Abstract
According to the conventional hypothesis, the high machinability of free cutting steels is attributed to both the easy cutting due to stress concentration at inclusions such as Pb and S (MnS) and their lubricating effects between cutting tool and chips. However, previous studies have not necessarily made clear the rela tionship between the strength characteristics and the cutting mechanism from the viewpoint of fracture caused by the crack nucleation due to the stress concentration at inclusions. If these inclusions behave as fracture origins, we cannot expect high fatigue strength of free cutting steels.
In the present paper, the effects of Pb and MnS on fatigue strength were studied by comparing the fatigue behavior of annealed 0.45%C Pb-free-cutting steel (SAE10L45) and 0.46%C steel (S45C). The effects of small fatigue cracks and small artificial holes(diameter: 40-500μm, depth: 40-500μm) on fatigue strength were also investigated by microscopic observations.
Experimental results showed no evident difference in both static and fatigue strengths between SAE10L45 and S45C; no evident decrease in strength of SAE10L45 due to the existence of inclusions of Pb and S (MnS) was confirmed. This result should be considered to explain the high machinability of SAE10L45, because the stress concentration at inclusions of Pb and MnS may not be the main factor for the high machinability.
