420 Stainless Steel | 1.2083 | 2Cr13

4. Heat Treatment

Martensitic stainless steels, including 420 stainless steel, are hardened by heating them above their transformation temperature to achieve a fully austenitic structure, followed by rapid cooling in oil or air. It has a low thermal conductivity, which can lead to uneven heating during rapid heating or quenching, resulting in distortion, warping, or cracking. Therefore, we strongly recommend preheating.

The heat treatment involves austenitizing, quenching, and then tempering to optimize its mechanical properties, particularly ductility and toughness.

4.1 Hardening(Austenitizing)

The hardening temperature for 420 steel is 975 °C to 1075 °C (1787 °F to 1967 °F), though some sources list it as 954 °C to 1010 °C (1750 °F to 1850 °F). However, based on our experience, the former range is more appropriate.

Soak time is typically about 30 minutes per inch (1.2 minutes per millimeter) of thickness at the hardening temperature.

To prevent decarburization, 420 stainless steel should be hardened in a controlled neutral atmosphere, vacuum, or neutral-salt furnace environment.

4.2 Quenching

For complex or irregularly shaped workpieces, we suggest air quenching to minimize deformation and reduce the risk of cracking. If a higher hardness is required, we opt for oil quenching. The quenching process involves cooling the material to 66 °C (150 °F) when air-quenched, or to 66-93 °C (150-200 °F) when oil-quenched. Please note that the tempering treatment should be performed immediately after quenching.

4.3 Tempering

Tempering can improve the toughness and strength of 420 and adjust its hardness. 420 steel material must undergo at least two tempering cycles, or even three. For most applications, the minimum tempering temperature is 204°C (400°F). The soaking time is 2 hours per inch (4.7 minutes per millimeter) of thickness at the specified temperature. Between each tempering cycle, the parts should be allowed to cool naturally to room temperature.

Effect of Tempering Temperature on Properties:

1. Hardness after different tempering temperatures: Hardness of 420 stainless steel is: 52 HRC minimum at 149 °C (300 °F), 50 HRC minimum at 204 °C (400 °F), and 48 HRC minimum at 316 °C (600 °F).
2. Temper Embrittlement and Corrosion Resistance: 420 steel has the characteristic of temper embrittlement. Do NOT temper at temperatures ABOVE 427°C (800°F), as this temperature range may result in a significant decrease in impact strength and corrosion resistance. This reduction is attributed to the precipitation of coarser chromium carbides (Cr23C6), which can create chromium-depleted regions around the carbides. This negative effect disappears when the tempering temperature is 593 °C (1100 °F) or higher. At temperatures above 600 °C, chromium diffusion is thought to “heal” these depleted zones, restoring corrosion resistance. Within this temperature range, the toughness of 420 increases, but its hardness decreases.

4.4 Stress Relieving (Unhardened Material)

For unhardened 420 stainless steel parts, stress relieving can be performed by heating slowly to 677 °C (1250 °F) and soaking for 2 hours per inch of thickness, followed by a slow cool in the furnace to room temperature.

[Reference: Leed, R. M. (2007). Tool and die making troubleshooter. Society of Manufacturing Engineers.]