HP Alloys Research Articles

Microstructural study of the NbC to G-phase transformation in HP-Nb alloys

The microstructure of a centrifugally cast HP alloy was studied in its as-received state and after ageing at 900 °C. A multi-scale approach combining X Ray Diffraction (XRD), advanced electron microscopy modes (scanning and transmission electron microscopies (SEM, TEM), together with focused ion beam/SEM nanotomography (FIB-nt)) has been carried out to characterize the evolution of niobium carbides during ageing. After thermal treatment, the carbides exhibit a complex microstructure, consisting of a core of untransformed NbC, an intermediate layer of G-phase (Ni16Nb6Si7) with embedded nanometric titanium carbide precipitates, and an outer shell of alternating chromium carbides Cr23C6 and G-phase. A simple diffusion model was used to explain the faster external growth of G-phase compared to the internal NbC dissolution, and to determine a diffusion coefficient of niobium in the G-phase at 900 °C.

Evaluation of electric and magnetic signals in heat-resistant HP alloys using scanning probe microscopy

Abstract HP steels are commonly used in steam reforming tubes, which are exposed to harsh operational conditions causing microstructural changes. These microstructural changes cause variations in the magnetic properties, which can be measured by different techniques. This work aims at the measurement of the magnetic and electric variations in HP-Nb modified alloys with different chemical composition using scanning probe microscopy (SPM) techniques. The results show that, despite the microstructural change due to variation of chemical composition, samples with aging state I exhibit a magnetic response at the carbides boundary, while for advanced aging states, the magnetic response is not evidenced.

On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

Experimental insights regarding bandgap evolution in hybrid perovskite alloys and the optimal small-bandgap absorber composition desired for next-generation perovskite tandems.

Microstructural evolution of a modified HP alloy: experimental and complementary computational study

In this work is presented results of the microstructural characterization of austenitic stainless steel (HP series) modified with Nb, aged at temperatures of 750, 800, 850, 900 and 950oC in air at different times. Microstructural changes were analyzed using optical microscopy, scanning electron microscopy (SEM) equipped with (EDS), x-ray diffraction and Vickers hardness. In the as-cast condition, the microstructure consists of an austenitic matrix and eutectic carbides network, Cr, Nb-rich carbides. The Cr-rich carbides are M7C3 type, whereas, those rich in Nb are NbC. During aging, there is a second precipitation in the matrix of very fine needle form of M23C6 carbides, which leads to an increase in hardness. After that, the decreasing in hardness is associated with a coalescence phenomenon of the secondary precipitates. Computational modeling shows that M23C6 is lesser stable than M7C3 carbide.

Fragmented structure of niobium carbide particles in as-cast modified HP alloys

The structure of niobium carbide particles in as-cast heat resistant modified HP40NbTi alloy have been studied by light and electron microscopy, electron microprobe and X-ray diffraction. The niobium carbide particles in the structure of the HP40NbTi alloys are a multiphase polycrystal clusters with inhomogeneous chemical composition and crystallographic form. The boundaries between the crystals of the carbide particle are large-angle; the disorientation angle between crystals is 30–60°. Polycrystalline character of carbides is, probably, connected with the high thermal stress arising at interphase boundaries in the alloy structure at primary cooling of an ingot. The explanation of the polymorphic character of a niobium carbide cluster requires an independent analysis on the basis of a structural and geometrical crystallography.

Microstructural Characterization and Image Analysis in Ex-Service HP Alloy Stainless Steel Tubes for Ethylene Pyrolysis

Carburization is the major degradation mechanism of the HP alloy stainless steel tubes used for ethylene pyrolysis. Due to the changes in the microstructure and magnetic properties of the heat-resistant HP grade (Fe-Cr-Ni) alloy tubes over their service life, the level of carburization can be detected non-destructively using eddy current methods. In the current project, the microstructure of a carburized ex-service Nb-modified HP alloy tube has been characterized via image analysis. The phase fractions and phase distributions have been measured in order to determine the microstructural features that contribute to the eddy current non-destructive testing response, to assist in improving the accuracy of remaining life estimates. Issues in differentiating the large number of phases were encountered when using electron backscattered images for image analysis, and thus, energy-dispersive x-ray spectroscopy maps have been used, in combination with the software programs ilastik, for automated image segmentation, and FIJI, for image processing. The chromium depletion of the austenite matrix of the tube, and thus the magnetic response, was observed to be predominantly influenced by the volume fraction of M7C3 and M23C6 chromium carbides, with potential contribution from the chromium-containing η-carbide intermetallic. Carbonitrides of the type Cr2(C,N) were seen to have negligible impact. A comparison between microstructures and magnetic response has determined that eddy current non-destructive testing can detect and measure carburization.

