Stainless steel

Family overview

Stainless steel is an alloy steel in which chromium is added to iron so that a thin, dense chromium oxide film (the passive film) forms on the surface. The film re-forms after scratching as long as oxygen is present, which is why the steel resists rust in air and water. In chloride-rich environments, in acids with little oxidizing power and at high temperature, however, the film may not hold, so stainless steel does not resist every environment.

Stainless steels are divided by their room-temperature structure into austenitic, ferritic, martensitic and precipitation-hardening types. Magnetism, whether the steel hardens by heat treatment, weldability and formability differ greatly between these structures, so choosing on corrosion resistance alone can cause problems later in processing.

The grade name itself offers clues. A number after SUS beginning with 3 generally indicates an austenitic grade and one beginning with 4 a ferritic or martensitic grade, while SUS630 is a precipitation-hardening grade. A final F often marks a free-machining grade with added sulfur and a final L a low-carbon grade; some free-machining grades, such as SUS303 and SUS416, carry no F.

Heat-resisting steel (SUH) shares chromium as its base element with stainless steel, but its purpose is to resist oxidation and keep its strength at high temperature rather than to prevent rust. Engine inlet and exhaust valves are repeatedly exposed to hot combustion gas and impact and rub against the valve seat, so oxidation resistance, high-temperature strength and wear resistance are needed together. On this page SUH3 and SUH11 are martensitic and SUH35 is austenitic.

As a rough guide, austenitic grades are the candidates when corrosion resistance and formability come first, and ferritic or martensitic grades when magnetism is needed. Martensitic grades are chosen when hardness and wear resistance must come from heat treatment, and precipitation-hardening grades when high strength is needed with little distortion from heat treatment. Heat-resisting steel is used where oxidation and strength at high temperature are the main concern. For parts with heavy machining, free-machining grades are compared as well, with attention to their lower corrosion resistance.

Role of the main elements

• Chromium (Cr): The basic element of stainless steel, forming the passive film on the surface. In general, more chromium improves corrosion resistance and high-temperature oxidation resistance, and chromium also stabilizes the ferrite structure. In martensitic grades, chromium carbides formed with carbon also contribute to hardness and wear resistance.
• Nickel (Ni): Stabilizes the austenite structure, making the annealed steel non-magnetic and raising toughness and formability. This is why austenitic grades keep their toughness at low temperature and are easy to work. In martensitic grades with a small nickel addition, such as SUS431, it reinforces corrosion resistance and toughness.
• Molybdenum (Mo): Helps the steel resist pitting and crevice corrosion in chloride environments, which is why the SUS316 family is more often considered than the SUS304 family for seawater and salt exposure. In heat-resisting steel it also raises high-temperature strength.
• Carbon (C): In martensitic grades it governs as-quenched hardness and wear resistance. More carbon makes the steel harder, but chromium is tied up as carbides, so corrosion resistance and toughness tend to fall. In austenitic grades the opposite applies: lower carbon reduces the risk of intergranular corrosion after welding.
• Sulfur (S): Added to improve machinability and present in free-machining grades such as SUS303, SUS416, SUS420F and SUS430F. Sulfides formed with manganese and other elements break the chips short, which helps tool life and the machined surface. In return the sulfides tend to become starting points for corrosion and to reduce formability, weldability and transverse toughness, so corrosion resistance is generally lower than that of the standard grade of the same family.
• Copper (Cu) and niobium (Nb): Used in SUS630. Copper forms fine precipitates during aging and raises strength; niobium ties up carbon as carbides and reduces the chance of chromium carbides forming.
• Nitrogen (N) and manganese (Mn): Used in place of part of the nickel to stabilize the austenite structure and raise strength. The austenitic heat-resisting steel SUH35 is a typical example.
• Silicon (Si): Forms a dense oxide film at high temperature and suppresses oxidation and scale formation. It is a typical element for oxidation resistance in the martensitic heat-resisting steels SUH3 and SUH11, although toughness and workability tend to fall as the content rises.

