1) What is stainless steel?
Stainless steel is a family of ferrous alloys that contain at least 10.5% chromium (Cr). Other alloying elements are also often present (Carbon, Nickel, Molybdenum, etc…).
2) Why is stainless steel generally more corrosion resistant than carbon steel?
The surface of stainless steel is covered by a very thin layer of chromium oxide (only a few nanometers – thousands of times thinner than a human hair).
3) When was stainless steel discovered?
Harry Brearley, from Sheffield in the United Kingdom, is generally considered to have discovered stainless steel in 1913. He was working on alloys for defense applications and accidentally discovered that the chromium-rich alloy was significantly more rust-resistant. The first application was for kitchen cutlery!
4) Are there several types of stainless steels?
Yes, there are five broad families, named after their microstructures:
Austenitic: Approximately 75% of the market, excellent corrosion resistance, ductile and easy to weld. Use: extremely varied (kitchen utensils, jewelry, architecture, hardware, chemical reactors). Most common grades: 304 (18/8) and 316.
Ferritic: Approximately 20% of the market, lower corrosion resistance and weldability but cheaper price. Use: automobile exhaust systems, appliances, industrial parts. Most common grades 409, 430 and 446.
Martensitic: Approximately 1-2% of the market, lower corrosion resistance and weldability but high hardness and abrasion resistance. Uses: reactors, valves, burners, cutlery. Most common grades: 410, 420 and 440.
Duplex: Approximately 1-2% of the market, high mechanical properties while keeping an excellent resistance to corrosion. Uses: oil and gas industry, heat exchangers, architecture and civil engineering (Nouveau Pont Champlain) Most common grades: 2304, 2205, 2507.
Precipitation hardening: Approximately 1-2% of the market, very high mechanical properties and good corrosion resistance. Uses: Aerospace and oil and gas industries. Most common grades: 17-4PH and PH15-7Mo.
5) How to determine if the alloy is stainless steel and identify grades?
The most well-known way is the magnet test: austenitic stainless steels are generally not ferromagnetic (not attracted to a magnet). However, other stainless-steel families are ferromagnetic, so this is not a perfect test.
Also, portable instruments (XRF) can generally identify alloys on-site and non-destructively. However, for very precise measurements, a laboratory test would be mandatory.
In part two, we will discuss failure modes for stainless steels and give examples of investigations performed by CEP Forensic.