Martensitic

What defines the martensitic family?

Martensitic stainless steels contain about 11.5 to 18 percent chromium and about 0.1 to 1.2 percent carbon. Heating into the austenitic range followed by quenching produces hard martensite. This stainless steel family hardens in the same manner as tool steel. It remains magnetic in every condition. A magnet can therefore distinguish a 400 series bar from a 300 series bar, but it cannot identify individual 400 series grades.

Carbon content governs the balance between hardness and corrosion resistance. Higher carbon produces harder martensite and improves edge retention. It also binds more chromium in carbides. This reduces the chromium available in the matrix to form the passive film. Hardened 440C therefore retains an edge better than 410 used for a valve stem, but provides less corrosion resistance.

The Laxcon Steels grade reference lists 46 martensitic grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Properties and applications of 410, 420, 431 and 440C

AISI 410, AISI 420, AISI 431 and AISI 440C represent the principal combinations of hardness, strength, toughness and corrosion resistance within the family.

AISI 410 contains 0.08 to 0.15 percent carbon and 11.5 to 13.5 percent chromium. It is a general-purpose hardenable grade with moderate hardness. It retains sufficient chromium in solution for corrosion resistance in mild environments. Common applications include valve and pump shafts, fasteners, turbine blades and machine parts that require strength with some corrosion resistance rather than edge retention.

AISI 420 contains at least 0.15 percent carbon and 12.0 to 14.0 percent chromium. It provides greater hardness and wear resistance than 410, with lower corrosion resistance after hardening. Applications include cutlery, blades and plastic moulds. Plastic mould applications use its combined hardness and polishability.

AISI 431 contains up to 0.2 percent carbon, 15.0 to 17.0 percent chromium and 1.25 to 2.5 percent nickel. Nickel maintains hardenability at a chromium content that would otherwise produce a partly ferritic structure. The grade combines the highest corrosion resistance among the common martensitic grades with high strength and good toughness. Applications include marine shafting, pump components and aerospace fittings.

AISI 440C contains 0.95 to 1.2 percent carbon, 16.0 to 18.0 percent chromium and up to 0.75 percent molybdenum. It develops the highest hardness of any stainless steel. Applications include bearings, ball valve components, surgical instruments and premium blades. Its corrosion resistance after hardening remains modest relative to its chromium content because a substantial proportion of the chromium is present in carbides.

Heat treatment and processing condition

The service properties of martensitic stainless steel depend on its heat treatment condition. The same bar may be supplied annealed and soft for machining, hardened and low-tempered for maximum hardness, or hardened and high-tempered for strength and toughness. Each condition produces different service properties.

Tempering at about 400 to 600 degrees Celsius causes a marked reduction in toughness. The exact window is grade-specific and is given on each grade page. It also reduces corrosion resistance in most grades. Tempering is therefore performed at either a low or a high temperature rather than within this intermediate range. Corrosion resistance also depends on the chromium retained in solution. Over-tempering or slow cooling can cause a part to rust in an environment that the same grade withstands after correct treatment. Rust on a hardened blade grade commonly indicates incorrect heat treatment rather than incorrect material.

AISI 416 and a resulphurised form of 420 contain deliberate sulphur additions for machining. AISI 416 is based on 410 and contains 0.15 percent sulphur. It is the free-machining martensitic grade. Sulphide inclusions assist chip breaking but also initiate pits and cracks. Both sulphur-bearing grades have substantially lower corrosion resistance than their parent grades and are effectively unweldable.

Questions about martensitic grades

Pitting resistance across the martensitic 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.

Martensitic 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 martensitic family is not the right answer

Martensitic stainless steel is unsuitable when high corrosion resistance is the principal requirement. Chromium present in carbides does not protect the surface. Even the most corrosion-resistant grades in this family perform below 304 in most environments and well below 316 in chloride environments. Applications requiring both corrosion resistance and hardness instead require a precipitation-hardening grade, a surface-treated austenitic part, or a lower hardness compatible with the environment.

The family is also unsuitable for welded fabrication without controlled thermal treatment. The heat-affected zone hardens during cooling and can crack unless preheating and post-weld tempering are applied. Free-machining grades cannot be welded. Martensitic grades are brittle at low temperature and are unsuitable for such service. A specification that omits the heat treatment condition defines only the chemical composition and leaves the relevant mechanical and corrosion properties undefined.

Martensitic grades in detail

Product forms Laxcon lists in martensitic steel