Stainless steels and heat resisting steels are ferrous alloys to which a minimum of 12% chromium is added. A 12% chromium stainless steel will resist corrosion or ‘rusting’ when exposed to weather. To obtain greater corrosion resistance for more severe applications, the chromium content may be raised to as high as 27%.
Stainless steels often contain other elements which are added to modify their properties. The effects of various elemental additions are reviewed in the following sections.
In the majority of stainless steel grades carbon is usually held to 0.08% maximum in the austenitic grades and preferably much less. For example, the grade sometimes specified for welding, 304L, has carbon restricted to 0.03% maximum. Higher carbon contents up to 1.00% render some of these steels amenable to conventional hardening and tempering heat treatment for the purpose of developing high strength and hardness levels e.g. the 440 grades.
Nickel is the most common element added to these steels and when added in quantities of 8.00% or greater, develops the austenitic series of grades. Lesser amounts of nickel will produce the duplex austenitic-ferritic grades.
Molybdenum improves passivity of the surface resulting in increased corrosion resistance, particularly pitting in chloride environments.
This element is a strong carbide former and is very effective in preventing precipitation of chromium carbide during welding. Chromium carbide precipitation adjacent to welds can result in intergranular corrosion or weld decay.
Similarly to nickel, manganese promotes the formation of austenite and in certain grades partially replaces nickel. It is also used in the free-machining grades to which sulphur and selenium additions have been made.
This element is added in quantities of around 1.00% to improve scaling resistance of austenitic grades when used at higher temperatures.
This element improves corrosion resistance in certain applications. An addition of 3.00 - 4.00% improves resistance to attack by sulphuric acid.
The sulphur content of these steels us usually kept below 0.03%. By increasing sulphur to around 0.2% there is an improvement in machinability but the corrosion resisting properties are severely impaired in many instances.
Niobium and Tantalum
These are carbide stabilising elements and act similarly to titanium. They are more commonly used in heavier sections. Niobium is used in stabilised welding rods and is preferred to titanium in this application.
As an alloying element nitrogen acts to promote the formation of austenite. It can dramatically improve the yield strength of austenitic grades.