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Hastelloy X is used in turbomachinery and petrochemical applications as it is designed for excellent oxidation and stress corrosion cracking resistance, strength, and stress rupture behavior. This alloy is now being printed via powder bed fusion processes as many industries have developed interests in the benefits additive manufacturing (AM) offers. However as-printed Hastelloy X suffers from material defect formation such as hot cracking. Hot isostatic pressing (HIP) is often applied to improve performance and reliability. Although the conventional HIP process has been shown to eliminate defects, the equipment is unable to cool at desired rates allowing the formation of excessive carbide precipitation, negatively influencing corrosion resistance and toughness. In turn the product is solution treated at a similar temperature while applying rapid gas cooling for performance requirements. With use of uniform rapid cooling available in modern HIP equipment, a high-pressure heat treatment can be applied offering the ability to perform both HIP and heat treatment in one piece of equipment. Microstructure and tensile properties are evaluated and compared to the conventional processing routes. The results demonstrate that the novel high pressure heat treatment approach offers a processing route that is equivalent to or better than conventional methods.

The use of molten halide salts as a primary coolant within next generation molten salt nuclear reactors (MSRs) has the potential to improve efficiency and nuclear reactor safety by operating at low pressures and high temperatures without boiling. However, these coolants will require the development of new corrosion resistant material systems that will have to meet or supersede existing standard codes for these systems. Therefore, new MSRs will require validation and testing of new material systems that can produce robust component structures and enable them to withstand these corrosive environments. Furthermore, any next generation process to apply protective coatings must allow for a conformal, thick, dense, and bonded material to be formed onto an ASME code approved material (e.g. type 316H stainless steel) that will comprise the structure of heat exchanger and containment vessel components. Within this context, Faraday Technology Inc. in collaboration with Oak Ridge National Lab and Quintus Technologies is working to develop and demonstrate the effectiveness of a low cost and high value corrosion-resistant alloy coating for next generation molten salt reactors. The manufacturing process involves the application of a functionally graded NiMo coating by electrodeposition onto 316H alloy substrates and their subsequent high temperature corrosion evaluation in molten fluoride salt at 700°C. After application the components were hot isostatically pressed (HIPped; at Quintus) to create a diffusion bond between the coating and alloy substrate. Initially, 316H alloy coupons were coated for evaluation (Figure 1 Left) in Mo capsules with static FLiNaK salt known to be corrosive due to metallic impurities. The next stage of testing involved evaluating coatings on the inner diameter of 316H tubes (25mm outer diameter; Figure 1 Middle). Coated tubes (75mm long) had coated endcaps welded to either end and were filled with the same FLiNaK salt and tested for 500 h at 700°C and 1000 h at 750°C. In addition, the microstructure and salt performance of butt welds were evaluated for joints between coated 316H tubes and joints between coated and uncoated 316H tubes. Results from the static capsule testing will be presented including the effect of coating composition and heat treating conditions on the depth of attack in the molten salt as feedback for the coating development process. The final phase of testing will include 200 mm long coated 316H tube sections in the hot and cold legs of a flowing FLiNaK thermal convection loop experiment which is currently being fabricated (Figure 1 R). Acknowledgements: The financial support of DOE Contract No. DE-SC0019602 is acknowledged. Figure 1: Corrosion characterization test plan. (Left) static coupon studies, (Middle) static pipe studies, and (Right) continuous flow loop pipe studies. Figure 1

The use of additive manufacturing (AM) technology is growing rapidly for offshore use, whilst established production technologies such as powder metallurgy near-net-shape (PM-NNS) continue to be used. New standards are being introduced and many of the oil and gas majors are now developing supply chains to ensure rapid supply of high-quality complex parts. Environmental concerns are helping to drive this with use of near-net-shape technologies to reduce carbon dioxide emissions through more efficient designs and metal forming methods. Hot isostatic pressing (HIP) has been used to remove shrinkage porosity and internal defects in cast products for many years, predominantly to improve mechanical properties and fatigue resistance. Recently, there has been an increasing focus on AM processes for demanding applications where localized corrosion and similar internal defects and porosity are a concern. HIP technology is being used to either produce PM-NNS parts, or for the post processing of AM produced components to ensure defect free material prior to service in demanding environments. This paper will present a broad overview in the use of HIP equipment to produce parts and components to the offshore industry.

With advancements in hot isostatic pressing (HIP) systems it is now possible to also perform heat treatment by achieving the desired microstructure during the HIP cycle. This modern approach offers the freedom to consider a combined HIP and heat treatment cycle known as high pressure heat treatment (HPHT™). There are many potential benefits from this processing route such as shorter process times, cost reductions, increased quality, and improved productivity. Material benefits also exist from this processing route. This paper will cover the fundamentals of HPHT™ highlighting key technology changes enabling this approach in modern HIP equipment. Then a transition to recent studies capturing the benefits for aerospace applications leveraging HIP and the HPHT™ methodology along with future possibilities with steered cooling will be reviewed.

Austenitic stainless steels are carburized or nitrided (i.e., surface hardened) at low temperatures in order to maintain their superior corrosion resistance. Treatment temperature must be low enough to prevent precipitation in the diffusion zone, yet high enough to allow sufficient diffusion depths to meet design specifications. At these temperatures, prior machining processes can have a significant effect not only on diffusion, but also the surface hardness and corrosion resistance achieved. This paper presents practical examples showing how cutting, grinding, honing, and polishing processes influence the results of low temperature surface hardening treatments for stainless steel parts. It also discusses the influence of surface deformation and finish.

In laser powder bed fusion (L-PBF), the mechanical performance and especially fatigue properties of fabricated parts are significantly improved by hot isostatic pressing (HIP) as the density increases (pores are closed) and the microstructure improves. HIP ensures consistent and defect-free material, and consequently, this high temperature and high pressure process is often a requirement for safety-critical aerospace applications. The use of HIP to directly consolidate intentionally-unmelted interior powder in a L-PBF part was recently demonstrated. By confining the laser melting to only the outer shell (contour) of the structure, L-PBF production times can be dramatically reduced. A subsequent HIP cycle, which may be mandatory for reliability reasons, and therefore does not add additional costs, is then used to densify the entire structure. Production rates and energy efficiencies can therefore be improved in this way. This study explores the effect of relying on the HIP process to consolidate interior sections of test coupons, for which micro computed tomography (microCT), process simulation and tensile tests were conducted. MicroCT of coupons with varying shell thicknesses identify the minimum shell thickness required; and provide indications of the shrinkage ratio as a function of powder content relative to shell thickness. Preliminary results are included in which the shrinkage can be compensated for during design, by way of a “bloated cube” which collapses to a nominal cube geometry after HIP. The mechanical evaluation of the consolidated shelled parts indicates that their tensile performance is equivalent to those observed on fully dense printed parts. Up to an order of magnitude faster build rates are possible and any resulting shrinkage or distortion can be offset at design.