Type 420 stainless steel

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

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

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

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

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

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:

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

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

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

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

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

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

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.