What is the hardness of 420 stainless steel plate?
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What 420 Stainless Steel Plate Is
420 is a martensitic stainless steel with a chromium content of roughly 12-14 % and sufficient carbon to respond to hardening. It is the classic cutlery, mould and wear-part grade: corrosion resistance is good but below that of the austenitic grades because of its higher carbon and lower chromium, while hardness and wear resistance are far higher. In the ASTM system it is UNS S42000, covered for plate, sheet and strip by ASTM A240/A240M and for bars and shapes by ASTM A276/A276M. The nearest EN grade is 1.4021, designated X20Cr13.
Hardness by Condition
The single most important point about 420 plate is that hardness is a function of heat treatment, not of the material as delivered. Plate leaves the mill in the annealed condition, soft enough to machine, cut and form, and the buyer then hardens and tempers the finished part.
| Condition | Typical hardness | Comment |
|---|---|---|
| Annealed, as supplied | about 200 HBW (roughly 20 HRC) | Machinable and formable; not a wear surface |
| As quenched from about 980-1010 degrees C | about HRC 55-58 | Fully martensitic, high residual stress, low toughness |
| Tempered at 150-250 degrees C | about HRC 54-58 | Maximum wear resistance; cutting edges and knives |
| Tempered at 400-550 degrees C | about HRC 45-52 | Balanced wear resistance and toughness |
| Tempered at 600-700 degrees C | about HRC 38-45 | Higher ductility for structural and shaft components |
These are typical commercial results from controlled quench and temper practice. The hardness actually achieved depends on austenitising temperature, soaking time, quenching rate, section thickness and the tempering cycle, so a production hardness range should always be verified on a trial piece of the same thickness rather than assumed from a table.
Why Section Thickness Matters
Martensitic grades harden by rapid cooling, and thick plate cools more slowly at its centre than at its surface. A heavy 420 plate may therefore show a hardness gradient through the section, with a softer core and harder faces. Where through-thickness hardness uniformity is a design requirement, the specification should state the hardness at a defined location, such as quarter thickness, and the test method to be used. Air-hardening behaviour in thin sections and oil or polymer quenching in heavy sections are both normal, and slow cooling can produce bainitic or ferritic transformation products that cannot reach the target hardness.
How Hardness Is Verified
Rockwell C is measured under ASTM E18 and is the usual method for hardened martensitic plate above about HRC 20. Brinell testing under ASTM E10 is common for annealed material and for coarser microstructures, while Vickers testing under ASTM E92 is used for thin sheet, case layers and small parts where the indentation of the other methods would be too large relative to the section. Conversion between scales should follow ASTM E140 rather than rule-of-thumb arithmetic. For thin plate, a Rockwell C test on an unsupported or too-thin section gives a misleadingly high reading, so the minimum test thickness for the chosen scale must be respected.
Hardness, Corrosion and Toughness Trade-offs
Increasing hardness in this grade family works against the other two properties. Tempering in the 400-550 degrees C range precipitates carbides and can reduce corrosion resistance compared with a low-temperature temper, and the high carbon content that enables the hardness also consumes chromium in the form of carbides, which lowers the effective chromium in solid solution. 420 should therefore not be selected for aggressive chloride or acid environments even though it is a stainless steel. Where both wear resistance and better corrosion resistance are needed, a higher-alloy martensitic or a precipitation hardening stainless grade is usually the correct answer.
Frequently Asked Questions
Q: What is the maximum hardness of 420 stainless steel?
A: After austenitising at about 980-1010 degrees C and quenching, 420 typically reaches HRC 55-58, which is the usual upper working range quoted for the grade.
Q: What hardness is 420 stainless steel plate supplied in?
A: Plate is normally supplied annealed, at a hardness around 200 HBW, so that it can be machined and formed before the buyer hardens it.
Q: Does tempering reduce the hardness of 420?
A: Yes. Hardness falls as tempering temperature rises, from roughly HRC 54-58 after a low-temperature temper to roughly HRC 38-45 after tempering at 600-700 degrees C.
Q: What is the EN equivalent of 420?
A: Grade 1.4021, designated X20Cr13, with the nearest ASTM designation being UNS S42000 under ASTM A240/A240M and ASTM A276/A276M.
Q: How is hardness measured on 420 plate?
A: Rockwell C under ASTM E18 for hardened plate, Brinell under ASTM E10 for annealed material, and Vickers under ASTM E92 for thin sections, with conversions taken from ASTM E140.
Q: Is 420 stainless steel corrosion resistant?
A: It offers moderate corrosion resistance. Its 12-14 % chromium is lower than the austenitic grades and its carbon content ties up chromium as carbide, so aggressive chloride or acid service requires a higher-alloy grade.
