High Cr Steels Research Articles

The Process of Steam Oxidation Degradation of High Cr Steel after 125 000 Hours at 568 °C under 16.5 MPa

AbstractThe aim of this work was to analyze the effect of steam oxidation when TP347HFG grade steel was exposed to industrial conditions for 125 000 hrs at 568 °C under 16.5 MPa pressure. The testing material was acquired from one of the coal power plants operating in Poland. Comprehensive investigations were carried out on the outer surface and cross section of the exposed sample using XRD, SEM/EDS and TEM. The analysis revealed the development of a fine grain oxide scale thickness spanning from 20 to 200 µm, consisting Fe2NiO4 (78 %), Cr1.3Fe0.7O3, Fe2O3 (0.8 %) and Fe3Co (0.8 %). Cross-sectional observations showed the formation of grains rich in Nb precipitates type MX carbonitrides surrounded by M23C6 carbides developed as a continuous net at the grain boundaries. The extent of material degradation under the influence of temperature, pressure and exposure time has been meticulously discussed. Graphical Abstract

Thermomechanical Processing for Improved Mechanical Properties of HT9 Steels.

Thermomechanical processing (TMP) of ferritic-martensitic (FM) steels, such as HT9 (Fe-12Cr-1MoWV) steels, involves normalizing, quenching, and tempering to create a microstructure of fine ferritic/martensitic laths with carbide precipitates. HT9 steels are used in fast reactor core components due to their high-temperature strength and resistance to irradiation damage. However, traditional TMP methods for these steels often result in performance limitations under irradiation, including embrittlement at low temperatures (<~430 °C), insufficient strength and toughness at higher temperatures (>500 °C), and void swelling after high-dose irradiation (>200 dpa). This research aimed to enhance both fracture toughness and strength at high temperatures by creating a quenched and tempered martensitic structure with ultrafine laths and precipitates through rapid quenching and unconventional tempering. Mechanical testing revealed significant variations in strength and fracture toughness depending on the processing route, particularly the tempering conditions. Tailored TMP approaches, combining rapid quenching with limited tempering, elevated strength to levels comparable to nano-oxide strengthened ferritic alloys while preserving fracture toughness. For optimal properties in high-Cr steels for future reactor applications, this study recommends a modified tempering treatment, i.e., post-quench annealing at 500 °C or 600 °C for 1 h, possibly followed by a brief tempering at a slightly higher temperature.

Effect of Nano‐TiC Ceramic Particles on Microstructures and Properties of High‐Cr Steel under Different Heat‐Treatment Conditions

Herein, to improve the service life of die steels, many methods are used to improve their properties. However, some properties may be weakened after different heat treatments. In this study, a novel method of nanoparticle treatment is used to improve the service performance of high‐Cr steel (HCS), and the effect of nano‐TiC ceramic particles on its microstructures and properties under different heat‐treatment conditions is investigated. After forging and isothermal spheroidization, the nano‐TiC ceramic particles treatment HCS has a mixed grain with equiaxed grain and striated grain. Under different heat‐treatment conditions, the nano‐TiC reinforced HCS has refined martensite laths, carbides, and a higher content of retained austenite. In contrast, the nano‐TiC ceramic‐particles‐reinforced HCS that have higher strength and strain, the yield strength (σ 0.2), tensile strength (σ b), fracture strain, product of strength and strain, and impact toughness of HCS‐51 with nano‐TiC (HCSN‐51) are 1349 MPa (1531 MPa), 1662 MPa (1770 MPa), 9.4% (12.1%), 12 405 MPa% (18 766 MPa%), and 266 J cm−2 (445 J cm−2), which is 13.5%, 6.5%, 28.7%, 51.3%, and 67.3% higher, respectively. The primary strengthening mechanisms included fine‐grain strengthening and precipitation strengthening. The main toughness mechanism was fine‐grain and transformation‐induced plasticity.

