How with alloying of steel it is possible to a achieve properties which can not be achieved with heat treatment ?

A prerequisite to the hardening of steels is that martensite should be formed on cooling, but this can only be achieved if the rate of cooling is great enough to suppress the formation of pearl¬ite or bainite and in plain carbon steels this can be achieved by quenching relatively small specimens in water. Larger specimens, however, cannot be cooled sufficiently rapidly through the whole section because of the limited rate at which heat can be extracted through the surface of the piece.

However, this difficulty can be overcome by alloying the steels to modify their transformation characteristics. For example, some alloying additions such as nickel and molybdenum reduce the rate of diffusion so that pearlite formation is suppressed in favour of the martensitic transformation.