Przewodnik po obróbce cieplnej stali nierdzewnej 420

What is 420 stal nierdzewna? 420 stainless steel is a prominent martensitic stainless steel, well-regarded for its hardenability and often classified as a stainless tool steel. It exhibits ferromagnetic properties. The heat treatment process for 420 stainless steel typically follows steps similar to those for carbon steel: austenitization, quenching, and tempering.

Austenityzowanie

The primary goal of austenitizing is to heat the steel to a temperature where its entire crystalline structure transforms into a homogeneous austenitic phase, allowing carbon and alloying elements to dissolve uniformly. The austenitizing temperature for 420 stainless steel is 954°C to 1010°C (1750°F to 1850°F). The soaking time is 30 minutes per inch (or approximately 1.2 minutes per millimeter) of thickness.

Gaszenie

Gaszenie involves rapidly cooling the austenitized steel to induce the transformation of austenit into martensite, a supersaturated body-centered tetragonal (BCT) phase that imparts high hardness to the steel. 420 stainless steel has high hardenability and can be effectively quenched using oil or air.

For parts with complex or irregular geometries, air quenching is often the preferred method of quenching. This method helps to minimize distortion and reduces the risk of cracking. If the component’s design allows and maximum hardness is desired, oil quenching can be employed. Additionally, oil quenching generally promotes optimal ductility and corrosion resistance.

For air quenching, we recommend cooling 420 stainless steel to 66°C (150°F). For oil quenching, the typical temperature range is 66-93°C (150-200°F). After cooling to the above temperature, tempering should be performed immediately. Delaying tempering can lead to increased internal stresses and potential cracking.

Odpuszczanie

Odpuszczanie is the process of reheating quenched steel to a temperature below its critical transformation temperature (Ac1) to reduce the inherent brittleness and high hardness of martensite, thereby improving toughness and impact resistance, and eliminating internal stress. A martensitic structure that has undergone proper tempering has a balance between tensile strength and impact strength.

420 stainless steel must undergo double tempering, or even triple tempering. 420 stainless steel must undergo double tempering, or even triple tempering.

The specific tempering temperature can be selected based on the target hardness level: for example, 150°C (300°F) can achieve a minimum of 52 HRC, 204°C (400°F) can achieve 50 HRC, and 315°C (600°F) can achieve 48 HRC.

Each tempering cycle requires a specific soak time, typically 2 hours per inch (or approximately 4.7 minutes per millimeter) of material thickness. We recommend air cooling the parts to room temperature between each tempering cycle.

420 stainless steel is prone to temper brittleness, so avoid tempering between 427°C and 566°C (800°F and 1050°F).

Tempering at temperatures between 400°C and 600°C may result in a decrease in corrosion resistance. At these tempering temperatures, coarser chromium carbides (Cr23C6) precipitate in the steel, and chromium-depleted regions form around these carbides, leading to a decrease in corrosion resistance. This phenomenon is called sensitization¹.

The above is based on our 20 years of experience in producing tool steel and our expertise in heat treating 420 stainless steel. Some of the information is also sourced from academic papers. If you have any questions regarding 420 stainless steel, please don’t hesitate to contact us.

1. Pramanik, A., & Basak, A. K. (Eds.). (2016). Stainless Steel: Microstructure, Mechanical Properties and Methods of Application (p. 125). Apple Academic Press. ↩︎