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AISI 420B / (1.4028) / UNS S42000 / AMS 5506 / AMS 5620

Corrosion resistance: The corrosion resistance of AISI 420B 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 420B is one of the martensitic stainless steels and is a suitable martensitic material for applications requiring high hardness, wear resistance and moderate corrosion resistance. The corrosion resistance of AISI 420B is limited, however.

What AISI 420B Is — and Where the Letter “B” Actually Comes From​‌​​‌​

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AISI 420B is the medium-carbon step of the 12–14 % chromium martensitic stainless family: in Europe EN 1.4028 / X30Cr13, with UNS number S42000. Its carbon band is 0.26–0.35 %. That single number is what defines it — the chromium band, the melting route, the heat-treatment logic and the corrosion behaviour are identical to its siblings. The difference between 420, 420B and 420C is carbon, and nothing else.

Let us settle the naming question honestly first, because this is where buyers go wrong. The common belief is that “420B is not an AISI grade, it is a label European stockists invented.” That belief is half wrong. The correct answer is this:​‌​​‌​

The honest sentence for the product page: “420B” is a genuine ASTM designation — but only within ASTM F899, that is, as a surgical-instrument steel. If you are buying general engineering bar (ASTM A276, A314, A484, A580), there is no box called 420B; there is only “Type 420”, and that name tells you nothing about carbon. This is why your purchase text must state either the EN number (1.4028 / X30Cr13) or the explicit carbon range (0.26–0.35 %). Writing “AISI 420B” on its own can bring you three different steels, depending on which document your supplier happens to consult.​‌​​‌​

All three grades carry the SAME UNS number — that alone is a trap

ASTM F899 assigns the same UNS number, S42000, to 420A, 420B and 420C alike. The UNS system does not distinguish these carbon steps. The consequence: a mill certificate that says “UNS S42000” gives you zero information about whether the material is 420A, 420B or 420C. You cannot know which step you have without reading the carbon analysis. Do not rely on the UNS number in your purchase specification; read the heat analysis carbon value.​‌​​‌​

The one-sentence positioning that follows from this table: 420B is the working middle of the 420 family. It does not stay soft like 420A and it is not glass-brittle like 420C. Where a part must hold an edge but also take impact and prying loads — surgical scissors, forceps, bone chisels, kitchen knives, shafts, mould components — this is the carbon step to specify.​‌​​‌​

Standards by Product Form

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The most important warning in this section: the grade name and the defined carbon band both change depending on whether you buy through ASTM or through EN. Do not mix the two routes.

ASME code acceptance — the short, clear answer

AISI 420B / 1.4028 has NO ASME pressure-vessel or piping code acceptance. The grade is not listed with an SA number in ASME Section II Part A, it cannot be used as pressure-boundary material in ASME Section VIII Div. 1 or Div. 2 design, and it carries no allowable stress in ASME B31.1 or B31.3. This is not an oversight: hard martensitic structure, low toughness and a strong tendency to post-weld cold cracking disqualify the grade as a pressure boundary. If a customer asks for “ASME-approved 420B”, the correct answer is that no such route exists. 420B is used in non-pressure-boundary internals — valve trim, stems, wear faces, seat rings — and those parts do not enter the code stress calculation.​‌​​‌​

The temperature ceiling is likewise a metallurgical ceiling, not a code ceiling: the limit on 420B is the obligation to stay below the tempering temperature. If the service temperature approaches the tempering temperature, the part keeps tempering in service and loses hardness. For a low-tempered part (150–200 °C) the realistic continuous service ceiling is about 200 °C. The continuous 650 °C / intermittent 750 °C figures on datasheets, and the mechanical 760 °C value quoted by one database, are scaling (oxidation) limits, not load-bearing limits, and must not be used as a design ceiling.

Product Forms With NO Standard — the Section Your Sales Team Should Memorise​‌​​‌​

This is where 420B causes the most commercial trouble: many of the product forms customers ask for simply have no product specification for this grade. Knowing that prevents promises you cannot keep.

