Influence of Single- and Double-Aging Treatments on the Mechanical and Corrosion Resistance of Alloy 625

2. Materials and Method

The material investigated in this work is a forged and untreated Alloy 625 rod with diameter of 60 mm. Heat treatments were carried out in a laboratory furnace. The tensile specimens were machined in the longitudinal direction. Samples for the microstructural analyses and the hardness tests were prepared according to the conventional metallographic procedure. After grinding and polishing, etching was carried out in five parts HCl diluted in one part 30 % H₂O₂ for 10 s [5,26,30]. Microstructural analyses were performed using a Leica (model DMR) light optical microscope and a Zeiss (model SIGMA 500) scanning electron microscope. The HV30 Vickers hardness tests were conducted using a Wolpert Testor 930 hardness tester in accordance with the EN ISO 6507 standard [31]. In each tested condition, at least five hardness measurements were carried out. Tensile tests were performed using round proportional specimens tested with an INSTRON model 4507 machine, according to the EN ISO 6892 standard [32]. The intergranular corrosion tests were carried out in a solution of ferric sulfate, Fe₂(SO₄)₃ and sulphuric acid, H₂SO₄ for 120 h, in accordance with the ASTM G28 – Method A standard [28]. The mechanical strength requirements considered for the evaluation of acceptability were taken from the ASTM B446 standard [12]. For the corrosion resistance, a maximum corrosion rate of 1.20 mm/year was considered. SEM and chemical EDXS analyses were carried out to better investigate the relationship between the microstructural constituents and the corrosion properties. The qualitative EDXS chemical analyses were performed using the OXFORD Altee Energy – Advanced detector. The bulk chemical composition was investigated by OES spectroscopy. The results are reported in Table 1. Based on the requirements of the ASTM B446 standard [12], the chemical composition is acceptable.

Before the aging treatment, all the samples and specimens were soft annealed at 1038 °C for 0.5 h and water quenched. The hardening response upon isothermal single-aging treatments was investigated at 732 °C and 621 °C. After this preliminary analysis, double-aging treatments were carried out changing the exposure times at both the primary aging temperature (732 °C) and the secondary aging one (621 °C). Hardness tests were performed in each aging condition investigated in this work. Successively, based on the hardness results, tensile and corrosion tests were carried out in a selection of aging conditions. SEM and EDXS analyses were performed to investigate the relationship between the microstructural constituents and the corrosion properties.

3. Results and Discussion

The as-forged material was soft-annealed at 1038 °C for 0.5 h in accordance with the ASTM B446-19 standard [12]. The microstructural investigation was performed by metallographic analysis using light-optical and scanning-electron microscopy. As reported in Figure 2, the austenitic microstructure showed presence of equiaxed and twinned grains without intergranular carbides. The average grain size measured according to the ASTM E112 standard was equal to 65 μm [33].

Then, the soft-annealed material was characterized by room-temperature tensile and hardness tests. The mechanical properties are reported in Table 2. The mechanical strength in the soft-annealed condition is not compatible with the minimum yield strength requirement (414 MPa) [12].

In the soft-annealed condition, the presence of a corrosion rate equal to 0.65 mm/year confirms the suitability of this heat treatment (1038 °C 0.5 h).

After soft-annealing at 1038 °C for 0.5 h, the age-hardening effect was investigated with isothermal single-aging treatments at both 732 °C and 621 °C up to 600 h starting from the results of a reduced set of conditions studied in our previous work [29]. The overall set of hardness data at both aging temperatures is reported in Figure 3 [29]. As shown in Figure 3, upon single aging at 621 °C, a steep hardness increase is obtained starting from 72 hours. Until such exposure, the hardness increment with respect to the soft-annealed condition is less than 10 %. At 732 °C, the precipitation hardening effect is slightly higher compared to that at 621 °C at least until the exposures tested in this work. At 621 °C, a satisfying hardening effect is obtained only after very long exposures and this behavior can limit its exploitation for most industrial applications. Such very slow age-hardening kinetic is also confirmed by the literature data of Moore et al. and Suave et al. about single aging from 550 °C to 900 °C [1,20,22]. The hardness values of our work are compatible with the literature [20]. The age-hardening phenomenon in this temperature range is determined by precipitation of the ${\gamma }^{\text{'}\text{'}}$ phase, as shown in the TTP curves of Figure 1. According to the TTP curves, at these aging temperatures, the formation of intergranular M₂₃C₆ carbides can occur together with the intermetallic hardening ${\gamma }^{\text{'}\text{'}}$ precipitates. However, compared to the ${\gamma }^{\text{'}\text{'}}$ phase, the hardening effect of the M₂₃C₆ carbides is negligible [15]. The simultaneous formation of Cr-rich M₂₃C₆ carbides was deeply investigated by EDXS analyses performed by scanning electron microscopy. The IDs of the tested positions are reported in Figure 4 and the results of the EDXS analyses are given in Table 3. As shown in Table 3 and Figure 4, at 732 °C, intergranular Cr-rich carbides were not present until 1 hour and the onset of their formation was detected in the 3 hours sample. According to the EDXS analyses reported in Table 3, the intergranular carbides observed in the samples aged at 732 °C were mainly enriched in chromium, as confirmed by the literature data [10,14]. The formation of such carbides is expected to be significantly detrimental to the corrosion resistance.

