# Type 420 stainless steel

Hardenable martensitic UNS S42000: carbon variants, heat-treated properties, surface condition and fabrication limits.

Reviewed: 2026-11-01

Tags: materials, stainless-steel, martensitic

## Identity and carbon-specific products

ATI tested **0.25% carbon Type 420 sheet**, hardened at **1038°C for 60 minutes per inch of thickness** and air cooled to room temperature. After a **two-hour temper**, it reports **48 HRC at 204°C** versus **23 HRC at 649°C**. These are typical results for the stated sheet and treatment. [ATI heat-treated mechanical tests](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/martensitic/ati_410_420_425_mod_440a_440c_tds_en2_v2.pdf#page=1)

Type 420, UNS S42000, is a heat-hardenable martensitic stainless steel. ATI's sheet/strip product page lists chromium 12–14% and carbon **at least 0.15%** by mass, and explicitly says mechanical properties vary with heat treatment. That broad carbon description does not define one universal hardness or one heat-treatment response. [ATI, 420 product page](https://www.atimaterials.com/Products/Pages/ati-420.aspx)

Outokumpu's Dura bulletin maps several products to S42000: Dura 420/4021, /4028, /4031 and /4034, with EN numbers 1.4021, 1.4028, 1.4031 and 1.4034 respectively. These are distinct products within a broad designation family, not interchangeable spellings. [Outokumpu, Dura data sheet, product table](https://www.outokumpu.com/-/media/files/products/dura/outokumpu_dura_datasheet.pdf#page=1)

The producer's web range illustrates the chemistry difference: Dura 420/4021 has typical carbon 0.2% and chromium 13%, while /4028 has typical carbon 0.3% and chromium 12.5%. These are named-product typical compositions, not acceptance limits for all Type 420. [Outokumpu, Dura product range](https://www.outokumpu.com/en/products/product-ranges/dura)

Specify the exact producer variant or bounded carbon range, form, dimensions, governing standard and required final condition. Avoid substituting a catalog's generic “420” for a drawing that requires a particular EN grade, carbon range or qualified manufacturing route.

## Supplied condition versus finished condition

A material record should distinguish annealed stock for fabrication from hardened-and-tempered material for service. ATI explains that hardening forms martensite and tempering restores some ductility. [ATI, martensitic stainless technical data sheet, structure and heat treatment](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/martensitic/ati_410_420_425_mod_440a_440c_tds_en2_v2.pdf#page=1)

A requested hardness alone does not define the entire property profile. Record the hardening and tempering route, section size, hardness test method/location and any toughness or corrosion requirements. Confirm that machining, grinding, welding or later heating does not invalidate the final condition. A heat-treatment temperature is not an allowable service-temperature rating.

## Bounded producer test example

ATI publishes **typical** results for sheet samples of its 420 containing **0.25% carbon**. The samples were hardened at 1900°F/1038°C for 60 minutes per inch of thickness, air cooled to room temperature, then tempered for two hours:

| Tempering temperature | Hardness | 0.2% yield strength | Tensile strength |
| --- | ---: | ---: | ---: |
| 400°F / 204°C | 48 HRC | 1311 MPa | 1759 MPa |
| 1200°F / 649°C | 23 HRC | 652 MPa | 838 MPa |

These are comparative results for the stated chemistry and sheet treatment, not guaranteed properties of arbitrary bars, thick sections or other carbon variants. [ATI, typical heat-treated mechanical properties and test description](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/martensitic/ati_410_420_425_mod_440a_440c_tds_en2_v2.pdf#page=1)

For a **comparison only**, the tabulated tensile-strength difference is `1759 − 838 = 921 MPa`, about `921 / 1759 × 100 = 52.4%` of the lower-tempering-temperature result. This does not establish a part's allowable load, toughness or wear life. Do not compare Rockwell numbers as a linear percentage of strength, or infer an unmeasured tensile value from hardness without an applicable validated relationship.

## Fabrication and joining

Carpenter's 420 product range includes bar, flats, hexagons, rounds, squares, billet, strip, wire and rod, with specifications including ASTM A276, A580 and AMS 5621. That range is a producer offering; confirm the actual specification/form combination and available condition for the purchased item. [Carpenter Technology, 420 forms and specifications](https://www.carpentertechnology.com/alloy-finder/420)

Outokumpu warns that traditional martensitic steels above 0.20% carbon are difficult to weld and recommends technical assistance. Its welding discussion identifies hydrogen-related cold-cracking considerations and dependence on composition and restraint. A procedure for a lower-carbon variant cannot simply be transferred to a higher-carbon 420 product. [Outokumpu, Dura fabrication guidance](https://www.outokumpu.com/-/media/files/products/dura/outokumpu_dura_datasheet.pdf#page=1)

Plan fabrication around the required final condition. Define rough machining, heat treatment, finish grinding and inspection allowances before release. For a joined component, qualify filler, thermal sequence, inspection and resulting properties rather than assigning the starting stock's hardness to the weld and heat-affected zone. Where distortion or cracking would be critical, use representative parts or process qualification rather than assuming a small test coupon predicts the full geometry.

## Surface condition and corrosion

ATI's guidance emphasizes removal of heat tint, oxide and surface decarburization for maximum corrosion resistance. Surface preparation affects delivered performance. Use an approved cleaning/passivation process appropriate to the component; a producer discussion is not a universal shop chemical recipe. [ATI, surface preparation](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/martensitic/ati_410_420_425_mod_440a_440c_tds_en2_v2.pdf#page=1)

Outokumpu describes Dura 420/4021 as corrosion resistant in water and steam for low-to-medium-hardness applications. This is a named-product screening statement, not approval for saline water, every steam condition or an arbitrary cleaning chemical. [Outokumpu, Dura 420/4021](https://www.outokumpu.com/en/products/product-ranges/dura)

Document fluid chemistry, chlorides, temperature, deposits, crevices and cleaning/drying practices. Specify the final finish and inspection criteria where staining, pitting or edge integrity matters. An application example such as a knife or surgical instrument does not establish food-contact, medical-device or sterilization qualification for a different finished product.

## Physical-property example and procurement

Outokumpu's Dura table 4 lists density 7.7 kg/dm³, elastic modulus 215 GPa at 20°C and thermal expansion 10.5 × 10⁻⁶/K over 20–100°C for Dura 420/4021. Keep those reference conditions with the data; they do not supply a mechanical design allowable. [Outokumpu, Dura physical properties](https://www.outokumpu.com/-/media/files/products/dura/outokumpu_dura_datasheet.pdf#page=1)

For procurement, retain grade/variant, standard edition, form and dimensions, supplied/final condition, heat identity and inspection certificate. Define final hardness range, test locations, surface acceptance, required toughness and corrosion tests where applicable. Record approved substitutions explicitly.

## Source editions

ATI's bulletin is printed **version 1, February 18, 2014**; the retrieved file name includes “v2.” Outokumpu's Dura sheet is printed **1564EN:4, September 2022**. No printed revision date was verified for the ATI, Carpenter or Outokumpu web product pages. Confirm governing specification editions and current supply separately.

See also [grades, tempers and conditions](../grades-tempers-and-conditions.md).