Chemical Composition

The table below shows how three separate documents describing the same grade diverge. This is the part of a certificate you should actually read.​‌​​‌​

What the carbon actually does — and why it decides everything​‌​​‌​

In a 12–14 % chromium steel carbon has two jobs, and the two jobs fight each other. First: carbon dissolves into austenite at the austenitising temperature and, on quenching, distorts the martensite lattice and produces the hardness. Achievable peak hardness depends almost entirely on how much carbon went into solution — the gap between 420A at 0.20 % and 420B at 0.30 % is worth several HRC points directly. Second: the carbon that does not dissolve remains as chromium carbide (M₂₃C₆), and those carbides pull chromium out of the matrix. Every carbon atom tied to chromium is chromium stolen from the passive film. 420B is harder than 420A but less corrosion-resistant at the same heat treatment; 420C is harder than 420B and less resistant still. In the 420 family there is a direct and unavoidable trade between hardness and corrosion resistance, and the currency of that trade is carbon.

Mechanical Properties​‌​​‌​

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Two separate families of numbers must not be mixed here: (1) the EN 10088-3 delivery-condition minima, which go on the certificate and are guaranteed; and (2) the mill cards’ tempering curve, which is typical, measured on a standard-diameter test bar, and NOT guaranteed.

The table below is the most valuable engineering data on this page. Ø10 mm round specimen, oil-quenched from 1000 °C, then tempered at the stated temperature. These are typical values, not guarantees, and they cannot be taken over unchanged into heavy sections.

Read that table once more — here is the story it tells. Between 200 °C and 500 °C the tensile strength barely moves (1700 → 1600 N/mm²). In other words, tempering at 450 °C instead of 200 °C buys you nothing at all in strength. What it costs you is impact energy: 20 J drops to 12 J, a loss of roughly 40 % of the toughness. Tempering in the 400–550 °C band means throwing away a third of the toughness in exchange for nothing. That is the clearest possible demonstration of why the band is forbidden, and it is a table you can put in front of a customer.

Physical Properties

Physical values are broadly consistent across mill cards; the divergence is confined to the elastic modulus and Poisson ratio.​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

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All of the engineering in 420B lives in this section. Buying this grade means buying a heat-treatment recipe; the same bar can yield a soft 700 MPa shaft or a glass-hard 1700 MPa blade.

The forbidden temper band — why, and by which mechanism

There are two distinct reasons this band is forbidden, and they should not be conflated.​‌​​‌​

(1) Temper embrittlement. In alloy steels, tempering in the 400–600 °C band embrittles the grain boundaries. In martensitic stainless steels the same phenomenon is reported in the 450–600 °C band. A published failure investigation of wrongly tempered 420 components found both brittle fracture and intergranular corrosion: the same parts showed a very high corrosion rate in 5 % nitric acid at 25 °C, and the attack was intergranular. So the band damages not only toughness but corrosion resistance as well — and both fail together, because both have the same cause: chromium carbide precipitation on the grain boundaries and chromium depletion in the neighbouring matrix.

(2) 475 °C embrittlement. This is a separate and slower phenomenon: it affects ferritic matrices with chromium above 12 %, appears between 250 and 550 °C, is most severe at about 475 °C, and works by spinodal decomposition of the ferrite into iron-rich and chromium-rich nanophases. Hardness rises; ductility and corrosion resistance fall. It can be partially reversed by a treatment at 550 °C. 420B is fully martensitic, so it is not the primary target of classical 475 °C embrittlement; nonetheless, because it occupies the same temperature band, mill cards carry the “avoid the 475 °C range” warning across to the 420 family. The practical conclusion is identical in both cases: stay out of the band.​‌​​‌​

Transformation temperatures and retained austenite

In the 420 family the Ms (martensite start) and Mf (martensite finish) temperatures fall as carbon rises. The higher the carbon, the more retained austenite remains after the quench. Because 420B carries less carbon than 420C, its retained-austenite problem is markedly smaller — a sub-zero (deep-freeze) step is unnecessary for most 420B applications. However, in parts where dimensional stability is critical (gauges, precision moulds, bearing faces), retained austenite transforms to martensite over time and grows the part. For such parts the correct practice is a sub-zero step between quench and temper, plus double tempering. Published Ms/Mf values for 1.4028 could not be independently verified; for the neighbouring grade 1.4034 a mill card gives Ms ≈ 280 °C, Mf ≈ 130 °C, and because of its lower carbon 420B is expected to sit higher than that.​‌​​‌​

