Metallurgy and specifications
Steel is defined as a material with a mass content of iron larger than the content of any other chemical element, and with a carbon content of usually less than two percent.
Different grades of steel are achieved by adding other alloying elements, such as manganese, nickel, chrome, titanium and molybdenum to the basic mix. By adding these alloys in varying quantities different grades of steel can be achieved that fulfil different needs. Also, most steel alloys can, after manufacture, be heat treated to various conditions to fulfil different applications.
Steel can be categorised into three basic groups:
- Non-alloyed steel
- Alloy steel
- Stainless steel
Non-alloyed steel – The basic steel with no significant amounts of alloy deliberately added.
Alloy steel – The basic steel with some alloying elements added. The most important alloying elements include Aluminum (AI), Boron (B), Cobalt (Co), Chromium (Cr), Copper (Cu), Manganese (Mn), Molybdenum (Mo), Niobium (Nb), Nickel (Ni), Phosphorus (P), Lead (Pb), Sulphur (S), Silicon (Si), Titanium (Ti), Vanadium (V), Tungsten (W).
Stainless steel – Stainless steels contain a mass fraction of at least 10.5 % chromium and a maximum of 1.2 % carbon. Other important alloying elements include Ni, Mo, Cu and many others, depending on the desired properties and application. Usually steels that do not form rust in the natural environment are named stainless steels. The most important group within the stainless steels are the austenitic stainless steels, which contain a minimum of 18 % chromium and 8 % nickel. Austenitic stainless steels are not magnetisable. Due to the austenitic structure they show a good ductility, and at low temperatures are very cold formable and show good weldability.
Effect of alloying elements in steel
The properties of steel can be widely influenced by alloying elements and heat treatment conditions. The most important element besides iron is carbon. The carbon is essential in steels which are to be hardened by quenching and tempering. The hardness of steel, as with all steel groups, is directly related to the amount of carbon alloyed.
Some alloying elements are of importance for the production of steel during melting, especially aluminum and silicon.
Aluminum is besides silicon, the most important deoxidising agent. In deoxidised steel the aluminum content is typically around 0.01 % Al. In nitriding steel, with typical contents of 0.8 to 1.2 % Al, aluminum increases the hardness of the hard-surface layer and its wear resistance.
Fine grained structural steel contains up to 0.015 % Al to induce the fine grained structure which increase the strength and ductility character of the steel.
Heat and scale resistant steels contain typically about 1.0 to 1.7 % Al to create a firmly adhering oxide layer on the surface.
In high-temperature steels low amounts of aluminum increase the high-temperature strength.
Chromium increases the ability to harden heat-treatable steels, and raises the tensile strength whilst the ductility is only slightly lowered. Used in heat-treatable steels up to 3.5 %, Cr increases the effective hardening depth where large diameters or thicknesses are heat treated. Decarburisation of high pressure hydrogenation steel is minimised by alloying up to 12 % Cr.
In stainless steels, chromium is the most important alloying element. Above 12 % Cr steel forms a chromium oxide layer on its surface that gives stainless steels its unique resistance against atmospheric corrosion; depending on the individual grade, up to 30 % Cr is alloyed. Heat and scale resistant steels contain up to 30 % Cr to create a chromium oxide layer on the steel surface that protects the steel from further scaling.
Cobalt is used as an alloying element for high-alloyed steels. Maraging steels contain up to 12 % Co. Tool steels, especially high speed steels, contain up to 10 % Co in order to achieve high hardness in elevated temperature applications.
Copper is used in carbon steels to increase resistance to atmospheric corrosion. In some high alloyed austenitic stainless steel grades copper is added to increase the creep resistance or the corrosion resistance.
Manganese is widely used for deoxidation and is therefore present in small quantities in nearly all steels. The introduction of manganese to an alloy is frequently an economic solution for increasing strength or improving the hardenability of steels. The maximum content can be up to 14 % in abrasion resistant austenitic manganese steels.
Molybdenum. Up to 2 % of Molybdenum is used as an alloying element in various grades of steels. In low alloyed steels it increases the ability to harden the steel and prohibits the temper brittleness of chromium and manganese alloyed heat treatable steels. In tool steels its ability to induce carbides is used to increase abrasion resistance and tempering resistance.
When added to stainless steel, molybdenum increases the resistance to corrosion, especially against pitting corrosion.
Nickel is used as an alloying element for various steel grades and applications.
With up to 2.2 % in case hardening steels and heat treatable steels, it increases ductility by inducing a fine grain structure, and allows full-diameter quenching and tempering.
The ductility at low temperatures of low and high alloyed steels is improved by alloying up to about 10 % Ni.
Austenitic stainless steels contain a minimum of 18 % chromium and 8 % nickel.
Silicon is a powerful deoxidiser and has additionally provides the following advantages
Spring steels use typically 0.6 to 1.8 % Si to increase yield and tensile strength and the limit of proportionality. Heat and scale resistant steels contain up to 4.5 % Si to help the formation of a protective layer. Transformer steels with up to 4.3 % Si have high electrical resistance and low hysteresis loss.
Sulphur is - similar to phosphorus - regarded as an undesirable element - except for special applications - that forms impurities in steel, and decreases ductility in different ways.
Increased sulphur contents of about 0.15 - 0.3 % S can however be used to improve cutting properties of steel (free cutting steel).
Titanium is used in fine-grain steels to create the fine-grain structure by precipitation of titanium carbides at high temperatures. This increases ductitlity, strength and weldability.
In stainless steels it is used to improve the resistance against intercrystalline corrosion by forming titanium carbides and thus preventing precipitation of chromium carbides.
Tungsten is alloyed up to about 8 % in various tool steels to improve hardness and wear resistance at normal and elevated temperatures.
Vanadium can be alloyed below 0.1 % to fine-grain steels with a similar effect to titanium, and is also alloyed in contents up to 4 % to tools steels to improve hardness and wear resistance.
