casting process, which basically consists of a homogenization heat treatment (also called solution annealing heat treatment) and a two-steps aging heat treatment (also called precipitation hardening heat treatment). In Figure 4, the typical γ/γ´- microstructures and segregation maps that are obtained before and after each heat treatment step are also shown. Figure 5 shows the so-called integrated HIP heat treatment (IHT), where the heat treatments steps of homogenization and precipitation hardening are carried out under high isostatic pressure in a single heat treatment cycle within a modern QIH9 HIP facility that provides rapid cooling rates [6]. In order to develop an IHT heat treatment, an extensive parametric study was carried to investigate the influence of temperature, pressure, holding time and cooling rate on the obtained microstructure (mainly porosity and γ/γ´ microstructure) [5-6]. From this study, it was concluded that the value of the HIP temperature has the main impact on porosity reduction. The HIP temperature value must be higher than the gamma prime solvus temperature (Tγ´–solvus) if the porosity must be completely close. Knowing this, the value of the isostatic pressure was set for a fixed super Tγ´–solvus value of 1300°C. In this regard, it was found that fully dense material could be obtained when 75MPa were applied for 3h above
Tγ´–solvus. Further
studies were conducted at 1300°C and 100MPa to investigate the effect of cooling rate on the γ/γ´ microstructure. High quenching rates gave rise to many γ´ nuclei due to limited time for diffusion and growth. Subsequent aging steps were then used to set the final γ’- precipitate sizes. Figure 6 shows the effect of three different cooling rate values on the γ/γ´ microstructure [6]. After the parametric study, the IHT heat treatment was carried out (see Figure 5) resulting in a dense material with a finer γ/γ´ microstructure, Figure 7b, regarding the obtained γ/γ´ microstructure after the conventional heat treatment route, Figure 7a. [7] Regardless the creep regime, the
30 ❘ September 2019 ®
Figure 5: Schematic of the integrated HIP heat treatment (IHT) developed for ERBO/1 [5]. Image courtesy of Ruhr Universität Bochum
Figure 6: The effect of cooling rate on the γ/γ´ microstructure of single crystal material ERBO/1 [6]. Image courtesy of Ruhr Universität Bochum
creep performance of the ERBO/1-IHT processed within the HIP facility showed extremely promising results, Figure 8. Both rupture strains and rupture lives were improved significantly with an improved creep rate minimum.
Conclusions
HIP has been successfully used to remove
porosity in cast for many years, improving
materials material
properties and the reducing the cost of non-conformity, yield losses and repair. Modern equipment is now
available on the market which can combine HIP and heat treatment in new
processes to further improve
material performance, opening new doors to develop the casting industry to compete more actively with other production methods such as forging or additive manufacturing.
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