The term “stainless steel” was actually coined during the metal’s early days when it was used predominantly for cutlery applications. It later became the generic name for a wide variety of steel types and grades containing a number of alloying elements that enhance the characteristics, structure and performance of the metal.
The most common stainless steel alloys are nickel, molybdenum, copper, chromium and titanium. Non-metal additions include carbon and nitrogen.
Carbon is the single most important alloying element in stainless steel. It is always present in all grades, and the amount of carbon in each grade is key to its performance. In all types of stainless steel except martensitic, carbon levels are kept quite low, but in martensitic grades, they are deliberately higher in order to increase hardness and strength. Low carbon content is particularly key in ferritic and austenitic stainless steels in order to avoid carbide precipitation when welding.
In all grades within the 300 series of stainless steel, nickel is the essential alloying element, creating an austenitic structure known for ductility, toughness and strength. Nickel also adds significant resistance to acids in general, but sulphuric acid in particular, and helps to reduce distortion and cracking during the quenching phase of heat treatment.
Molybdenum adds resistance to localised pitting, and, when mixed with chromium-nickel austenitic steels, gives better resistance to crevice corrosion. It also protects against the negative effects of chlorides, slows the critical quenching speed, and increases high temperature tensile strength.
Copper is often present as a residual element in stainless steel, but can also be intentionally added to improve corrosion resistance, especially in acidic or marine environments. Copper is not added for cutlery applications as it has a detrimental effect on surface quality and hot working behaviour.
Titanium merges with carbon to form titanium carbides that are stable and not easily dissolved. This helps to reduce inter-granular corrosion that could affect the formation of the passive (protective) layer of stainless steel.
In a similar way to nickel, the addition of nitrogen to stainless steel increases the austenite stability of the metal, helping to protect against pitting and inter-granular corrosion – particularly in duplex stainless steel. Nitrogen also helps raise the yield strength.
This highly reactive element accounts for the “passive” nature of all stainless steels, protecting against chemical corrosion and the typical oxidation (rusting) that is common in unprotected steel. When stainless steel contains a minimum of 10.5% chromium, an instant and insoluble surface film is filmed, preventing oxidation.
There are many other elements that are added in varying amounts to stainless steel to enhance it characteristics and performance. Steelmor has been manufacturing all grades of stainless steel for over 40 years, and we’d welcome the opportunity to further explain the role of alloying elements in the functionality of stainless steel so that you know which grade is best for your particular application. Get in touch today.