Hot isostatic pressing (HIP) is a process used to eliminate internal voids and defects such as porosity in castings, additive-manufactured (AM) parts and MIM parts to achieve a 100%-dense material. The elimination of defects results in improved material properties such as fatigue, creep, ductility and fracture toughness.[1-7]
The HIP process uses a high isostatic gas pressure (up to 30,000 psi/207 MPa) and elevated temperature (up to 3632°F/2000°C) to achieve the material densification. HIP is used within many different industries – most commonly aerospace, medical implant and oil and gas – for a variety of applications. In Figure 1, the densification by HIP of a powder-bed-fusion Ti-6Al-4V material analyzed with X-ray CT is presented.
In-situ Heat Treatment in HIP
HIP has historically been used only for densification and defect elimination of material, and any modification and optimization of a material’s microstructure was usually performed after the HIP process in a separate heat-treatment step in separate equipment (e.g., vacuum furnace). The main reason that these processes have been performed separately is that the achievable cooling rates in HIP systems have traditionally been relatively low – lower than what many materials require for heat treatment.
One important development within HIP equipment during recent years is the HIP furnace with ultrahigh-pressure gas quenching, which enables high cooling rates in the HIP unit. The high cooling rates are achieved by a forced-convection cooling of the highly pressurized argon gas that is used in the HIP process. Fast cooling and quenching systems make it possible to perform many of the conventional heat treatments for metals directly in the HIP furnace. This enables the combination of the HIP and heat-treatment process to be performed in the same cycle in one piece of equipment, the HIP.
The main purpose of integrating the heat treatment into the HIP process is to eliminate process steps in the post-processing of a component to achieve a more time- and cost-effective post-processing. Presented in Figure 2 is a schematic visualization of how the conventional thermal post-processing for a metal AM component could look, where the different thermal-treatment steps are performed separately.
These steps are often performed in different equipment and sometimes even at different physical sites. The possibility of doing rapid cooling and quenching directly in the HIP unit enables the combination of the HIP and solutionizing step to be performed at the same time in the HIP furnace. Other treatments, such as stress relieve, aging or tempering, etc., can also be incorporated into the HIP cycle. In
Figure 3, a potential combined post-processing route is shown for the same case as shown in Figure 2.
Benefits of In-situ Heat Treatment in HIP
By combining several steps into one cycle performed directly in the HIP, the downtime when moving components from one piece of equipment to another can be eliminated. The total effective cycle time is also reduced when combining the steps together by eliminating soak times and time for heating and cooling. Another benefit of the combined process is that the number of times the components have to be heated up (and cooled down) can be reduced, which saves energy.
One benefit from a metallurgical perspective is that the time at elevated temperature can be reduced with the combined process, which can minimize grain growth. For example, if a two-hour soak HIP cycle and a two-hour solution anneal is combined into one cycle, the material will spend two hours at elevated temperature with the combined process instead of four hours with the conventional process.
Rapid Cooling and Quenching HIP Furnaces
The high cooling rates in the HIP furnace are achieved by circulation of gas and mixing of hot and cold gas inside the pressure vessel. During the heating and soak time of the HIP cycle, the hot gas is kept inside the hot zone of the furnace where the components are processed. As a part of the furnace surrounding the hot zone, there is an insulating thermal barrier that protects the pressure vessel from the heat. Therefore, the gas is much cooler outside the hot zone.
During the forced-convection cooling, the gas inside the pressure vessel is circulated around so the cold gas outside the furnace is pushed into the hot zone at the same time as the hot gas in the hot zone is moved out. The hot gas is cooled down by the cold pressure-vessel walls that are water-cooled from the outside, so the pressure vessel is used as a heat exchanger during the cooling. A schematic picture of a forced-convection-cooled HIP furnace and the gas flow during forced-convection cooling is shown in Figure 4.
With forced-convection cooling technology, cooling rates up to 8000°F/min (4444°C/min) in the gas can be achieved in a modern HIP system.
High cooling rates are not the only thing that makes a modern HIP system suitable for heat treatment. A modern HIP system is also very flexible, where HIP and heat-treatment cycles can be created with infinite heating, soaking, cooling and quenching steps. This makes it possible to tailor-make heat-treatment recipes for optimized microstructures and material properties. The logged data from a combined HIP and hardening treatment of a tool steel is shown in Figure 5.
Example of In-situ Heat Treatment in HIP
HIP+HT for SX Ni-Based Superalloy and Effect on Creep Properties
Ruhr-Universität Bochum, Germany, has studied a combined HIP and heat-treatment cycle for an SX Ni-based superalloy ERBO 1, an alloy of type CMSX-4 that has a specific heat treatment, including a solution anneal and dual-step aging. In this study, a conventionally heat-treated variant and a variant processed with an integrated HIP and heat treatment have been compared and evaluated for microstructure and creep properties.
The details of the heat treatment are presented in Table 1, and the measured thermal profile of the integrated HIP and heat-treatment cycle can be seen in Figure 6. The cooling rate for the conventional process performed in a vacuum furnace was around 270°F/min (150°C/min) from the solution-anneal temperature and down. The same cooling step in the HIP had a significantly higher cooling rate around 2700°F/min (1500°C/min). In the HIP, the quenching was stopped at the first aging temperature for a direct aging without having to cool down to room temperature first.
The resulting microstructures of the two variants analyzed by SEM can be seen in Figure 7 a and b. The γ/γ′ microstructure achieved by HIP+HT is finer than the microstructure of the conventional variant, which is a result of the significantly faster cooling in the HIP system. The γ/γ′ volume fraction is similar for both variants.
The amount of porosity in the material was measured by image analysis of the metallographic cross section. The conventional, only heat-treated, variant had 0.244% porosity, while most of the porosity had been eliminated in the HIP+HT variant with only 0.002% remaining porosity.
Creep testing of the two variants was performed in two temperature/stress regimes – 1382°F / 116 ksi (750°C / 800 MPa) and 1922°F / 23 ksi (1050°C / 160 MPa) – and the results are presented in Figure 8 a and b. The creep life is increased significantly with HIP+HT, and the creep rate at the high-stress regime is lower compared to the conventional variant. This is a result of the much-reduced porosity content in the HIP+HT material and the finer γ/γ′ microstructure obtained by the higher cooling rate in the HIP.
Modern HIP technology offers the possibility of performing rapid cooling and quenching in the HIP furnace, which makes it possible to combine HIP with conventional heat treatment into one cycle.
There are several benefits of integrated heat treatment in HIP. Process steps can be eliminated, which will reduce total lead time and result in a more cost-effective process. The process energy consumption can also be reduced since less heating up and cooling down of the parts is needed. Time at elevated temperature can potentially be reduced, which will minimize grain growth and microstructural coarsening.
A modern HIP system today has great flexibility and can cool up to 8000°F/min (4444°C/min) under pressures up to 30,000 psi (207 MPa). These capabilities make it possible to heat treat a variety of different materials.
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