Metallurgical investigation of direct drive friction welded joint for austenitic stainless steel (AISI 316)

The present study investigates the metallurgical behavior of direct-drive friction-welded (DDFW) joints for high Cr, Ni, and Mo steel (AISI 316). Macro-microscopic, microhardness, and X-ray diffraction (XRD) were conducted. The results for the macroscopic view showed that the narrow welding center did not exceed 600 µm and there were no macro cracks or defects, indicating a uniform structure. The microstructure exhibited a significant welded joint interface with microcavities and a grain refinement of 10 µm, about three times smaller than the grain size of AISI 316, due to the creation of a highly plastically deformed zone (HPDZ), while the thermo-mechanically affected zone (TMAZ) formed in the neighboring area with a grain growth of about 100 µm, relatively enlarged three times compared to AISI 316 and time time related HPDZ. Thus, the high level of microhardness was investigated at the welding center and low values in the neighboring area were caused by the formation of the HPDZ and TMAZ, respectively. XRD also illustrated the concentration of gamma iron at the 111 level due to the grain refinement resulting from high plastic deformation.

11–23% Cr steels for solid oxide fuel cell interconnect applications at 800 °C – How the coating determines oxidation kinetics

The present work investigates the low-cost steels AISI 441, AISI 430, and AISI 444 against the tailor-made high Cr steel Crofer 22 APU (22.9 wt% Cr) at 800 °C in simulated solid oxide fuel cell (SOFC) cathode conditions. Furthermore, a low Cr steel, AISI 409 (11.4 wt% Cr) is included in the study. The oxidation, chromium evaporation, and area-specific resistance (ASR) of the uncoated and Ce/Co-coated steels are studied for up to 3000 h. Ce/Co-coated steels showed significant improvement in behaviour compared to their uncoated counterparts. The oxidation and chromium evaporation behaviour between the uncoated steels varied substantially while the Ce/Co coated steels exhibited highly similar behaviour. The area-specific resistance of the coated low-cost steels was on par with Crofer 22 APU. However, 430 formed a continuous silica layer, resulting in a higher ASR after 3000 h. Cross-sections of the uncoated and Ce/Co-coated steels were analysed using a scanning electron microscope and energy dispersive X-ray spectroscopy.

Tribological Study of Simply and Duplex-Coated CrN-X42Cr13 Tribosystems under Dry Sliding Wear and Progressive Loading Scratching

CrN coatings are widely used in the industry due to their excellent mechanical features and outstanding wear and corrosion resistance. Using scratch and ball-on-disk wear tests, the current study deals with the tribological characterisation of CrN coatings deposited onto an X42Cr13 plastic mould tool steel. Two surface conditions of the secondary-hardened substrate are compared—the plasma nitrided (duplex treated) and the un-nitrided (simply coated) states. The appropriate combination of secondary hardening providing the maximum toughness and the high-temperature nitriding of this high Cr steel is a great challenge due to the nitrogen-diffusion-inhibiting effect of Cr. The beneficial effect of the applied duplex treatment is proven by the 34% improvement of the adhesion strength and the 43% lower wear rate of the investigated duplex coatings. Detailed morphological analyses give insight into the characteristic damage mechanisms controlling the coating failure processes during scratching and wearing. For the simply CrN-coated sample, a new type of scratch damage mechanism, named “SAS-wings”, is identified, providing useful information in predicting the final failure of the coating. The tribological results obtained on tribosystems with the investigated high Cr steel/CrN constituents represent a novelty in the given field.

Early Spalling Analysis of Large Particles in High-Cr Steel during Thermal Fatigue: Relevant Mechanisms.

