Alloy 800H Research Articles

Effect of lanthanum on inclusions and solidification structure of alloy 800H

Alloy 800H is commonly used in high temperatures and corrosive environments due to its exceptional strength and resistance to high temperatures, but the ingots are prone to longitudinal cracking during the hot working process because of its coarse solidification structure. Based on the heterogeneous nucleation effect of rare earth inclusions, a strategy is proposed to refine the solidification structure of alloy 800H by La treatment to solve the crack problem. In this study, four kinds of alloy 800H ingots (0, 120, 260 and 470 ppm La) were prepared to investigate the effect of La on the solidification structure. After adding La, the types of inclusion were changed, and the modification path of inclusions was Al2O3 (0 ppm La) → LaAlO3 (120 ppm La) → LaAlO3·La2O2S and La2O2S (260 ppm La) → La2O2S and La2O3 (470 ppm La). After adding La, the inclusion morphology tended to be more spherical and the average size smaller. The lattice misfit calculation indicated that La-containing inclusions can serve as heterogeneous nucleation cores, and the number of effective heterogeneous nucleation also increased with La content increased. The columnar to equiaxed transition occurred early with La treatment, the ratio of the equiaxed zone increased from 2.1% to 36.5%, and the average equiaxed grain size decreased from 2.56 to 0.43 μm with La content increased from 0 to 470 ppm. The solidification structure of alloy 800H was significantly refined by adding La, and can thus provide a new strategy to solve the crack problem.

Improvement of fatigue life by aluminizing of additive manufactured Fe- and Ni-base alloy

Laser powder bed fusion (PBF-LB/M) is used in various industries to manufacture complex parts with high precision. High surface roughness and porosity have a negative impact on fatigue resistance. This work presents aluminum pack cementation as a method to reduce surface roughness and improve fatigue resistance of AM parts. An Fe-base alloy (Alloy 800H) and a Ni-base alloy (Alloy 699XA) are selected. Some of the specimens made from PBF-LB/M were treated by pack cementation with aluminum, while others were subjected to the vibratory finishing process. The surface roughness, microhardness distribution and the microstructure of the interface zone were measured and analyzed for the as-built, vibratory finished and aluminized specimens. Rotating bending fatigue test was performed at room temperature. Conventionally fabricated specimens of both investigated alloys were tested under the same conditions to evaluate the effects of pack cementation. The aluminized samples (PBF-LB/M) show a significant reduction in surface roughness with a decrease of 45 % for Alloy 800H and 65 % for Alloy 699XA. This leads to an improvement in fatigue life for both Alloy 800H and Alloy 699XA. In contrast, the conventionally fabricated specimens exhibited increased surface roughness after pack cementation compared to their initial condition and showed a significant reduction in fatigue resistance.

Role of different density negative micro-pillars textured H13 surfaces on enhanced wettability and tribological performance

The present study investigates how varying negative micro-pillar densities on H13 alloy, used in hot-forming dies, affect wettability with different fluids and tribological performance, alongside detailed morphological analysis. Recast materials, pockmarks, micro-voids, micro-cracks, and globules of debris are observed on EDM textured surfaces, and challenges are form accuracy and debris entrapment with higher textured density. The functional stressed surfaces, evident from XRD analysis, display hydrophobic properties with water and hydrophilic characteristics with mineral-oil-based emulsion. Increased texture density enhances these properties. Wettability properties transition of fabricated functional surfaces occurs within fluid surface energies of 48 to 72.8 mN/m. The coefficient of friction (COF) of the lower-density patterned surface is 20.42 % lower than the untextured surface, and the higher-density pattern shows a 13.48 % COF reduction compared to the lower-density surface. Key wear mechanisms include abrasion, adhesion, sharp edges and adhered debris deformation, and localized plastic deformation for untextured and textured surfaces.

