The Behaviour of Graphitized Steels in Machining Processes

Abstract

Abstract. Graphitized steels are claimed to perform excellent in machining processes. They therefore can be considered as environmental friendly alternatives to widely used Pb-alloyed steels. Due to liquid metal embrittlement and in-situ lubrication Pb improves machinability in a narrow tool-chip interface temperature window corresponding to low machining speeds. Although graphite inclusions are also supposed to generate in-situ lubrication, the mechanism and the corresponding optimum working zone is not very clear. The present work applies a new test methodology (including in-situ tribology, analysis of material flow and chip formation, optimum working zone analysis) to investigate the effect of graphite inclusions in turning and drilling operations. Pballoyed low carbon free-cutting steel and Pb-alloyed case hardening steel were used as reference steels. IntroductionUsing free cutting steels in mass production machining performance is the most important property. It is the main factor influencing part production costs. Widely spread low carbon free cutting steels like 11SMn30 exhibit low hardness resulting in low machining forces. Addition of sulphur leads to the formation of manganese sulphides, which protect the machining tool by in-situ layer formation and reduces friction forces in the chip-tool interface. As a consequence high productivity and long tool lifetimes are reached. However the ferrite matrix of such steels is soft and tends to stick on the tool surface if cutting speeds are low. This so called built-up edge formation (BUE) can be a severe problem producing small parts or in machining operations with partially low speeds like cutting-off or drilling. BUE formation can be efficiently suppressed by adding lead to the steel. Liquid lead then acts as lubricant in the chip-tool interface. Due to its low melting point this effect already occurs at low machining speeds. At higher speeds this benefit disappears (which might be caused by a change in viscosity). As a rule of thumb considering all kind of machining conditions productivity of leaded steel 11SMnPb30 is estimated to be 20% higher comparing to non-leaded steel 11SMn30. This explains the popularity if this steel which is consumed in large quantities in machining workshops all over the world.

Challenges and Innovation in Steel Wire Production

“…Steel 50SiB8 exhibits the lowest friction coefficients in a wide range of cutting speeds, proving the in-situ lubrication of graphite. Additional machining tests reveal the realistic potential of graphite to replace lead if the hardness of the compared steels is similar [11].…”

Johnson Cook Flow Stress Parameter for Free Cutting Steel 50SiB8

“…The relatively new material 50SiB8 [4,11] was developed by Swiss Steel c as a lead free alternative for classical free cutting steels, e.g. 11SMnPb30 and 16MnCrS5Pb.…”