JOURNAL OF MATERIALS SCIENCE LETTERS 20, 2 0 0 1, 333 – 335 Effect of aging on the hardness and impact properties of Hastelloy C-276 J. I. AKHTER, M. A. SHAIKH, M. AHMAD, M. IQBAL, K. A. SHOAIB, W. AHMAD Nuclear Physics Division, Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad, Pakistan E-mail: pinstech@paknet2.ptc.pk; akhterji@usa.net Nickel base alloys constitute an important class of materials, which are used under demanding condi- tions of high corrosion resistance and high tempera- ture strength. A wide range of these alloys exists due to the ability of nickel matrix to accommodate large amounts of elements, such as chromium, iron, copper, molybdenum, etc., while retaining essentially a single phase face-centered-cubic structure [1]. Ni-Cr-Mo-W alloys, named as Hastelloys, are known for their good mechanical and physical properties and resistance to a wide variety of corrosive environments. The alloys have found widespread use in chemical, aerospace and nuclear industry because of these properties. Hastelloy C-276 has been previously studied due to its many applications. Effect of aging treatment on the microstructure and corrosion properties of this alloy has been investigated by a number of researchers [2–5]. Precipitation of intermetallic phases (μ and P) and carbides has been reported in most of these stud- ies. The μ phase is known to be hard and brittle [5]. On the other hand this alloy has been considered to be non-hardenable by conventional aging treatment [6]. Present study has been carried out to investigate the effect of aging treatment on the hardness and impact energy of the material, as no detailed results are avail- able on these aspects. The results would be correlated with the precipitates produced due to heat treatment. Hastelloy C-276 was received in the form of a sheet having thickness 3 mm. The composition of the alloy is given in Table I. Samples were solution treated at 1200 ◦ C for 1 h and water quenched. Aging was car- ried out at two temperatures 650 ◦ C and 850 ◦ C for time varying up to 240 h. Specimens were etched in a so- lution of 10–12%. HCl in ethanol and then they were examined in scanning electron microscope (SEM). El- emental analysis was done using X-ray energy disper- sive microanalysis system (EDS) attached with SEM. Precipitates were extracted for analysis using carbon replica technique. Samples were prepared for impact testing with size 3 × 10 × 55 mm 3 and having notch of 45 ◦ angle. These samples were tested with 50 J hammer. Hardness of the alloy was measured in three con- ditions, i.e. as-received, solution treated and aged by Rockwell hardness tester (HRB). Hardness was found to remain constant for the samples aged at 650 ◦ C. Its value is the same as that of the solution treated sam- ples. It suggests that no hardening phases are produced at 650 ◦ C for aging time up to 240 h. According to the previously reported [2] time-temperature-precipitation diagram one of the hardening phases, i.e., μ phase is not formed at 650 ◦ C up to 500 h of aging. However M 6 C carbides may be produced after about 10 h of aging. In the present case no precipitates have been observed by SEM examination in the samples aged at 650 ◦ C for aging period up to 240 h. It may be that precipitation of carbides may take much longer than the aging time period in the present case. Room temperature absorbed energies of the samples were attempted by impact test- ing after various aging treatments. At 650 ◦ C, no sample could be broken showing again that hardening was not produced by aging at this temperature. Hardness of the samples aged at 850 ◦ C increases with aging time up to 96 h and then remains constant. These results are plotted in Fig. 1. These measurements suggest that aging at 850 ◦ C hardens the alloy. This ob- servation is contrary to the previously quoted statement TABLE I X-ray microanalysis (wt.%) of matrix and precipitates. Numbers within the brackets indicate ±σ . K lines for Ni, Cr, Fe, Co, L line for Mo, and M line for W were used Element Matrix Precipitates Previous results Ni 54.49(0.37) 25.45(0.75) 28.0(2.4) Mo 17.26(0.50) 46.30(0.80) 49.5(2.2) Cr 16.25(0.30) 11.77(0.42) 11.0(0.4) Fe 5.49(0.30) 3.42(0.34) 2.0(0.2) W 4.62(0.37) 11.38(0.53) 7.5(2.6) Co 1.67(0.31) 1.68(0.38) 1.5(0.1) Figure 1 Variation of hardness and impact energy as a function of tem- perature. 0261–8028 C  2001 Kluwer Academic Publishers 333