Select the correct option for the given processes.

(P) Process of heating steel to redness and then cooling it very slowly.
(Q) Process of heating steel in presence of ${\mathrm{NH}}_3$ and producing hard coating of iron nitride on the surface of steel.
(R) Process of heating steel to redness and then cooling it suddenly by plunging it into water or oil.

(S) Process of heating quenched steel to a temperature well below redness and then cooling it slowly.
 

While tempering requires heating the metal precisely below the critical point and is frequently carried out in the air, vacuum, or inert atmospheres, annealing includes heating the steel to a defined temperature and then cooling at a very slow and controlled pace.

Adding nitrogen (N) to steel, where it reacts with the iron and other alloying elements to generate hard metallic nitrides, hardens the surface of steel objects. This process is known as nitriding.

Hence, option D is correct.