"The most significant effect of AlN in steel is on grain size control, which directly influences hardenability, hot ductility, texture development, and mechanical properties."I have read this statement somewhere.However what I do not understand is how is grain size control a determining parameter here.A detailed explanation to this is most welcome.I feel the effect of Aluminium in alloying as far as nitrding is concerned is one of the most sought for.
Your question can be boiled down to why does the grain size affect the strength.
Under an applied stress, dislocations will move through a crystalline lattice until encountering a grain boundary. As more dislocations propagate to a boundary, dislocation 'pile up' results in diffusion along the grain boundary.
Decreasing grain size decreases the amount of possible pile up at the boundary, increasing the amount of applied stress necessary to move a dislocation across a grain boundary. The higher the applied stress needed to move the dislocation, the higher the yield strength.
The higher the yield strength, the higher the hardness of the material.
You are mixing two different things. Aluminum is added to many steel for deoxidation as a final step before casting. The Al does two things , it scavenges traces of oxygen and, more significantly is makes many particles of aluminum oxide which nucleate steel grains producing a fine ( small ) grain product. This use of aluminum is called "fine grain practice" ( FGP) . Many steel specs call for fine grain practice : For carbon plate steels it is the difference between A 515 and A 516 ( FGP). FGP is the first step to producing a steel with good low temperature toughness . The second use of alumium is an small alloy addition to low alloy steels that are intended for nitriding , a treatment which produces a very thin , very hard layer on steels, typically done with ammonia gas. Any steel can be nitrided but the Al addition is supposed to be an advantage .