420 Stainless Steel
Also: AISI 420, UNS S42000, 1.4021 (X20Cr13), 1.4028 (X30Cr13), 1.4031, 1.4034 (X46Cr13), 420J2, SUS420J2, 420 mold steel, 1.2083 (420 ESR mold grade), cutlery stainless, CA-40 (cast)
420 is the cutlery and mold stainless: 12-14% chromium with enough carbon to harden to 50-54 HRC and hold an edge. Corrosion resistance needs the hardened, polished state.
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At a glance
| Density | 7.75 g/cm³ (0.28 lb/in³) |
|---|---|
| Tensile strength | Hardened and tempered at 205 °C (400 °F) about 1600-1720 MPa (230-250 ksi); annealed about 655 MPa (95 ksi) |
| Yield strength | Hardened and tempered at 205 °C (400 °F) about 1350-1380 MPa (195-200 ksi); annealed about 345 MPa (50 ksi) |
| Hardness | Hardened and tempered low 50-54 HRC (up to 56 HRC in the 0.4-0.45% C variants); annealed 190-240 HB (ASTM A276 maximum 241 HB) |
| Elongation | Hardened and tempered low about 8%; annealed about 25% |
| Modulus of elasticity | 200 GPa (29 Msi) |
| Thermal | Melts at 1450-1510 °C (2640-2750 °F); thermal conductivity ~25 W/m·K; expansion ~10.3 um/m·K; hardened parts hold hardness to about 300 °C (570 °F) |
| Composition | Fe, 12-14% Cr, 0.15% min C (typically 0.30-0.40%), up to 1% Mn, up to 1% Si |
Typical values, Hardened from 1010-1065 °C (1850-1950 °F), oil or air quenched, tempered at 205 °C (400 °F); annealed figures as noted. Confirm against the mill certificate and the purchase spec.
- MachinabilityFair
- About 45% of B1112 annealed, similar to 304 but with shorter chips and less work hardening; hard-milling of 30 HRC pre-hardened mold blocks is routine with coated carbide. Above 50 HRC it is a grinding and EDM material.
- WeldabilityPoor
- The high-carbon martensite cracks on cooling. Mold repair welds are done with matching 420 filler after preheating to 300-400 °C (570-750 °F), followed by a temper, and even then the polished surface can show the weld. Do not design welds into 420 parts.
- FormabilityFair
- Annealed strip blanks, coins and takes gentle bends (about 2t inside radius) for blades and instrument parts; harden after. Nothing forms once hardened.
- Corrosion resistanceFair
- Resists atmosphere, fresh water, steam, food acids and mild chemicals only when fully hardened, tempered low and polished; annealed 420 rusts faster than 410. Chlorides, salt water and dishwasher detergents pit it. Passivate after final polishing.
420 is 410 with more carbon: at least 0.15% and typically 0.3-0.4% in the grades sold for blades and molds, with higher-carbon European variants (1.4028, 1.4031, 1.4034) reaching 0.45%. That carbon lets it harden to 50-54 HRC, which is where knives, surgical instruments, scissors and shear blades live. It is also the standard stainless injection-mold steel: 420 ESR (1.2083, "420 stainless mold quality") is polished to optical finishes for lens, medical and PET-preform molds and resists corrosive plastics like PVC.
Annealed it is soft enough to machine, blank and form; hardened from about 1010-1065 °C (1850-1950 °F) and tempered low it reaches around 1700 MPa (250 ksi) with 8% elongation. Corrosion resistance is a step below 410 in the annealed state, because much of the chromium is tied up in carbides, and is only respectable when the steel is fully hardened (carbides dissolved), tempered below 370 °C (700 °F) and polished. It is magnetic in every condition.
Choose 420 for an edge, a wear face or a polishable mold that must resist rust and water, not for salt water or chemicals. It is not weldable in any practical sense, and it must not be tempered in the 425-540 °C (800-1000 °F) band, which drops both toughness and corrosion resistance. For higher hardness and better wear, 440C; for toughness at moderate hardness, 410; for corrosion, 17-4 PH or 316.
Heat treatment
Anneal at 840-900 °C (1550-1650 °F) with a slow furnace cool. Harden by austenitizing at 1010-1065 °C (1850-1950 °F), oil quench or air cool (sections under about 25 mm / 1" air harden), then temper at 150-370 °C (300-700 °F) for 50-54 HRC and the best corrosion resistance. Tempering at 595-650 °C (1100-1200 °F) gives about 30-38 HRC for tougher parts but reduces corrosion resistance. Never temper at 425-540 °C (800-1000 °F). Sub-zero treatment at -75 °C (-100 °F) converts retained austenite in high-carbon variants. Mold blocks are commonly supplied pre-hardened at 29-33 HRC.
Tempers and conditions
- Annealed
- Pre-hardened mold block, 29-33 HRC
- Hardened and tempered, 50-54 HRC
- Hardened and tempered, 30-38 HRC (high temper)
Stock forms
- Round and flat bar
- Strip and sheet (blade and instrument gauges)
- Mold plate and blocks (420 ESR, 1.2083)
- Forgings
- Castings as CA-40
- Metal 3D printing powder (binder jet)
Relative cost
Tier 3 of 5. Similar to 304 per pound for bar and strip; 420 ESR mold blocks carry a remelt and machining premium of 1.5-2x. Cheaper than 440C and much cheaper than powder-metallurgy stainless blade steels.
