Stainless steels-The core application material of industries

Iron and iron alloy such as steel are poor materials as they rust in air, corrode in acids and scale in furnace conditions. There is a group of iron-base alloys, the iron-chromium-nickel alloys are called as stainless steels, that do not rust in sea water, are resistant to concentrated acids and that do not scale at temperatures up to 1100oC.

Good mechanical properties and manufacturing features give the stainless steels their raison and make them vital tool for the designer. The usage of stainless steel is nominal as compared to carbon steels that attain a steady growth, unlike to the constructional steels.

Stainless steels as a family is certainly more non-uniform than the constructional steels and their properties are in diverse cases relatively unfamiliar to the designer. In some ways, the stainless steels are an undetermined world however to take the benefit of these materials will need a better understanding of their basic properties.

Use of stainless steel

Steel is the major industrial constructional material. Dominant product form for stainless steels is cold rolled sheet. The applications are dominated by major sectors: consumer products, equipment for oil and gas plants, chemical process plants, food and beverage industry.

The commonly used stainless steel grades are SS 304 and SS 304l that form more than 50% of the global production of stainless steel. In order to receive a perspective of the development of stainless steels, it is right to consider the background.

Alloying elements have a specific influence on the properties of the steel. It is the combined influence of the allying elements, to some level, the contaminants that determine the feature of a specific steel grade. The influence of alloying elements on the important material properties is evaluated.

The most important feature of stainless steel is their corrosion resistance. The reason for the good corrosion resistance of stainless steels is that they develop a thin, invisible surface film in oxidizing conditions.

Aqueous corrosion refers to corrosion in liquids or moist conditions at temperatures up to 300oC, often in water-based conditions. The corrosion process is electrochemical and needs the presence of an electrolyte in the form of a liquid.

High temperature corrosion allows corrosion in hot gases at temperatures up to 1300oC.

Stainless steels are significantly different from carbon steels in some respects. There are also significant differences between the different categories of stainless steels.  Heat conductivity for stainless steels is usually lower than for carbons steels and reduces with increasing alloying level for every stainless steel category.

Austenitic steels are featured by supreme corrosion resistance, good toughness and excellent weldability, they are the most common stainless steels. The properties of Hastelloy wire grades include resistance to general corrosion, pitting and crevice corrosion and stress corrosion cracking. Low carbon grades attain supreme resistance to intergranular corrosion and as a result higher alloyed steels are only available with low carbon concentrations. Austenitic stainless steels are used in all types of applications and industries. Common areas include piping systems, heat exchangers, tanks and process vessels for food, chemical, pharmaceutical, pulp and paper and other process plants.