9Cr-1Mo Ferritic Steel Research Articles

INVESTIGASI LAJU REGANGAN CREEP BAJA TAHAN KARAT FERRITIC PADA TEMPERATUR 570 °C

The probabilistic assessment of failure due to the creep phenomenon must be determined with the minimum deviation. Since the creep failure took years before rupture, laboratory experiments were conducted to represent the failure time. However, the laboratory results are only an empirical approach; therefore, the probabilistic assessment can be deviated. A huge number of variables in creep experiments must be conducted to minimize the deviation. This study investigated the creep strain rate for the 9Cr-1Mo ferritic stainless steel. The lever-arm creep machine was used for the creep rupture test, and the test followed the standard of ASTM E139. The constant loads were applied in the range of 250 – 170 MPa with the homologous temperature of 570 °C. The curve of strain to time was recorded from each constant load condition. The creep strain rate was then calculated and presented in the curve of the creep strain rate to the applied load. The linear regression was drawn from the curve by applying Norton’s equation. By obtaining the creep strain hardening coefficient and exponent, an initial probabilistic assessment of creep failure at a temperature of 570 °C can be predicted.

Mechanical and Metallurgical behavior of Manual Metal Arc Welded P91 Ferritic Martensitic Steel Joints

In the nuclear power plant industry, Cr-Mo ferritic steels are indispensable due to their high-temperature tensile strength, creep strength, and resistance to stress corrosion cracking. This study evaluated and analyzed the mechanical properties and metallurgical behavior of indigenously developed filler (over matched with base metal) manual metal arc welded Cr-Mo ferritic steel (hereafter referred as P91 steel). The analysis revealed that the ultimate tensile properties of the weld joint exceed those of the unwelded metal, being 6% higher than the base metal. Consequently, the joint efficiency for the weld joint is 106%. However, the impact toughness of the weld pad is significantly lower compared to the unwelded metal, nearly 2.5 times less than the base metal. The weld metal region’s microstructure is characterized by untempered lath martensite pinned with dense dislocations, which is attributed to rapid cooling from the liquidus range. Furthermore, a distinct Heat-Affected Zone (HAZ) was identified adjacent to the weld metal region, which results from the elevated temperature experienced in this area.

Influence of Anions on the Corrosion Behavior of 9Cr-1Mo Ferritic Steel in Aqueous Media

Modified 9Cr-1Mo steel is used as steam generator material in prototype fast breeder reactors, owing to its good corrosion resistance, creep, and thermal conductivity characteristics. In the present study, the corrosion behavior of modified 9Cr-1Mo steel in aqueous environments containing various corrosive ions at potentiodynamic polarization conditions is reported. In chloride-exposed samples, randomly distributed dense shallow pits were seen, whereas mixed anions (, Cl−, and ) in fresh water resulted in deeper pits uniformly distributed on the entire surface. Laser Raman spectroscopy studies revealed the formation of Fe-oxides/hydroxides and Cr(VI) species, except in alkaline solution. A very thin film of only Fe-oxides (Fe3O4, γ-Fe2O3, and α-Fe2O3) was identified in an OH– ion-dominated solution. X-ray photoelectron spectroscopy results confirm the corroded surfaces comprised of Fe, Cr, and Mo-oxides of varying composition and enrichment of Mo-oxides in alkaline solution. Chloride ions present in the corroded layer influenced the pitting corrosion in neutral chloride and fresh water solutions. Field emission scanning electron microscopy images revealed abundant crystalline lepidocrocite with laminar morphology and doughnut-type magnetite together with pits on the samples exposed in neutral chloride and fresh water media.

Microstructural characterization of dissimilar weld joint between ferritic steel and stainless steel

In this investigation, microstructural variation across the dissimilar weld joint between modified 9Cr-1Mo ferritic steel and 316LN stainless steel with nickel-based filler metal was fabricated and characterized by various techniques. It was observed that the heterogeneous microstructure and non-uniform microhardness were developed across the dissimilar weld joint. Carbon-depleted soft zone, carbon-enriched hard zone, type I and type II boundaries are evident at the P91 heat-affected zone (HAZ) – buttering layer interface. An unmixed zone was observed at the weld metal-316 LN stainless steel HAZ interface. Distinct microstructures were developed across the dissimilar weld joint are due to the mismatch in chemical composition, crystal structure and melting temperature difference between base metals and filler metal.