Increasing Weldability of Service-Aged Reformer Tubes by Partial Solution Annealing

A dissimilar joint of 25Cr-35Ni/30Cr-48Ni (HP/HV) heat-resistant steels was evaluated. The investigations indicated that the as-cast HP alloy contained M7C3, M23C6, and NbC carbides and HV alloy with 5 wt.% tungsten, contained M23C6 and M6C carbides embedded in an austenitic matrix. After 8 years of ex-service aging at 1050 °C, the ductility of HP/HV reformer tubes was decreased dramatically, and thus, the repair welding of the aged HP/HV dissimilar joint was at a risk. In order to repair the aged reformer tubes and increase weldability properties, a new partial solution annealing treatment was designed. Mechanical testing results showed that partial solution annealing at 1200 °C for 6 h increased the elongation and toughness of the aged HP and HV alloys drastically. Also, a mechanism for constitutional liquation cracking in the heat-affected zones (HAZ) of the HP/HV dissimilar joint was proposed. In the HAZ of the aged HP/HV welded joint, the cracks around the locally melted carbides were initiated and propagated during carbides solidification at the cooling cycle of welding associated with the decrease in the ductility of the aged HP and HV alloys. In addition, Varestraint weldability test showed that the susceptibility to hot cracking was decreased with partial solution annealing.

Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich η-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.

Influence of niobium addition on the high temperature mechanical properties of a centrifugally cast HP alloy

The influence of niobium addition on the mechanical properties at high temperature of HP alloy has been investigated. Two HP alloys were centrifugally cast with a similar chemical composition differing only in the niobium content. Low strain rate high temperature tensile tests and creep-rupture tests were performed in the range of 900–1100°C, and the results compared between the alloys. According to the results, the high temperature mechanical behavior of both alloys is controlled by several factors like solid solution, network of eutectic carbides, intradendritic precipitation and dendrite spacing. A significant increase in the mechanical properties for the HP alloy with niobium addition was found within the temperature range of 900–1050°C. Beyond this temperature the mechanical behavior of both alloys is basically the same.

Crystallography and Morphology of MC Carbides in Niobium-Titanium Modified As-Cast HP Alloys

The microstructures of two as-cast heats of HP alloy stainless steels modified with niobium and titanium were examined with particular attention paid to the interdendritic niobium-titanium-rich carbides formed during solidification of these alloys. Generally, these precipitates obtain a blocky morphology in the as-cast condition. However, the (NbTi)C precipitates may obtain a nodular morphology. To provide further insight to the origin of the two different morphologies obtained by the (NbTi)C precipitates in the HP-NbTi alloy, the microstructure and crystallography of each have been studied in detail using scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (EBSD, SAD, and CBED), and energy-dispersive X-ray spectroscopy.

Spall failure of aluminum materials with different microstructures

The effects of microstructure on the spall failure were studied for four aluminum materials by a series of plate-impact spall experiments, including the real-time measurements of the free surface velocity profiles and the microscopic postimpact examination of the soft-recovered samples. Spall strength values are calculated by using the free surface velocity measurements. The high density high purity aluminum (Al HP) exhibits a higher spall strength than the low-porosity pure aluminum. The metallographic examination revealed that it could be attributed to the less impurity in the grain boundaries in the Al HP samples, having a better resistance for void nucleation. The 2024-T4 aluminum alloy exhibits a stronger spall failure resistance than the 7075-T6 aluminum alloy, which is associated with the stronger plastic strain hardening behavior. Comparison among the Al HP, 2024-T4 and 7075-T6 alloys indicates that the differences observed in the rise rate of pull-back are linked with the different active mechanism and growth rate of damage evolution.

Effect of Heat Treatment Conditions on Microstructure and Mechanical Properties of Long Term Serviced Fe25Cr-35Ni Alloy

Abstract HP alloy Fe-25Cr-35Ni is widely used in petrochemical industry, because of its good corrosion and oxidation resistance at high temperature. As-cast microstructure of HP alloy consists of eutectic carbide in discontinuous network. After service in carburizing atmosphere at high temperature, the microstructure of serviced HP alloy, which consisted of carbide network that was thicker and coarser than as-cast specimens as well as irregular shape carbide, lost its tensile strength and fracture elongation properties. However, the hardness increased. The serviced HP alloys were heat treated in 12 conditions, which could be classified in four main categories. The first one was solutionized at three different temperatures — 1,373, 1,423 and 1,473 K. The second was solutionized at 1,373 K and aged at 1,073, 1,173 and 1,273 K. The third was solutionized at 1,423 K and aged at 1,073, 1,173 and 1,273 K. The last one was solutionized at 1,473 K and aged at 1,073, 1,173 and 1,273 K. The process time for solution was 21.6 ks and aging is 86.4 ks for all conditions. The result showed that the amount of Cr rich carbide vastly decreased after heat treatment in all conditions. The Vicker hardness values (HV) of all heat treated specimen decreased comparing to that of serviced alloy. In conclusion, heat treatment provided new microstructures with lower amount of carbide networks of serviced alloy which resulted in improvement of tensile strength and fracture elongation properties.

