Ferritic

What defines the ferritic family?

Ferritic stainless steel is a stainless steel family that retains a body-centred cubic ferrite structure from room temperature to melting. It contains 11 to 30 percent chromium and little or no deliberate nickel. It is magnetic in every condition. Quenching cannot harden it because no transformation occurs. It also work hardens much less than an austenitic grade.

The low nickel content gives ferritic grades a price that follows chromium and iron rather than the substantially more volatile nickel market. The ferritic structure is also largely immune to chloride stress corrosion cracking, which can cause failure in austenitic components exposed to hot chlorides. Its main disadvantages are reduced toughness, especially in thick sections and at low temperatures, and limited weldability. Welding causes permanent grain growth in the heat-affected zone.

The Laxcon Steels grade reference lists 26 ferritic grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

AISI 409, 430, 439 and 444

Ferritic stainless steel grades form a general progression of increasing chromium content and added stabilisation. Four grades cover most specified uses.

AISI 409 contains 10.5 to 11.75 percent chromium, no more than 0.03 percent carbon and up to 0.5 percent titanium. It is the least expensive useful stainless steel. Its principal application is automotive exhaust systems, where it must resist condensate and road salt for the service life of a vehicle. It can develop surface rust in outdoor exposure and is not an appearance grade.

AISI 430 contains 16.0 to 18.0 percent chromium and up to 0.12 percent carbon. It is the general-purpose ferritic grade and is often substituted for 304 in appearance applications. It is not stabilised. Welding sensitises AISI 430 and allows corrosion in the weld zone. The grade is therefore used for formed and fastened parts rather than welded parts.

AISI 439 contains 17.0 to 19.0 percent chromium, no more than 0.03 percent carbon and up to 1.1 percent titanium. Stabilisation makes it the weldable equivalent of AISI 430. It is specified when an application comparable to that of AISI 430 requires welding.

AISI 444 contains 17.5 to 19.5 percent chromium, 1.75 to 2.5 percent molybdenum and no more than 0.025 percent carbon. Titanium and niobium provide dual stabilisation. The molybdenum content gives it chloride pitting resistance in the region of the 316 group. Its ferritic structure avoids the chloride stress corrosion cracking that restricts 316 in hot water. Applications include hot water tanks, solar thermal storage and heat exchangers.

Comparison with austenitic stainless steel

Ferritic grades generally resist chloride stress corrosion cracking better than austenitic grades. Hot chlorides under stress can crack 304 and 316 but generally do not crack ferritic grades. This difference supports the use of AISI 444 in hot water cylinders. Ferritic grades also provide greater price stability because they lack the nickel content that dominates austenitic cost. Their thermal expansion is close to that of carbon steel and lower than that of austenitic stainless steel. Their thermal conductivity is higher. Ferritic exhaust and heat exchanger components therefore distort less during thermal cycling and transfer heat more effectively.

Austenitic grades provide greater toughness, better formability into deep shapes and better performance in welded thick sections. Ferritic grades have a ductile to brittle transition. The transition temperature rises with section thickness and grain size. Ferritic products are therefore used mainly as sheet and strip rather than heavy plate. Welding permanently coarsens grains in the heat-affected zone. Welded ferritic components consequently use stabilised grades and thin sections. Heavy welded ferritic construction is generally avoided.

Questions about ferritic grades

Pitting resistance across the ferritic family

Computed by Laxcon Steels from its own grade reference using 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 relationship is published for austenitic and duplex stainless steels, so these figures extend it, and the same source says the number is useful for ranking grades within one family rather than across families. Read the pitting resistance reference before acting on any of these figures: it states what the number does not predict.

Ferritic equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the ferritic family is not the right answer

Ferritic stainless steel is unsuitable when low-temperature or thick-section toughness is required. Its ductile to brittle transition rises with thickness. Cryogenic service, pressure-retaining heavy plate and impact-loaded structures therefore use austenitic or duplex grades instead.

Ferritic stainless steel is also unsuitable for welding in substantial thickness because heat treatment cannot reverse grain growth in the heat-affected zone. It is unsuitable when high hardness is required because quenching produces no transformation. Martensitic stainless steel serves that requirement. Unstabilised AISI 430 is unsuitable for welded components because the weld zone is likely to corrode first. AISI 439 or AISI 444 is used when welding is required.

Ferritic grades in detail

Product forms Laxcon lists in ferritic steel