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http://www.leonghuat.com/articles/elements.htm

Effects of Mn, P, S, Si & V on the Mechanical Properties of Steel where A is the atomic number indicating the number of protons exist in the nucleus of the atom; N is the atomic mass unit, defined as the ratio of the average mass per atom to 1/12 of the atomic mass of carbon-12 in its nuclear and electronic ground state; and Z is the chemical symbol of the element…

Manganese, 25Mn54.938049

Manganese increases hardenability and tensile strength of steel, but to a lesser extent than carbon. It is also able to decrease the critical cooling rate during hardening, thus increasing the steels hardenability much more efficient than any other alloying elements. Manganese also tends to increase the rate of carbon pe*******on during carburizing and acts as a mild deoxidizing agent. However when too high carbon and too high manganese accompany each other, embrittlement sets in. Manganese is capable to form Manganese Sulphide (MnS) with sulphur, which is beneficial to machining. At the same time, it counters the brittleness from sulphur and is beneficial to the surface finish of carbon steel.

For welding purposes, the ratio of manganese to sulphur should be at least 10 to 1. Manganese content of less than 0.30% may promote internal porosity and cracking in the weld bead, cracking can also result if the content is over 0.80%. Steel with low Manganese Sulphide ratio may contain sulphur in the form of iron Sulphide (FeS), which can cause cracking (a “hot-short” condition) in the weld

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Porosity
Porosity is the presence of pockets which do not contain any solid material. They differ from slag inclusions in that the pockets contain gas rather than a solid.

The gases forming the voids are derived form:

Gas released by cooling weld because of its reduced solubility temperature drops.
Gases formed by the chemical reactions in the weld.
Porosity is best prevented by avoiding:

Overheating and undercutting of the weld metal.
Too high a current setting.
Too long an arc.

Slag Inclusions
Slag inclusions are elongated or globular pockets of metallic oxides and other solids compounds. They produce porosity in the weld metal. In arc welding, slag inclusions are generally made up of electrode coating materials or fluxes. In multilayer welding operations, failure to remove the slag between the layers causes slag inclusions. Most slag inclusion can be prevented by:

Preparing the groove and weld properly before each bead is deposited.
Removing all slag.
Making sure that the slag rises to the surface of the weld pool.
Taking care to avoid leaving any contours which will be difficult to pe*****te fully with the arc.

Undercutting

Undercutting is the burning away of the base metal at the toe of the weld. Undercutting may be caused by the following conditions:

Current adjustment that is too high.
Arc gap that is too long.
Failure to fill up the crater completely with weld metal.