p20 tool steel | 1.2311

4. Heat Treatment

P20 steel is commonly supplied in a pre-hardened state, with a hardness of 28-40 HRC. It can be directly machined into complex mold cavities and put into use without further high-temperature heat treating. The pre-hardened state helps to avoid deformation and dimensional changes that may occur during subsequent heat treatment.

Although P20 material is always used directly in a pre-hardened state, its surface properties can be enhanced through further heat treatment (particularly carburizing) to meet specific application requirements, such as plastic molding.

4.1 Carburization

For plastic molding applications, P20 material is frequently carburized and then hardened after the mold impression has been machined. The as-quenched surface hardness of carburized P20 molds can reach up to 65 HRC, with the core hardness typically around 45 HRC. Case depths can be controlled, generally varying from 0.030 inches for smaller molds to 0.065 inches for large molds. We would like to remind you that the resistance of carburized P20 material to softening during tempering is relatively low, similar to that of plain carbon steel.

The carburizing temperature should NOT exceed 870°C (1600°F). Exceeding this temperature will adversely affect its polishing performance. For instance, carburizing at 845°C (1550°F) resulted in a surface hardness of 65.6 HRC and a case depth of 0.36 mm, whereas at 900°C (1650°F), the surface hardness was slightly lower at 62.5 HRC with a case depth of 0.41 mm.

Carburized P20 steel has a good combination of wear resistance, toughness, and polishability.

4.2 Preheating

We strongly recommend preheating P20 tool steel to a temperature of 1200°F (650°C).

The purpose of preheating is to precondition the molecules within the part for transformation and to relieve stresses caused by machining. It also helps to equalize heat within the part, allowing stresses to dissipate before the metal becomes too soft at higher temperatures.

4.3 Hardening (Austenitizing)

The hardening temperature is 1525°F (830°C). The soaking time is 45–60 minutes per inch of thickness. For parts under 1 inch in thickness, we recommend a minimum soaking time of 45–60 minutes. [For information about austenite, please refer to the explanation of austenite on Wikipedia.]

4.4 Quenching

Material P20 steel is oil quenched. Oil quenching from a relatively low temperature results in less dimensional change compared to water-hardening steels. After quenching, parts should be cooled to a minimum of 150°F (66°C) if air-hardened, or 150-200°F (66-93°C) if liquid-quenched, and then tempered immediately.

4.5 Tempering 

P20 tool steel usually requires double tempering. The standard tempering temperature is 400°F (205°C).

For a P20 steel that has been oil quenched from 845°C (1550°F) and tempered for 2 hours at 205°C (400°F), the resulting hardness is 36-38 HRC, with a tensile strength of 1310 MPa, a yield strength of 1172 MPa, 13% elongation, and 51% reduction of area. If the as-quenched microstructure is bainitic, the hardness after tempering may be 4 HRC lower than standard tempering diagrams.

[Reference: Roberts, G., Krauss, G., & Kennedy, R. (1998). Tool Steels: 5th Edition. ASM International.]