Earlier investigations of secondary shear zone have been carried out mostly with the use of carbide tools. In this work investigation has been carried out with machining mild steel with H.S.S. tool having 5 per cent cobalt. The emphasis is made on the metallurgical behaviour of the work material in the extreme conditions of strain and temperature in the secondary shear zone as it is an important factor in machinability. Changes in structural features in different types of built-up-edge and the secondary shear zone are discussed. © 1971.