The main reason for the presence of manganese in all steels is to control sulfur problems.
Sulfur is present as an impurity in all steels. It has two unique properties, which lead to embrittlement of steel even, if the sulfur content in the iron is very low:
1) Sulfur solubility as in austenitic iron, and in ferritic iron is extremely small - almost zero.
2) Sulfur forms a chemical compound with iron - iron sulfide (FeS), which melts at a temperature 1190 ° C.
Iron sulfide - the reason for the hot brittleness of steel
How these two properties in combination lead to the problem of embrittlement of steel?Consider steel with a sulfur content as an impurity in an amount of only 0,04 % by weight. Since the solubility of sulfur in iron grains - and austenite, and ferrite - almost zero, all sulfur will be present as iron sulfide at any temperature. Since the temperature of hot rolling and forging steel is often higher 1190 ° C, then iron sulfide will be present at these temperatures as a liquid. Since the sulfur content in steel is very low - only 0,04 %, then, seemingly, such a small amount of liquid will have no effect on the hot working of steel.
Liquid iron sulfide
However, there is one physical phenomenon, which changes the whole picture - wettability. Imagine a small ball of liquid iron sulfide, which lies on the grain boundary in steel at its hot working temperature. In a drop of liquid iron sulfide, when it contacts the solid surface of the grain boundary, a surface tension force arises. This force causes molten iron sulfide to wet the boundaries of the austenite grains.. Iron sulfide is literally drawn into the "gap" between the grains - grain boundaries - in the form of a thin liquid film. This negates the grain boundary strength of the steel..
Since the thickness of this liquid film is very small, then even a very small amount of iron sulfide is enough, to hit most of the grain boundaries of steel. This leads to, that steel breaks brittle along these grain boundaries during hot rolling or forging. Such embrittlement, which occurs during deformation of steel at high temperatures is called hot fragility. Another steel impurity - phosphorus - can also lead to embrittlement, but, unlike sulfur, only with a large content in steel.
Manganese sulfide instead of iron sulfide
The role of manganese in the fight against hot brittleness of steels is as follows. Like iron, manganese forms a chemical compound with sulfur - manganese sulfide. It has the chemical formula MnS and melts at temperatures much higher, than iron sulfide - at 1655 ° C. In real steels, in which iron is also necessarily present, and manganese, this sulfide has the formula (MnFe)S. Its melting point is lower, than manganese sulfide, but still higher, than the temperatures of hot rolling and forging steels. Thereby, the problem of hot brittleness in steels is removed. These sulfides are commonly referred to as manganese sulfides., although they often contain significant amounts of iron.
Sulfide inclusions in steel
Although manganese supplementation alleviates the problem of hot brittleness, the presence of these particles, who call inclusions, can often lead to embrittlement problems when machining steel at room temperature. Since sulfide inclusions at hot working temperatures are often quite plastic, then as a result of such processing they are lengthened into lines, as shown in the picture 1.
Such sulphides lead to varying degrees of embrittlement of steel in sheets and rods, depending on the direction of the applied load..
This type of embrittlement often leads to problems with breaking steel sheet products.. On the picture 2 showing sulfide inclusions, elongated in the rolling direction of the steel sheet. Also shown are the different orientations of Charpy impact toughness samples..
clear, that inclusions will contribute much more to the formation of cracks in transverse specimens, in which they lie along the entire V-notch, than in longitudinal samples, where inclusions cross the notch at only one point. The difference in the magnitude of the impact energy upon fracture of steel samples in different directions can differ significantly: eg, 60 J in the longitudinal direction and 20 J - in the transverse.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007