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AISI 420F (UNS S42020) is the free-machining variant of grade 420 martensitic stainless steel, developed specifically for high-volume, precision machining applications where cut quality, dimensional consistency, and cycle time matter as much as the final part properties. The defining modification from standard 420 is a controlled sulfur addition (minimum 0.15%), which promotes the formation of manganese sulfide inclusions that act as internal chip-breakers — dramatically improving machinability while retaining the core characteristic of the 420 family: the ability to reach high hardness through heat treatment.
In the hardened and stress-relieved condition, 420F achieves 48–55 HRC, making it viable for precision-ground components and wear-resistant surfaces. The tradeoff is a modest reduction in corrosion resistance compared to 420 or 420 Modified, a consideration that is fully manageable in the environments where this grade is routinely deployed.
UNS S42020 Chemical Composition
All limits per ASTM A582 / ASTM A276, UNS S42020.
| Element | Specification Limit |
|---|---|
| Carbon (C) | 0.26 – 0.40 % |
| Chromium (Cr) | 12.0 – 14.0 % |
| Manganese (Mn) | ≤ 1.25 % |
| Silicon (Si) | ≤ 1.00 % |
| Sulfur (S) | ≥ 0.15 % (minimum — free-machining addition) |
| Phosphorus (P) | ≤ 0.060 % |
| Molybdenum (Mo) | ≤ 0.60 % |
| Iron (Fe) | Balance |
The carbon range (0.26–0.40%) is what separates 420F from lower-carbon martensitic grades: it allows the martensite formed on quenching to reach high hardness, but also means this grade requires careful preheat and post-weld heat treatment when welding is involved. The mandated minimum sulfur content is the critical distinction from standard 420 — it is not simply a permissible impurity but a deliberate alloying addition that determines the machinability behaviour.
Governing standard: ASTM A582 (bar, shapes), ASTM A276 (bar)
Martensitic 420F Physical Properties
| Property | Value | Conditions |
|---|---|---|
| Density | 7.7 g/cm³ | Room temperature |
| Elastic Modulus | 190 – 210 GPa | Room temperature |
| Poisson’s Ratio | 0.27 – 0.30 | — |
| Shear Modulus | ~76 GPa | — |
| Thermal Expansion Coefficient | 10.3 µm/m·°C | 0 – 100 °C |
| Thermal Conductivity | 25 W/m·K | ~27 °C |
| Specific Heat Capacity | ~480 J/kg·K | — |
| Electrical Resistivity | ~57 µΩ·cm | — |
| Magnetic Behaviour | Magnetic (ferromagnetic after hardening) | All temperatures |
| Melting Range | 1390 – 1440 °C | Solidus to liquidus |
Mechanical Properties Of 420F (UNS S42020)
Annealed Condition (per ASTM A276 / A582)
| Property | Value |
|---|---|
| Tensile Strength (R_m) | ≥ 655 MPa (95 ksi) |
| Yield Strength 0.2% (R_p0.2) | ≥ 345 MPa (50 ksi) |
| Elongation (50 mm gauge) | ≥ 20 % |
| Hardness | ≤ 241 HB (max, annealed) |
Hardened and Tempered Condition (typical values)
| Property | Value |
|---|---|
| Hardness | 48 – 55 HRC |
| Tensile Strength | 1550 – 1900 MPa (depending on temper temperature) |
| Yield Strength | 1380 – 1750 MPa |
| Elongation | 4 – 8 % |
| Fatigue Strength | ~270 MPa |
The hardened condition is where 420F delivers its principal value. Depending on the temper temperature selected, hardness can be tuned across the 48–55 HRC range, allowing designers to trade peak hardness against residual toughness based on service requirements. Lower temper temperatures (150–175 °C) retain maximum hardness; higher temper temperatures (300–370 °C) sacrifice some hardness in exchange for improved resistance to brittle fracture.
Heat Treatment Specifications
Annealing: 815 – 900 °C, hold until uniform temperature, furnace cool or slow air cool. Purpose: Soften to ≤241 HB for machining, forming, or cold work.
Hardening (Austenitising): 980 – 1035 °C, hold to achieve uniform temperature throughout section. Quench medium: Oil or pressurised air; air quench acceptable for smaller sections.
