420 Stainless Steel: Properties and Applications

420 stainless steel is a versatile material known for its hardenability and corrosion resistance. Understanding its characteristics is key to determining if it’s the right choice for your application. At Aobo Steel, with our extensive experience in tool steels, we can help you evaluate its suitability.

1. Composition of 420 Steel

420 steel belongs to the martensitic stainless steel family. This means its microstructure can be transformed through heat treatment to achieve high hardness and strength. Like other martensitic grades, 420 steel is magnetic.

2. Key Properties of 420 Stainless Steel

• Hardness and Wear Resistance. One of the main advantages of 420 steel is its ability to achieve high hardness after proper heat treatment, typically reaching 46–52 HRC. This hardness provides good resistance to abrasion and wear.
• Corrosion Resistance. 420 stainless steel offers very good corrosion resistance in mild atmospheric, domestic, and industrial environments. Its chromium content forms a passive layer protecting the surface.
• Polishability. This grade exhibits excellent polishability, making it a preferred choice for applications requiring a high-quality surface finish, such as plastic injection molds and optical components.
• Toughness and Weldability Considerations. While the carbon content enhances hardness, it consequently reduces toughness compared to lower-carbon grades. It also makes welding 420 steel challenging. If welding is necessary, significant preheating and post-weld heat treatments (like annealing) are often required to prevent cracking. Using specialized filler metals (e.g., ERNiCr-3) might be considered, though this can affect the final strength and hardness of the weld area.
• Machinability. In its annealed condition, 420 steel offers fair machinability. For applications demanding more complex machining, a free-machining variant, Type 420F (with added sulfur), is available. Note that the sulfur addition can slightly decrease notch toughness.

3. Heat Treatment

Understanding the heat treatment of 420 stainless steel is critical to achieving the specific mechanical properties required for your applications. At Aobo Steel, with over two decades specializing in tool and alloy steels, we know that precise control during heat treatment unlocks the full potential of this material. This guide outlines the key processes.

3.1 Annealing 420 Steel

If your manufacturing process requires softening 420 steel for improved machinability or formability, annealing is the necessary first step.

Annealing Procedure

1. Heating: Slowly heat the steel to 1,450°F (788°C).
2. Soaking: Ensure the steel soaks thoroughly at this temperature for a uniform internal structure.
3. Cooling: Slow cooling is crucial for softness. A recommended rate is 25°F (14°C) per hour down to 900°F (482°C). After this, the steel can be air-cooled to room temperature.
4. Result: This process typically yields a maximum hardness of 221 HB.

Important Note: To prevent decarburization (loss of carbon from the surface), perform annealing in a controlled neutral atmosphere, vacuum, or neutral-salt furnace.

3.2 Hardening 420 Steel

A hardening heat treatment is essential to achieve the maximum strength and wear resistance inherent in 420 steel. This involves austenitizing followed by quenching.

Austenitizing

1. Heating: Heat the steel to within the range of 1,750–1,850°F (954–1,010°C).
2. Soaking: Soak thoroughly at temperature to ensure complete transformation to austenite. A general guideline of thickness is 30 minutes per inch (1.2 minutes per mm). Use the higher end of the temperature range for maximum hardness and corrosion resistance.
3. Atmosphere: To minimize the risk of decarburization at these high temperatures, use a controlled neutral atmosphere, vacuum, or neutral salt furnace.

Quenching

1. Purpose: Rapid cooling transforms the austenite into hard martensite.
2. Methods:

1. Air Quenching: Preferred for complex shapes to minimize distortion and cracking risk. Cool in air down to about 150°F (66°C).
2. Oil Quenching: Can be used for higher hardness if the part geometry allows. Quench in oil to 150–200°F (66–93°C).
   
   3. Crucial Next Step: Temper the steel immediately after quenching to relieve internal stresses in the martensite.

3.3 Tempering 420 Steel

Tempering is a mandatory step after hardening. It reduces brittleness, relieves stress from quenching, and allows you to fine-tune the balance between hardness and toughness.

Tempering Procedure

1. Cycles: Typically requires a double temper; demanding applications might need a triple temper.
2. Process: For each cycle, heat to the desired tempering temperature, soak, and then air cool to room temperature.
3. Soaking Time: A standard soak time of thickness for each temper is 2 hours per inch (4.7 minutes per mm). Ensure air cooling to room temperature between cycles.

Selecting Tempering Temperature

• General Use: A minimum temperature of 400°F (204°C) is recommended.
• Critical Avoidance Zone: Do not temper between 800°F (427°C) and 1,100°F (593°C). Tempering in this range causes temper embrittlement, significantly reducing impact strength and corrosion resistance.
• Higher Toughness: Tempering at 1,100°F (593°C) or higher restores impact strength and corrosion resistance but reduces hardness.
• Higher Hardness (Above 50 HRC): Lower temperatures (around 482°F / 250°C) or higher temperatures (around 932°F / 500°C) can be used.
• Corrosion Resistance Consideration: Tempering at higher temperatures (~932°F / 500°C) can precipitate chromium carbides, potentially reducing corrosion resistance. Lower tempering temperatures are often preferred for 420 steel for the best combination of high hardness and corrosion resistance.

3.4 Stress Relieving 420 Steel

A stress relief cycle can minimize distortion during the later hardening process if 420 steel parts undergo significant forming, straightening, or machining before hardening.

Stress Relieving Procedure (Unhardened Material)

1. Heating: lowly heat to around 1,250°F (677°C).
2. Soaking: Soak for approximately 2 hours per inch (4.7 minutes per mm) of thickness.
3. Cooling: Slow cool (furnace cool if possible) to room temperature.

Note: If stress-relieving hardened material, the stress relief temperature must not exceed the final tempering temperature used.

4. Common Applications for 420 Steel

Thanks to its balance of properties, 420 steel is used in various industries:

• Cutlery: Knives, pocket knives, surgical blades.
• Tools: Hand tools, dental and surgical instruments.
• Molds: Plastic injection molds, especially for corrosive resins like PVC, or where high polish is required. It can also be used to repair certain mold tool steels.
• Valves & Components: Valve parts, nozzles, shafts, and gears requiring moderate corrosion resistance and high strength.

5. Equivalent Grades and Availability

You may encounter 420 steel or similar grades under different international standards:

• JIS (Japan):SUS420J1, SUS420J2
• DIN (Germany):4021, 1.4028, 1.2083 (X42Cr13 – often used for molds)

Aobo Steel can supply 420 steel in various forms, including bars, plates, and sheets, to meet your specific requirements.