Impurities in steel

Manganese, silicon, sulfur, phosphorus and others

Most common grade steels ("Common" or "common" steels) contain fairly high amounts of manganese, which remains from the deoxidation process. Impurities, such as silicon, sulfur and phosphorus, also almost always present in finished steel. The influence of these impurities on mechanical properties depends mainly on the fact, how these impurities are distributed in the steel structure. If an impurity is present in the structure in the form of a strong liquation, then it has a much more harmful effect, than, if this impurity was evenly distributed over the structure. Excessive segregation concentrates impurity at grain boundaries, which in this case often looks like a very fragile intergranular film.

Between liquidus and solidus

The degree of manifestation of liquation of a given impurity element depends on the distance between the solidus and liquidus lines on the corresponding equilibrium diagram (picture 1):

  • In the equilibrium diagram of Figure 1a, the relative content of the impurity element (sulfur or phosphorus) in solid (S) and liquid (L) glands are very far from each other at any temperature. This can lead to severe liquation.. When a relatively pure metal solidifies, the bulk of the impurity element will concentrate there, where the metal is the last to solidify - at the grain boundaries.
  • In the equilibrium diagram of Figure 1b, the content of the impurity element (silicon or manganese) in liquid (L) and solid (S) the glands remain close to each other throughout the hardening time. This gives a relatively uniform distribution of this impurity element over the microstructure and the absence of a dangerous concentration of brittle impurities at the grain boundaries..

Picture 1 - Equilibrium diagrams:
a - iron - (sulfur or phosphorus)
b - iron - (silicon or manganese)

Liquidation, associated with impurity elements

There is never a strong segregation in solid steel., associated with silicon and manganese. Since these elements have high solubility in solid solution of steel, then they can hardly be found in the form of separate components of the microstructure. Therefore, in a solid solution with an amount of up to 0,3 % their direct impact is minimal.

On the other hand, sulfur and phosphorus give significant segregation and, if they are present in large numbers, then they can precipitate during solidification in the form of corresponding compounds with iron at the boundaries of austenite grains. This phenomenon is aggravated by the relatively low solubility of these elements in steel..

Main impurities in steel

Manganese

Manganese is not only soluble in austenite and ferrite, but also forms stable Mn carbide3C. It is known from practice, that manganese increases the depth of hardening of steel. It also increases the strength and toughness of steel. In high-carbon steels, the manganese content should not exceed 0,3 % due to the tendency to form hardening cracks, especially when water quenched.

Silicon

Silicon increases the fluidity of steels, which are intended for the manufacture of castings. In such steels, it is present in an amount up to 0,3 %. In high carbon steels, the silicon content must be kept low, so it reduces the stability of cementite and promotes its decomposition into graphite (as discharge) and ferrite.

Sulfur

Sulfur is the most harmful impurity commonly found in steel.. If you do not take measures to convert sulfur into harmless forms, it tends to form brittle FeS sulfide. Sulfur is completely soluble in molten steel, but when solidified, its solubility drops to 0,03 %. In addition to the phenomenon of liquation, which was described above, need to take into account, that even with such a low sulfur content as 0,01 %, it can cause sulfide precipitation along grain boundaries. In this case, the austenite grains become literally enveloped in brittle films of iron sulfide (II). Since this sulfide has a rather low melting point, then during hot plastic processing, steel can literally crumble. At ambient temperature, iron sulfide (II) is fragile, what makes the steel unsuitable for cold plastic working or generally unsuitable for any subsequent service in service.

In most steels, it would be very difficult and very expensive to reduce the sulfur content to less than, than 0,05 %. At present, to eliminate the effect of sulfur in steel, an excess content of manganese is introduced into it during deoxidation.. When adding manganese in an amount of about five times more, what is required theoretically sulfur forms manganese sulfide MnS instead of iron sulfide (II). The resulting manganese sulfide is insoluble in molten steel and part of it goes into the slag. The rest of the manganese sulfide looks like rather large globules, which are distributed over steel. Since they are insoluble, then when the steel solidifies, they do not participate in the formation of its structure. Besides, manganese sulfide at the forging temperature is ductile and therefore the embrittlement tendency of steel is eliminated. In subsequent rolling operations, the manganese sulfide globules elongate into fibers (Figures 2a and 2b).

