Not All Steels Are Alike

To the average layperson, “stainless steel” is a straightforward descriptor for a particular kind of alloy. In industry, though, it’s rarely this simple. The term “stainless steel” refers to a whole class of steel alloys, which share many properties but still differ in other ways. When a product specification refers to something like custom 465 stainless steel, what does that mean?

What Makes Steel Stainless?

Broadly speaking, stainless steels are steel (i.e. iron-carbon) alloys with a minimum of 10.5% chromium content by mass, as well as a maximum of 1.2% carbon content by mass. Among the common properties of stainless steels, high corrosion resistance is most notable, stemming from the formation of a chromium oxide layer preventing conventional rust formation. This trait alone makes stainless steel a favored material for many applications from cutlery and surgical tools to large-scale construction and equipment in chemical treatment plants.

Grades and Specifications

Stainless steels vary by the ratio of iron, chromium and carbon, as well as the inclusion of other elements like titanium, molybdenum, nickel and niobium—all of these influence the physical, mechanical and chemical properties demonstrated. Primarily, these steels are divided into four families based on the crystalline structure caused by metal inclusions:

  • Austenitic, often with moderately high nickel or manganese content;
  • Ferritic, which arises from higher chromium and no nickel;
  • Martensitic, a broad family that includes many compositions;
  • Duplex, a austenitic/ferritic mix with a high chromium/nickel ratio, as well as around 5% molybdenum.

These families are also divided into over 150 grades of stainless steel, with 15 making up the majority of usage. The grade of stainless steel, denoted by a number, corresponds to specific values for mechanical properties and varying resistance to particular corrosive agents. While the sheer variety of stainless steels can seem boggling, these established systems inform manufacturing and allow easy reference to desired alloys, making it possible for suppliers, manufacturers and clients to communicate effectively.