AISI 420 MARTENSITIC STAINLESS STEEL (UNS S42000) — COMPREHENSIVE TECHNICAL DATASHEET
AISI 420 (UNS S42000) is a hardenable, martensitic stainless steel characterized by a nominal 12% chromium content combined with an elevated carbon range. This chemical configuration makes the alloy a higher-carbon modification of the baseline martensitic grade, AISI 410. The elevated carbon concentration increases hardenability, allowing the material to reach the highest peak hardness of any 12% chromium stainless steel grade, up to 50 - 56 HRC depending on the sub-grade and heat treatment.
The alloy is widely produced in several product forms, including hot-finished or cold-finished round bars, square and hexagonal sections, flat sheets, thick plates, forgings, and wire. It is standardly specified under ASTM A276 for general-purpose bars, ASTM A582 for free-machining variants, and military/aerospace specifications such as AMS 5621, AMS 5620, and QQ-S-763.
■ Chemical Composition and Sub-Grade Variations
The performance of AISI 420 is determined by the balance between its carbon and chromium content. The stepping of carbon content across these sub-grades directly controls their mechanical behavior. Lower-carbon variants exhibit higher as-quenched toughness, whereas higher-carbon variants maximize peak hardness and edge retention at the cost of ductility and localized pitting resistance.
| Standard / Grade | UNS / EN Number | Carbon (C) | Chromium (Cr) | Manganese (Mn) | Silicon (Si) | Sulfur (S) | Others / Additives |
|---|---|---|---|---|---|---|---|
| AISI 420 | UNS S42000 | ≥ 0.15 | 12.00 - 14.00 | ≤ 1.00 | ≤ 1.00 | ≤ 0.030 | P ≤ 0.040 |
| EN X20Cr13 | EN 1.4021 | 0.16 - 0.25 | 12.00 - 14.00 | ≤ 1.50 | ≤ 1.00 | ≤ 0.030 | P ≤ 0.040 |
| EN X30Cr13 | EN 1.4028 | 0.26 - 0.35 | 12.00 - 14.00 | ≤ 1.50 | ≤ 1.00 | ≤ 0.030 | P ≤ 0.040 |
| EN X39Cr13 | EN 1.4031 | 0.36 - 0.42 | 12.50 - 14.50 | ≤ 1.00 | ≤ 1.00 | ≤ 0.030 | P ≤ 0.040 |
| EN X46Cr13 | EN 1.4034 | 0.43 - 0.50 | 12.50 - 14.50 | ≤ 1.00 | ≤ 1.00 | ≤ 0.030 | P ≤ 0.040 |
| SUS 420J1 | SUS 420J1 | 0.16 - 0.25 | 12.00 - 14.00 | ≤ 1.00 | ≤ 1.00 | ≤ 0.030 | Ni ≤ 0.60 |
| SUS 420J2 | SUS 420J2 | 0.26 - 0.40 | 12.00 - 14.00 | ≤ 1.00 | ≤ 1.00 | ≤ 0.030 | Ni ≤ 0.60 |
| AISI 420F | UNS S42020 | ≥ 0.15 | 12.00 - 14.00 | ≤ 1.25 | ≤ 1.00 | ≥ 0.150 | Mo ≤ 0.60 |
| 420 ESR Mold | Specialty | 0.35 - 0.45 | 13.00 - 14.00 | 0.40 - 0.65 | 0.70 - 0.95 | ≤ 0.003 | V: 0.25 - 0.35 |
| 420 MQ Mold | Specialty | 0.38 | 13.60 | 0.45 | 0.40 | — | V: 0.30 |
| 420 MOD Tube | 13Cr Well | 0.18 - 0.22 | 12.50 - 14.00 | 0.40 - 1.00 | ≤ 0.80 | ≤ 0.005 | V ≤ 1.00, Mo ≤ 0.50 |
■ Proprietary Datasheet Download
For application detailers, toolmakers, and procurement officers requiring high-precision dimensional tolerances, dynamic tool-wear coefficients, and complete continuous cooling transformation (CCT) curves, the proprietary guide must be accessed.
AISI 420 / 420S / 420H — Complete Mechanical Hardening & Tooling Datasheet
Contains empirical data for stress simulations, laser powder bed fusion (LPBF) parameters, and advanced passivation guidelines. Engineering credentials required.