High-temperature electrochemical corrosion evaluation of 2.25Cr–1Mo alloy in eutectic LiCl–KCl molten salt

ABSTRACT The Cr–Mo ferritic steels are candidate structural materials for the pyrochemical reprocessing applications due to desirable high-temperature strength, resistance to oxidation, and etc. The corrosion resistance of the alloys in molten salts depends on the moisture, impurities, oxidising nature of the electrolyte and temperature. In the present study, in order to monitor the corrosion behaviour of 2.25Cr–1Mo steel in LiCl–KCl molten salt under inert argon atmosphere at 500°C for 200 h duration, electrochemical measurements like open-circuit potential (OCP), linear polarisation resistance, and electrochemical impedance spectroscopy have been carried out. On increasing the duration of exposure to molten salt, the OCP is shifted to the noble direction, and the polarisation resistance increased. The electrochemical impedance spectra exhibited three time constants attributed to the intermittent oxide layer, which was also evident from the surface morphology. The XRD analysis did not reveal the formation of oxide films.

In-Situ Detection of Early Corrosion of Ferritic Cr-Mo Steel in Aqueous Solutions of different Anions using Laser Raman Spectroscopy

We report the early-stage corrosion of modified 9Cr-1Mo ferritic steel in aqueous environments at natural corroding conditions. Uniform, pitting, and crevice corrosion was observed in acidic sulphate, neutral chloride, and fresh water environments, respectively. In-situ laser Raman spectroscopy (LRS) studies revealed the formation of Fe3O4, γ−Fe2O3, γ−FeOOH phases, and stable heterogeneous corrosion products of γ−FeOOH and α−FeOOH in all media, except in an alkaline solution. A stable passive film, composed of oxide and oxy-hydroxides of chromium and iron, is formed in an alkaline solution. X-ray photoelectron spectroscopy (XPS) results confirm the presence of Cr and Fe oxide and oxy-hydroxides in all corrosion products and enrichment of Mn and Nb oxides on the corroded surface in neutral chloride solution, but only Mn oxy-hydroxide in acidic solution. Chloride ion in the corroded surface in neutral chloride solution indicates a chloride-induced corrosion attack. In-situ LRS, together with ex-situ XPS enabled the identification of all corrosion products formed on modified 9Cr-1Mo steel. The presence of laminar γ−FeOOH and acicular α−FeOOH phases are confirmed from the FESEM images. Our results indicate that except in alkaline solution, the corrosive ions deteriorate the integrity of native film on modified 9Cr-1Mo steel.

Generation of creep-fatigue interaction diagram for modified 9Cr–1Mo steel

The selection of structural materials for elevated temperature applications relies on the availability of proper design codes based on adequate data pertaining to the properties at service-relevant loading conditions. Two of the most widely used high temperature design codes, viz, the ASME Boiler and Pressure Vessel Code and the RCC-MRx code are at variance with each other on the bilinear interaction point in the creep-fatigue interaction diagram for modified 9Cr–1Mo ferritic steel. In view of this, a detailed investigation was carried out on the creep-fatigue interaction behavior of this alloy with special emphasis on the evaluation of the creep-fatigue damage and construction of the interaction diagram. Tests were performed on two chemical variants of the steel at two temperatures (773 K and 823 K) using the strain amplitudes of ±0.25%, ±0.4%, ±0.6% and ±0.8%. Hold times of different durations in the range, 1–60 min, were imposed in the tensile and compressive peak strains to introduce the creep damage. The ASME creep-fatigue interaction diagram was observed to be a more conservative design approach for the alloy.