The aim of this study was to investigate the surface deterioration of high-Cr roll steel caused by the spalling of larger particles during thermal fatigue. The mechanisms of surface deterioration due to spalling of larger particles are discussed. Using a laboratory thermal fatigue test that replicates hot rolling conditions, samples were tested cyclically (up to 4500 times) at maximum cycle temperatures of 500, 600 and 700 °C, followed by water cooling. Specimens with surface deterioration were selected for analysis, revealing important influencing parameters, i.e., the combination of test temperatures, chemical composition, thermal stress and microstructural properties, leading to oxidation-assisted crack growth in different directions and consequent surface deterioration due to early spalling of larger particles. Here, we describe the mechanisms of crack propagation, especially in the lateral direction, and their relation to the subsequent spalling of larger particles, which depend on the influence of the local chemical composition on the microstructural constituents, as well as their distribution and properties. The results obtained in this study can be used in the development of roll steel microstructures with improved resistance to the identified mechanisms of surface degradation.

Long-Term Corrosion Behavior of Strong and Ductile High Mn-Low Cr Steel in Acidic Aqueous Environments.

To expand the industrial applicability of strong and ductile high Mn-Low Cr steel, a deeper understanding and mechanistic interpretation of long-term corrosion behavior under harsher environmental conditions are needed. From this perspective, the long-term corrosion behaviors of 24Mn3Cr steel under acidic aqueous conditions were examined through a comparison with conventional ferritic steels using the electrochemical measurements (linear polarization resistance and impedance), and immersion test followed by the metallographic observation of corrosion morphologies. In contrast to conventional ferritic steels, 24Mn3Cr steel, which had the lowest corrosion resistance at the early immersion stages (i.e., the highest corrosion current density (icorr) and lowest polarization resistance (Rp)), showed a gradual increase in corrosion resistance with prolonged immersion. Owing to the slow formation kinetics of (Fe,Cr)-enriched oxide scale, a longer incubation time for ensuring a comparatively higher corrosion resistance is required. On the other hand, conventional ferritic steels had an oxide scale with less densification and a lower elemental enrichment level that did not provide an effective anti-corrosion function. From the results, this study can provide significant insight into the industrial applicability of the high Mn-low Cr steel by providing the mechanistic interpretation of corrosion behaviors in acidic aqueous environments.

Wear behaviour of C/C–SiC composites sliding against high-Cr steel discs

Abstract An experimental investigation was conducted to examine the wear behaviour of cross-ply carbon– carbon – silicon carbide composites (C/CαSiC) under dry sliding against a chromium steel disc at various speeds and loads. Results indicated that under identical conditions, C/C– SiC composites with weak fibre resin interfaces at all times had a larger weight loss than those with a strong bond. In the first few minutes of wear, weight losses were approximately the same for both types of composites. Also, the wear rates of both the composites decreased sharply with sliding distance up to 7 km, and then gradually tended to become constant. Scanning electron microscopy showed that the graphite film was transferred on the test pin side, which stabilizes the increase in test pin temperature after the formation of a thick film of graphite debris.

Root Cause of Degradation in the Creep Strength of Martensitic Steel

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q), activation volume (V), and Larson&amp;ndash;Miller constant (C) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q, V, and C even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 &amp;deg;C compared to Fe2W in Grade 92 steel.

High-Temperature Oxidation of Steels in Direct-Fired CO2 Power Cycle Environments

Future direct-fired supercritical CO2 power cycles require steels resistant to oxidation/corrosion in high-temperature CO2 environments containing various impurities. Herein we studied the oxidation behavior of 14 candidate steels in a simulated direct-fired CO2 power cycle environment consisting of 95% CO2, 4% H2O, 1% O2 with/without 0.1% SO2 at 1 atm and 550 °C, 600 °C, 650 °C for up to 2500 h. Steels with ≥ 11.5 wt% Cr exhibited at least partial coverage by Cr-rich oxide scales leading to a significant decrease in the oxidation rates in both gases. While SO2 had little effect on low-Cr steels that formed Fe-rich oxides, it generally worsened performance of high-Cr (> 11.5 wt%) steels by hindering the establishment of a protective Cr-rich oxide. This effect was most pronounced at the lowest temperature of 550 °C, which was attributed to strong preferential adsorption of sulfur-containing species within the oxide at relatively low temperatures.

