Debasis Double tempering is done with two purposes : One is to reduce the presence of un-wanted or un-tempered martensite and in some steels a portion of retained austenite from quenching and tempering processes. Second tempering : is done with the purpose of changing un-tempered phases (mainly martensite or some retained austenite also in some cases) + more from improving Impact strength i.e, to have dramatic improvement in Mechanical properties ( especially Impact strength) + improvement in magnetic properties and thermal fatigue properties of the materials heat treated for a second tempering operation. Second tempering temp : need not be done at the same temp of first tempering temperature but can also be done at a lower temp and hold for a longer time. This may be needed for vessels which are heavy walled low alloy steel components. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 10, 2007 E mail: nishi@vsnl.com >Double tempering is generally applied to CA6NM, CA15 type >Martensitic cast Item after austenitising treatment. The >2nd tempering treatment is done at lower temperature than >the 1st stage tempering. > >One of the reason behind this 2nd temper is to temper any >untempered martensite (if any) from the 1st temper. > >If any one has any other technical input behind the theory >of double tempering please share. >regards