Microstructural Evolution of Dissimilar Metal Welds Involving Grade 91

Dissimilar metal weld failures between low alloy Cr-Mo ferritic steels and austenitic stainless steels made with Ni-base filler metals are typically observed along the fusion line. Such DMW failures often exhibit the onset of damage well before their expected service life. Failure is typically associated with a carbon-depleted region in the ferritic steel and formation of creep voids along a row of so-called Type I carbides. More recently, the formation of Type I carbides adjacent to a carbide-free ferrite band has also been observed in DMWs where the ferritic steel was a 9 pct Cr creep strength-enhanced ferritic steel. However, it has not been completely clarified whether these microstructural features formed during welding, post-weld heat treatment or service. In this study, single pass bead-on-plate welds were prepared on a Grade 91 steel substrate using commonly specified Ni-base weld metals; ENiCrCoMo-1, ENiCrMo-3, ENiCrFe-3, ENiCrFe-2 and EPRI P87 (ENiFeCr-4) and were characterized in the as-welded, post-weld heat treatment (PWHT) and aged conditions. The evolution of the ferrite band was found to form by a diffusion-controlled process and was observed after PWHT and aging associated with regions exhibiting steep concentration gradients in the partially mixed zone (PMZ). In this study, steep concentration gradients were observed along the toe of the weld whereas relatively shallow gradients were observed at the weld bottom-center, resulting in a discontinuous ferrite band along the fusion line during PWHT and aging. The variation in chemical gradients in the PMZ corresponded to different chemical potential gradients, which lead to differences in carbon diffusion during high-temperature PWHT and aging. The results of this study provide insight into potential solutions for minimizing the risk of failure in future applications.

Effect of aging on coarsening- and creep resistance of a Ti-modified Fe–Ni–Al–Cr–Mo ferritic steel with L21/B2 composite precipitates

The FBB8-ferritic superalloy (Fe- 10Ni- 6.5Al- 10Cr- 3.4Mo- 0.25Zr- 0.005B, wt.%) modified with 2 wt.% Ti additions, was aged between 700 and 860 °C, leading to coarsening of its coherent L21-Ni2TiAl/B2–NiAl composite precipitates. The temporal evolution of these submicron precipitates upon aging, studied via scanning electron microscopy (SEM), is consistent with the diffusion-limited Lifshitz-Slyozov-Wagner (LSW) and Philippe-Voorhees (PV) coarsening models with an activation energy Q = 253 ± 72 kJ/mol and interfacial energies for the precipitates of 83, 78 and 75 (±26) mJ/m2 for 700, 780 and 860 °C, respectively. Creep tests performed at 700 °C reveal threshold stresses ranging from σth = 94 MPa (after over-aging at 860 °C, with significant coarsening) to σth = 187 MPa (after peak-aging at 700 °C). Creep resistance is reduced with increasing aging temperature and/or creep temperatures (from 700 to 860 °C) and the following mechanisms are identified: (i) composite L21/B2 precipitate coarsening, (ii) partial loss of coherency of precipitates by misfit dislocations at the L21/B2 precipitate/matrix interface, (iii) absence of nano-sized secondary precipitates. Nevertheless, the alloy exhibits a high creep resistance when aged and creep-tested at 780 °C, with a threshold stress σth = 67 MPa, which is similar to that exhibited at 700 °C, σth = 69 MPa, by the same alloy without 2% Ti addition (FBB8), which contains weaker, single-phase B2–NiAl precipitates.

Modelling carburisation in 9Cr-1Mo ferritic steel tube substrates in experimental CO2 atmospheres

By combining modelling and experimental work, new insights have been gained into the combined oxidation/carburisation of 9Cr-1Mo steels in CO2 rich environments. The breakaway-oxidation process is preceded by the rejection of carbon into the scale due to the poor solubility of carbon in the metal. Experimental TEM work reveals the formation of a carbide rich layer near the substrate surface which forms a further barrier against carbon ingress. The eventual oxidation of this layer could contribute to breakaway oxidation. A 2D finite-difference based diffusion model developed in combination with Thermo-Calc software underlines the role of specimen geometry on breakaway initiation.