Effect of grain boundaries on the helium degradation mechanisms of alloy 800H: A molecular dynamics study

Alloy 800H is currently used as structural material in light-water cooled nuclear reactors, and it is also considered as candidate materials for various advanced reactor designs. Under operation, the exposure of alloy 800H to neutron irradiation results in the formation of helium (He), mainly from Ni by the transmutation reaction (n, α). In this work, we present a molecular dynamics (MD) simulation study of the behavior and effects of He on the microstructure and mechanical properties of alloy 800H. Our results show that the population of clusters made of 5 to 9 He atoms is nearly constant throughout the simulation time (10 ns), while the population of larger clusters increases as the simulation time increases. The growth of clusters is controlled by either the dissociation and diffusion of smaller clusters towards nearby larger clusters or the merging of larger clusters initially located nearby to each other. A significant accumulation of He is observed at the grain boundaries (GB), while a depletion zone is found at the neighboring regions. As a result, the density of He cluster is significantly higher at the GBs as compared to the intra-granular regions. The nucleation and growth of He clusters also results in the formation of Frenkel pairs (FP), whose associated self-interstitial atoms (SIA) agglomerate into interstitial clusters in the alloy 800H matrix. As a consequence, dislocation segments, mostly of the Shockley type, are generated in the microstructure, and often located next to He clusters. The combination of the aforementioned defect structures and the high density of He clusters at the GBs results in a substantial degradation of the mechanical properties of alloy 800H single crystal and bicrystals.

Oxide Growth Behaviour of 800H and HR-120 Series Ni-based Alloys on Isothermal Oxidation

Oxide Growth Behaviour of 800H and HR-120 Series Ni-based Alloys on Isothermal Oxidation

High temperature mechanical properties of diffusion welded alloy 800H

High temperature mechanical properties of diffusion welded Alloy 800H was investigated to fabricate a printed circuit heat exchanger (PCHE) for high temperature reactor (HTR) systems. Surface treatment was performed on Alloy 800H to transform the solubility product (Ksp) to exceed the reaction quotient (Q). The surface treatment facilitated the grain boundary migration across the interface. The yield strengths exceeded the values described in ASME Section III Division 5 Table HBB-I-14.5 up to 760 °C, while the tensile strengths were comparable to Table HBB-3225-1 up to 700 °C. At 760 °C, the tensile strength was ∼30 MPa lower than the code. The stress-to-rupture values exceeded the expected minimum stress-to-rupture values of Alloy 800H described in Table HBB-I-14.6C. The ductility of the diffusion weldment acquired from the tensile test was comparable to the as-received Alloy 800H. Meanwhile, the formation of the secondary precipitates on the interface during the stress-to-rupture test deteriorated the ductility of the diffusion weldment, which induced intergranular fracture.

Microstructure Evolution of Alloy 800H during Cold Rolling and Subsequent Annealing

Microstructure Evolution of Alloy 800H during Cold Rolling and Subsequent Annealing

Sample size effect phenomenon in microcompression testing of unirradiated and ion-irradiated 800H steel at high temperatures

In situ microcompression testing is used to quantify the mechanical properties of materials and directly observe the deformation mode. It allows to be site specific and probe specific microstructural regions of interest. Sample size effect, where the strength of materials increases with decreasing sample size, remains a challenge. This phenomenon has been studied widely at room temperature but not at elevated temperature. This work here sets out to evaluate and understand the scaling effects at elevated temperature on irradiated and unirradiated austenitic (FCC) Fe-Cr-Ni alloy 800H. Micro-compression samples were fabricated in a 〈111〉 grain containing both the unirradiated and the irradiated volumes for direct comparisons of yield strength at room temperature and 300 °C. TEM lift-out of the compressed micro-pillars at room temperature were fabricated for microstructural characterization. The influence of irradiation and temperature on sample size effect are independently investigated. It was observed that sample size effects increased with increasing temperature in only the unirradiated material.

Effects of Deformation Temperature on Dynamic Recrystallization and Twinning of Incoloy 800H Alloy

Effects of Deformation Temperature on Dynamic Recrystallization and Twinning of Incoloy 800H Alloy

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Solidification Characteristics and TiC Formation Behaviour in Alloy 800H

Solidification Characteristics and TiC Formation Behaviour in Alloy 800H

Micro bi-prism bi-telecentric single lens Stereo-DIC system using one-step calibration method

