Square kellys is made from AISI 4145H alloy steel acc. to API Spec.7 and SY/T6509. It is formed by hot forged and heat-treated by intermediate frequency induction heat treatment or 20m horizontal resistance furnace to gain high physical properties in length. All threads are phosphate, plate copper or cold worked.
Kelly is at the top of drill string, which has two types, the square and the hexagonal.