Impurities in steels: harmful and useful

Impurities: permanent, hidden and random

Manganese, silicon, aluminum, sulfur and phosphorus refer to permanent impurities. Aluminum, together with manganese and silicon, is used as a deoxidizer, and therefore, in small quantities, they are always present in deoxidized steels.. Iron ores, as well as fuels and fluxes always contain a certain amount of phosphorus and sulfur, which remain in the cast iron, and then go to steel.

Nitrogen called hidden impurity - it enters steel mainly from the air.

TO random impurities include copper, arsenic, lead, zinc, antimony, lead and other elements. They get into steel with a charge - with ores from various deposits, and also from scrap iron.

All impurities are permanent, hidden and accidental - to varying degrees are inevitable due to the technology of steel production. So, calm steel usually contains these impurities in the following ranges: 0,3-0,7 % manganese; 0,2-0,4 % silicon; 0,01-0,02 % aluminum; 0,01-0,05 % phosphorus, 0,01-0,04 % sulfur, 0,-0,2 % copper. In these quantities, these elements are considered as impurities, and in larger quantities, which are intentionally introduced into steel, they are already considered alloying elements.

Effect of phosphorus on the properties of steels

Phosphorus (Р) segregates when the steel hardens, but to a lesser extent, than carbon and sulfur. Phosphorus dissolves in ferrite and thereby increases the strength of steels. With an increase in the phosphorus content in steels, their ductility and impact strength decreases and the tendency to cold brittleness increases..

The solubility of phosphorus at high temperatures reaches 1,2 %. With decreasing temperature, the solubility of phosphorus in iron drops sharply to 0,02-0,03 %. This amount of phosphorus is typical for steels, that is, all phosphorus is usually dissolved in alpha gland.

Phosphorus has a strong tendency to segregate at grain boundaries, which leads to temper brittleness of alloy steels, especially in manganese, chromium, magnesium-silicon, chromium-nickel and chromium-manganese steels. Phosphorus, Besides, increases the hardenability of steels and slows down, like silicon, decomposition of martensite in steels.

Increased phosphorus content is often set in low alloy steels to improve their machining., especially automatic.

In low-alloy structural steels with a carbon content of about 0,1 % phosphorus increases strength and resistance to atmospheric corrosion.

In austenitic chromium-nickel steels, phosphorus additions increase the yield strength. In strong oxidants, the presence of phosphorus in austenitic stainless steels can lead to corrosion along the grain boundaries. This is due to the phenomenon of phosphorus segregation along grain boundaries.

Influence of sulfur on the properties of steels

Sulfur content (S) in high quality steels does not exceed 0,02-0,03 %. In general-purpose steels, the permissible sulfur content is higher - 0,03-0,04 %. By a special treatment of liquid steel, the sulfur content of the steel is brought to 0,005 %.

Sulfur does not dissolve in iron, therefore, any amount of it forms iron sulfide FeS with iron. This sulfide is part of the eutectic, which is formed at 988 ° C.

The increased sulfur content in steels leads to their red brittleness due to low-melting sulfide eutectics, which arise along grain boundaries. Phenomenon redbrittleness occurs at a temperature 800 ° C, that is, at a temperature red hardening of steel.

Sulfur has a detrimental effect on plasticity, impact strength, weldability and surface quality of steels (especially in steels with low carbon and manganese content).

Sulfur has a very strong tendency to segregate along grain boundaries. This leads to a decrease in the ductility of steels in the hot state.. However, sulfur in quantities from 0,08 to 0,33 % deliberately added to steels for automatic machining. Known, that the presence of sulfur increases the fatigue strength of bearing steels.

The presence of manganese in steel reduces the harmful effect of sulfur. The reaction of manganese sulfide formation takes place in liquid steel. This sulfide melts at 1620 ° С - at temperatures much higher, than the hot working temperature of steels. Manganese sulphides are ductile at hot working temperatures of steels (800-1200° C) and therefore easily deformed.

Effect of aluminum on the properties of steels

Aluminum (Al) widely used for deoxidizing liquid steel, as well as for grain grinding of steel ingots. The harmful effects of aluminum include that, that it contributes to the graphitization of steels. Although aluminum is often considered an impurity, it is actively used as an alloying element. Since aluminum forms solid nitrides with nitrogen, it is usually an alloying element in nitrided steels. Aluminum increases the resistance of steels to scale formation, and therefore it is added to heat-resistant steels and alloys. In precipitation-hardened stainless steels, aluminum is used as an alloying element, accelerating the dispersive release reaction. Aluminum increases the corrosion resistance of low carbon steels. Of all the alloying elements, aluminum is the most effective for controlling grain growth when heating steels for hardening.

Effect of nitrogen on the properties of steels

The harmful effects of nitrogen (N) is that, that the rather large, fragile non-metallic inclusions - nitrides - worsen the properties of steel. A positive property of nitrogen is that, that it is able to expand the austenitic region steel condition diagrams. Nitrogen stabilizes austenitic structure and partially replaces nickel in austenitic steels. Vanadium nitride-forming elements are added to low-alloy steels, niobium and titanium. With controlled hot working and cooling, they form fine nitrides and carbonitrides, which significantly increase the strength of steel.

Effect of copper on the properties of steels

Copper (With) has a moderate tendency to segregation. The harmful effects of copper include a decrease in the cold brittleness of steel. With an increased copper content, it adversely affects the surface quality of the steel during hot working.. However, if the content is more 0,20 % copper increases its resistance to atmospheric corrosion, as well as strength properties of alloyed and low-alloyed steels. Copper in quantity over 1 % increases the resistance of austenitic stainless steels to sulfuric and hydrochloric acids, as well as their resistance to stress corrosion.

Effect of tin on the properties of steels

Lead (Sn) already in relatively small quantities is harmful to steels. It has a very strong tendency to segregate to grain boundaries and cause temper brittleness in alloy steels.. Tin has a detrimental effect on the surface quality of continuously cast ingots, and can also reduce the hot ductility of steels in the austenitic-ferritic region of the state diagram.

The effect of antimony on the properties of steels

Antimony (Sb) has a strong tendency to segregate when the steel solidifies and therefore adversely affects the surface quality of continuously cast steel ingots. In the solid state of steel, antimony readily segregates to grain boundaries and causes temper brittleness in alloy steels..

Sources:
Steel Heat Treatment: Metallurgy and Technologies, ed. G. E. Totten, 2006.
Gulyaev A. P. Metallurgy, 1986.