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.
Low‐cycle fatigue and corrosion fatigue mechanisms of ultra‐high‐purity (UHP) Ni and Ni‐base alloys in H2SO4 solutions are analysed. UHP materials are used to eliminate the impurity effects, including those on grain boundaries which can promote hydrogen embrittlement. Particular attention is paid to the conditions for hydrogen absorption and its effect on damage processes. Hydrogen embrittlement is analysed in terms of persistent slip band–grain boundary (PSB–GB) interactions and hydrogen diffusion short circuits at grain boundaries. The effect of chromium on the kinetics of the PSB formation as well as on both anodic dissolution and hydrogen absorption is emphasised.
The interest in ultra-high-purity (UHP) metals and alloys (including single crystals and bicrystals) in studying damage mechanisms is presented through three practical cases of environment or precipitation-induced damages in nuclear power plants. First, stress corrosion cracking mechanisms are assessed by direct measurements of hydrogen-dislocation interactions in pure nickel single crystals in low-cycle fatigue tests under cathodic potential. Hydrogen-induced softening is observed, that confirms an important assumption on the corrosion-enhanced plasticity model. Second, mechanisms of high-temperature intergranular cracking of bimetallic welds are analysed on a series of model materials including 302 H stainless steel and UHP stainless steel selectively doped with carbon. The key role of carbide precipitation and localised shearing in a chromium-depleted zone is pointed out. Finally, the analysis of intergranular penetration of liquid Bi-rich films in a polycrystalline solid Ni inducates the presence of very long and brittle films of nanometric thickness, which have to be taken into account in any model of liquid-metal embrittlement.
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