatment matters. After oil quenching from 1050 °C the steel holds 1703 MPa at 400 °C and 1482 MPa at 540 °C, then falls to 531 MPa at 650 °C and 124 MPa at 760 °C, with elongation rising from 12 to 71 per cent over the same span of temperature. The air cooled condition is softer to begin with at 1358 MPa at 540 °C, and it is the only H12 condition for which the source reports a yield strength, 1086.7 MPa at 540 °C and 396.5 MPa at 620 °C.

Reading the four sets of figures together

The four sets make the same point at different temperatures. Strength falls with temperature in every case and ductility rises to meet it, so a hot work tool that is run above its working temperature does not become brittle, it becomes soft and distorts. H12 oil quenched from 1050 °C starts stronger than H11 or H13 at the same testing temperature, and H12 keeps 53 HRC and H13 keeps 52 HRC before testing where H11 carries 50 HRC. Selection between the three grades follows the working temperature of the tool first and the shock loading it sees second.

Two cautions apply to all four sets. The first is that the figures belong to the exact heat treatment named in the caption above the table, so a die austenitized or tempered differently will sit somewhere else on these scales. The second is that the values are from one published source on one heat of each condition, so confirm the working figures against the test certificate of the heat you buy.

Related reference data

The two grades most used in hot work tooling are listed with their forms, section ranges and delivery conditions on H11 tool steel and H13 tool steel. Their hardness ranges and performance at working temperature are set out on H11 tool steel properties and H13 steel properties, and the hardness each grade keeps at 315 to 650 °C is charted on tool steel hot hardness. Physical properties such as thermal conductivity and expansion are on hot work tool steel physical properties, the heat treatment of H12 is on H12 tool steel heat treatment, and the choice of grade for a specific die is worked through on hot extrusion die steel selection.

Source: Tool Steels (G. A. Roberts and G. Krauss).