Surface Integrity and Structural Stability of Broached Inconel 718 at High Temperatures
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TRODUCTION
INCONEL 718 is a Ni-based superalloy that is widely used as a disk material in the hot sections of turbine engines, owing to its superior mechanical properties and its high resistance to oxidation/corrosion at elevated temperatures. The alloy is predominantly strengthened by the precipitate gamma double prime (c¢¢-Ni3Nb) which appears as disk-shaped particles with a coherent relationship with the c matrix.[1] The microstructure of Inconel 718 degrades when exposed to temperatures above 923 K (650 C). The c¢¢ phase transforms to incoherent d phase and the mechanical properties deteriorate rapidly.[2] Meanwhile, for long-term applications, other precipitates such as a-Cr and r could also form.[3] The formation and growth of a-Cr has been reported to be partly responsible for the significant drop in impact strength of Inconel 718.[4] The good oxidation/corrosion resistance of the alloy at high temperatures is provided by a high chromium content which allows a protective surface chromia scale to be developed in most atmosphere.[5–7] Z. CHEN, Doctoral Student/Candidate, R. LIN PENG, Associate Professor, J. MOVERARE, Professor and Head, and S. JOHANSSON, Professor, are with the Division of Engineering Materials, Linko¨ping University, 58183 Linko¨ping, Sweden. Contact e-mail: [email protected] P. AVDOVIC, Specialist in Cutting Technology, is with the Siemens Industrial Turbomachinery AB, 61283 Finspa˚ng, Sweden. J.M. ZHOU, Professor, is with the Division of Production and Materials Engineering, Lund University, 22100 Lund, Sweden. Manuscript submitted May 15, 2015. Article published online April 22, 2016 3664—VOLUME 47A, JULY 2016
Machining of Inconel 718 has always been a challenge. Due to its high strength, the cutting forces attain high values, while the low thermal conductivity leads to high cutting temperatures being developed in the cutting zone. The high chemical affinity of the alloy to many tool materials and its sensitivity to strain rate can also cause rapid wear of the cutting tools and work hardening of the machined workpiece. Ulutan and Ozel[8] published a review paper with a focus on the machining-induced surface integrity in titanium and nickel alloys in which they pointed out that the need for attention in machined Inconel 718 mainly involves surface tearing, cavities, micro-cracks, dynamic recrystallization, plastic deformation, work hardening, and the formation of residual stresses. High intensity of plastic deformation is generally perceived to be a main threat to surface integrity.[9] Bushlya et al.[10] suggested that in high speed turning of Inconel 718, the giant temperature gradients imposed on the greatly deformed surface material could even lead to the formation of a white layer. Imran et al.[11] also observed the white layer for a micro-drilling process for Inconel 718 and the thickness increased slightly with increasing feed rate and cutting speed. The presence of a surface white layer has been found to significantly reduce the fatigue life because of the loss of ductility in this layer.
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