Achetez de l'acier inoxydable en ligne : Découvrez les propriétés des différents alliages d'acier inoxydable

Stainless steel came into being in the early 20th Century because of an urgent need for better materials. Chemical processing, oil refining, and other new manufacturing activities of the time were quickly surpassing the performance barriers imposed by conventional engineering materials.

It was an English metallurgist, Harry Brearley, who discovered stainless steel while searching for an improved alloy to protect cannon bores. He found that by adding chromium to low carbon steel, the steel became stain resistant. Further research showed that the protection mechanism that inhibited this rust was the formation of a microscopically-tight, self-healing, protective-oxide film on the surface of the metal.

This film has proven resistant to corrosives such as water, air, foods, and alkalis. The oxide is so thin and transparent that it escapes detection by the unaided eye. When scratched, nicked, or otherwise penetrated, a fresh film forms almost instantly on the exposed portion of the metal.

Stainless steel is defined today as a steel alloy containing at least 10% chromium, plus other elements — especially nickel. Generally speaking, stainless steel may be subdivided into four basic families: Austenitic, Ferritic, Martensitic, and Precipitation Hardening.

Austenitic Family

Austenitic stainless steels are iron-chromium-nickel alloys which are hardenable only by cold working. Nickel is the main element varied within the alloys of this class while carbon is kept to low levels. The nickel content may be varied from about 4% to 22% — higher values of nickel are added to increase the ductility of the metal. When chromium is increased to raise the corrosion resistance of the metal, nickel must also be increased to maintain the austenitic structure.

These alloys are slightly magnetic in the cold-worked condition, but are essentially non-magnetic in the annealed condition in which they are most often used.

The austenitic types feature adaptability to cold forming, ease of welding, high-temperature service, and, in general, the highest corrosion resistance.

Austenitic Summary:

• Not hardenable by heat treatment or cold working
• Moderate strength
• Magnetic
• Low resistance to corrosion
• Does not contain nickel

Ferritic Family

Ferritic stainless steels are iron-chromium alloys which cannot be hardened significantly by heat treatment.

The ferritic types are intermediate in their ability to withstand corrosion. Increasing the amount of chromium raises the corrosion resistance of the metal. A chromium content of about 10% is necessary to ensure maximum corrosion resistance. Additional amounts of chromium (up to about 20%) are utilized to further increase the resistance of the metal to oxidation and scaling at elevated temperatures.

Ferritic types are highly resistant to atmospheric oxidation and strong oxidizing solutions. Qualities include adaptability to high-temperature, chemical, and outdoor use. Ferritic stainless steels are magnetic in all conditions.

Ferritic Summary:

• Not hardenable by heat treatment or cold working
• Moderate strength
• Magnetic
• Low resistance to corrosion
• Does not contain nickel

Martensitic Family

Martensitic stainless steels are iron-chromium alloys that contain from 10% to 18% chromium and can be hardened by heat treatment to high strength levels. Type 410 stainless steel is the basic alloy in this grouping. The martensitic types are the lowest in their ability to withstand corrosion.

Adding more carbon to the basic martensitic alloy increases hardness. But, as carbon is increased, chromium content is also increased to as high as 18% to maintain no less than 10% free chromium for corrosion resistance.

Other modified martensitic alloys contain additional elements, such as sulfur or selenium.

Martensitic varieties find major applications in products that must resist atmospheric oxidation, mildly corrosive chemicals, and wet or dry corrosion environments found in steam and gas turbine parts, bearings, and cutlery. The martensitic types are magnetic in all conditions.

Martensitic Summary:

• Hardenable by heat treatment
• Magnetic
• High strength
• Moderate resistance to corrosion
• Contains no nickel

Precipitation Hardening Family

This group of iron-chromium-nickel alloys has a corrosion resistance approaching that of the austenitic types and can be heat treated to high strength levels—approaching that of the hardenable martensitic types—through a special heat-treating cycle.

Type 17-4 is normally supplied from the mill in the solution heat-treated condition (Condition A) when fabrication calls for machining, welding, or cold forming prior to hardening.

The precipitation-hardening types are magnetic in the hardened condition.

Precipitation Hardening Summary:

• Hardenable by heat treatment and aging
• Medium to high strength
• Magnetic
• High resistance to corrosion
• Contains nickel

Stainless Steel Bar Finishes

Hot Rolled, Annealed and Pickled

These are bars that have been hot rolled to shape; they are not cold worked. These are sometimes called "true bars."

Hot Rolled, Annealed and Rough Turned

These are typically, large diameter bars (3 inches and larger). These bars are hot rolled or forged and then the OD is turned. All tolerances are on the plus (+) side. This is an excellent finish for customers who will be machining the bar OD in subsequent operations.

Cold Finished

Cold-finished bars are produced from hot-finished bars by additional operations at room temperature (cold finishing) to improve tolerances, surface finish, and mechanical properties. Because of their shape, cold-finished square, flat, hexagonal, octagonal, and special shape bars are produced from hot-finished bars, usually by cold rolling or cold drawing in straight lengths. When cold-finished bars are required to have higher strength and hardness, they are cold drawn or heat treated, depending upon the composition, cross section, and properties indicated. These bars, in the case of round sections, can subsequently be centerless ground, polished, or smooth turned to improve surface finish or tolerance. Cold drawing, smooth turning, and centerless grinding are all examples of cold finishing operations.

Cold Drawing

Rod, bar, or wire in straight lengths or in coil is pulled through a die to attain the desired diameter or shape.

Turning / Peeling

Sometimes called "peeling," this method removes material from the bar with a stationary cutting tool while the bar spins. This is a very cost effective process to cold finish material to commercial ASTM standards. Typically there is a slight spiral mark left by the tool down the length of the bar, but this can be burnished away if desired.

Centerless Grinding

Centerless grinding is an OD grinding process for long bars. It differs from other cylindrical processes in that the work piece is not mechanically constrained. Instead, the work piece is supported on its own outer diameter by an angular-top work rest blade located between a high-speed grinding wheel and a slower speed regulating wheel of smaller diameter. The angle of the work rest blade helps keep the work piece in contact with and under the control of the slower regulating wheel.

Centerless grinding is able to actually improve the roundness of out-of-round bars. This makes it a popular pre-processing option for bars in many screw machine applications. Centerless grinding allows for very tight finish tolerances, typically ±0.0005 inches.