It has been a conspicuous problem that overproof defects of heavy disc forgings found with supersonic inspection.The ignot
and waste forgings have been dissected and analysed.The principle of metal flowwith a large reduction ratio was studied by the modelling of room-
temperature upsetting.It was concludedthat(1)the size and shape of dead metal zone almost did not change when △H/H0 was less than 60%,and(2)the size of upper and lower dead metal zone decreased,and not intersected or inserted into each otherwhen △H/H, was larger than 60%. Furthermore,it was thought that one of main reasons of crack occurrencewas due to a very narrow shear band formed in the centre of forgings.The deformation behavior of inclusionwas studied at high temperature.It was observed that(1)plastic inclusion was compressed to thin slice,caused stress/strain concentration and made microcrack nucleate around the inclusion,(2) there were threetypes of microcrack nucleation in
plastic deformation at high temperature because the formability of inclusionand matrix changed with the temperature increasing,and(3)plastic inclusions and microcracks coalesced inupsetting deformation process.It was the main reason for crack originated from inclusion and overproof defects of heavy disc forgings with supersonic inspection. Some measures have been put forward and tested.