Dislocation imaging through mapping based on the combination of an electron energy-loss spectroscope with a scanning transmission electron microscope

ABSTRACT This study analysed the electronic structures of dislocations in 9Cr steel using a spherical aberration-corrected scanning transmission electron microscope equipped with a monochromated electron energy-loss spectroscope. This is the first study to report that dislocations broaden a zero-loss peak (ZLP) and also induce an absorption phenomenon at approximately 0.5 eV in the ZLP tail, which are interpreted to be related to phonon scatterings by dislocations and interference effect of multiple beams, respectively. This work also experimentally demonstrates that the use of a spectrum-imaging method allows imaging of dislocations through ZLP broadening. The approach proposed herein is a promising technique for detecting dislocations in high Cr steel.

The Morphology and Microstructure of Oxide Scale Grown on Austenitic Steel during Steam Oxidation at 700 °C for 500 h.

The modern power generation industry needs materials able to withstand severe conditions, such as high temperatures, steam pressure, and an aggressive environment, to create more electric power out of a decreasing amount of fuel. Therefore, new metallic materials are continuously being developed. In order to gain knowledge about modern materials, the investigation of high Cr and Ni austenitic steel oxidized in 100% steam at 700 °C for 500 h was performed. The morphology, the phase composition, and the chemical composition of the oxidation products were investigated through methods of advanced electron microscopy techniques. Moreover, precipitates present in bulk material were identified. The material developed a continuous and complex oxide scale, consisting of Fe2O3, Cr2O3, and spinel phases. Very fine MX, fine ε-Cu, and M23C6 precipitates were found in the bulk material. The creation of iron oxide is induced due to the coarse grain size of the material. Cr2O3 forms due to the internal oxidation process.

Study on serration flow and dynamic strain aging of Cr–Ti–B low-carbon steel

Compression tests were carried out for hot-rolled Cr–Ti–B low-carbon steels with different Cr contents (low Cr and high Cr) at three strain rates (0.01, 0.1 and 1 s−1) and temperatures ranging from 250 to 650 °C. The serrated flow and dynamic strain aging (DSA) behaviors during deformation were systematically analyzed. The results showed that the effect of the strain rate on serration flow is more obvious than that of the deformation temperature, and the serration type changes from type C to type A with increasing strain rate. The occurrence temperature range of DSA in high Cr steel (350–450 °C) is higher than that in low Cr steel (300–400 °C) at a strain rate of 1 s−1. The activation energy for DSA close to that for the diffusion of carbon, which is considered to produce DSA. A Cr–C cluster is observed in high Cr steel, which clarified that the delaying mechanism throughout the DSA regime is attributed to the pinning of carbon diffusion by Cr atoms.

FTMP-based Hierarchical Modeling and Creep Analyses of Lath Martensite Structure for High Cr Steels

FTMP-based Hierarchical Modeling and Creep Analyses of Lath Martensite Structure for High Cr Steels

Microstructural Characterisation of Austenitic Heat Resistant Sanicro 25 Steel after Steam Oxidation.

Microstructural and morphological observations of the surface scale on a high Cr and Ni austenitic heat resistant steel oxidised in water vapour at 700 °C are reported. Analysis of microstructure was carried out by analytical techniques of transmission- and scanning electron microscopy. Investigation of M23C6 nucleated at the interface between matrix and the Z-phase precipitates after exposing to high temperature showed semicoherency between M23C6 and the matrix and no coherency with the Z-phase. Plates developed on the oxide scale surface consist of Cr2O3 crystals separated by amorphous SiO2.