Picture 2 - Segregation of impurities in steel:
a - Segregation of iron sulfide (FeS) along grain boundaries in steel. ×750
b - Formation of isolated globules of manganese sulfide (MnS),
when manganese is present in the steel
c - Light stripes (ghost bands) or areas without pearlite,
which indicate the presence of phosphorus in steel

Phosphorus

Phosphorus is soluble in solid steel up to almost 1 %. When this amount is exceeded, brittle phosphide Fe is released3P. In solid solution, phosphorus has a significant strengthening effect on steel. However, this should be tightly controlled to contain about 0,05 % or less due to the risk of steel embrittlement, special, if Fe sulfide3P is present in the structure as a separate component.

In rolled or forged steel, light streaks indicate phosphorus («ghost bands») (Figure 2c). These sites (which got an elongated shape during rolling) do not contain perlite, and instead have a high concentration of phosphorus. The presence of phosphorus and the absence of pearlite reduce the strength of these areas., special, if they also contain other impurities.

Nitrogen

Atmospheric nitrogen is absorbed by molten steel during the manufacturing process. It doesn't matter, whether this nitrogen will combine with iron in the form of nitrides or will it remain dissolved in the atomic lattice of iron after solidification (picture 3) - it causes strong embrittlement and makes the steel unsuitable for intensive cold plastic working. For this reason, low carbon steel (mild steel) for deep landing (deep-drawing) must have a low nitrogen content.

Due to the nature of the smelting method, Thomas steel (Thomas steel) and has a nitrogen content up to 0,02 %, what, usually, leads to the presence of fragile F4N (picture 3). That's more than four times that, than the average nitrogen content in steel, obtained by the open-hearth method, which is suitable for cold heading operations. Naturally, that with the use of modern "oxygen" technologies it is possible to obtain low-carbon steel with a very low nitrogen content (below 0,002 %), since in oxygen, which blows the melt with very little or no nitrogen. This steel is ideal for cold heading. However, it is very difficult to prevent the melt from absorbing some atmospheric nitrogen., so during ladle casting molten steel is in contact with the atmosphere.

Picture 3 - Part of the iron-nitrogen equilibrium diagram

Hydrogen

Hydrogen ions dissolve in between the nodes of the atomic lattice in the solid solution of steel and therefore they are able to migrate inside the metal, which leads to embrittlement and loss of ductility. This hydrogen can dissolve during steelmaking, but more likely to get into steel during welding - from moisture, which is in the flux mixture (covering) electrodes. Another source is the release of hydrogen on the surface of electrolytic coating. Surface corrosion also produces hydrogen, which can be absorbed by steel.

The presence of hydrogen in steels can lead to the so-called "delayed fracture" – destruction under static load, which has been going on for some time. Such destruction can occur after several hours at a voltage of no more than 50 % from the yield point 0,2 %. The manifestation of this type of destruction is very dependent on the rate of deformation.. So, plasticity indicators, which are obtained in tensile test (slow loading), are significantly reduced, and the impact test results remain virtually unchanged.

Is considered, that the mechanism of hydrogen embrittlement of steels is associated with the movement of hydrogen ions between the sites of the atomic lattice to atomic lattice defects, as well as in areas with high triaxial stresses. These accumulations of hydrogen ions cause crack initiation and subsequent premature failure.. This explains why, why destruction more often occurs after some time, which is required for hydrogen ions to accumulate on atomic lattice defects. Most of the dissolved hydrogen can be dispersed during low-temperature (200 ºS) annealing in a dehydrated atmosphere.

Source:

Engineering Metallurgy – Part I – Applied Physical Metallurgy / R. A. Higgins – 6th ed. – 1999