Welding

Short answer: 420B is not a steel designed to be welded. Mill cards say so plainly — one European producer states it is “not suitable for manual metal arc welding”, another simply writes “welding is not recommended”. If welding is unavoidable, the discipline below must be applied in full; applying half of it is no better than applying none.​‌​​‌​

Machining​‌​​‌​

The governing rule is one sentence: 420B is machined in the annealed condition, and hardening comes AFTER machining. Trying to machine hardened 420B (49–55 HRC) with conventional cutting tools is not economic; at that stage only grinding, honing and EDM remain.

Corrosion — the Most Misunderstood Part of 420B

This is the most important section on the page and it should be given to the customer as it stands. 420B belongs to the “stainless” family, but its corrosion resistance depends not on what you bought but on the final condition of the delivered part. The same steel, given two different processing histories, behaves like two different materials.​‌​​‌​

Frequently Asked Questions​‌​​‌​

My customer wrote “AISI 420B” but my supplier sends an “ASTM A276 Type 420” certificate. Is that acceptable?

Do not accept it as-is — read the carbon value on the certificate. ASTM A276 defines Type 420 only as 0.15 % carbon minimum, with no upper limit. A material conforming to A276 may therefore be at 0.16 % C (the 420A / 1.4021 band) or at 0.47 % C (the 420C / 1.4034 band); both are legally compliant with A276. What you need for 420B is the 0.26–0.35 % band. If the heat analysis on the certificate falls inside that band, the material is in fact 420B and you may accept it — but because the certificate does not say so by name, you should add a note to your traceability file reading “heat analysis C 0.3X %, complies with the ASTM F899 420B and EN 1.4028 bands”. If it falls outside, reject it. To avoid a repeat, change the order text: instead of “AISI 420B” write “EN 1.4028 / X30Cr13, C 0.26–0.35 %”, or, if you are buying surgical instrument material, “ASTM F899 Type 420B”. Either phrase binds the supplier to a defined carbon band; “AISI 420B” on its own does not.​‌​​‌​

I make knives. Should I buy 420B or 420C? Does the difference really matter that much?

It matters, and which way it points depends on how the knife is used. 420C (1.4034, C 0.43–0.50 %) gives higher hardness — EN ISO 7153-1 working hardness 50–58 HRC, and one producer states that 57 HRC is reachable under ideal conditions — and higher hardness means longer edge retention. 420B (1.4028, C 0.26–0.35 %) gives lower hardness (49–55 HRC) but is markedly tougher and has better corrosion resistance, because less chromium is tied up in carbides. The decision rule is this: if the edge is thin and sees prying or impact loads, choose 420B; if the edge is thick, the load is low and all you want is cutting life, choose 420C. Kitchen slicers, razors and craft blades sit on the 420C side. Bone chisels, surgical scissors, outdoor knives and hard-use blades sit on the 420B side. If edge retention alone is the concern, 440C (C 0.95–1.20 % plus Mo) gives higher wear resistance still, at the cost of yet lower toughness and harder sharpening.​‌​​‌​

We tempered the part at 480 °C because a 500 MPa yield was enough. Is that a problem?

Yes, a serious one — on two fronts. First: tempering at 480 °C gained you nothing. The producer’s tempering curve gives 1620 N/mm² tensile at 450 °C and 1700 N/mm² at 200 °C. In other words that band gave you less strength; for a 500 MPa target you should have tempered in the 650–700 °C band in the first place (at 650 °C, Rp0.2 is 650 N/mm²). Second, and genuinely serious: the 400–550 °C band is where toughness bottoms out — impact energy falls from 20 J at 300 °C to 12 J at 450–500 °C, a loss of roughly 40 %. On top of that, a published failure investigation found intergranular corrosion in 420 parts that had entered this band. In short, your part is now weaker, more brittle and more corrosion-prone at the same time. What to do: do not put the parts into service. The correct route is to re-austenitise (1000–1050 °C), oil quench and temper at 650–700 °C — that gives you the target yield and brings toughness up into the 32–40 J range. If you want high hardness instead, drop to the 150–200 °C low temper. The rule: never stop anywhere between 350 °C and 600 °C.​‌​​‌​

Are 420B and 1.4028 definitively the same steel? Which should I specify on the certificate?

