Cast iron with flake graphite
There are, however, some significant differences between machining cast iron with flake (lamellar) graphite (EN-GJL) and machining cast iron with spheroidal (nodular) graphite (EN-GJS). For both material types, the shape of the graphite inclusions determines the shape and type of chips that are produced when machining. As a general rule, a tool’s service life will be shorter if it is used to machine SG iron, in comparison with machining iron containing flake graphite. Wear and tear on the tool itself is heavily dependent on the amount and the form of the graphite, as well as on the form of the metallic material structure.
In cast iron containing flake (lamellar) graphite, the basic metallic structure of the iron is interrupted by the graphite flakes. The flakes act as the material’s preferred slip planes and as internal notches; at the tips of these notches are regions of increased stress, at which cracks can form under external loading. This leads to crack stress and (as the cutting speed increases) increased shearing; the result is short, compact chips. The result is minimized wear and cutting pressure on the tool; however, small areas may chip and break away from the edges of the part during machining. The surface quality is heavily dependent on the production technique, the machining conditions, and the casting quality. The specified cutting force should always be in the range of 800 N/mm2 (for lower–strength materials) to 1,350 N/mm2.