Effect of soft additives (Pb/Bi) on machinability of low carbon resulphurised free machining steels

The effect of soft additives (Pb, Bi) on the machinability of low carbon resulphurised free machining steels has been investigated using the plunge machinability test. The cutting and thrust forces and chip thickness were measured as a function of cutting speed in the steels having various contents of Pb and Bi. Unexpected results, such as very thick chips and formation of groove wear, were observed for 0·2%Bi at low cutting speeds. Such results have been attributed mainly to the lack of a stable built-up edge (BUE). As a result of these unexpected results, the effects of soft additives on various aspects of machinability differ depending on cutting speed. At high speeds, the effects of soft additives are in accordance with general understanding, i.e. machinability improves with increasing Pb or Bi content, with Bi being more effective. At low cutting speeds, some results very different from those at high cutting speeds were observed. It is obvious from these results that machinability test results may misrepresent the overall capability of steels when only a limited number of machining conditions are used. Finally, the observations made on the chips and theoretical considerations have suggested that liquid metal embrittlement by Pb or Bi could play a significant role in reducing the force in the secondary shear zone and in enhancing chip breaking. However, no definite conclusions can be made concerning whether or not the effective shear strength in the primary shear zone is reduced by liquid metal embrittlement. Solid metal embrittlement has been excluded from a possible mechanism in the primary shear zone.MST/870