At 621 °C, the precipitation of the hardening ${\gamma }^{\text{'}\text{'}}$ phase is faster than that of the intergranular Cr-rich carbides. In fact, according to the TTP diagram of Figure 1, at this temperature, the formation of M₂₃C₆ carbides requires longer exposures compared to 732 °C. This difference was confirmed by the SEM and EDXS analyses performed on the samples aged at 621 °C. The results are reported in Figure 5 and Table 4. In this work, Cr-rich carbides were observed at the grain boundaries of the sample aged at 621 °C for 592 h, and they were not detected on shorter exposures. So, upon aging at 621 °C, it is possible to exploit the precipitation of the hardening intermetallic ${\gamma }^{\text{'}\text{'}}$ phase before worsening the corrosion resistance.

After the investigation of single-aging response, the possibility of accelerating the precipitation kinetic by double-aging treatments was studied considering different time-temperature conditions with primary aging at 732 °C (up to 48 h) and secondary aging at 621 °C (up to 592 h). Double-aging treatments significantly accelerate the ${\gamma }^{\text{'}\text{'}}$ precipitation. In fact, fixing the duration of the secondary aging, the precipitation kinetic is accelerated increasing the prior exposure at 732 °C. In this case, short primary aging times are sufficient to obtain the peak hardness. The adoption of longer primary aging time is not associated with a further improvement in the mechanical strength. In fact, when the peak hardness is reached, the hardness curve remains almost flat. Considering the TTP diagram reported in Figure 1, the major drawback of such double-aging procedure is represented by the primary aging temperature. In fact, even though the primary aging enables significant acceleration of the ${\gamma }^{\text{'}\text{'}}$ formation, such aging temperature is also associated with a faster precipitation of intergranular carbides which are extremely detrimental to the corrosion resistance. For this reason, it is required to limit as much as possible the primary aging exposure. Regarding the double aging, Eiselstein and Tillack [7] found an increase of hardness from 170 HV to 210 HV adopting a primary treatment at 760 °C for 1 h prior to aging at 621 °C for 72 h. In our research work, the hardness increases from 195 HV to 280 HV with double aging at 732 °C for 1 h and 621 °C for 72 h. As described in the Introduction, this double-aging procedure was investigated starting from that of the alloy CarTech® Custom Age 625 PLUS® [26,27] for which the suggested double-aging treatment resulted in a hardness increase from 188 HV to 353 HV. Regarding our research work, the same heat treatment was associated with a poorer hardening response (from 194 HV to 220 HV) because of the lower titanium content.

Successively, in selected double-aging conditions, room-temperature tensile tests were performed. As shown in Figure 7, the tensile properties show a trend similar to that observed for the hardness curves reported in Figure 6. In fact, after a certain exposure, the increase in the primary aging time with fixed secondary aging does not provide any further improvement for both the yield strength and the ultimate tensile strength. The hardening effect obtained by double aging is obtained at the expense of tensile deformability. In fact, a reduction of both A% and Z% was observed. However, the A% values remained well above the minimum standard requirement (30 %) [12]. In all the tested double-aging conditions, except 732 °C 3 h plus 621 °C 8 h, both the yield strength and the ultimate tensile strength were compatible with the minimum standard requirements [12]. Therefore, when insufficient tensile properties characterize the soft- or solution-annealed condition, this aging procedure can be exploited to obtain satisfying strength improvements with reasonable heat treatment times compared to single aging. In fact, as previously described, compared to single-aging treatments, the double aging allows to significantly accelerate the hardening kinetic. As reported in Table 5, with double aging at 732 °C 3 h and 621 °C 72 h, it is possible to obtain the same mechanical properties of single aging at 621 °C for 260 h with a conspicuous acceleration of the aging response and consequent reduction of the heat treatment time and cost.

