Have you ever wondered why magnets only work on some stainless steels? There are in fact five distinct families that make up the more than 150 grades of stainless steel, and magnetism is just one characteristic that differs among them.
In this article, we take a look at the five different categories – Austenitic, Ferritic, Martensitic, Duplex, and Precipitation-Hardening (PH) – and identify how and why they’ve been developed.
Introduction to the families of stainless steel
In the early 1900s, metallurgists discovered that when at least 10% chromium was added to steel, it produced excellent corrosion resistance. Further, by adding higher levels of chromium together with other alloys such as nickel, different types of stainless steel can be produced.
Each of the five types of stainless steel has its own unique metallurgical composition with properties that make them ideal in specific working environments. The five categories of stainless steel are as follows:
Austenitic stainless steels
Austenitic stainless steels are the most commonly used. All 200 and 300 series steels (see Grade 304 and Grade 316) are austenitic and contain 15% to 30% chromium and 2% to 20% nickel for enhanced surface quality, formability and increased corrosion and wear resistance. They are generally non-magnetic, and are used for automotive trim, cookware, processing equipment and a variety of industrial applications.
Ferritic stainless steels
This group of steels in the 400 series contains 10.5% to 20% chromium and are highly resistant to scaling and cracking at elevated temperatures. They cannot be hardened by heat treating, however, they can be cold worked and softened by annealing. They are magnetic and are typically used in applications where corrosion resistance is important, such as sinks and automotive applications.
Martensitic stainless steels
Martensitic grades, which are also of the 400 series, are straight chromium steels (11.5% to 18% chromium). They were developed to be corrosion resistant and hardenable by heat treating. They are magnetic and are used where hardness, strength and wear resistance are required, such as in cutlery and multipurpose tools.
Duplex stainless steels
The newest stainless steel group is a combination of austenitic and ferritic materials, and produces a higher strength and superior resistance to stress corrosion cracking. Nitrogen is added to second generation duplex alloys and provides strength and increased weldability.
Precipitation-Hardening (PH) stainless steels
This group of stainless steels provides a unique combination of fabricability, strength, ease of heat treatment and corrosion resistance not found in any other class of material. Precipitation-hardening grades are primarily used for bars, rods, wire and forgings, but increasingly in flat rolled form.
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