Effects of Multiple Rejuvenation Cycles on Mechanical Properties and Microstructure of IN-738 Superalloy
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JMEPEG (2018) 27:2524–2533 https://doi.org/10.1007/s11665-018-3335-5
Effects of Multiple Rejuvenation Cycles on Mechanical Properties and Microstructure of IN-738 Superalloy Cosimo Monti, Alessandro Giorgetti, Leonardo Tognarelli, and Francesco Mastromatteo (Submitted November 11, 2017; in revised form January 31, 2018; published online April 6, 2018) The scope of this work is to show the effects of multiple applications of a rejuvenation treatment studied for IN-738 on both the microstructure and the mechanical properties of the creep-damaged superalloy and to check the recovery obtained after one and two rejuvenation cycles through creep and tensile tests, whose results will be compared with the performance of the virgin material. This work will show that this rejuvenation treatment is able to recover the microstructure of creep-damaged specimens after one and two applications and that the mechanical properties of the rejuvenated alloy are very similar to the virgin material even after two rejuvenation cycles. Keywords
creep, IN-738, microstructure, rejuvenation, tensile
1. Introduction IN-738 is an equiaxed nickel-based superalloy widely used for the fabrication of engine parts in gas turbine applications such as blades, vanes and integral wheels, thanks to its excellent high temperature strength and hot corrosion resistance (Ref 1, 2). The alloy studied in this paper, whose chemical composition is shown in Table 1, is the low carbon version of IN-738, called IN-738LC (Ref 3). The base material used in this work is the same already studied in (Ref 4). The main hardening effect of IN-738, as for many other nickel-based superalloys, is given by the precipitation of the c¢ phase that is highly coherent with the matrix and impedes the dislocations motion. According to the chemical composition of the alloy, the c¢ volume fraction of IN738 can vary between 40 and 50% (Ref 5-8). The standard heat treatment of IN-738 is composed by two main steps (Ref 3, 8): Solution heat treatment for 2 h at 1120 C ± 10 in vacuum/air followed by cooling at the rate equivalent to air cooling to 650 C; • Aging for 4 h at 845 C ± 5 in vacuum/air followed by air cooling to room temperature. •
The material obtained after the standard heat treatment (which will be considered as the virgin material in the following) shows a bimodal distribution of c¢ particles, characterized by cuboidal and larger primary precipitates and by smaller and spherical secondary particles. Primary MC carbides are present both in the grain cores and at the grain boundaries, while secondary M23C6 carbides usually precipitate along the grain boundaries. Depending on their configuration, secondary carbides can strengthen or weaken the
Cosimo Monti and Alessandro Giorgetti, Department of Innovation and Information Engineering, Guglielmo Marconi University, via Plinio 44, 00193 Rome, Italy; Leonardo Tognarelli and Francesco Mastromatteo, BHGE (TPS) – Nuovo Pignone Tecnologie SRL, Via F. Matteucci, 2, 50127 Florence, Italy. Contact e-mail: [email protected].
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