Most people probably see the world built around them as being just a handful of different materials – metal, wood, plastic etc. But for anyone involved in construction or any kind of metallurgy, the difference between stainless steel and mild steel can be quite profound, with the particular composition of each having massive ramifications on their use, advantages and longevity. For those outside of industry and the many professional fields surrounding steel, we’ve drawn up this layman’s guide to help you better understand why stainless steel and mild steel are so different, and why it matters.
The Steels Defined
In a simple description, Mild Steel is a low carbon variety of steel – containing as little as 0.05–0.25%. Widely manufactured and supplied as flat-rolled products (either sheets or stripes), the low carbon content makes mild steel remarkably idea for use in automobile body panels, tin plates and wire products. Suitable for cutting, drilling and welding, mild steel box sections and round bars are also all popular choice of material in the construction of frames and semi-permanent structures. Within all carbon steels, it’s important to note that steel that has higher carbon counts have higher melting points, and are therefore less easy to weld.
Stainless steel, conversely, has a minimum of 10.5% chromium. It’s composition means it does not stain, corrode or rust as easily as ordinary steel, plus there are various grades of stainless steel, and surface finishes to apply, that help it better suit the environment in which it will be used in. Whilst unprotected carbon steel will rust when exposed to air and moisture, the high chromium within stainless steel creates a protective film of ‘chromium oxide’ that prevents oxygen from spreading into the metal’s internal structure.
Why use mild steel over stainless steel, and vice-versa?
The difference between stainless steel and mild steel in a practical sense really comes down to purpose. Stainless steel comes in 150 grades, and thus can be seen used in architecture, jewellery and even dentistry tools, whilst the stiffer, stronger mild steel also exhibits ferromagnetism, meaning its extensively used in motors and electrical appliances. A common way to visualise the difference is to picture two knives – one mild steel and one stainless steel. The mild steel knife is going to be stronger and easier to sharpen, and in fact it will stay sharper for longer. However the stainless steel knife, whilst not as strong, will be sufficient for its use as a kitchen utensil, and its chromium content gives it the extra quality of being anti-corrosive and more aesthetically pleasing.
There’s also the question of treatments. In the case of mild steel, or other metals with low carbon counts, it’s quite common to apply a case-hardening process which infuses additional carbon into the case once the part has been formed into its final shape. Because their corrosive resistance is poor, this is often necessary when one wants to use mild steel in a corrosive heavy environment. Stainless steel on the other hand, with its inherent corrosion resistance, doesn’t always need this kind of treatment; only when it is at a low SAE steel grade. In this case more carbon is added for greater hardness and strength, with the additional heat treatment often uses to make such stainless steel products as razor blades, cutlery, and tools.