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AISI 420 / (1.4021) / UNS S42000 / AMS 5506 / AMS 5620

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Corrosion resistance: The corrosion resistance of AISI 420 is lower than that of the austenitic group and it can rust in very demanding environments. Grade 304 is recommended for more demanding environments and 316L where acids are present.

Weldability: Weldability is poor and hardening and annealing are generally required as a final step after welding. With average weldability, this material can nevertheless be used in an environment involving extensive welding. The most important point to observe when welding this material is what the other material being welded to is. When joining 420 to 304 stainless by welding, for example, greater care and attention are required.​‌​​‌​

Machinability: In terms of machinability, the martensitic stainless steels are the most suitable group for high hardness. For the material to perform, it should not be used at sub-zero temperatures (where it loses its softness) or at high temperatures (where its strength falls). Its machinability in the annealed condition is very good, but machining becomes difficult once it has been hardened, particularly above 30 HRC.

Heat treatment: Grade 1.4021 (420) stainless can be hardened by heat treatment like many high carbon steels. It contains at least 12% chromium, and that alloy content is sufficient for corrosion resistance. It has good softness in the annealed condition but is capable of reaching 50 Rockwell hardness by heat treatment. For the best corrosion resistance it should be hardened and the surface cleaned or polished.​‌​​‌​

Applications: It is frequently used in the machinery industry, in the oil and petrochemical industries, in food and food production plants (forks, knives, spoons, blade edges), in decorative work, in transport, and in shafts, pistons and valves.

AISI 420 is one of the martensitic stainless steels and is a suitable material for applications requiring high hardness, wear resistance and moderate corrosion resistance. It does not offer corrosion resistance as high as the austenitic steels, however, and there are limitations in terms of weldability.​‌​​‌​

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Standards by Product Form

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AISI 420 is the high-carbon derivative of 410 and the readily hardenable martensitic workhorse. The single most important fact on this page: “420” is not one chemistry. ASTM A276 / A314 / A484 define Type 420 with only a 0.15% carbon minimum and no maximum, so one ASTM “420” mill certificate can legitimately correspond to anything from EN 1.4021 to EN 1.4034 carbon levels.

Sub-grade / EN number mapping — the most frequently confused point for this grade:

ASTM A276 Type 420 composition: C 0.15% min (no ASTM maximum) · Cr 12.00–14.00% · Mn ≤1.00% · Si ≤1.00% · P ≤0.040% · S ≤0.030%. Typical annealed values: tensile ~655 MPa, yield ~345 MPa, elongation ~25%, hardness 192–241 HB. Hardened and low-tempered: the 48–52 HRC class, tensile of the order of 1600–1760 MPa. This is precisely why European and UK suppliers split into narrow carbon bands what ASTM keeps under one open-ended heading.

Welding, Heat Treatment and Machining​‌​​‌​

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Welding

420 is not a preferred grade for welding — its air-hardening nature makes weldability poor. Where welding is unavoidable, GTAW and GMAW with matching ER420 filler are used, chiefly for repair and surfacing. Preheat and interpass run 150–320 °C; the ER420 wire manufacturer specifies a minimum of 204 °C (400 °F) preheat/interpass with slow cooling. A high-temperature post-weld temper or anneal is mandatory — sources give figures across the 600–790 °C band, so the exact value must follow the fabricator’s welding procedure. Where preheat or PWHT cannot be applied, austenitic AWS 309 / 312 filler gives a ductile joint at the cost of colour and corrosion match. The governing risk is cold (hydrogen) cracking in the hard, untempered martensite of the HAZ.​‌​​‌​

