The grades are determined by a chemical formula. Of all the stainless steel alloys, the 300 series grade is the most commonly used, of which, the 304 alloy austenitic steel is the most common type. Its versatility and strength make it the most widely used, and is available in more forms and finishes than any other grade of stainless steel. Approximately 70% of all stainless steel made is austenitic, a nonmagnetic, solid solution of mostly iron and carbon which relates to its primary crystalline structure.
The 300 series grade is the most corrosion resistant, most ductile, and can be easily formed and welded. The 304-grade stainless steel is the ideal grade to use not only because of its incredible welding characteristics, but also because of its balanced austenitic structure. It's widely used in many industrial, architectural, and transportation related applications.
The level of corrosion resistance, or what makes stainless steel stainless, is related to the amount of carbon and chromium used in the finished alloy. The corrosion resistance of the many grades of stainless steels stems from alloying the base iron with at least 10.5 % chromium and a maximum of .15% carbon. The chromium provides a passive film of chromium oxide that rises to the surface, coats the steel, and blocks corrosion from spreading into the metal's internal structure. Therefore, increasing the amount of chromium gives an increased resistance to corrosion.
