Dear Mr Kulkarni, 

Chemical affinity Mo or Mn to S depends on temperature and amount of element in steel (thermodynamic activity).

If we consider same amount of Mn and Mo (1 mol) with a certain amount of S in steel, Gibbs Free Energy changes in MnS and MoS2 reactions will change according to temperature as figure in attachment.

Thanks for the answer.

Can you please provide any research article or literature to support the answer?Gibbs free energy signifies the easiness for the compound formation, right? Please help me to understand the graph. 

gibbs free energy dictates the stability of compounds at particular temperature.  more negative free energy is more stable.

Means, an alloy containing Mn and Mo, along with S, behaves like at high temp. MnS is present, and as the metal cools down to room temp. MnS will be disintegrated and MoS2 will be formed? 

 Dear Mr. Emre Alan, Is the diagram what you have shared is called as Ellingham Diagram?

yes you are correct... some where around 850 MnS loosing its stability

You may call this diagram as ellingham diagram for sulphides.

Currently, I am referring two kinds of literature:

1. Influence of alloying elements on the sintering thermodynamics, microstructure and properties of Fe-MoS2 composites, by Keline Furlan 

2. Thermodynamic aspects during the processing of sintered materials, by Renon Schroeder 

Both articles have referred the Ellingham Diagram for Sulphides, but interaction between MoS2 and MnS is not seen there. They are available on Research Gate 

How can be this discrepancy is addressed?

Ellingham diagram is a common name for oxide reactions of metals; plot of Gibbs Free Energy vs Temperature. You may consider that the reaction with lower gibbs free energy can proceed preferentially. 

It should be noted that, the reactions in Ellingham diagram represent metal with 1 mole of O2. Thus, comparison must be made with same amount of reactant; oxygen, sulphur, etc. 

Similarly, you may compare gibbs free energies of sulphide reactions at a specific temperature. 

ellingham diagram never explain interaction between the oxides in a system. They are standard and hypothetical conditions. You have to make thermodynamic calculations may be using thermocalc or factsage to study its interactions.

What can one understand by lines crossing each other in the Ellingham diagrams? Even, this kind of evidence is supported by the research articles (referred above), using the Ellingham Diagrams  

stability ... more negative free energy more stable. 

point of intersection dictates the stability of different sulphides above and below the temperature.