Heat treatment

Classic martensitic hardening: austenitise, quench, temper. Austenitising: 1010–1066 °C. Quench: warm oil; air or pressurised gas is acceptable for thin sections. Tempering: maximum hardness at 149–204 °C (48–54 HRC); as the temperature rises toward 649 °C hardness falls into the low thirties and then the mid-twenties HRC. Tempering between 427 and 552 °C (800–1025 °F) is forbidden — two independent specialty-steel producers report that both toughness and corrosion resistance drop in this band. Full anneal: 843–900 °C, furnace cool (~179 HB). Sub-critical anneal: 732–788 °C, furnace or air cool (~196 HB) — this condition gives the best machinability. Standard delivery is annealed. Corrosion resistance is best at a low temper (149–204 °C) and falls as temper temperature rises.​‌​​‌​

Machining

Machinability is quoted on two different scales, which must not be mixed: roughly 30% against B1112, and roughly 50% against Type 416. Both indicate that 420 machines considerably harder than the free-machining grades. Machine in the annealed condition and harden afterwards; sub-critically annealed material gives the best machinability. It behaves like a high-carbon tool steel and produces tough, stringy chips. Carbide tooling allows 2–3× the surface speed of HSS and 50–100% higher feed. Coolant: sulphur-chlorinated mineral oil for HSS, heavy-duty emulsifiable oil for carbide. 420F (S42020) is the free-machining variant; the sulphur addition improves machinability and the material should be used hardened for its best corrosion resistance.​‌​​‌​

Where 420 Belongs — and Where It Does Not

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420 sells on hardness: at 12–14% chromium it is the grade that holds a cutting edge while still being genuinely stainless. Its corrosion resistance, on the other hand, is basic — there is no molybdenum and no nickel.​‌​​‌​

Service limits: the governing mechanical and corrosion ceiling is the tempering limit — continuous service above about 427 °C is not recommended. The scaling (oxidation) limit is higher; sources give figures in the 650–760 °C band for intermittent service. The grade is magnetic.​‌​​‌​

Frequently Asked Questions

I need about 55 HRC for a blade — which sub-grade should I order?​‌​​‌​

Ask for a carbon-controlled grade rather than generic “ASTM 420”: EN 1.4034 (X46Cr13, C 0.43–0.50%), or at minimum 1.4031 (X39Cr13, C 0.36–0.42%). The reason is that ASTM A276 / A314 define Type 420 with only a 0.15% carbon minimum and no maximum, so a mill may legally ship anything from about 0.16% C (EN 1.4021 territory, which tops out around 48–52 HRC) up to 0.5% C. The more carbon, the more carbide dissolves into the martensite during austenitising, and the higher the attainable hardness. Order “ASTM A276 Type 420” without a carbon range or EN designation and you risk receiving material that cannot be hardened past the mid-forties HRC — a common and costly surprise for buyers who assume 420 is one fixed chemistry.

Is 420 actually stainless in the annealed condition, or must it be hardened first?​‌​​‌​

It is stainless annealed: at 12–14% chromium it forms a passive chromium-oxide film and will not rust like plain carbon steel, and every manufacturer datasheet reviewed lists corrosion resistance for the annealed condition (fresh water, steam, mild atmospheres, food acids). That resistance is nevertheless noticeably lower than in the hardened and polished condition: annealed 420 has coarse, undissolved carbides and typically a duller, rougher surface. If the part will see humid, wet or mildly acidic service, order it hardened, low-tempered at 149–204 °C and ground or polished. Annealed 420 stock is fine for machining and for dry indoor components, but is not the delivery condition to specify for a corrosion-critical end use.

Is 1.2083 the same steel as AISI 420? Which should I buy for a mould?​‌​​‌​

1.2083 (X40Cr14) is widely marketed as an “AISI 420 equivalent” and sits in the same carbon band as EN 1.4031 (X39Cr13) — that is, at the upper end of the 420 family, not the low end. For plastic injection moulds 1.2083 is the right call: it gives good polishability, resistance to the humid and corrosive mould environments created by PVC and flame-retardant plastics, and hardness in the 48–52 HRC class after heat treatment, and it is often ESR-remelted for cleanliness. Do not, however, treat it as interchangeable with a generic low-carbon ASTM “420” bar order — if cavity finish and consistent hardening response matter, specify 1.2083 / DIN or the ESR mould grade by name on the order.

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