Influence of heat treatment on the microstructure and tensile properties of Ni-base superalloy Haynes 282

Abstract

The effect of heat treatment on the microstructure and mechanical properties of Ni-base superalloy Haynes 282 was investigated. Applying a standard two-step ageing (1010 °C/2 h + 788 °C/8 h) to the as-received, mill-annealed, material resulted in a the presence of discrete grain boundary carbides and finely dispersed intragranular g´, with an average size of 43 nm. This condition showed excellent room temperature strength and ductility. The introduction of an additional solution treatment at 1120 °C resulted in grain growth, interconnected grain boundary carbides and coarse (100 nm) intragranular g´. The coarser g´ led to a significant reduction in the strength level, and the interconnected carbides resulted in quasi-brittle fracture with a 50 % reduction in ductility. Reducing the temperature of the stabilization step to 996 °C during ageing of the mill-annealed material produced a bi-modal g´ distribution, and grain boundaries decorated by discrete carbides accompanied by g´. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary g´ and a bi-modal intragranular g´ distribution can be used to tailor the mechanical properties to suit specific applications. The yield strength of all three conditions could be accurately predicted by a unified precipitation strengthening model.

Precipitation Kinetics and Morphology of Grain Boundary Carbides in Ni-Base Superalloy Haynes 282

“…interconnected ''brick wall'' network at the second aging temperature (788°C) and the resulting loss of ductility. Notably, the formation of a ''brick wall'' morphology at 788°C when carbide precipitation at 1010°C was suppressed [5] is consistent with the present observations. There are, however, some noticeable differences between our observations and the TTT curves proposed in Reference 6.…”

“…No c¢ particles could be observed by SEM, and the grain boundaries were mainly free from carbides, although occasional isolated grain boundary carbides could be found. The hardness of the mill-annealed material was measured to be 241 HV, higher than for the same material in the solutiontreated and water-quenched state, [5] which indicates the presence of c¢ with size below the SEM resolution limit as reported for the mill-annealed state in Reference 6. Figure 1 shows micrographs of samples aged for 2 hours at different temperatures.…”

“…However, a systematic study of the grain boundary carbide morphology has not yet been reported, which appears to be warranted given the previously observed effect on tensile ductility [5] and known reduction in creep resistance for superalloys. [8][9][10] It is also important for the development of alternative heat treatments, such as the single-stage aging proposed in Reference 11, where the stabilization treatment was omitted and carbides were instead precipitated together with c¢ at 800°C.…”

“…A recent study showed that the morphology of grain boundary carbides can significantly affect the tensile ductility of Haynes 282. [5] When carbides were present in an interconnected ''brick wall'' structure, the ductility decreased by almost 50 pct compared to material with discrete carbides in the grain boundaries. Fahrmann and Pike [6] studied the kinetics of secondary carbide formation in Haynes 282 and showed that the precipitation of M 6 C was significantly slower than expected.…”

“…The Vickers macro-hardness of samples was measured with a 10-kg load, and an average of five measurements is reported in this study. The microstructure of the mill-annealed sheet consists of equiaxed grains of the order of~100 lm with MC carbides (rich in Ti and Mo) and carbonitrides distributed in the rolling direction both inter-and intragranularly (see [5] for details). No c¢ particles could be observed by SEM, and the grain boundaries were mainly free from carbides, although occasional isolated grain boundary carbides could be found.…”