Heat treatment and delivery condition

• Austenitic grades (SUS303, SUS304, SUS304L, SUS316, SUS316L): The standard condition is solution annealed, that is, heated to dissolve carbides and then cooled rapidly. These grades do not harden by quenching. Strength is raised by cold working such as cold drawing, and heavy working can produce weak magnetism depending on the grade.
• Low-carbon (L) grades and sensitization: When an austenitic grade is welded or held in an intermediate temperature range for a long time, chromium carbides can precipitate at the grain boundaries, leaving the steel prone to intergranular corrosion; this is called sensitization. SUS304L and SUS316L lower the carbon content to reduce this risk and are widely used in welded structures, although their room-temperature strength can be somewhat lower than that of the standard grades.
• Ferritic grade (SUS430F): Used in the annealed condition and not hardened by quenching. It is magnetic, and its formability and weldability are poorer than those of austenitic grades. Prolonged exposure to an intermediate temperature range can embrittle it, so the service temperature is checked as well.
• Martensitic grades (SUS416, SUS420F, SUS420J1, SUS420J2, SUS431, SUS440C): Heated to the austenitizing temperature, quenched and then tempered to balance hardness and toughness. The lower the tempering temperature, the harder and more wear resistant the steel; a higher tempering temperature lowers hardness but improves toughness. In some tempering ranges toughness and corrosion resistance can fall together, so tempering conditions are confirmed against the standard and manufacturer data.
• Effect of carbon content: SUS420J1 and SUS420J2 are both martensitic, but SUS420J2 has the higher carbon content and can reach a higher hardness after quenching. SUS440C has a still higher carbon content and can reach high hardness and wear resistance, at the cost of lower toughness.
• Precipitation-hardening grade (SUS630): The usual sequence is to receive it solution treated, machine it and then raise strength by aging. A lower aging temperature moves the properties toward higher strength and a higher one toward toughness and stress corrosion cracking resistance. Dimensional change on aging is small, so the grade is used for precision parts that are heat treated after machining; the aging condition is specified by the condition designation in the standard.
• Heat-resisting steels (SUH3, SUH11, SUH35): SUH3 and SUH11 are martensitic and are used quenched and tempered after austenitizing; SUH35 is austenitic and gains its strength from solution treatment followed by aging. For engine valves, room-temperature hardness alone does not show high-temperature performance, so the heat-treatment condition, hardness and structure are specified by the drawing and customer approval requirements.
• Delivery condition: The delivery condition of a bar is set by a combination of heat-treatment condition (annealed, solution treated, quenched and tempered, etc.) and surface finish (hot rolled, cold drawn, peeled, ground, etc.), and dimensional accuracy, straightness and surface condition differ even within one grade. Martensitic and precipitation-hardening grades that are heat treated after machining are often received in the condition before heat treatment. The heat-treatment condition and test items are confirmed on the mill certificate.

Standards cross-reference

Composition range

Confirm the composition range against the applicable standard.

Typical applications

• SUS304 and SUS304L: General-purpose corrosion-resistant parts such as food and beverage equipment, piping and vessels for chemical and pharmaceutical plants, building exteriors and interiors, kitchen and sanitary equipment, and fasteners such as bolts, nuts and screws.
• SUS316 and SUS316L: Parts for coastal installations, chemical and pharmaceutical equipment with chloride exposure, pump and valve parts and medical devices, where the SUS304 family may not give enough corrosion resistance.
• SUS303: Precision machined parts cut on automatic lathes, such as bolts, nuts, screws, shafts, bushings, pins and fittings. It is generally not suited to parts that require welding or severe forming.
• SUS430F, SUS416 and SUS420F: Shafts, pins, screws and pump and valve parts that involve heavy machining. All three are magnetic and therefore unsuited to parts that must be non-magnetic; SUS416 and SUS420F are hardened by heat treatment when required.
• SUS420J1 and SUS420J2: Parts that need hardness and corrosion resistance together after heat treatment. SUS420J1 suits general machine parts such as shafts, valves and nozzles; SUS420J2 is used where higher hardness is needed, such as blades, medical instruments and mold parts.
• SUS431: High-strength shafts, pump shafts, fasteners such as bolts, marine parts and valve parts that need corrosion resistance and toughness together.
• SUS440C: Bearing parts, valve seats and balls, nozzles, blades and mold parts that need high hardness and wear resistance.
• SUS630: Shafts, valve and pump parts, aerospace structural parts, chemical and petroleum equipment parts, mold parts and fasteners that need high strength and corrosion resistance together.
• SUH3, SUH11 and SUH35: Inlet and exhaust valves of gasoline and diesel engines. In general the martensitic SUH3 and SUH11 are used for inlet valves and for exhaust valves at comparatively low temperature, and the austenitic SUH35 for exhaust valves exposed to higher temperature. The grade actually applied is set by the engine type, fuel, valve temperature conditions and the drawing.

Products covering these grades

• Stainless steel · specialty sourcingSUS303 · SUS304 · SUS304L · SUS316 · SUS316L · SUS430F · SUS416 · SUS420F · SUS420J1 · SUS420J2 · SUS440C · SUS630
• Valve materials & heat-resistant steel sourcingSUS316L · SUS416 · SUS431 · SUS630 · SUH3 · SUH11 · SUH35