Typical applications
- Knife blades, cutlery and scissors
- Surgical, dental and veterinary instruments
- Injection molds for optical, medical and PVC parts (420 ESR)
- Shear blades and food-processing cutters
- Needle valves, seats and pump plungers
- Ball bearings and races in mild service
- Gauges, pins and wear parts
Process compatibility
How 420 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | FairFair | Machine annealed or pre-hardened (29-33 HRC) stock, then harden; above 50 HRC use grinding and EDM. Coated carbide, rigid setups, positive geometry. |
| CNC Milling | FairFair | Mold cavities are milled in pre-hardened 420 ESR at 29-33 HRC, then polished; hard milling of small features at 50 HRC is possible with ball-nose carbide. |
| Heat Treating | ExcellentExcellent | Oil or air quench from 1010-1065 °C (1850-1950 °F) and temper below 370 °C (700 °F) for 50-54 HRC; state the target HRC and avoid the 425-540 °C band. |
| Vacuum Heat Treating | ExcellentExcellent | The standard for molds and instruments: bright, scale-free hardening with gas quench and low distortion; add sub-zero for high-carbon variants. |
| CNC Grinding | ExcellentExcellent | Hardened blades, plungers and pins grind to size and edge; the normal finishing route above 50 HRC. |
| Sinker EDM | GoodGood | Cavities and ribs in hardened mold blocks; polish out the recast layer before texturing. |
| Wire EDM | GoodGood | Profiles hardened blades and mold inserts without softening them; temper afterward to relieve the recast layer. |
| Metal Stamping | GoodGood | Annealed strip blanks and coins into blades and instrument parts; harden and temper after forming. |
| Passivation | GoodGood | ASTM A967 citric after final polishing removes free iron from grinding; essential on surgical instruments. |
| Electropolishing | FairFair | Used on surgical instruments for a smooth, passive surface, but hardened 420 comes out less bright than 316; mechanical polish first. |
| Nitriding | FairFair | Gas or plasma nitriding hardens mold surfaces for glass-filled plastics, but it lowers corrosion resistance; skip it on PVC molds. |
| Metal 3D Printing | FairFair | Binder-jet 420 infiltrated with bronze is a common printed material for fixtures and non-critical parts; it is not equivalent to hardened wrought 420. |
| Welding | Not recommendedNot recommended | High-carbon martensite cracks on cooling; mold repairs only, with matching filler, 300-400 °C preheat and a post-weld temper. |
| Hot-Dip Galvanizing | Not recommendedNot recommended | Zinc does not bond to the passive oxide and the bath softens hardened parts. |
Design and sourcing tips
- Specify the carbon level you need: "420 per ASTM A276" allows 0.15% minimum, which will not reach 54 HRC; call a 0.3-0.4% C grade (1.4028 / 1.4034, 420HC or 420 ESR) for blades and molds.
- State the final hardness and the temper: "harden and temper to 52-54 HRC, temper below 370 °C (700 °F)". The low temper is what keeps the corrosion resistance.
- Machine molds in pre-hardened 29-33 HRC 420 ESR when finish and corrosion matter; harden inserts only when the plastic is abrasive.
- Do not design welds into 420 parts; press, pin or screw inserts instead.
- For salt water, dishwasher or chemical exposure use 440C only if hardness is essential, otherwise 17-4 PH or 316; 420 will pit.
- Polish and passivate (ASTM A967) after heat treatment; a ground, unpassivated 420 surface will freckle in humid storage.
Frequently asked questions
- Is 420 stainless steel magnetic?
- Yes, strongly, in the annealed and the hardened state. It is a martensitic chromium steel and a magnet holds to it as it would to carbon steel.
- How hard can 420 stainless get?
- It depends on carbon. The ASTM minimum of 0.15% C reaches about 50 HRC; the 0.3-0.4% C grades sold for cutlery and molds reach 52-54 HRC, and the 0.45% C European variant (1.4034) about 56 HRC. Beyond that you are into 440C at 58-60 HRC.
- Is 420 stainless good for knives?
- It is the basic budget stainless blade steel: inexpensive, easy to blank and grind, hardens to 52-54 HRC (as 420HC or 1.4034) and resists rust in the kitchen if kept clean. It gives up edge retention to 440C and to powder-metallurgy steels, and it will spot in a dishwasher. Most inexpensive kitchen and pocket knives are 420-class steel.
- Can 420 stainless be welded?
- Only as a repair. Its carbon content makes the weld and heat-affected zone crack on cooling unless the part is preheated to 300-400 °C (570-750 °F), welded with matching 420 filler and tempered immediately afterward. Mold repair shops do this; product designs should not rely on it.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A276 and ASTM A240
- AMS 5621; ISO 7153-1 for surgical instrument grades
- ASM Handbook, Volume 4D: Heat Treating of Irons and Steels
- Producer data sheets for 420 and 420 ESR mold steel (Crucible, Uddeholm Stavax equivalents, Carpenter)
