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AISI 420C / (1.4034) / UNS S42000 / AMS 5506 / AMS 5620

Corrosion resistance: The corrosion resistance of AISI 420C 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. The high carbon content makes the steel harder and more wear resistant, while its corrosion resistance is lower than that of other stainless steel types.​‌​​‌​

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 420C is one of the martensitic stainless steels and is used in applications requiring high hardness, wear resistance and moderate corrosion resistance.

What AISI 420C Is — the Hard End of the 420 Family​‌​​‌​

AISI 420C is the highest-carbon step of the 12.5–14.5 % chromium martensitic stainless family: in Europe EN 1.4034 / X46Cr13, with UNS number S42000. Its carbon band is 0.43–0.50 % (ASTM F899 gives the same step as 0.42–0.50 %). The EN ISO 7153-1 surgical working-hardness band is 50–58 HRC (530–675 HV), and one producer states that 57 HRC is reachable under ideal conditions. The difference between 420, 420B and 420C is one element: CARBON. The chromium band is almost the same, there is no nickel and no molybdenum, and the melting route and heat-treatment logic are shared.

The honest answer on naming: “420C” is a genuine ASTM designation — but only within ASTM F899 (stainless steels for surgical instruments). In the general engineering bar specifications ASTM A276, A314, A484 and A580 there is no grade called 420C; there is only “Type 420”, whose carbon is defined as 0.15 % minimum with no upper limit. Europe uses no letters at all, only numbers: 1.4034. EN ISO 7153-1 uses a third system entirely — it assigns 1.4034 the reference letter “C”; the fact that this coincides with the ASTM “C” is a coincidence, and the two systems are independent of one another.​‌​​‌​

Positioning 420C in one sentence​‌​​‌​

420C is the steel for parts that must cut but will not be abused. When carbon reaches the 0.43–0.50 % band two things happen at once: (1) more carbon dissolves into the matrix during austenitising, the martensite is distorted further and hardness can reach the 55–57 HRC band; (2) the carbon that does not dissolve raises the chromium carbide volume, pulls chromium out of the matrix, and lowers both toughness and corrosion resistance. These two outcomes cannot be separated. Choosing 420C means buying edge retention and paying for it with toughness and corrosion resistance. If the part will see impact, prying or torsion, 420B is the right choice; if it will only cut and will be kept dry, 420C is right.

Standards by Product Form​‌​​‌​

ASME code acceptance

AISI 420C / 1.4034 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 serve as pressure-boundary material in ASME Section VIII Div. 1 or Div. 2 design; it carries no allowable stress in ASME B31.1 or B31.3. This applies to the whole family and is even clearer for 420C: the 50–58 HRC hardness obtained from carbon approaching 0.5 % is far outside the toughness and ductility levels the code accepts. If a customer asks for “ASME-approved 420C”, the correct answer is that no such route exists.​‌​​‌​

The temperature side is equally clear. Mill cards give continuous 650 °C / intermittent 750 °C; those are scaling (oxidation) limits, not load-bearing limits. The real ceiling is the tempering temperature: the whole point of 420C is hardness, and that hardness comes from a 150–200 °C low temper. Such a part must not exceed about 200 °C in service, or it will keep tempering in service and lose its edge.

Product Forms With NO Standard​‌​​‌​

Chemical Composition​‌​​‌​

Why things change at 0.5 % carbon​‌​​‌​

In a 13 % chromium steel the behaviour changes qualitatively once carbon reaches about 0.45 %. There is a ceiling on how much carbon the matrix can dissolve at the austenitising temperature; carbon above that ceiling remains undissolved and sits in the matrix as primary chromium carbide (M₂₃C₆). The consequences: (1) Those carbides are hard and brittle; they raise wear resistance but behave as crack initiation sites — toughness falls. (2) Every carbide pulls chromium out of the surrounding matrix; less free chromium remains to feed the passive film, and corrosion resistance drops visibly relative to 420B. (3) As carbon rises, Ms and Mf fall: a mill card gives Ms ≈ 280 °C, Mf ≈ 130 °C for 1.4034. Even though Mf appears to lie above room temperature, in practice appreciable retained austenite survives the quench, and in 420C this is a far bigger problem than in 420B. (4) Raising the chromium band to 12.5–14.5 % partly offsets the carbide effect — but only partly.