A comparison of two high spatial resolution imaging techniques for determining carbide precipitate type and size in ferritic 9Cr-1Mo steel

Two high spatial resolution imaging techniques, focused gallium ion beam imaging in conjunction with XeF2 gas (FIB/XeF2) and high-speed atomic force microscopy (HS-AFM), were used to analyse 9Cr-1Mo ferritic steel samples, which had been exposed for extended periods to hot CO2 gas containing traces of CO, H2, H2O and CH4. The carbide precipitates embedded in the metal matrix were observed and their morphology, size and spatial distribution were quantified using these two techniques. The lower resolution of the FIB/XeF2 imaging technique suggested that small carbide precipitates (<50 nm) may be missed, while the existence of a limited flow layer introduced by sample preparation may influence the HS-AFM results. The gallium ion beam was used to remove a thin oxide layer of approximately 50 nm from sample surfaces prior to FIB/XeF2 imaging, avoiding the influence of surface contamination. HS-AFM provided higher resolution (∼5 nm) than FIB/XeF2 imaging. A quantitative comparison of the experimental data confirmed the value of both FIB/XeF2 and HS-AFM for imaging carbide precipitates, while clarifying their strengths and limitations.

Tensile Properties Variation Across the Dissimilar Metal Weld Joint Between Modified 9Cr–1Mo Ferritic Steel and 316LN Stainless Steel at RT and 550 °C

In liquid metal cooled fast breeder reactors (LMFBR), modified 9Cr–1Mo ferritic steel (P91 or Grade 91) is a preferred material for constructing steam generators due to its creep strength and stress corrosion cracking resistance. The austenitic stainless steels (SS 316LN and SS 304LN) are widely used for primary and secondary piping systems because of its oxidation resistance and excellent creep strength. So, the dissimilar metal weld joint (DMWJ) between P91 and SS 316LN is inevitable. Nickel-based consumables (Alloy 82 and Alloy 182) are preferred to join these materials. The DMWJ will experience the temperature up to 550 °C. For accurate integrity assessment, the mechanical properties of individual regions are to be evaluated at room temperature (RT) and 550 °C. Hence, the present investigation is focused on evaluating the mechanical properties of various regions of DMWJ at RT and 550 °C. From this investigation, it is understood that the tensile properties are heterogeneous across the DMWJ at RT and 550 °C. The high-temperature (550 °C) tensile properties are significantly lower with respect to the RT properties. The development of complex microstructures at the interfaces will alter the mechanical properties across the DMWJ.

Tensile and impact toughness properties of various regions of dissimilar joints of nuclear grade steels

Abstract Modified 9Cr-1Mo ferritic steel is a preferred material for steam generators in nuclear power plants for their creep strength and good corrosion resistance. Austenitic stainless steels, such as type 316LN, are used in the high temperature segments such as reactor pressure vessels and primary piping systems. So, the dissimilar joints between these materials are inevitable. In this investigation, dissimilar joints were fabricated by the Shielded Metal Arc Welding (SMAW) process with Inconel 82/182 filler metals. The notch tensile properties and Charpy V-notch impact toughness properties of various regions of dissimilar metal weld joints (DMWJs) were evaluated as per the standards. The microhardness distribution across the DMWJs was recorded. Microstructural features of different regions were characterized by optical and scanning electron microscopy. Inhomogeneous notch tensile properties were observed across the DMWJs. Impact toughness values of various regions of the DMWJs were slightly higher than the prescribed value. Formation of a carbon-enriched hard zone at the interface between the ferritic steel and the buttering material enhanced the notch tensile properties of the heat-affected-zone (HAZ) of P91. The complex microstructure developed at the interfaces of the DMWJs was the reason for inhomogeneous mechanical properties.