High-Temperature Galling Characteristics of TI-6AL-4V with and without Surface Treatments

Galling is a severe form of surface damage in metals and alloys that typically arises under relatively high normal force and low sliding speed and in the absence of effective lubrication. It can lead to macroscopic surface roughening and seizure. The occurrence of galling can be especially problematic in high-temperature applications like diesel engine exhaust gas recirculation system components and adjustable turbocharger vanes, because suitable lubricants may not be available, moisture desorption promotes increased adhesion, and the yield strength of metals decreases with temperature. Oxidation can counteract these effects to some extent by forming lubricative oxide films. Two methods to improve the galling resistance of titanium alloy Ti-6Al-4V were investigated: (a) applying an oxygen diffusion treatment and (b) creating a metal–matrix composite with TiB2 using a high-intensity infrared heating source. A new oscillating three-pin-on-flat, high-temperature test method was developed and used to characterize galling behavior relative to a cobalt-based alloy (Stellite 6B, HP Alloys, Windfall, IN). The magnitude of the oscillating torque, the surface roughness, and observations of surface damage were used as measures of galling resistance. Due to the formation of lubricative oxide films, the galling resistance of the Ti alloy at 485°C, even nontreated, was considerably better than it was at room temperature. The infrared (IR)-formed composite displayed reduced surface damage and lower torque than the substrate titanium alloy.

EBSD Phase Identification and Modeling of Precipitate Formation in HP Alloys

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.

The aluminizing and Al–Si codeposition on AISI HP alloy and the evaluation of their carburizing resistance

Heat resistance casting alloy ACI HP was aluminized or silicon-aluminized for investigating their carburization resistance in a high-carbon potential atmospherized-fluidized-bed which simulated the carburizing environment of the pyrolysis tubing in a ethylene production line. The results showed that the coating layer prepared by the high-activity aluminizing pack was superior to that of the low-activity one. On the contrary, the coating layer provided by the low-activity silicon-aluminizing pack was superior to that of the high-activity one. Experimental results evidenced that silicon could effectively retard the degeneration rate of NiAl and reduce the formation of porous band, which located at the interface between aluminide layer and interdiffusion zone. Moreover, the kinetics of the reaction, D ̃ = D ̃ exp(− Q ̃ /RT) , was empirically determined by the method of Al pack and Al–Si pack. The results showed that, in a low-activity Al–Si pack, Si can effectively reduce both Q̃ and D ̃ 0 . As a consequence, it increased the formation rate of aluminide layer, in contrast, it reduced the formation rate of interdiffusion zone.

Effect of ultra-purification on the microstructure of Fe-0.8mass%C alloy

An ultra-high purity Fe-0.8 mass%C alloy (UHP alloy) was melted in a newly designed ultra-high vacuum induction melting furnace equipped with a copper cold crucible. Reference materials were also prepared, consisting of a high-purity Fe-0.8 mass%C alloy (HP alloy) and a commercial Fe-0.8 mass%C alloy (Com alloy), which were melted in a high vacuum induction melting furnace at research laboratory. The purity of the UHP, HP and Com alloys were approximately 99.998, 99.943 and 99.033 mass %, respectively. The effect of purification on the microstructure was revealed after various heat-treatments applied to the UHP alloy. It was clarified that the microstructure of Fe-0.8 mass%C alloys, especially the morphology of carbide, is strongly affected by purity. In the UHP alloy, lamellar carbide in pearlite easily dissolved, and spheroidized carbide precipitated.

KHR35H-HiSi

Abstract KHR35H-HiSi is an improvement over HP alloy in oxidation resistance and creep strength. This composition also has high silicon for resisting carburizing environments in ethylene pyrolysis coils. This datasheet provides information on composition, physical properties, and tensile properties as well as creep. It also includes information on corrosion resistance. Filing Code: Ni-567. Producer or source: Kubota Metal Corporation.

KHR35H

Abstract KHR35H is a high-temperature heat resistant alloy with a molybdenum addition to increase creep and rupture strength to levels better than those of HP alloy. Typical applications are in heat treatment equipment. This datasheet provides information on composition, physical properties, and tensile properties as well as creep. It also includes information on corrosion resistance as well as joining. Filing Code: Ni-564. Producer or source: Kubota Metal Corporation.

UCI-198

Abstract UCI-198 is best described as an HP alloy modified with niobium and with microalloying by titanium and rare-earth elements. The alloy was developed to produce increased creep resistance in furnace tube applications. This datasheet provides information on composition, physical properties, hardness, and tensile properties as well as creep. It also includes information on high temperature performance and corrosion resistance as well as casting, machining, and joining. Filing Code: SS-770. Producer or source: Ultra-Cast Inc.

KUBOTA ALLOY KHR35C

Abstract Kubota alloy KHR35C is similar to HP alloy with the addition of niobium to increase its creep-rupture strength. Typical applications include components and assemblies for severe carburizing environments, such as ethylene pyrolysis coils. This datasheet provides information on composition, physical properties, elasticity, and tensile properties as well as creep. It also includes information on high temperature performance as well as casting and joining. Filing Code: SS-753. Producer or source: Kubota Metal Corporation.