The same in practice, not identical in law. The carbon band overlaps (both 0.26–0.35 %), and the chromium band overlaps (12–14 %). But two differences exist and can cause certificate disputes: (1) Manganese — EN 10088 allows 1.50 % max for 1.4028, while ASTM F899 420B sets 1.00 % max. (2) Sulphur — the EN 10088 base ceiling is 0.015 % max (with 0.015–0.030 % separately permitted for machinability), whereas ASTM F899 420B simply gives 0.030 % max. A heat at 0.025 % S therefore complies with F899 420B but exceeds the base EN 1.4028 sulphur limit. Decide like this: if you make surgical or dental instruments, specify ASTM F899 420B (or EN ISO 7153-1) — those documents were written for that use. If you make general engineering parts, shafts, moulds or blades, specify EN 1.4028 / X30Cr13 and add S ≤ 0.015 % and Mn ≤ 1.00 % as separate requirements. In either case, do not rely on the phrase “UNS S42000”: that number is identical for 420A, 420B and 420C, and tells you nothing.​‌​​‌​

Common Datasheet Errors and Ordering Traps

1) “420B is not an AISI grade, it is a made-up name.” Half wrong. 420B is a real ASTM designation, defined with a written carbon band, in ASTM F899. What is wrong is looking for a grade called 420B in the general bar specifications ASTM A276/A314/A484 — it is not there.
2) “A UNS S42000 certificate confirms 420B.” It does not. ASTM F899 gives the same UNS number to 420A, 420B and 420C alike. Read the carbon analysis.
3) “1.4028 contains molybdenum.” It does not. The molybdenum-bearing, resulphurised free-machining variant is 1.4029 (X29CrS13). No Mo is specified in 1.4028.
4) “420B is stainless, so it can be used in seawater.” It cannot. There is no molybdenum and 13 % chromium is inadequate against chloride pitting.
5) “Annealed 420B is stainless too.” Technically yes, practically no. In the annealed condition the carbides are undissolved and the surface is rough; resistance is markedly lower. Where corrosion matters, take delivery hardened + low tempered + polished.
6) “Continuous service 650 °C.” That is a SCALING limit, not a load-bearing limit. The real ceiling is the tempering temperature; for a low-tempered part the practical ceiling is around 200 °C.
7) “We can get ASME-approved 420B.” You cannot. This grade has no ASME pressure-vessel or piping code acceptance.
8) “We ordered 420B pipe.” It is not a standard product. No verified ASTM or EN pipe/tube specification could be found for 1.4028.
9) “1.4028 = BS 420S45.” Contradictory. Some sources map 420S45 to 1.4028, others to 1.4034. Do not use that BS number alone as an identity.
10) “We stress relieved after welding, so we are fine.” Not enough. A 200–300 °C stress relief does not fix the untempered martensite in the HAZ. Either apply a 732–788 °C anneal before the part cools, or re-run the full heat-treatment cycle.
11) “1.4028 has the same mechanical properties as 420.” It does not. Because “420” carries no defined upper carbon limit, there is no single set of mechanical values for it; the 1.4028 figures belong to the 0.26–0.35 % carbon band.
12) “Modulus of elasticity is 190 GPa.” Contradictory. Mill cards give 200–215 GPa; the 190 GPa figure comes from a database. For stiffness calculations use 215 GPa and ask your supplier to confirm.
13) “We need a non-magnetic stainless, give us 420B.” You cannot. 420B is ferromagnetic in every condition (μr ≈ 700–1000). For non-magnetic stainless go to the 304 or 316 family.​‌​​‌​

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