Mechanical Properties of Mold Steels P4, P20 and P21

The condition behind each set of figures

Mold steels are not sold with one set of mechanical properties, because the figures move with the heat treatment. P4 is a low carbon grade that is carburized to give the cavity surface its wear resistance, and the core figures below are for the core alone, air cooled from 955 °C and tempered at 425 °C. P20 is normally bought pre-hardened and machined into the cavity at about 300 HB, and the figures below are for the grade taken through a full hardening and tempering cycle instead, oil quenched from 845 °C and tempered 2 h at 205 °C. P21 is a different route again. It is solution treated, machined while it is soft, and then aged to its working hardness, so it has two columns of figures rather than one.

All the figures are from one published source, listed under each table with the printed page it comes from. Tensile and yield strength are given in megapascals with the ksi figure in brackets as the source prints it, and the two hardness scales are kept as published, so P4 is reported on the Brinell scale and P20 and P21 on the Rockwell C scale with a Brinell figure alongside.

Mechanical properties of P4 mold steel

Core properties after air cooling from 955 °C and tempering at 425 °C

Source, Table 15-3, Core mechanical properties of P4 mold steel air cooled from 955 °C (1750 °F) and tempered at 425 °C (800 °F) of Tool Steels (G. A. Roberts and G. Krauss).

P4 is the deepest hardening of the carburizing mold steels, which is why it can be air cooled rather than quenched and still reach core strength in the same band as the quenched grades. The core figure of 1034 MPa yield against 1289 MPa tensile leaves a wide gap between yield and ultimate, and the reduction of area of 53 per cent with an elongation of 15 per cent describes a core that will take shock without cracking. That combination is the reason the grade is chosen for large cavities and for hubbing. The CVN impact figure of 43 J is measured on a standard V notch specimen, and the 390 HB core is the hardness the cavity walls carry behind the carburized case.

Mechanical properties of P20 mold steel

Figures after oil quenching from 845 °C and tempering 2 h at 205 °C

Source, Table 15-5, Mechanical properties of P20 mold steel after oil quenching from 845 °C (1550 °F) and tempering 2 h at 205 °C (400 °F) of Tool Steels (G. A. Roberts and G. Krauss).

P20 is the grade most machined cavities are cut from, and the figures show why a low tempering temperature is used when the grade is hardened rather than bought pre-hardened. Tempering at 205 °C leaves 36 to 38 HRC, which is above the 300 HB pre-hardened condition but still low enough to machine and to hold a cavity shape. Yield strength at 1172 MPa sits close to the tensile figure of 1310 MPa, and the elongation of 13 per cent with a reduction of area of 51 per cent is the ductility that keeps a mold from cracking when it is pulled off a part. Raising the tempering temperature trades this hardness away, and the tempering response of the grade is charted on the P20 tool steel heat treatment page linked below.

Mechanical properties of P21 mold steel

The solution treated and the aged condition

Source, Table 15-6, Mechanical properties of P21 mold steel of Tool Steels (G. A. Roberts and G. Krauss). The solution treated column is the steel quenched from the solution temperature and machined in that condition. The aged column is the same steel after holding 20 to 24 h at the aging temperature.

P21 hardens by precipitation of a nickel aluminium compound, so it is machined in the solution treated condition at 26 HRC maximum and then aged at a lower temperature to reach its working hardness. The aged column carries 1234 MPa tensile against 862 MPa in the solution treated condition, and yield strength climbs from 586 MPa to 1138 MPa. Ductility falls in step, with elongation dropping from 24 per cent to 16 per cent and reduction of area from 59 per cent to 40 per cent. The hardness range of 36 to 39 HRC after aging is narrower than the range on the quenched and tempered grades, which is the usual reason a mold shop accepts the extra furnace step.

Hardness left by each aging temperature

Source, Table 15-7, Effect of aging temperature on hardness of P21 mold steel of Tool Steels (G. A. Roberts and G. Krauss). Aging time was 20 to 24 h at each temperature.

The aging temperature sets the hardness, and the table runs from 510 °C down to 565 °C with the hardness falling the whole way. Aging at 510 °C gives 38 to 41 HRC, and moving the furnace up to 565 °C drops the range to 27 to 32 HRC. The step between one temperature and the next is 15 °C, and each step costs the steel a little of its hardness, so the aging temperature is the one control that sets the working hardness of the mold.

Reading the three grades together

The three sets of figures answer the same question at different points in the process. P4 gives its core strength only after carburizing and heat treatment, and the case that sits over that core is what carries the wear. P20 reaches a comparable tensile figure in a single hardening and tempering cycle, which is why it is used where the whole block has to be strong rather than just the surface. P21 reaches the highest tensile figure of the three, but only after two furnace steps, one to solution treat the blank and one to age it after machining.

Two cautions apply to all three columns. The first is that the figures belong to the exact heat treatment named in the caption, so a mold run at a higher tempering temperature or aged at a different temperature will sit somewhere else on these scales. The second is that the values are from one published source on one heat of each grade, so confirm the working figures against the test certificate for the heat you buy. A carbide pin or a spacer block that is not loaded in the same way as the cavity may be run well outside these figures.

Related reference data

P20 is supplied pre-hardened at about 300 HB and is listed with its forms and section range on P20 tool steel, while the hardening, tempering and carburizing cycle for the grade is on P20 tool steel heat treatment. The corrosion resistant mold steels worked in the same chapter correspond to an AISI 420 martensitic stainless steel and are listed on 420 stainless steel. The choice between a mold steel and a hot work steel for a mold that sees higher temperature is set out on H13 and P20 steel, and the properties of the wider tool steel families are on tool steel properties and plastic mold steel selection.

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