What Is Stainless Steel? Part I

Part I: An Introduction to the World’s Most Popular Metal

“Stainless” steel is actually a generic term referring to a variety of steel types. Like all other kinds of steel, stainless steel is made primarily from iron and carbon in a two-step process. What makes stainless steel different is the addition of chromium (Cr) and other alloying elements such as nickel (Ni) to create a corrosion-resistant product.

Steel corrodes because iron, the metal used to make steel, occurs in nature in combination with other elements. When iron ore is artificially manipulated into a pure form to make steel, it becomes unstable and will readily recombine with oxygen.

When chromium is added to steel, it forms chromium oxide, which acts as a protective surface to prevent air and moisture from causing rust, as happens with ordinary steel. Chromium is added in quantities ranging from 10.5 to 30%, depending on the application or environment in which the steel is to be used. There are more than 100 different grades of stainless steel but they can be grouped into five major types:

Austenitic is the most widely used type of stainless steel. It has excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. Austenitic steel is used in housewares, industrial piping and vessels, construction, and architectural facades.

Ferritic stainless steel has similar properties to mild steel (the most common steel), but better corrosion, heat, and cracking resistance. Ferritic steel is commonly used in washing machines, boilers and indoor architecture.

Martensitic stainless steel is very hard and strong, though it is not as resistant to corrosion as austenitic or ferritic grades. It contains approximately 13% chromium and is used to make knives and turbine blades.

Duplex stainless steel is a composite of austenitic and ferritic steels, making it both strong and flexible. Duplex steels are used in the paper, pulp, shipbuilding, and petrochemical industries. Newer duplex grades are being developed for a broader range of applications.

Martensitic or semi-austenitic steels can also be classified as precipitation hardening stainless steels. These steels are made to be extremely strong with the addition of elements such as aluminum, copper and niobium.

Corrosion resistance is the main advantage of stainless steel, but it certainly isn’t the only one. Stainless steel is also:

• High and low temperature resistant

• Easily fabricated

• Strong and durable

• Easy cleaned and maintained

• Long lasting, with a low lifecycle cost

• Aesthetically attractive

• Environmentally friendly and recyclable.

In addition to chromium, stainless steels are made with alloys of silicon, nickel, carbon, nitrogen, and manganese. Nitrogen, for example, improves tensile properties like ductility. Nickel is added to austenitic steel to improve flexibility. These alloys are added in varying amounts and combinations to meet specific end-use applications, which is why it’s very important for stainless steel manufacturers to verify that the correct percentages of each alloy are being used. There are two technologies that provide the elemental analysis needed to produce high quality stainless steel: X-Ray Fluorescence (XRF) and Optical Emission Spectroscopy (OES).

Handheld XRF and LIBS are highly valuable technologies in the scrap metal market. Stainless steel is 100% recyclable and therefore a huge amount of stainless steel must be evaluated at the scrap yard. Handheld XRF analyzers bring immense value to scrap metal recyclers because it is a highly accurate, nondestructive testing technique that can analyze a metal sample in seconds with little to no need for sample preparation.  With XRF, stainless steel scrap can be quickly analyzed and sorted according to grade and type. Laser Induced Breakdown Spectroscopy (LIBS) is better for carbon analysis for metals and alloys.

OES is a robust, reliable, and widely-used technology for the analysis of metals and alloys in the lab. Compared with traditional combustion analyzers, OES provides faster elemental analysis with high precision and accuracy in iron and steel, aluminum, copper, magnesium, precious metals and other specialty metals/alloys. OES has demonstrated its capability to provide more efficient control of steel production by providing accurate sample analysis during the manufacturing process.