Casting has inherent defects like segregation and formation of unfavourable dendrites. The property of a cast product is not uniform and it is susceptible to brittle fracture. Forging, causes plastic deformation to occur and helps to uniformly re-distribute defects if they exist. By forging, you break up any inherent segregations and obtain far more uniform properties. This property is good to resist impact loading. Cast products have poor impact resistance and not good path for producing hammers that are meant to receive impact loading.

The fracture toughness of the forged parts are notably higher than that of the cast parts. The reason for that is a combination of overall strain hardening and grains rearrangement through plastic deformation assisted processes. Since one of the main requirements for hammers is just high fracture toughness combined with hardness, a forging is a favorite production process for them.

Alexander Katz-Demyanetz I agree, more structural integrity in forging responsible for good mechanical properties.

thanks for your answrs.

Note: Alexander Katz-Demyanetz

Fracture toughness can be regarded as the amount of energy a material can absorb before fracture. It is a property that describes the ability of a material containing a crack to resist further fracture. It is a function of crack length and applied stress. Fracture toughness is achieved by reduction in hardness and strength when such strength is achieved by martensitic phase ( in a tempering process). Hence, the stringent requirement for hammer head is impact wear resistance and not fracture toughness. Fracture toughness could be a consideration for the hammer handle.

The grain orientation derived from a casting technique cannot be compare to the orientation gotten by the use of forging. Forging technique gives a better grain orientation. These orientations influence the response generated under certain service condition.

In forging you don’t get the defects that you do within castings.

Beyond fracture toughness needed for your hammer because of the nature of its service application, you also want to build significant strength into your material. A good and cheap way to do this is called Strengthening by strain hardening. During forging, the hammer head is made to undergo significant stress which creates dislocation ring around the paths of dislocation movements. This explains the increased fracture toughness and strength induction via forging. You will spend more if you decide to cast the material and later want to increase its fracture toughness and strength

Casting of hammer produces grains in it. Grains have grain boundaries which helps in fracture. Cooling rate also influences the grain size. Coarser grains in hammer casting have inferior mechanical properties.

Forged hammer has fibrous structure, do not have grains inside. It has better mechanical properties and crack propagation is lesser.