Mechanical Properties​‌​​‌​

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The table below is the real engineering map of 420C. Ø10 mm round specimen, oil-quenched from 1000 °C. Typical values, not guarantees. Placed beside the 420B curve, it shows exactly where the difference comes from.​‌​​‌​

Comparing the two curves settles the 420B-versus-420C decision on its own. At a 200 °C temper 420C gives 100 N/mm² more tensile strength than 420B (1800 against 1700). At that same point, however, elongation drops from 9 % to 6 % and impact energy from 18 J to 14 J. In other words, at the low temper — which is the blade and surgical-instrument condition — 420C is about 6 % stronger but roughly 25 % less tough. And that comparison shows only strength; the real gain is in HARDNESS: at the low temper 420B sits in the 49–55 HRC band while 420C sits in the 50–58 HRC band and its top end reaches 57 HRC. Those 3–4 extra points of hardness translate into a disproportionate gain in cutting life.​‌​​‌​

Physical Properties​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

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The two separate reasons for the forbidden band​‌​​‌​

(1) Temper embrittlement. In alloy steels the 400–600 °C band, and in martensitic stainless steels the 450–600 °C band, embrittles the grain boundaries. A published failure investigation of 420 components that had entered this band found both brittle fracture and intergranular corrosion; the same parts showed a very high corrosion rate and intergranular attack in 5 % nitric acid at 25 °C. (2) 475 °C embrittlement. In matrices above 12 % chromium this appears between 250 and 550 °C, most severely at about 475 °C, through spinodal decomposition of the ferrite; hardness rises while ductility and corrosion resistance fall, and it can be partially reversed at 550 °C. Because 420C is fully martensitic the first mechanism dominates, yet mill cards still carry the “avoid the 475 °C range” warning. The practical rule is identical either way: never stop anywhere between 350 °C and 600 °C.

Retained austenite — the quiet problem in 420C​‌​​‌​

Everyone who heat treats 420C needs to know this. As carbon rises, the martensite finish temperature (Mf) falls; a mill card gives Mf ≈ 130 °C for 1.4034. That value appears to lie above room temperature, but when a high austenitising temperature is chosen (1050 °C and above) more carbon dissolves into the matrix, Ms and Mf shift further down, and untransformed austenite survives the quench. The consequences: (1) Hardness comes out below expectation — if the laboratory says 55 HRC and you measure 51 HRC, this is the first place to look. (2) Dimensions change over time — retained austenite transforms to martensite over months and the part grows; unacceptable in precision moulds, gauges and bearing faces. (3) The edge micro-cracks — late-transforming martensite creates internal stress around itself. The fix: keep the austenitising temperature mid-window (1000–1030 °C), apply a sub-zero step after quenching, and double temper.

Welding​‌​​‌​

420C is not welded — it really is that simple. One medical-alloy producer marks its weldability directly as “limited”. The reason is straightforward: in a martensitic steel at 0.45 % carbon, the HAZ transforms on cooling into extremely hard, completely untempered martensite, and that zone is practically an invitation to hydrogen-assisted delayed cracking. The risk that can be barely managed in 420B becomes unmanageable in 420C.

Machining

Corrosion — the Weak Side of 420C

Let us be plain: 420C is the least corrosion-resistant member of the 420 family, and that follows directly from the carbon. Even so, delivered in the right condition, it has served in surgical instruments and kitchen cutlery for decades. What decides the outcome is not the chemistry but the final condition.​‌​​‌​

420 · 420B · 420C · 440C — an Honest Comparison​‌​​‌​

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This table can be shown to a customer directly. None of them is “better”; each is a different trade-off point.

Frequently Asked Questions

What is the real difference between 420C and 440C? Both are sold as “hard stainless”.​‌​​‌​

They are different families doing different jobs. 420C has 12.5–14.5 % chromium and 0.43–0.50 % carbon; 440C has 16–18 % chromium, 0.95–1.20 % carbon and up to 0.75 % molybdenum. That roughly twofold difference in carbon means a far larger primary carbide volume in 440C. The consequences: (1) Wear resistance: 440C wins clearly. For bearing races, valve faces and cutting in abrasive environments, 440C is the right choice. (2) Hardness: both can reach the 58–60 HRC band; 440C holds it more easily. (3) Toughness: 440C is worse — the larger the carbide volume, the more crack initiation sites. (4) Corrosion: this is the surprising one. Despite its higher chromium, so much of 440C’s carbon locks chromium into carbides that the free chromium in the matrix is not as high as the nominal figure suggests; in practice the corrosion resistance of 440C and 420C is comparable, and neither resists chlorides. (5) Sharpenability: 420C is far easier to sharpen in the field; 440C’s carbides wear out abrasives. Decision rule: if wear and cutting life come first, 440C; if easy sharpening, lower cost and slightly better toughness matter more, 420C.

I ordered 420C and heat treatment produced 51 HRC, but I expected 56 HRC. What went wrong?​‌​​‌​

There are three possibilities, in this order of likelihood. First and most likely: retained austenite. If you held the austenitising temperature high (1050 °C and above), more carbon dissolved into the matrix, Ms and Mf shifted downwards, and untransformed austenite survived the quench. Retained austenite is soft and drags the measured hardness down. The fix: pull the austenitising temperature back to the 1000–1030 °C band, apply a sub-zero step after quenching, and double temper. Second: insufficient austenitising. The opposite case — if the temperature or soak time was too low, the carbides never dissolved and not enough carbon entered the matrix; in that case the as-quenched (untempered) hardness is already low. Measuring the as-quenched hardness distinguishes the two: the expected value for 420C is the ~50–53 HRC band (sources diverge). Third: the wrong material. Read the carbon value on the certificate — if it is below 0.43 % you do not have 420C, most likely 1.4031 (X39Cr13) or a lower step. With material supplied against an “ASTM A276 Type 420” certificate this is entirely possible, because that specification sets no upper carbon limit and its lower limit is only 0.15 %.