In recent years, the bi-prism bi-telecentric single lens stereo digital image correlation (BBSL Stereo-DIC) system has emerged as a viable approach for measuring stereo surface deformation. However, the reliance on a bi-prism that needs to be precisely aligned perpendicular to the optical axis of the bi-telecentric lens using a laser beam, as well as the need to remove the bi-prism during the calibration phase, impose limitations on the practical application of the existing method. To overcome these challenges, we propose a convenient one-step calibration method that eliminates the positional dependence on the bi-prism. This method incorporates the bi-prism’s attitude parameter and principal point coordinates as additional intrinsic parameters. Similar to traditional binocular calibration, the calibration process involves capturing a series of images of a calibration target at various positions and orientations, making it convenient and straightforward for DIC users. A numerical simulation calibration experiment is performed to illustrate the difference between the proposed method and the existing method. The result shows that the bi-prism’s attitude parameter has a significant impact on 3D reconstruction. A rigid-body translation experiment and a tensile experiment of an 800H alloy micro-specimen with a notch are conducted on a micro BBSL Stereo-DIC system. The results and the comparison with finite element simulation prove the accuracy of the proposed method, which demonstrates that the one-step calibration method can be easily employed for micro stereo surface deformation measurement.

Prediction of Hot Deformation Behavior for Inconel 740H Alloy Based on Ensemble Learning

Prediction of Hot Deformation Behavior for Inconel 740H Alloy Based on Ensemble Learning

High-temperature corrosion model of Incoloy 800H alloy connected with Ni-201 in MgCl2–KCl heat transfer fluid

Abstract Long-term dipping corrosion experiments on Incoloy 800H (800H) in molten eutectic MgCl2–KCl salt at 1,073 K with and without connection with Ni-201 alloy were carried out. While connecting with Ni-201 alloy, the corrosion rate of 800H was 4.47 ± 0.26 mg·cm−2·day−1 (10−3.55±0.02 A·cm−2). However, the corrosion rate for the 800H that was not connected to the Ni-201 alloy was 3.00 ± 0.27 mg·cm−2·day−1 (10−3.71±0.04 A·cm−2). Therefore, Ni-201 alloy accelerates the corrosion rate of 800H in the molten MgCl2–KCl salt while connecting to each other. In addition, based on the calculation of Gibbs energy of Ni–Fe–Cr alloy and exchange current density of 800H in MgCl2–KCl salt, a new Tafel model was developed to predict the corrosion rate of 800H alloys. The corrosion rate calculated by the new model is 10−3.74 A·cm−2, which agrees with the experimental data.

Diffusion Welding of Surface Treated Alloy 800H

Diffusion welding of heat-resistant alloys has attracted interest in the manufacturing of components with complex configurations. Controlling secondary precipitates along the interface is necessary to enhance the quality of the diffusion welding. Surface treatment to increase the solubility product (Ksp) of Ti-rich carbide is proposed to accomplish such an enhancement. The reduction of secondary precipitates along the interface induced grain boundary migration across the interface. The chemical compositions at/near the interface satisfied the material specifications. The mechanical properties of the diffusion weldment were similar to those of Alloy 800H that underwent the same thermo-mechanical processes in the range of 25–700 °C. At 25 °C, the tensile strength was 553 MPa, which satisfied the minimum specified tensile strength described in ASTM B: 409-22. The location of failure was random in the gauge section, and dimples, the evidence of the macroscopic plastic deformation, were observed on the fracture surface.

Thermal Deformation Characteristics and Dynamic Recrystallization Mechanism of Incoloy 800H Alloy under Different Deformations

Incoloy 800H alloy has excellent high‐temperature mechanical properties and broad application prospects. However, the dynamic recrystallization (DRX) behavior of this alloy at different deformations has been little studied. The thermal deformation characteristics and DRX mechanisms of Incoloy 800H alloy are studied by thermal compression tests. Quantitative analysis of grain size, DRX fraction, misorientation distribution, and twin boundaries under different deformation degrees is conducted using electron‐backscattered diffraction. Furthermore, the evolution characteristics of dislocations and grains are observed through transmission electron microscopy. It is found that the proportion of DRX improves with increasing deformation significantly. In the DRX process, discontinuous DRX (DDRX) is the main nucleation mechanism, while continuous DRX (CDRX) serves as an auxiliary nucleation mechanism. As the deformation increases, the CDRX + DDRX mechanisms can also be observed simultaneously in the high‐density dislocation region near the grain boundary. In addition, the ratio of ∑3 boundaries first decreases and then increases as the deformation increases. The initial twins gradually disappear due to the increase in deformation, and new twins are formed within the DRX grains. Twins can promote grain boundary bulging, provide more nucleation sites for DRX, and have a significant promoting effect on DRX.