Creep deformation and fracture behaviour of modified 9Cr-1Mo steel in flowing liquid sodium environment

The creep deformation and rupture behaviour of modified 9Cr-1Mo ferritic steel in flowing liquid sodium has been studied at 873K over a stress range of 130–160MPa and compared with those tested in air environment. In the flowing sodium environment, the steel exhibited better creep deformation and rupture strength than those in air environment. The onset of tertiary stage of creep deformation has delayed in sodium environment over that in air environment, which led to the decrease in minimum creep rate. Also the rate of tertiary creep strain accumulation was lower in sodium environment than that in air, leading to the increase in rupture life. The steel failed in ductile dimple fracture mode in both the testing environments. The steel exhibits relatively higher creep damage tolerance in sodium environment than in the air environment. Minimal oxidation was observed in the specimens creep tested in sodium environment. The near absence of oxidation of the steel in sodium environment delayed on onset of tertiary stage of creep deformation and decrease the rate of tertiary creep strain accumulation, leading to the increase in creep deformation and rupture strength.

Tensile properties of shielded metal arc welded dissimilar joints of nuclear grade ferritic steel and austenitic stainless steel

AbstractIn nuclear power plants, modified 9Cr-1Mo ferritic steel (Grade 91 or P91) is used for constructing steam generators (SG’s) whereas austenitic stainless steel (AISI 316LN) is a major structural member for intermediate heat exchanger (IHX). Therefore, a dissimilar joint between these materials is unavoidable. In this investigation, dissimilar joints were fabricated by Shielded Metal Arc Welding (SMAW) process with Inconel 82/182 filler metals. Transverse tensile properties and Charpy V-notch impact toughness for different regions of dissimilar joints of modified 9Cr-1Mo ferritic steel and AISI 316LN austenitic stainless steel were evaluated as per the standards. Microhardness distribution across the dissimilar joint was recorded. Microstructural features of different regions were characterized by optical and scanning electron microscopy. The transverse tensile properties of the joint is found to be inferior to base metals. Impact toughness values of different regions of dissimilar metal weld joint (DMWJ) is slightly higher than the prescribed value. Formation of a soft zone at the outer edge of the HAZ will reduce the tensile properties of DMWJ. The complex microstructure developed at the interfaces of DMWJ will reduce the impact toughness values.

Impurity Antimony-Induced Creep Property Deterioration and Its Suppression by Rare Earth Ceriumfor a 9Cr-1Mo Ferritic Heat-Resistant Steel

The high temperature creep properties of three groups of modified 9Cr-1Mo steel samples, undoped, doped with Sb, and doped with Sb and Ce, are evaluated under the applied stresses from 150 MPa to 210 MPa and at the temperatures from 873–923 K. The creep behavior follows the temperature-compensated power law as well as the Monkman-Grant relation. The creep activation energy for the Sb-doped steel (519 kJ/mol) is apparently lower than that for the undoped one (541 kJ/mol), but it is considerably higher for the Sb+Ce-doped steel (621 kJ/mol). Based on the obtained relations, both the creep lifetimes under 50 MPa, 80 MPa, and 100 MPa in the range 853–923 K and the 105 h creep rupture strengths at 853 K, 873 K, and 893 K are predicted. It is demonstrated that the creep properties of the Sb-doped steel are considerably deteriorated but those of the Sb+Ce-doped steel are significantly improved as compared with the undoped steel. Microstructural and microchemical characterizations indicate that the minor addition of Ce can stabilize the microstructure of the steel by segregating to grain boundaries and dislocations, thereby offsetting the deleterious effect of Sb by coarsening the microstructure and weakening the grain boundary.

Advanced materials for structural components of Indian sodium-cooled fast reactors

The performance of fast reactor (FR) working under high temperature, moderate pressure and intense neutron irradiation depends on its structural materials. Type 316 LN austenitic stainless steel with 0.02–0.03 wt.% carbon and 0.06–0.08 wt.% nitrogen is generally used for structural components of FR along with modified 9Cr–1Mo ferritic steel for steam generator. With an aim of enhancing the life of FR, efforts towards the enhancement of strength of the materials are been described. Nitrogen content in the 316 LN steel has been increased for optimizing the tensile, creep, fatigue and creep–fatigue interaction strengths. Both the tensile and creep strength were found to increase with nitrogen content; whereas the fatigue and creep–fatigue interaction strength have optimum values at around 0.12 wt.% of nitrogen. Based on the study, nitrogen content in 316 LN steel has been optimized at 0.11–0.13 wt.% for optimum combination of strengths. Weldability study has been performed and matching welding consumable has been developed.Creep strength enhancement of modified 9Cr–1Mo steel and its fusion welded joint has been addressed through microalloying the steel with boron along with the restriction on nitrogen content. The microalloying not only improves the creep strength of the steel also reduces the type IV cracking susceptibility of the fusion welded joint of the steel. The optimum contents of boron and nitrogen in the steel have been identified as around 60 ppm boron and 100 ppm nitrogen for better creep strength and type IV cracking resistance. The paper describes the challenges in developing the materials.

