In short: Type 420 is the cutlery-grade martensitic stainless steel — a 12–14% chromium alloy with carbon ≥ 0.15% that quench-hardens to ~50–55 HRC, noticeably harder than 410. Its crystal structure after quenching is BCT martensite (α′); it is permanently magnetic and derives all its strength from the austenitize → quench → temper cycle. The higher carbon that enables superior hardness also promotes chromium-carbide formation, depleting the matrix of chromium and reducing corrosion resistance relative to austenitic grades. Best as a step up from 410 where sharper edges and greater hardness matter; step further up to 440C for maximum hardness in bearing applications.
What 420 Stainless Steel Is
Type 420 is the cutlery-grade martensitic stainless steel — a direct evolution of 410 with elevated carbon for harder, sharper edges. It contains 12–14% chromium and ≥ 0.15% carbon (with commercial grades running 0.15–0.40%, some sub-grades reaching higher), in an iron base with near-zero nickel.[1]
In the Unified Numbering System it is UNS S42000; in European EN it is 1.4021 (X20Cr13, for the lower-carbon end) or 1.4028/1.4031 for higher-carbon sub-grades. In Japanese practice: SUS 420J1 (C ≤ 0.28%) and SUS 420J2 (C ≤ 0.40%). The minimum carbon threshold of ≥ 0.15% is what differentiates 420 from 410.[1]
In the Chinese GB system, 420 corresponds to 20Cr13 (GB/T 20878), still commonly called 2Cr13 — the numeral prefix encodes carbon (2Cr13 ≈ 0.2% C, 13% Cr). It sits one rung up the carbon ladder from 410 / 12Cr13 (1Cr13); rungs further up are the higher-carbon GB martensitics 30Cr13 (3Cr13) and 40Cr13 (4Cr13), then the high-Cr bearing grade 95Cr18 (9Cr18). Chinese buyers searching *2Cr13* or *20Cr13* are looking for this alloy.
The essential difference from 410: more carbon → harder martensite → better edge retention — at the cost of more chromium carbide formation and slightly reduced corrosion resistance. Research comparing heat-treated AISI 410 and 420 confirms this relationship: 420 develops a more complex microstructure of martensite plus Cr-rich carbide precipitates, translating directly into higher peak hardness.[2]
Chemical Composition
Composition limits for Type 420 (per ASTM A240/A276 and Carpenter 420 specification). The carbon level — the critical differentiator from 410 — is the key hardening element, raising the tetragonality of the hardened martensite and the peak HRC achievable.[1]
| Element | Symbol | Content (wt%) | Role |
|---|---|---|---|
| Chromium | Cr | 12–14 | Forms the passive film; minimum for stainlessness — partially consumed by carbides |
| Carbon | C | ≥ 0.15 | Higher than 410 — the key hardening element; increases BCT tetragonality and peak HRC |
| Manganese | Mn | ≤ 1 | Deoxidiser; forms MnS inclusions (pit-initiation sites) |
| Silicon | Si | ≤ 1 | Deoxidiser; assists oxidation resistance |
| Iron | Fe | Balance | Base metal |
Per ASTM A276 and Carpenter Technology 420 specification.[1]
Crystal Structure: BCT Martensite with Greater Tetragonality
Stainless steel is an alloy — a solid solution of elements in iron — so it has no molecular formula. The correct description is by crystal structure.
420 shares the same martensitic transformation pathway as 410, but the higher carbon content produces greater BCT tetragonality and therefore greater hardness. On austenitising (~980–1065 °C) and quenching, the austenite transforms displacively into BCT martensite (α′) — a body-centred lattice distorted along one axis by trapped interstitial carbon atoms. More carbon → more distortion → harder martensite.[1]
The as-quenched 420 is very hard but brittle, with retained austenite alongside the martensite. Tempering at 205–370 °C converts retained austenite and relieves stresses — maintaining high hardness while reducing brittleness. The BCT lattice is ferromagnetic throughout all heat-treatment states. Studies on AISI 420 confirm that austenitizing at ~1198 K followed by oil quenching yields a martensite + retained austenite + chromium carbide microstructure, with tempering adjusting carbide size and distribution.[2]
Corrosion Resistance: Good When Hardened and Polished
420 is moderately corrosion resistant in mild atmospheres, fresh water, and dilute acids when in the hardened and polished condition. Its resistance is lower than 410 and substantially lower than 304 or 316, because higher carbon intensifies two degradation mechanisms.[4]
Vulnerability 1 — Carbide precipitation and Cr depletion. Research on tempered type 420 stainless steel shows that chromium-rich carbides (Cr₂₃C₆, Cr₇C₃) precipitate during tempering. The matrix chromium concentration falls: at 550 °C tempering, the Cr in the matrix drops to ~10.5% — below the threshold for a continuous passive film — causing intergranular corrosion. At 700 °C tempering, localised corrosion occurs at carbide-adjacent Cr-depletion zones, initiating pitting.[4]
Vulnerability 2 — Lower base Cr with more C. Even at the higher end of the Cr range (14%), the effective chromium available for passivation is reduced by carbide formation. The grade is less resistant than 410 despite having a slightly higher Cr ceiling, because the C level is higher.