⬇ DOWNLOAD DATASHEET■ Physical, Thermal, and Magnetic Parameters
The physical properties are determined by its body-centered cubic (BCC) structure in the annealed state and its strained body-centered tetragonal (BCT) lattice upon quenching. The lower thermal expansion coefficient minimizes thermal distortion relative to austenitic grades.
| Physical Constants | Value in Metric Units | Value in Imperial / US Units |
|---|---|---|
| Density | 7.73 - 7.80 g/cm³ | 0.278 - 0.282 lb/in³ |
| Melting Range Boundaries | 1450 - 1510 °C | 2650 - 2750 °F |
| Modulus of Elasticity (20°C) | 200 - 215 GPa | 29.0 - 31.2 × 10⁶ psi |
| Poisson's Ratio / Shear Modulus | 0.28 / 83 GPa | 0.28 / 12.0 × 10⁶ psi |
| Electrical Resistivity (20°C) | 55.0 μΩ·cm | 25.6 μΩ·in (at 68°F) |
| Specific Heat Capacity (0-100°C) | 460 J/kg·K | 0.110 Btu/lb·°F |
| Thermal Conductivity (100°C / 500°C) | 24.9 W/m·K / 28.7 W/m·K | 14.4 Btu/hr·ft·°F / 16.6 Btu/hr·ft·°F |
| Mean Coeff. of Thermal Expansion (0-100°C) | 10.3 × 10⁻⁶ K⁻¹ | 5.70 - 5.83 μin/in·°F |
| Mean Coeff. of Thermal Expansion (0-500°C) | 12.2 × 10⁻⁶ K⁻¹ | 6.80 × 10⁻⁶ in/in·°F |
Magnetic State Transformations: Fully annealed AISI 420 is highly ferromagnetic, displaying a high saturation magnetization ($180.2\text{ A}^2\text{m/kg}$) and a low coercive force ($3.87\text{ Oe}$). Upon hardening, high dislocation density and lattice distortion pin domain wall movement, causing the saturation induction to decrease to $105.0\text{ A}^2\text{m/kg}$ and the coercive force to increase to $55.5\text{ Oe}$.
■ Mechanical Properties and Heat-Treated Tempers
Mechanical properties vary significantly based on the chosen tempering profile. Hardening requires thorough preheating to bypass localized thermal gradients before passing the critical transformation limits ($Ac_1 = 802^\circ\text{C}$ and $Ac_3 = 863^\circ\text{C}$).
| Thermal Conditioning / Temper State | Tensile Strength Rm | 0.2% Yield Strength Rp0.2 | Elongation in 50 mm | Hardness Boundaries | Charpy V-Notch Impact |
|---|---|---|---|---|---|
| Annealed (Condition A) | 655 MPa (95.0 ksi) | 345 MPa (50.0 ksi) | 25% | ≤ 241 HBW / ≤ 96 HRB | — |
| Tempered at 204°C (400°F) | 1600 MPa (232.0 ksi) | 1360 MPa (197.3 ksi) | 12% | 444 HBW / 48 - 52 HRC | 20 J |
| Tempered at 316°C (600°F) | 1580 MPa (229.2 ksi) | 1365 MPa (198.0 ksi) | 14% | 444 HBW / 45 - 48 HRC | 19 J |
| Tempered at 427°C (800°F) | 1620 MPa (235.0 ksi) | 1420 MPa (206.0 ksi) | 10% | 461 HBW / 46 HRC | EMBRITTLED |
| Tempered at 538°C (1000°F) | 1305 MPa (189.3 ksi) | 1095 MPa (158.8 ksi) | 15% | 375 HBW / 36 - 38 HRC | SENSITIZED |
| Tempered at 593°C (1110°F) | 1035 MPa (150.1 ksi) | 810 MPa (117.5 ksi) | 18% | 302 HBW / 23 - 30 HRC | 22 J |
| Tempered at 650°C (1200°F) | 895 MPa (129.8 ksi) | 680 MPa (98.6 ksi) | 20% | 262 HBW / ≤ 23 HRC | 42 J |
| EN 1.4021 +QT700 Condition | 700 - 850 MPa | ≥ 500 MPa | 13% min | ≈ 220 HBW / ≈ 20 HRC | 25 J min |
| EN 1.4021 +QT800 Condition | 800 - 950 MPa | ≥ 600 MPa | 12% min | ≈ 255 HBW / ≈ 25 HRC | 20 J min |
High-Temperature Stiffness & Yield Decay: Thermal activation decreases atomic bonding forces, reducing structural load limits. For the QT700 state, yield strength falls from a baseline down to ≥ 395 MPa at 300°C and ≥ 330 MPa at 400°C. Service operations exceeding 400°C trigger severe over-tempering and rapid microstructural softening.