Superhydrophobic coating on modified 9Cr – 1Mo ferritic steel using perfluoro octyl triethoxy silane

Lotus leaves are a typical example of superhydrophobic surface. Numerous studies have confirmed that surface morphology possessing micro- and nanoscale roughness along with a low surface energy material coating leads to apparent water contact angle (WCA) ⩾150°. In nuclear power plants, modified 9Cr–1Mo ferritic steel is the favoured steam generator tubing material. During transit, storage and installation, SHP surface on modified 9Cr–1Mo ferritic steel can impart good corrosion resistance to retain the integrity of the specimen during operation. In this study, SHP surface of modified 9Cr–1Mo ferritic steel with a WCA of 150±1° was successfully achieved by polishing, etching, perfluoro octyl triethoxy silane coating and baking. The WCA and contact angle hysteresis were measured. The surface morphology and the composition were characterised by atomic force microscopy and attenuated total reflection–infrared spectroscopy respectively. Superhydrophobicity and its related theories are also discussed in this paper.

Nanocontainer Impregnated Self-Healing Coatings for Active Corrosion Protection of Modified 9Cr-1Mo Ferritic Steel

This study displays the influence of TiO2 nanocontainers loaded with corrosion inhibitor 2-mercaptobenzothiazole into hybrid organic-inorganic coatings on the active corrosion protection of modified 9Cr-1Mo ferritic steels. The corrosion resistance and the self-healing effects of this hybrid passive barrier coating with and without the addition of inhibitor-loaded TiO2 nanocontainers were studied using electrochemical impedance spectroscopy (EIS). EIS analysis revealed that inhibitor-loaded nanocontainer impregnated coatings showed enhanced corrosion protection as a result of the release of inhibitor molecules from the nanocontainers. Later these titania nanocontainers were used to fabricate inhibitor-loaded nanocontainer mixed hybrid coating with epoxy topcoating. Along with the reference (plain top epoxy) coating, these samples were exposed to neutral salt spray test in a salt spray cabinet in order to investigate the long-term corrosion protection and the self-healing efficiency of the nanocontainer mi...

Development of sol–gel alumina coating on 9Cr–1Mo ferritic steel and their oxidation behavior at high temperature

Sol–gel derived alumina coating was developed on 9Cr–1Mo ferritic steel and their oxidation behaviour was studied at 750–900 °C in air atmosphere. The elemental composition of the coating was analyzed through energy dispersive X-ray analysis (EDX) while microstructure of the oxidized substrates was examined through scanning electron microscopy. EDX analysis confirmed the presence of alumina in the developed coating. Oxidation kinetics study indicated the occurrence of two orders of magnitude lower oxidation rate constant for the coated substrate as compared to uncoated substrate at 700 and 800 °C. However, an increase in oxidation rate was measured for the coated substrates at 850 °C and above temperature. Microstructural examination of the oxidized substrates indicated that comparatively thin, compact and less porous scale forms at the coated surfaces. In the present work, sol–gel derived alumina coating developed successfully on 9Cr–1Mo steel substrates. ES analysis through the coating confirmed the presence of alumina coating. Oxidation kinetics study of the coated and uncoated substrates up to 900 °C in air atmosphere indicated a decrease of nearly two orders of magnitude lower oxidation rate for the coated substrate. Based on the elemental X-ray mapping and SEM of the oxidized sample, oxidation kinetics is discussed.