The effect on the mechanical response of Cr and Ni segregation on dislocation lines in bcc Fe

The understanding of irradiation-induced strengthening in ferritic-martensitic high-Cr steels remains an essential issue in the assessment of materials for fusion and next generation fission reactors. Recent advanced experimental studies revealed the importance of solute-rich clusters, containing Cr, Ni, Si, P and Mn, which often are created as a consequence of segregation at dislocation lines. In this work, Metropolis Monte Carlo (MMC) is applied to investigate segregation of Cr and Ni atoms around edge dislocation lines in bcc Fe. The content of Cr is varied between 7% and 10%, with and without 0.25% of Ni. Molecular dynamics has subsequently been applied to the enriched dislocation lines to study the critical resolved shear stress at 300 K and 600 K for different concentrations of solutes in the alloy. It is found that even small amounts of Ni significantly increase the hardening of the material.

A Diffusion–Reaction Continuum Damage Model for the Oxidation Behaviors of High-Cr Steels in Supercritical Water

In this paper, a diffusion–reaction continuum damage model is proposed to study the oxidation behavior of high-Cr steels in supercritical water. The model is formulated based on a three-dimensional (3-D) setting. By considering the diffusion and reaction effects of free oxygen in a domain of solid material, a partial differential equation named as the diffusion–reaction equation is derived, which contains an oxidation damage variable representing the fraction of the oxidized metal at a local material point. The evolution equation of the oxidation damage variable can also be derived, which together with the diffusion–reaction equation formulates the governing system of the model. To show the efficiency of the model, a 1-D problem is first studied. By taking P92 steel as an example, the values of the parameters in the governing system are determined. Based on the numerical solutions to the governing system, some key features of the oxidation behavior of high-Cr steel sample are analyzed. The influences of the parameters on the oxidation process have also been discussed. The model is further implemented into the ANSYS UserMatTh subroutine. Then, some typical examples of FEM simulations are introduced. It can be seen that our current model is suitable to simulate the global oxidation behaviors of high-Cr steel specimens with different geometrical shapes and subject to different boundary conditions. Therefore, the model would be helpful for the design and residual lifetime evaluation of high-Cr steel components in practical applications.

On the Effect of Pre-formed Scales in Mitigating Corrosion of Steels in CO2 Environments

Chromium (Cr) containing steels were tested to analyse corrosion behaviour in carbon dioxide saturated water of varying salinities with extended exposure time. Both potentiodynamic and mass loss data were collected to gain a better understanding of the corrosion mechanisms. It was found that both the high Cr steels displayed degradation in the form of pitting with increasing salinities. However, the low alloy steel reference material showed uniform iron carbonate (FeCO3) precipitation. The use of high salinity precipitated layers to aid corrosion protection in lower salinity seawater environments was then established as an interesting area for greater examination. Subsequently, samples of the low alloy steel previously corroded in solutions of 7, 14 and 28% sodium chloride (NaCl) concentration were then tested in seawater salinities of 3.5% NaCl. It was found that both the 7 and 14% NaCl pre-corroded samples resulted in a significant reduction in the corrosion rate when compared with non-pre-corroded samples. The 7% NaCl pre-corroded sample showed the greatest reduction in corrosion rate, and through SEM analysis of the layer both on the surface and cross-section it was found to display an iron carbonate layer more densely packed and defect free. This indicated the potential benefits of high salinity pre-corrosion techniques to aid protection in seawater environments.

The development of creep damage constitutive equations for high Cr steel

ABSTRACTThis paper reports (1) an application of the modified hyperbolic sine law to P92 for the minimum creep strain rate over a wider range of stress level, and 2) the calibration of the creep cavity fracture model and its applications to such as P92, E911 and MARBN alloys. It was motivated by the original development of creep cavitation based creep rupture model and application to other alloys, and the need of creep strain and stress relation over a wider range of stress. The results include: (1) the calibration of creep cavity fracture model for E911, and the pre-required cavity nucleation and growth models, for E911, (2) the creep lifetime and stress relation coefficient U’ for P92, and (3) the relationship of the cavity nucleation rate coefficient A2 and stress level for MARBN cross-welds. This paper contributes to the specific knowledge and the methodology. It anticipates that this methodology will find its value and application in modelling other fractures such as ductile and fatigue.