Best corrosion performance from 420 is achieved in the hardened (low-temperature temper ≤ 370 °C), polished, and passivated condition — this keeps Cr in solid solution and maximises passive film integrity.[4]
MnS inclusions from residual sulfur act as pit-initiation sites, documented across stainless grades.[3]
Mechanical & Physical Properties
420 offers two distinct property profiles depending on heat treatment — the defining advantage of hardenable martensitic grades. In the annealed condition it is soft and machinable; quenched and tempered, hardness in HRC becomes the primary metric.[1]
| Tensile strength (MPa) | ≈655 |
| Yield strength (MPa) | ≈345 |
| Elongation (%) | ≈25 |
| Hardness | ≤241 HB |
| Magnetic response | Magnetic |
| Hardness | ≈50–55 HRC |
| Density (g/cm³) | 7.75 |
| Elastic modulus (GPa) | 200 |
| Magnetic response | Magnetic |
The 50–55 HRC achievable at low tempering temperatures (205–370 °C) is the defining property of 420 in commercial cutlery, surgical blades, and fine tooling — noticeably harder than 410's ~40–45 HRC. As-quenched and untempered the steel reaches ~56–58 HRC but is too brittle for service; a high temper (600–650 °C) trades hardness back down to ~28–35 HRC for greater toughness, and the 370–565 °C range is avoided.[1][2]
Studies comparing AISI 410 and 420 confirm that 420 — with its higher carbon — shows a more complex martensitic matrix with chromium carbide precipitates that strongly affect both hardness stability and localised corrosion susceptibility after tempering.[2]
Key Characteristics
- Heat-treatable to 50–55 HRC. The highest usable hardness range among common cutlery-grade stainless steels — harder than 410, softer than 440C.
- Mirror-polishable. The martensitic structure with fine carbides takes a high-lustre mirror polish, critical for surgical and dental instruments.
- Permanently magnetic. The BCT lattice is ferromagnetic through all heat-treatment states.
- Moderate corrosion resistance. Adequate for mild atmospheres, fresh water, and food contact when hardened and polished — inferior to austenitic grades in chloride environments.
- Poor weldability. High carbon makes the heat-affected zone very hard and brittle; pre-heat and post-weld heat treatment required.
How 420 Is Made
Production follows the same route as 410 — EAF melting, AOD decarburisation, hot and cold rolling, annealing, pickling — but the higher target carbon demands tighter AOD control to land precisely in the ≥ 0.15% C window without overshooting into 440 territory. For service as a cutting or precision-tool part, the critical step is the hardening cycle.
Heat treatment sequence: Preheat at 760–790 °C → austenitize at 980–1065 °C → quench in air or oil → temper immediately at 205–370 °C for maximum hardness, or 600–650 °C for improved toughness.[1]
420 vs 410 — Edge Hardness vs. General Service
Both are martensitic and hardenable. The choice hinges on whether peak hardness or corrosion resistance is the priority. 410 keeps carbon below 0.15% for slightly better corrosion resistance and toughness; 420 raises it for a finer, harder edge.
Applications by Industry
420's combination of maximum hardness and corrosion resistance in a stainless steel makes it the standard for sharp-edge and wear-critical components.[1]
Cutlery and Kitchen Knives
Table knives, steak knives, kitchen scissors — the main grade for mass-market knife blades (420J1/J2) and budget consumer cutlery. The 50–55 HRC hardness provides edge retention that carbon steel offers, with the corrosion resistance stainless demands.[1]
Surgical and Dental Instruments
Scalpels, scissors, forceps, and dental probes. 420 hardens sharply and survives repeated steam sterilisation cycles while holding a fine edge. The mirror-polish capability is critical for instruments in sterile environments.[1]
Moulds and Dies
Plastic injection moulds where moderate corrosion resistance and high surface hardness are both required. 420 provides the combination of polishability and wear resistance needed in mould cavity surfaces.
Valve Seats and Pump Components
Harder than 410 for more erosive service. Valve seats, pump internals, and nozzles subjected to abrasive fluid flow, where the extra 10–15 HRC over 410 meaningfully extends service life.
Forms & Finishes
Common product forms:SheetPlateBar (round / flat)CoilForging
Surface finishes:2B (annealed)No.4Mirror / BAHardened + ground
For hardened cutlery and instrument applications, the mirror or near-mirror polish (BA/No.8) applied before hardening is the functional surface — improving both edge quality and corrosion resistance.
References
- Stainless Steel – Grade 420 (UNS S42000). AZoM (materials datasheet based on the ASTM/AISI grade system). azom.com/article.aspx?ArticleID=972
- Investigating the structural properties and wear resistance of martensitic stainless steels. (AISI 410 & AISI 420 heat treatment, hardness, microstructure). Heliyon / PMC11575766, 2024. pmc.ncbi.nlm.nih.gov/articles/PMC11575766/
- Pitting corrosion characteristics of sintered Type 316L stainless steel: pores and MnS. (Saito et al.). npj Materials Degradation 8, 2024. — pool P5 doi.org/10.1038/s41529-024-00482-6
- Accessing the full spectrum of corrosion behaviour of tempered type 420 stainless steel. (Zhou & Engelberg). Materials and Corrosion, 2021. doi.org/10.1002/maco.202112442