AISI 420

What is AISI 420?

AISI 420 is a high-carbon martensitic stainless steel containing 12.0 to 14.0 percent chromium. Its carbon content is 0.15 percent minimum, with no upper limit in the composition table normally provided to purchasers. The grade is primarily a hardening steel. AISI 410 has a maximum carbon content of 0.15 percent, while AISI 420 begins at that level. The higher carbon content allows quenched martensite to attain greater hardness.

ASTM A276 distinguishes AISI 420 from AISI 410 through their responses to heat treatment. AISI 410 must attain at least 35 HRC after austenitising and quenching. AISI 420 must attain at least 50 HRC after austenitising from a minimum temperature of 995 degrees Celsius and air quenching. This value is the highest minimum hardness among the 12 percent chromium grades.

AISI 420 has lower corrosion resistance and toughness than lower-carbon alternatives. Its carbon binds a significant proportion of the chromium in carbides. This reduces the chromium available in the matrix to maintain the passive film. The grade therefore has slightly lower corrosion resistance than AISI 410 and appreciably lower resistance than the 17 percent chromium ferritic grade AISI 430.

Laxcon Steels lists AISI 420 in its grades reference as a martensitic grade. The same steel is written 420, SUS 420J1, 1.4021 and S42000.

What is the chemical composition of AISI 420?

Composition limits in weight percent are carbon 0.15 minimum, manganese 1.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum and chromium 12.0 to 14.0.

What is AISI 420 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 420?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The published relationship is given for austenitic and duplex stainless steels, so this figure extends it: read it against other martensitic grades rather than across families. The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties in annealed and hardened conditions

ASTM A276 specifies no tensile strength or yield strength for AISI 420 in Condition A. It specifies a maximum hardness of 241 HB for hot-finished annealed bar and 255 HB for cold-finished bar. Atlas Steels reports typical annealed values of about 655 MPa tensile strength, 345 MPa 0.2 percent proof stress and 25 percent elongation in 50 mm. These values are informational and are not guaranteed properties.

Heat treatment produces a wide range of hardened properties. Atlas Steels reports about 1,600 MPa tensile strength, 1,360 MPa proof stress, 12 percent elongation, 444 HB and 20 J Charpy V after tempering at 204 degrees Celsius. Tempering at 593 degrees Celsius produces about 1,035 MPa tensile strength, 810 MPa proof stress, 18 percent elongation, 302 HB and 22 J. Tempering at 650 degrees Celsius produces about 895 MPa tensile strength, 680 MPa proof stress, 20 percent elongation and 262 HB. The strength curve peaks near 427 degrees Celsius at roughly 1,620 MPa tensile strength and 461 HB. This temperature is within the prohibited tempering range.

Heat treatment and tempering range

Full annealing consists of holding at 840 to 900 degrees Celsius, cooling slowly in a furnace to 600 degrees Celsius and then cooling in air. Process annealing between forming operations uses 735 to 785 degrees Celsius followed by air cooling. Hardening requires heating to 980 to 1,035 degrees Celsius and then quenching in oil or air. Heavy sections require an oil quench. The working tempering range is 150 to 370 degrees Celsius.

Tempering between 425 and 600 degrees Celsius must be avoided. Impact resistance declines sharply in this range while hardness remains high. Atlas Steels identifies its 427 and 538 degrees Celsius data rows as unsuitable for this reason. Blades and dies are commonly tempered at 150 to 250 degrees Celsius. This treatment relieves quench stress while retaining the required hardness.

Corrosion behaviour and surface condition

AISI 420 has a pitting resistance equivalent number of 12.0 at the specified composition minima and 14.0 at the maxima. In the hardened condition, it has good resistance to the atmosphere, foods, fresh water and mild alkalis or acids. Its resistance is distinctly lower in the annealed condition. Annealing precipitates carbon as chromium carbide and reduces the chromium available to support the passive film.

Surface condition also affects corrosion resistance. The best resistance is obtained by hardening followed by surface grinding or polishing. Cutlery blades are therefore polished rather than left as-ground. Atlas Steels states that AISI 420 can resist food and normal washing. Prolonged contact with unwashed food residues can cause pitting, including contact with acidic residue left on a blade overnight.

Machining, welding and forming

AISI 420 has reasonable machinability in the annealed condition. Machining becomes markedly more difficult above about 30 HRC. Components are therefore normally machined in the soft condition, hardened and then ground to final dimensions. Free-machining variants with added sulphur are available for high-volume turned work, with a corresponding reduction in corrosion resistance.

Welding requires preheating to 150 to 320 degrees Celsius and post-heating at 610 to 760 degrees Celsius. Matching 420 coated electrodes are used for high-strength joints that will subsequently be hardened and tempered. Austenitic 309 filler produces a ductile joint when the part will remain in the as-welded condition. AISI 420 is not a fabrication steel, and designs normally avoid welded joints.

Applications and designations

AISI 420 is used for cutlery and knife blades, surgical and dental instruments, scissors and shear blades, needle valves, plastic injection moulds and dies, bearing components, and general wear parts that also require resistance to mild corrosion. Its European designation is material number 1.4021, with the name X20Cr13. Its UNS number is S42000, and its Japanese designation is SUS 420J1. The related SUS 420J2 has a higher carbon content and provides greater hardness. Purchasers also use the unspaced forms SUS420J1 and SS420.

Where AISI 420 is not the right choice

AISI 420 is unsuitable where chlorides or standing moisture are present. Its PREN range of 12.0 to 14.0 is among the lowest for stainless grades. It pits readily in seawater, coastal air, brine and chlorinated water. The annealed condition is also unsuitable for corrosion service because it is measurably less resistant than the hardened condition. A specification that omits heat treatment and surface finish does not adequately define the expected corrosion performance.

AISI 420 must not be tempered between 425 and 600 degrees Celsius because impact resistance decreases without a corresponding reduction in hardness. It must not operate above its tempering temperature because tempering continues during service and reduces hardness. Its scaling temperature is only about 650 degrees Celsius. The ductile to brittle transition of martensitic stainless steels also excludes sub-zero service.

The grade is unsuitable when hardness above its available range is required. The 440 series contains more carbon and reaches 55 to 58 HRC. AISI 431 is more suitable when corrosion resistance must increase without loss of hardenability because it contains 15.0 to 17.0 percent chromium with nickel. AISI 416 is more suitable when machining time controls grade selection because it machines much faster. Martensitic stainless steel with the carbon level of AISI 420 is also unsuitable when welding is a primary requirement.

Other martensitic grades