■ Additive Manufacturing Component Anisotropy
Laser powder bed fusion (LPBF/SLM) utilizes extreme solidification rates ($10^5 - 10^6\text{ K/s}$) to forge dense, non-equilibrium structures. However, layer-by-layer laser tracks generate distinct mechanical anisotropy based on build orientation parameters.
| LPBF Printing Parameter (XM200C System) | Reference Value | Orientation Specimen (As-Printed) | Tensile Strength Rm | 0.2% Yield Strength | Elongation at Break |
|---|---|---|---|---|---|
| Layer Thickness Sizing | 30 μm | Horizontal (XY-Axis) | 820 MPa (118.8 ksi) | 447 MPa (69.1 ksi) | 14.1% |
| Laser Power Input | 100 W | Vertical (Z-Axis) | 667 MPa (96.7 ksi) | 440 MPa (63.8 ksi) | 6.33% |
| Theoretical Density Achieved | 99.9% (7.74 g/cc) | Z-axis loading induces premature interlayer separation due to heat pass self-tempering. | |||
■ Machining Feed and Speed Guidelines (Annealed State)
In the soft annealed condition, AISI 420 maintains a machinability rating of 36% - 56% relative to the free-machining carbon steel standard (B1112). Spindle speed calculation follows $N = (1000 \times V_c) / (\pi \times D)$.
| Machining Operation | Tooling Material | Cutting Speed, Vc (m/min) | Feed Rate, fz (mm/rev) | Tooling Setup Recommendations |
|---|---|---|---|---|
| Turning (Rough) | Carbide Tooling | 120 - 180 | 0.30 - 0.60 | P20-P35 Grade, CVD Coated, thin TiAlN matrix |
| Turning (Fine) | Carbide Tooling | 180 - 240 | 0.05 - 0.30 | P10-P15 Grade, PVD Coated, sharp edge geometries |
| Turning (Fine) | HSS Tooling | 25 - 30 | 0.05 - 0.20 | Rigid tool block, active sulfurized cutting oil |
| General Milling | Carbide Tooling | 105 - 140 | 0.03 - 0.08 | Coated end mills (AlTiN), mandatory climb milling |
| Drilling | Carbide Tooling | 160 - 200 | 0.08 - 0.12 | Short overhang tool length, high-pressure coolant |
| Reaming | HSS Tooling | 10 - 15 | 0.15 - 0.30 | Spiral flute reamer design, sulfurized mineral oil dabs |
■ Processing Controls and Engineering Restrictions
- The Tempering Danger Window: Thermal treatment within the 425°C to 600°C (800°F to 1100°F) envelope is strictly proscribed. This region induces severe temper embrittlement, cutting impact energies to a fraction of standard targets. Simultaneously, chromium carbides ($Cr_{23}C_6$) precipitate continuously along grain boundaries, lowering local chromium levels below the 12% passivation floor (sensitization) and causing severe intergranular corrosion and rapid pitting.
- Passivation Bath Guidelines: Standard nitric descaling methods will trigger a catastrophic "flash attack"—chemically blackening and pitting the matrix surrounding raw inclusions. Post-machining passivation must utilize specialized chemical retardants: either ASTM A967 Nitric 1 parameters (20-25 v% $HNO_3$ + 2.5 w% Sodium Dichromate inhibitor) at 50°C, or a 4-10 w% Citric Acid chelation fluid. For complex tooling parts, enforce the full Alkali-Acid-Alkali (A-A-A) sequence to neutralize trapped residual acid spots.
- Structural Welding Restrictions: Welding generates brittle untempered martensite in the heat-affected zone (HAZ), making joints exceptionally susceptible to cold cracking. Welding is prohibited for structural or high-pressure pressure-retaining configurations. If welding is mandatory, enforce a 150°C - 260°C preheat, utilize matching E/ER 420 or ductile E/ER 309L electrodes, and follow immediately with a subcritical anneal at 732°C - 788°C for 6 to 8 hours.
- Marine Exposure Exclusion: Possessing a basic pitting resistance equivalent number ($PREN = \%Cr \approx 12.0 - 14.5$), AISI 420 has low overall halide passivity. Do not specify this material for marine flow lines, seawater joints, or environments containing high chloride concentrations. Surface finishes must be mirror-polished to remove crevice initiation lands.
- Service Hardening Mandate: Never specify AISI 420 for continuous operational boundaries in the soft annealed state. Annealed microstructures contain coarse, isolated chromium carbides that deplete the surrounding matrix of free chromium, severely lowering corrosion protection. This material must always be deployed in the fully hardened and tempered condition.