I make kitchen knives. Is 420C enough, or should I move to 1.4116?​‌​​‌​

Look at the use case. 420C (1.4034) is perfectly adequate for a knife that is kept dry, hand washed and sharpened regularly: it gives good edge retention in the 50–58 HRC band, it is inexpensive and it sharpens easily. It falls short in two scenarios. First, the dishwasher: machine detergents contain chlorides and run hot; 420C pits and stains in that environment. Second, salty or acidic food contact: prolonged contact with brine, lemon, tomato or meat juices produces staining and pitting on 420C. If either scenario applies, 1.4116 (X50CrMoV15) is the right answer: it sits in the same carbon band (0.45–0.55 %) but carries 14–15 % chromium, 0.5–0.8 % molybdenum and 0.1–0.2 % vanadium. Molybdenum gives direct protection against chloride pitting; vanadium refines the grain and delivers better toughness at the same hardness (quenched and tempered 55–57 HRC). The price is a higher material cost and slightly harder machining. A third option: if edge retention is not critical and the knife will see impact, stepping down to 420B is also sensible — tougher, easier to sharpen and a little more corrosion-resistant.

The certificate says “UNS S42000”. Does that prove it is 420C?​‌​​‌​

No, it proves nothing. ASTM F899 assigns the same UNS number (S42000) to 420A, 420B and 420C alike. The UNS system does not distinguish these carbon steps. Material bearing “UNS S42000” may be at 0.18 % carbon (420A) or at 0.47 % carbon (420C). The only thing to do is read the carbon value of the heat analysis on the certificate. If it falls in 0.43–0.50 % it is EN 1.4034; if it falls in 0.42–0.50 % it meets ASTM F899 420C (the 0.01-point difference at the lower bound matters on borderline heats). Outside the band, reject it. To avoid the problem in future, change the order text: instead of “AISI 420C” write “EN 1.4034 / X46Cr13, C 0.43–0.50 %, S ≤ 0.015 %”; if you are buying surgical instrument material write “ASTM F899 Type 420C” or “EN ISO 7153-1, 1.4034”. Those phrases bind the supplier to a defined carbon band.

Common Datasheet Errors and Ordering Traps​‌​​‌​

1) “A UNS S42000 certificate confirms 420C.” It does not. The same UNS number is used for 420A, 420B and 420C alike. Read the carbon analysis.
2) “420C and 1.4034 are completely identical.” Almost — but the lower carbon bound differs. EN 1.4034 sets 0.43 % as the floor, ASTM F899 420C sets 0.42 %. On borderline heats that difference changes an accept/reject decision.
3) “420C is the same as 1.2083.” It is not. 1.2083 (X40Cr14) is a mould steel at 0.36–0.42 % C, below 1.4034, and is usually supplied ESR-refined.
4) “420C contains molybdenum and vanadium.” It does not. The Mo- and V-bearing blade grades are 1.4116 (X50CrMoV15) and 1.4112 (X90CrMoV18).
5) “420C is more stainless because it is harder.” The opposite is true. As carbon rises the chromium carbide volume rises, the free chromium in the matrix falls and corrosion resistance DROPS. 420C is the least corrosion-resistant member of the family.
6) “Continuous service 650 °C.” That is a SCALING limit. The real ceiling is the tempering temperature; for a low-tempered blade the practical ceiling is ~200 °C.
7) “420C can be welded.” In practice, no. One producer marks its weldability directly as “limited”; a HAZ at 0.45 % carbon is wide open to hydrogen cracking.
8) “We ordered 420C pipe.” It is not a standard product. No verified pipe/tube specification could be found for 1.4034.
9) “We bought cast 420C.” No such standard grade exists. The cast martensitic grades are CA-15 and CA-40, and neither reaches the 420C carbon band.
10) “1.4034 = BS 420S45” or “1.4034 = SUS420J1”. Both are contradictory. Some sources map 420S45 to 1.4028, and SUS420J1 carries less carbon than 1.4034 (the usual JIS equivalent is given as the top of SUS420J2).
11) “57 HRC is guaranteed.” It is not. 57 HRC is one producer’s “maximum achievable under ideal conditions”. The standard working band is 50–58 HRC, and the value you actually get depends on the austenitising temperature, retained austenite and the temper step.
12) “Polishing is cosmetic.” It is not — it is a corrosion countermeasure. The producer states explicitly that the best corrosion resistance is obtained on a high-gloss polished, metallic bright surface and that resistance depends on the quality of the polish.
13) “We need non-magnetic surgical instruments.” 420C cannot provide that. μr ≈ 700; it is ferromagnetic in every condition. For MR-compatible instruments go to 316L or the titanium family.

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