On the Effect of Grain Size and Hardness on the Machinability of Superalloys and Chip Deformation

Licentiate thesis, 2009

Superalloys are used in applications such as gas turbines, steam power plants and nuclear power systems that require good high temperature properties and/or exceptional corrosion resistance. Superalloys are classified as materials that are “difficult to machine”. In production machining is a most commonly occurring procedure. Thus, the concept of machinability – the ease with which a material will be machined – becomes important. In particular since machinability problems tend to be increasingly costly with the toughness of the work material. This work aims to study the microstructural influence on the machinability of the two wrought superalloys Alloy 718 and Waspaloy. The most obvious reason is the apparent scarcity of knowledge about how the microstructure affects the machining procedures. Through heat treatment four different conditions of each alloy were studied; fine and coarse grain size that were solution annealed or aged. The machining operation chosen was transverse turning. The machinability was examined through actual progression of tool wear, burr formation, surface deformation and chip microstructure. These investigations showed less flank wear when machining fully age hardened Waspaloy compared to solution annealed Alloy 718. The hardness had a significant effect on the flank wear within each alloy, while the effect of grain size was much more limited. The grain size was found to affect the deformation behaviour, the large grain size material exhibits more inhomogeneous deformation then the same material in the small grain size condition. Inhomogeneous deformation occurs when the size of the grains was in the same order of magnitude as the feed rate and the inhomogeneous deformation was connected with large notch wear and also with burr formation. The deformation of the machined surface was also larger (deeper) in the large grain size material. Inhomogeneous deformation also affects the chip formation, the chips from the coarse grain microstructures are much more serrated, with larger segmentation depth.