SUS420J2 is a martensitic stainless steel defined by the Japanese Industrial Standard (JIS) G4303. It is part of the 420-series stainless steels, closely related to SUS420 but with a higher carbon content, which enhances its hardenability, hardness, and wear resistance. This makes it a popular choice for applications requiring a balance of moderate corrosion resistance, high strength, and durability. Below is a detailed overview:
Chemical Composition
The typical chemical composition of SUS420J2 is as follows, with stricter controls on carbon to optimize hardenability:
Element Content Range Role
Carbon (C) 0.26–0.40% Higher than SUS420; critical for achieving high hardness after heat treatment and forming wear-resistant carbides.
Chromium (Cr) 12.00–14.00% Forms a passive oxide layer for corrosion resistance and improves hardenability.
Silicon (Si) ≤1.00% Aids deoxidation during production and enhances mechanical stability.
Manganese (Mn) ≤1.00% Improves hardenability and reduces brittleness.
Phosphorus (P) ≤0.040% Minimized to prevent embrittlement and machining issues.
Sulfur (S) ≤0.030% Controlled to balance machinability and corrosion resistance.
Nickel (Ni) ≤0.60% Optional trace additions to enhance toughness (not a primary alloying element).
Key Properties
SUS420J2’s performance is shaped by its higher carbon content, distinguishing it from lower-carbon 420-series steels like SUS420:
1. Corrosion Resistance
Similar to other 420-series steels, it offers moderate corrosion resistance due to its 12–14% chromium content, which forms a protective oxide film. It performs well in mild environments (air, fresh water, and non-aggressive chemicals) but is less resistant than austenitic stainless steels (e.g., SUS304) due to lower chromium and no nickel. It is not suitable for highly corrosive environments (e.g., saltwater or strong acids).
2. Heat Treatability and Hardness
As a martensitic stainless steel, SUS420J2 is highly heat-treatable. Through quenching and tempering, it achieves hardness levels of 55–58 HRC (significantly higher than SUS420’s 50–55 HRC), making it ideal for wear-intensive applications.
3. Wear Resistance
The combination of high carbon (which forms hard chromium carbides) and post-heat-treatment hardness gives SUS420J2 excellent wear resistance, outperforming many low-carbon stainless steels and even some carbon tool steels in abrasive conditions.
4. Machinability
In the annealed state (hardness ≤235 HBW), it has fair machinability, allowing for cutting, drilling, and grinding. However, its machinability is lower than austenitic stainless steels (e.g., SUS304) due to higher carbon content and potential carbide formation.
5. Toughness
While high hardness improves wear resistance, it can reduce toughness. Proper tempering balances hardness and toughness, making it suitable for applications requiring both strength and impact resistance.
Physical Properties
Density: ~7.75–7.85 g/cm³
Melting Point: ~1450–1510°C
Thermal Expansion Coefficient: ~10.5×10⁻⁶/K (at 20–100°C)
Thermal Conductivity: ~25 W/(m·K) (room temperature)
Magnetic Properties: Magnetic (due to its martensitic crystal structure).
Mechanical Properties (After Heat Treatment)
Property Typical Value
Hardness 55–58 HRC (quenched and tempered)
Tensile Strength (Rm) ~1200–1700 MPa
Yield Strength (Rp0.2) ~1000–1500 MPa
Elongation (A) ~8–12%
Impact Toughness (Charpy V-notch) ~10–20 J/cm² (room temperature)
Heat Treatment Process
Heat treatment is critical to maximize SUS420J2’s hardness and wear resistance while managing brittleness:
1. Annealing
Purpose: Soften the steel for machining, relieve internal stress, and improve ductility.
Process: Heat to 800–850°C, hold for 2–4 hours, then furnace cool slowly (≤50°C/hour) to below 500°C before air cooling.
Result: Hardness ≤235 HBW, facilitating machining.
2. Quenching
Purpose: Harden the steel by transforming austenite to martensite.
Preheating: Heat to 600–700°C to prevent thermal shock.
Austenitizing: Heat to 1000–1050°C, hold for 30–60 minutes (based on section thickness) for uniform carbon diffusion.
Cooling: Quench in oil (for deeper hardness penetration) or air (for minimal distortion).
3. Tempering
Purpose: Reduce quenching stress, lower brittleness, and adjust hardness-toughness balance.
Process: Temper at 150–200°C for maximum hardness (55–58 HRC) or 250–300°C for improved toughness (with slight hardness reduction to 50–55 HRC). Hold for 1–2 hours per 25mm thickness, then air cool.
Application Fields
SUS420J2’s high hardness, wear resistance, and moderate corrosion resistance make it suitable for:
Cutting Tools: Knives, scissors, blades, and surgical instruments (where sharpness retention and corrosion resistance are critical).
Mechanical Components: Bearings, shafts, valves, and bushings (requiring wear resistance and strength).
Molds and Dies: Small plastic molds, stamping dies, and forming tools (due to good dimensional stability after heat treatment).
Hardware: Fasteners, springs, and precision parts in mild environments.
Comparison with SUS420
Feature SUS420 SUS420J2
Carbon Content 0.16–0.25% 0.26–0.40%
Max Hardness (HRC) 50–55 55–58
Wear Resistance Good Excellent
Toughness Slightly higher Slightly lower (but adjustable via tempering)
Corrosion Resistance Similar (moderate) Similar (moderate)
In summary, SUS420J2 is preferred when higher hardness and wear resistance are required, while SUS420 is chosen for applications needing slightly better toughness with lower hardness.