Comparison of Wrought and Cast Superalloys
A wrought alloy normally starts from cast billets but is deformed and reheated several times to achieve the final stage. The wrought alloys are more uniform as compare to cast alloys that often have segregation due to solidification processing. The segregation is resulted by solidification of alloys but it may be more vigorous in few conditions as compare to others.
Wrought alloys are normally considered to have higher ductility than cast alloys. Therefore mill product shapes like bar are wrought formed since these can be formed best by processing and for immediate application. Forgings are also actually wrought materials and use ductility of wrought material to form specific bigger size products like gas turbine disks.
All alloys cannot be produced in wrought form. Some alloys can be produced and employed in cast forms only. Wrought alloys are very tough to process and these can be treated through powder metallurgy, to prepare them for ultimate forging. In the moderate temperature applications of gas turbines where big size disks are widely essential, standard wrought or wrought powder metallurgy disks are used. Powder metallurgy method has been used to straight produce the parts for final machining but this type of processing is infrequent.
Cast superalloys are used in warm areas of gas turbines particularly as airfoils, blades and vanes. Many castings are polycrystalline equiaxed however others are directionally solidified. The polycrystalline castings comprise of various grains that may have different sizes for one part to another. Directionally solidified castings may comprise of several grains whole arranged parallel to each other.
Castings are internally stronger as compare to forgings for use at the high temperature. The coarse grain size of powder metallurgy castings provide higher strength at the elevated temperatures. Moreover, chemical composition of casting can be made suitably for the high temperature strength,as much as forgeability properties are not significant. For instance, the maximum creep rupture strength at the high temperatures can be obtained in nickel base superalloy castings for large stress and elevated temperature turbine blade uses. The suitable grain structures present in forgings adds higher yield strengths and enhances low cycle fatigue strengths at the low to moderate temperatures hence, the use of forgings in disk applications.