The possibility of estimating the tensile properties from the results of hardness tests represents a fast and useful tool in the analysis and comparison of different time-temperature aging conditions. The relationship between the hardness and the tensile strength was investigated by regression analysis of a dataset obtained combining the results reported in this work with those previously published in the literature [11,29]. The linear regression analysis of this larger set of experimental data is reported in Figure 8 together with the determination coefficient $R^2$ which was adopted to assess the goodness of fit. Considering the results of this analysis and the requirements of the ASTM B446 standard [12], 192 HV and 202 HV represent the minimum hardness values for acceptable ultimate tensile strength and yield strength, respectively.

The intergranular corrosion resistance [28] was analysed in a selection of the conditions investigated in this work. The variation of the corrosion rate as function of the aging time is depicted in Figure 9 for both single and double aging. The single-aging treatment at 732 °C shows a rapid increment in the corrosion rate starting from 3 hours and, until this exposure, the hardening effect is very poor. This experimental result is confirmed by the SEM and EDXS analyses reported in Figure 4 and Table 3. In fact, single aging at 732 °C for 3 hours represents the onset of intergranular Cr-rich carbides formation which is extremely detrimental to the corrosion resistance. The rapid increase in the corrosion rate at longer exposures is associated with a more copious formation of such carbides at the grain boundaries. At 732 °C, such undesired precipitation phenomenon occurs very quickly and the reduction in corrosion resistance becomes critical much earlier than the onset of ${\gamma }^{\text{'}\text{'}}$ precipitation, which represents the primary hardening phase. In fact, as confirmed by the experimental results and the literature [15], the hardening effect of the intergranular carbides is very poor and the effect of sensitization is predominant. At 621 °C, the precipitation of intergranular M₂₃C₆ carbides is obtained only after very long exposures. This peculiarity allows to exploit the hardening effect of the ${\gamma }^{\text{'}\text{'}}$ precipitates without worsening the corrosion resistance. Based on the experimental results reported in Figure 9, at 621 °C, the rapid increment in the corrosion rate is no longer obtained from 3 hours, as occurs at 732 °C, but only after 130 hours. Therefore, in this case, it is possible to exploit the hardening effect reported by the hardness curves in Figure 3 before worsening the intergranular corrosion resistance. This experimental result is confirmed by the SEM and EDXS analyses reported in Figure 5 and Table 4. Regarding the double aging, the corrosion tests were performed in selected conditions to evaluate the effect of an increasing primary exposure at 732 °C with fixed secondary aging time. As shown in Figure 9, increasing the time at 732 °C, the corrosion resistance is quickly reduced because of a very rapid and copious precipitation of intergranular carbides. As shown by the results of the corrosion tests, the thermal exposure at 732 °C is surely the most critical and it should be restricted as much as possible to maintain the corrosion resistance within acceptable limits, but provide sufficient acceleration of the precipitation response. Therefore, in the presence of double aging, the limitation of the primary aging time is extremely important also considering that above a certain duration there is no further strength improvement.

Considering the corrosion rate limit (1.20 mm/year) and the minimum requirement for the mechanical strength (202 HV), a performance map was built with the experimental conditions tested by hardness and corrosion tests in this work and in our previous paper [29], as shown in Figure 10. The hardness limit of 202 HV was calculated considering the minimum standard requirement for the mechanical strength [12] and the relationship between the hardness and the yield strength reported in Figure 8.a. In the soft-annealed condition, only the corrosion resistance is acceptable. Single aging at 621 °C for 130 h provides good corrosion resistance and excellent mechanical strength. Longer aging times at 621 °C allow further strength improvement at the expense of a dramatic loss in the corrosion resistance. Regarding the double aging, even reducing the primary aging time down to 1 hour, the corrosion rates remain above the threshold limit. In this case, a reduction of both the primary and secondary aging times is still not sufficient to obtain acceptable corrosion resistance. Moreover, further limitation of the primary aging exposure would be not feasible in the presence of large components.