AISI 303

What is AISI 303?

AISI 303 is a free-machining austenitic stainless steel based on an 18/8 composition. It contains 17.0 to 19.0 percent chromium, 8.0 to 10.0 percent nickel and at least 0.15 percent sulphur. Carbon may reach 0.15 percent, and phosphorus may reach 0.20 percent. The sulphur produces manganese sulphide inclusions within the steel. These inclusions initiate chip fracture and lubricate the cutting edge. Atlas Steels gives the machinability of 303 as approximately 78 percent of free-cutting carbon steel B1212, which serves as the benchmark for stainless grades.

AISI 303 is intended mainly for components machined from bar on lathes and automatic screw machines. In this type of production, its advantage over AISI 304 is shorter cycle time and longer tool life.

The sulphur addition distinguishes 303 from AISI 304. Manganese sulphide inclusions interrupt formation of the passive film. The grade therefore has lower corrosion resistance, poorer weldability and lower formability than AISI 304. The addition improves machining performance rather than stainless steel performance.

Laxcon Steels lists AISI 303 in its grades reference as an austenitic grade. The same steel is written 303, SUS 303, 1.4305 and S30300.

What is the chemical composition of AISI 303?

Composition limits in weight percent are carbon 0.15 maximum, manganese 2.0 maximum, sulphur 0.15 minimum, phosphorus 0.2 maximum, silicon 1.0 maximum, chromium 17.0 to 19.0, nickel 8.0 to 10.0 and zirconium plus molybdenum 0.6 maximum.

What is AISI 303 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 303?

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 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 and physical properties under ASTM A582

ASTM A582 is the specification for free-machining stainless steel bar. For AISI 303 in annealed Condition A, it specifies a maximum hardness of 262 HB. It does not specify guaranteed tensile or yield strength.

Published strength figures are typical values rather than specification requirements. Atlas Steels gives a typical tensile strength of 650 MPa, a typical 0.2 percent proof stress of 300 MPa and a typical elongation of 45 percent in 50 mm for annealed 303 bar. These values are for information only and are not guaranteed. Drawn bar, generally up to about 25.4 mm in diameter, has higher strength. Its proof stress is significantly higher, while its elongation is lower. A guaranteed minimum yield strength must therefore be agreed with the mill when required on a drawing.

Typical physical properties include a density of about 7,900 kg per cubic metre and an elastic modulus of 193 GPa. Thermal conductivity is around 16.3 W per metre kelvin at 100 degrees Celsius. The mean coefficient of thermal expansion is roughly 17.3 micrometres per metre per degree Celsius from 0 to 100 degrees Celsius.

Corrosion behaviour

The pitting resistance equivalent number ranges from 17.0 at the specified composition minima to 19.0 at the maxima. This calculation does not account for manganese sulphide inclusions. These particles initiate pits, so pitting occurs at lower chloride concentrations and lower potentials than the composition alone indicates.

Corrosion resistance varies with bar orientation. Rolling elongates the sulphide inclusions along the bar. A machined cross-section exposes more inclusion ends per unit area than the bar surface. Cut faces therefore have particularly low corrosion resistance.

Atlas Steels describes 303 as resistant to mildly corrosive atmospheres but significantly less resistant than 304. Marine and similar environments cause rapid pitting corrosion. The grade is also susceptible to chloride stress corrosion cracking above about 50 degrees Celsius. Its unrestricted carbon content permits sensitisation and subsequent intergranular corrosion.

Machining, welding and heat treatment

Solution treatment consists of heating to 1,010 to 1,120 degrees Celsius followed by rapid cooling. Thermal treatment cannot harden the grade. Cold work is the only means of increasing its strength.

Welding is not generally recommended. When welding is unavoidable, 308L or 309 consumables are used. The welds require subsequent annealing for maximum corrosion resistance. Atlas Steels states that poor mechanical and corrosion properties will remain. Sulphide inclusions promote solidification cracking in the weld metal and heat affected zone, and welding procedures cannot remove them.

AISI 303 permits higher surface speeds and heavier feeds than AISI 304. Light dwelling cuts allow the surface to work-harden ahead of the tool and reduce this machining advantage. Several mills produce improved-machinability variants through controlled inclusion shape and steelmaking practice. These variants permit higher machining rates during long repetition runs.

Applications and designations

Applications include nuts, bolts, studs, machined fasteners, bushings, shafts, gears, electrical switchgear components, instrument parts and valve parts. The grade is used for heavily machined components when its corrosion and fabrication properties are acceptable.

The European designation is material number 1.4305, with the name X8CrNiS18-9. The UNS designation is S30300. The Japanese designation is SUS 303, also written as SUS303 or SS303. The selenium-bearing variant 303Se, designated UNS S30323, replaces sulphur with selenium. This improves hot and cold forming and machined surface finish but produces slightly lower machining rates. The variant is not widely stocked.

In which product forms does Laxcon list AISI 303?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where AISI 303 is not the right choice

AISI 303 is unsuitable for marine and coastal exposure, brine, chlorinated water and salt-laden air. Chlorides cause rapid pitting at the sulphide inclusions, and surface treatment does not remove this vulnerability. AISI 304 is used instead for general service, while AISI 316 is used where chlorides are present. An improved-machinability form of 304 is generally more suitable when a component requires both extensive machining and corrosion resistance.

AISI 303 is unsuitable for welded or formed components. Sharp bends should not be attempted. Its sulphur content reduces ductility and promotes hot cracking in the weld pool. AISI 304 accepts both welding and forming.

The carbon content may reach 0.15 percent. The grade therefore sensitises readily when held between 425 and 860 degrees Celsius. It is unsuitable for elevated-temperature service followed by aqueous exposure. AISI 416 provides higher machinability when hardenability is more important than corrosion resistance. This martensitic free-machining grade can be hardened by heat treatment, while AISI 303 cannot.

Other austenitic grades