Alloying elements, which are used in steels, there are more than two dozen. Here we look at the effect on steel of the most common (often inevitable) alloying elements - carbon, manganese and silicon.
Effect of carbon on the properties of steels
Carbon is the main hardening element in all steels, except for austenitic stainless steels and some other high alloy steels. The hardening effect of carbon consists of hardening solid solution and hardening due to dispersed precipitation of carbides. With an increase in the carbon content in steel, its strength increases, but ductility and weldability are reduced.
Carbon has a moderate tendency to macrosegregation during crystallization. Macrosegregation of carbon is usually more significant, than all other alloying elements. Carbon has a strong tendency to segregate at defects in steels, such as grain boundaries and dislocations. Carbide-forming elements can interact with carbon and form "alloyed" carbides.
Effect of manganese on the properties of steels
Manganese is present in almost all steels in quantities from 0,30 % and more. Manganese is used to remove oxygen and sulfur from steel. It has less tendency to segregate, than any other alloying element. Manganese favorably affects surface quality throughout the entire carbon range, excluding very low carbon steels, and also reduces the risk of red brittleness. Manganese favorably affects the ductility and weldability of steels.
Manganese does not form its own carbide, but only dissolves in cementite and forms alloyed cementite in steels. Manganese promotes the formation of austenite and therefore expands the austenitic region of the phase diagram. High content of manganese (more 2 %) leads to an increased tendency to cracking and warping during quenching. The presence of manganese in steels encourages impurities such as phosphorus, lead, antimony and arsenic segregate to grain boundaries with the appearance of temper brittleness.
The effect of silicon on the properties of steels
Silicon is one of the main deoxidizers, which are used in steel making. Therefore, the silicon content determines the type of steel produced.. Calm carbon steels can contain up to a maximum of silicon 0,60 %. Semi-quiescent steels may contain moderate amounts of silicon, eg, 0,10 %.
Silicon completely dissolves in ferrets with silicon content up to 0,30 %. It increases the strength of ferrite, almost without reducing its plasticity. With a silicon content above 0,40 % in general purpose carbon steel there is a significant decrease in ductility.
In combination with manganese or molybdenum, silicon provides higher hardenability of steel. The addition of silicon to chromium-nickel austenitic steels increases their resistance to stress corrosion. In heat-hardened steels, silicon is an important alloying element, increases the ability of steels to heat hardening and their wear resistance, increases the elastic limit and yield strength. Silicon does not form carbides and does not contain cementite or other carbides. It dissolves into the martens and slows down the decay of doped martensite to 300 ° C.
Source: Steel Heat Treatment: Metallurgy and Technologies, ed. G. E. Totten, 2006