Tempering: 150 – 370 °C, hold 1 – 2 h minimum per 25 mm of section, air cool.
| Temper Temperature | Approximate HRC | Notes |
|---|---|---|
| 150 – 175 °C | 52 – 55 HRC | Maximum hardness, reduced toughness |
| 200 – 250 °C | 50 – 52 HRC | Balanced hardness/toughness |
| 300 – 370 °C | 48 – 50 HRC | Improved toughness, slightly lower hardness |
Important: Tempering between 400 – 580 °C produces a temper embrittlement region and is not recommended for 420-family grades. Always temper either below 400 °C or above 600 °C.
Pre-heat for welding: 200 – 300 °C. PWHT at 750 – 800 °C followed by controlled cool is strongly recommended to restore ductility in the heat-affected zone.
Machinability
Machinability is the defining attribute of this grade. The sulfur addition creates a machinability rating of approximately 85–90% relative to AISI B1112 (the free-machining carbon steel baseline), making 420F significantly easier to machine than standard 420 (~55% rating). Practical benefits include:
- Clean chip break, reducing chip management issues in automated turning
- Higher feed rates and cutting speeds achievable without tool degradation
- Improved surface finish quality on precision-turned and ground components
- Reduced tool wear in high-volume production runs
For tight-tolerance components — pump shafts, dental burs, valve stems — 420F is often preferred precisely because the machined surface quality meets finish requirements without supplementary operations.
Corrosion Resistance
420F provides moderate corrosion resistance — adequate for many industrial, medical, and consumer environments, but less than standard 420 or 420 Modified due to the sulfide inclusions reducing the continuity of the passive film.
| Environment | Performance |
|---|---|
| Dry atmosphere, mild indoor service | Excellent |
| Fresh water, neutral pH | Good; passive film stable |
| Weak acids, dilute alkalis | Limited; dependent on concentration and temperature |
| Humid or outdoor environments | Adequate with surface finishing (polishing, plating) |
| Chloride-containing media | Limited; not recommended for sustained chloride exposure |
| Aggressive chemical environments | Not suitable |
PREN (Pitting Resistance Equivalent) ≈ 13 – 14 — lower than 316L (~25) or duplex grades, and lower than standard 420 for equivalent chromium content. Where corrosion resistance is the primary concern, specifying 420 or 420 Modified is the better choice; 420F is selected when machinability is the primary requirement and corrosion exposure is controllable.
Equivalent and Related Designations
| Standard | Designation |
|---|---|
| AISI / ASTM | 420F |
| UNS | S42020 |
| EN (European) | ~1.4005 (X12CrS13) — nearest equivalent; not identical |
| JIS (Japan) | SUS 420F |
Typical Applications
420F occupies a clearly defined niche: precision-machined components operating in controlled environments where dimensional accuracy, hardness, and surface finish are the primary performance drivers.
Medical and dental instruments: Surgical scissors, dental burs, probes, and handpiece components. The combination of achievable hardness and machinability to tight tolerances is well-matched to the production requirements of these parts.
Cutlery and blades: Knife blades, scissors, and cutting tools where edge retention through hardness (48–55 HRC) is required and the production process favours free-machining stock.
Automotive components: Fuel injector components, valve stems, shift mechanism parts, and small shafts where automated high-volume machining defines the manufacturing economics.
Pump and valve components: Small pump shafts, spindles, actuator parts, and fittings used in non-aggressive fluid systems.
General industrial tooling: Gauges, jig components, pins, fasteners, and precision-machined structural parts where hardness and dimensional stability after heat treatment are essential.
Available Product Forms
Round bar and hexagonal bar (most common for machining stock), seamless tube, pipe fittings (elbows, tees, reducers, caps), flanges, and custom-machined precision components. Material is supplied in the annealed condition as standard; hardened and tempered to customer specification is available on request.
Why C&N
At C&N (Hongcheng Pipe Fittings Limited), we stock 420F to ASTM A582 / A276, supplied with full mill test reports traceable to heat number. We work with manufacturers, fabricators, and procurement teams across the medical, automotive, and industrial sectors — and we’re direct about what a material can and can’t do for your application.
If you’re evaluating 420F for a project, or comparing it against 420 or 416, contact us and we’ll give you a straight answer.
