Physical Properties of Stainless Steel

Stainless steel is also known as inox steel, or simply inox, which comes from the French word inoxydable, which literally translates to or is interpreted as stainless.

Stainless steel, commonly known as SS in the engineering parlance, is a steel alloy with a minimum of 10.5% chromium content by mass. It is highly resistant to corrosion, rusting, and staining and thus, the name "stainless" steel. Despite the name, it is not fully stain-proof, most notably under low-oxygen, high-salinity, and/or poor ventilation conditions.

This variety of steel differs from the regular steel in the amount of chromium present in it, which also alters its properties considerably. Some of the grades of SS also contain other elements―most commonly nickel, molybdenum, and silicone―to impart different physical and chemical properties to it.

Stainless steel is not a single material, but rather the name given to a family of corrosion-resistant, iron-based alloys. This material was invented in 1912 by Harry Brearley of the Brown Firth Research Laboratory in Sheffield, England while trying to find a corrosion-resistant alloy for gun barrels.

The material that Brearley invented was a martensitic steel alloy containing 13% chromium that was subsequently industrialized. The first non-industrial application of this material was in cutlery, for which Sheffield became famous the world over. Simultaneous research work in France led to the development of austenitic stainless steel.

Stainless steel has a broad range of physical properties, but we will see here the ones that are markedly different than the plain carbon steel. This difference in properties is due to the chromium content in SS.

SS is an iron-based alloy containing a minimum of 13% (by weight) chromium to achieve high oxidation resistance at ambient air temperatures. The chromium content may be increased up to 26% for achieving corrosion resistance in a harsh environment.

The chromium in the alloy forms a protective layer of chromium(III) oxide (Cr₂O₃) on the surface when it is exposed to oxygen in the environment. This layer is too thin to be visible, so the surface remains smooth and lustrous. Even though it is thin, the layer is impervious to water and air, protecting the metal beneath.

This layer quickly reforms when the surface is scratched, thus, protecting the alloy regardless of the method of fabrication employed. In case of plain carbon steel, this corrosion resistance has to be achieved by painting the surface or by other methods, like galvanizing.

SS is generally highly resistant to attack from acids, though this property depends heavily on the type and concentration of acid, the ambient temperature, and the type of steel. Type 304 is resistant to sulfuric acid (H₂SO₄) at room temperatures even in high concentrations, but Types 316 and 317 are resistant to it only at low concentrations.

All types of SS are resistant to phosphoric acid (H₃PO₄) , while Types 304L and 430 are resistant to nitric acid (HNO₃). Any type of stainless steel gets damaged by hydrochloric acid (HCl), and hence, the two should never be used together.

The 300 series of SS grades is highly resistant to any of the weak bases, like ammonium hydroxide (NH₄OH), even in high concentrations and at extreme temperatures. But, the same types of stainless steel, when exposed to stronger bases, such as sodium hydroxide (NaOH), or the ones containing chlorides will experience etching and cracking at high temperatures and concentrations.

This property of any material is measured by its ductility or toughness at sub-zero temperatures. The tensile strength of austenitic steels is markedly higher at cryogenic temperature range than at ambient temperatures. The toughness of the steel is also maintained at an excellent level.

Ferritic, martensitic, and precipitation hardening steels should not be used at sub-zero temperatures though, as their toughness drops significantly. In some cases, the drop in toughness could also occur at room temperature.

Work hardenable grades of SS have the advantage that a significant increase in strength can be achieved through cold working. A combination of cold working and annealing can be employed to give a specific strength to the fabricated component.

An example of this is drawing of a stainless steel wire. If the wire is to be used as a spring, it is work hardened to a specific required strength; while if it is to be used as a bendable tie wire, it is annealed to achieve a softer material.

Austenitic stainless steels retain high strength at elevated temperatures. This is particularly true with grades that contain high amount of chromium and/or high amounts of silicone, nitrogen, and rare earth elements. Ferritic grades containing high amount of chromium, like 446, also show a high hot strength. The high chromium content also helps the stainless steel resist scaling at high temperatures.

Ductility of a solid material is its ability to deform under tensile stress; in other words, it's the ability of the material to be drawn into a wire. The ductility of austenitic stainless steel is quite high. This property, combined with high work hardening rates, allows this material to be drawn into a wire using severe processes, such as deep drawing.

When compared to the various mild steels, stainless steels have higher tensile strength. The duplex stainless steel has an even higher tensile strength than the austenitic grades. The highest tensile strengths are seen in martensitic (431) and precipitation hardening grades (17-4 PH). These grades have strengths almost double than that of 304 and 316, which are the most commonly used grades of SS.

Stainless steel is a relatively poor conductor of electricity, similar to plain carbon steel, with only a few percent electrical conductivity of copper. Also, ferritic and martensitic SS are magnetic, while austenitic grades are non-magnetic.

Stainless steel's resistance to corrosion and staining, low maintenance, and luster have made it ideal for many applications, industrial or otherwise.

The alloy is milled into coils, sheets, plates, bars, wires, and tubing to be used in cutlery, cooking utensils, household hardware, surgical instruments, industrial equipment, as an automotive and aerospace structural alloy, and as a construction material in large buildings.

Some of the most famous examples of the use of this material are:

★ Cala Galdana Bridge in Menorca, Spain, which was the first road bridge to be made entirely from SS.

★ The aeration building at Edmonton Composting Facility in Edmonton, Alberta, Canada, which is the largest stainless steel building in North America