Microstructural evolution of 800H alloy after long-time creep rupture test at 675 °C

This study investigated the microstructural evolution of 800H alloy tubes and their welding through creep rupture test at 675 °C with different applied stresses. Vickers hardness testing was used to determine microhardness and optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to examine the microstructure of the tubes. The microhardness of the base metal, heat-affected zones, and welding seams of the tubes after testing were all significantly higher than the as-received 800H tubes due to the combination of Cr23C6 precipitates, dislocation multiplication, and work-hardening effect. The base metal of all tubes had a coarse and equiaxed austenitic structure with large Ti-N-rich precipitates, parallel needle-like Cr23C6 precipitates within twins, and discontinuously dispersed chain-like Cr23C6 precipitates along grain boundaries. As testing progressed, spherical Cr23C6 were precipitated within grains then coarsened, and the intervals between parallel needle-like Cr23C6 within twins were increased due to their low thermodynamic stability and synergistic effect of time and stress. The heat-affected zones exhibited a low correlation with test time, with all samples displaying dense spherical and needle-like Cr23C6, whereas the welding seams showed insignificant microstructural evolution. γ΄ (Ni3(Al, Ti)) precipitates were observed along the sides of the Cr23C6, further coarsening of γ΄ was observed as the creep rupture test progressed.

SAGBO on Inner and Outer Surface of Large Alloy 800H Pipe

Abstract One of the objectives of this series of case studies presented in section Failure Analysis of Practical Metallography is to educate colleagues who are new in the field, to inform them about failure mechanisms, not least about the rarer, more exotic ones and about metallurgical causes of failure that were utterly unexpected to occur in the particular failure case studies presented here. The subject of this contribution is a failure mechanism called Strain-assisted Grain Boundary Oxidation (SAGBO). This failure mechanism is not rare at all in hot-going nickel-base components in the aerospace industries. However, SAGBO was certainly unexpected to occur in large pipes that remained well below 400 °C in service.

Investigation on regression model for the force of small punch test using machine learning

The analytical solution for the force-deflection curve of the small punch test (SPT) is still a challenge. Machine learning is employed in the present study to establish a model for estimating SPT forces by using the strength of a material. The yield strength and ultimate tensile strength of materials and corresponding SPT force-deflection curves generated by finite element simulations are the training and testing data for machine learning. Pearson correlation analysis was performed first to measure the statistical association between the SPT force at a given deflection and the strength of materials. It was shown that a binary linear regression model was capable of correlating SPT forces to the yield strength and ultimate tensile strength of a material. The model parameters were determined after training, and then this model was tested by testing data. The accuracy of the regression model was verified by hypothetical materials, X70 steel and Incoloy 800H alloy. This study gives a new insight into the relationship between force responses of SPT and material properties, and can promote the development of novel approaches for determining material tensile properties using SPT.

Effects of aging and nitridation on microstructure and mechanical properties of austenitic stainless steel

Understanding the effects of in-service microstructural changes in metal alloys on the mechanical performance and remaining life of equipment is a critical part of integrity management in industrial plants. However, the effects of processes such as carburization or nitridation are not accounted for in industry-standard life assessment methodologies such as those provided in the API 579-1/ASME FFS-1: Fitness-For-Service standard. This is problematic for the austenitic stainless steel Alloy 800H, which typically operates in the creep regime and has been reported to suffer nitriding during high-temperature service in air. Despite this being one of the most common service environments for 800H, the impact of nitriding on in-service performance and implications for remaining life assessment have not been well-studied for this alloy. In this work, we characterize the microstructures of as-received, aged, and nitrided Alloy 800H tube material, and correlate observations with room-temperature tensile properties and high-temperature creep behavior. We show that while the creep properties of aged 800H material can be captured by the widely-used MPC Project Omega creep model and API 579-1 Omega properties for 800H, nitrided material properties fall outside of the expected bounds, and therefore remaining life cannot be reliably predicted using this method without experimental data.