AISI 430

What is AISI 430?

AISI 430 is a standard ferritic stainless steel containing 16.0 to 18.0 percent chromium, no deliberate nickel and no more than 0.12 percent carbon. The absence of nickel prevents austenite from becoming stable. The steel therefore retains a body-centred cubic ferritic structure at all service temperatures. Heat treatment cannot harden it. The grade is ferromagnetic in every condition, which is relevant to many appliance and trim applications.

The absence of nickel also makes its price dependent on chromium rather than on both chromium and the volatile nickel market. Its physical properties differ from those of austenitic grades. The mean coefficient of thermal expansion from 0 to 100 degrees Celsius is about 10.4 micrometres per metre per degree Celsius, compared with 17.2 for AISI 304. Thermal conductivity at 100 degrees Celsius is about 23.9 W per metre kelvin, compared with 16.3 for AISI 304. A ferritic panel therefore undergoes less thermal movement and transfers heat more rapidly than an austenitic panel of equal size.

AISI 430 differs from AISI 304 mainly in toughness and ductility rather than corrosion resistance, which is only slightly lower. Ferritic steels undergo a ductile to brittle transition and work-harden slowly. AISI 430 cannot tolerate the deep-drawing depths possible with AISI 304. It is used where moderate formability and lower cost are required.

Laxcon Steels lists AISI 430 in its grades reference as a ferritic grade. The same steel is written 430, SUS 430, 1.4016 and S43000.

What is the chemical composition of AISI 430?

Composition limits in weight percent are carbon 0.12 maximum, manganese 1.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum and chromium 16.0 to 18.0.

What is AISI 430 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 430?

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 ferritic 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 of AISI 430

ASTM A276 and ASTM A240 specify different properties because they cover different product forms. ASTM A276 covers bars and shapes. For annealed Condition A 430, it specifies a minimum tensile strength of 415 MPa, a minimum 0.2 percent yield of 207 MPa, a minimum elongation of 20 percent in 50 mm and a minimum reduction of area of 45 percent. These values apply to both hot-finished and cold-finished products. ASTM A240 covers flat rolled plate, sheet and coil. It specifies a minimum tensile strength of 450 MPa, a minimum yield of 205 MPa and a minimum elongation of 22 percent. It limits hardness to 89 HRB or 183 HB.

The density is about 7,700 kg per cubic metre, compared with 7,900 to 8,000 kg per cubic metre for austenitic grades. The elastic modulus is about 200 GPa, which makes the grade slightly stiffer. Its low expansion, high conductivity and high stiffness support its use in element supports, stove trim and heat-exchange hardware. An AISI 304 component with the same geometry would undergo more distortion.

Corrosion behaviour of AISI 430

The pitting resistance equivalent number ranges from 16.0 at the specified composition minima to 18.0 at the maxima. Chromium provides the full value. Atlas Steels describes the grade as resistant to a wide range of corrosive media, including nitric acid and some organic acids. Its resistance to pitting and crevice corrosion is slightly lower than that of AISI 304. Maximum corrosion resistance occurs in a highly polished or buffed condition. The grade performs well in mild indoor environments but is not usually recommended for exterior exposure.

AISI 430 has very high resistance to chloride stress corrosion cracking. This property is common to ferritic grades. The mechanism that affects austenitic steels above about 60 degrees Celsius essentially does not operate in a ferritic structure. In hot, stressed components exposed to chlorides, a ferritic grade can therefore outlast an austenitic grade with a higher PREN.

Heat treatment, embrittlement and welding

Solution annealing requires heating to 815 to 845 degrees Celsius and holding for half an hour per 25 mm of thickness. The material is then cooled slowly in a furnace to 600 degrees Celsius and cooled rapidly in air. Rapid cooling during the final stage prevents the embrittlement caused by slow cooling from 540 to 400 degrees Celsius. A sub-critical anneal at 760 to 815 degrees Celsius, followed by air cooling or a water quench, can be used when full annealing is unnecessary.

Prolonged service at 400 to 600 degrees Celsius makes AISI 430 brittle at room temperature. This effect occurs within the ferrite phase and can be removed by annealing. Oxidation resistance is good in intermittent service up to about 870 degrees Celsius and in continuous service up to about 815 degrees Celsius. At elevated temperatures, AISI 430 develops heavier scale than AISI 304. Its scale is also more difficult to remove by pickling.

Welding requires preheating to 150 to 200 degrees Celsius. A post-weld anneal at 790 to 815 degrees Celsius can reduce embrittlement in the weld metal and heat affected zone. Atlas Steels notes that this treatment does not refine the grains. Grain growth in the ferritic heat affected zone is permanent because no phase transformation occurs to restore the structure, as it does in carbon or martensitic steel. This limits the suitability of AISI 430 for heavy-section fabrication.

Machining and forming

AISI 430 is easier to machine than standard austenitic grades such as AISI 304. It can still gall and adhere to the cutting tool. Lightly drawn bar machines better than fully annealed material. Its lower work-hardening rate makes some bending and forming operations easier than with AISI 304, but its lower ductility restricts severe forming. Where possible, severe bends should place the bend axis at right angles to the rolling direction. The free-machining variant 430F contains added sulphur for high-speed automatic screw machine work. The addition reduces corrosion resistance.

Applications and designations

Applications include dishwasher linings, domestic appliance panels, refrigerator cabinet panels, automotive trim, element supports, stove trim rings, chimney liners, lashing wire and fasteners. Under EN 10088, the European material number is 1.4016 and the grade name is X6Cr17. The UNS designation is S43000. The Japanese designation is SUS 430, also written as SUS430 or SS430. The free-machining variant 430F has the UNS designation S43020 and the European material number EN 1.4105.

Where AISI 430 is not the right choice

AISI 430 is unsuitable for exterior architectural and marine exposure. Its PREN range of 16.0 to 18.0 is below that of AISI 304 and well below that of any molybdenum grade. Outdoor exposure can produce rust staining. AISI 304 is used for general exterior applications, while AISI 316 is used where chlorides are present.

AISI 430 is unsuitable for heavy welded fabrication. Heat treatment cannot reverse grain growth in the ferritic heat affected zone. A thick welded joint therefore remains less tough than the parent metal. Post-weld annealing reduces embrittlement but does not restore grain size. Thin-gauge welded fabrication is routine, but structural thicknesses are not.

AISI 430 is also unsuitable for severe cold forming and for sub-zero or impact-loaded service. Ferritic steels have a ductile to brittle transition near room temperature in ordinary sections. Deep drawing to the depths tolerated by AISI 304 can cause splitting. Prolonged service between 400 and 600 degrees Celsius embrittles the grade. This excludes many warm-service applications unless periodic annealing is practical. Where machining volume has priority over corrosion resistance, 430F provides substantially faster cutting. Where chloride resistance and formability are both required, an austenitic grade is more suitable.