9Cr 1Mo Research Articles

Microstructure and Mechanical Properties of Nb- and Nb + Ti-Stabilised 18Cr–2Mo Ferritic Stainless Steels

To explore the optimum use of stabilised elements and study the influences of stabilisation in 18Cr–2Mo grades, the Nb and Nb + Ti microalloying investigation focused on the relationships of the microstructure and mechanical properties of the microalloyed 18Cr–2Mo ferritic stainless steel thick plates. Thermo-Calc calculation was performed to predict the equilibrium phase diagrams. Afterwards, the microstructure, i.e. grain size and precipitation, of as-annealed specimens was analysed by means of optical microscopy, scanning electron microscopy and transmission electron microscopy, X-ray diffraction and energy-dispersive spectroscopy. Also, electron backscatter diffraction mapping was constructed to characterise grain boundary. The mechanical properties, including tensile strength and impact toughness, were tested to correlate with the microstructure. The results show that the grain sizes of Nb-stabilised steel are comparatively smaller, which is related to the fine precipitation at the grain boundaries and beneficial to the impact toughness. The increase in its strength is not apparent due to the inhomogeneous grain sizes. The grain boundary characters are similar, which is not the main factor related to their mechanical properties. When Ti is added, TiN forms above the liquidus, and large TiN particles evidently impair impact toughness.

Controlling crystallographic ordering in Mo–Cr–Ti–Al high entropy alloys to enhance ductility

Refractory high entropy alloys (RHEAs) from the Mo–Cr–Ti–Al system exhibit promising strength at elevated temperatures. However, at room temperature they possess a B2-ordered crystal structure and, therefore, exhibit reduced ductility. Guided by thermodynamic calculations, the Al concentration was systematically reduced in MoCrTi-xAl (x = 25, 15, 10, 5, 3 at%) in order to achieve a disordered, solid solution with A2-type crystal structure even at room temperature. The alloys were manufactured by arc melting followed by a homogenization treatment. To evaluate the chemical homogeneity, backscatter electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS) were performed. Dynamic differential scanning calorimetry (DSC) and selected area diffraction utilizing transmission electron microscopy (TEM-SAD) were performed for the: (i) detection of the characteristic heat signatures of the disorder-order transformations and (ii) determination of the ordering state at room temperature of the synthesized alloys, respectively. In case of MoCrTi–3Al, a single-phase A2 crystal structure without indications of a disorder-order transformation was obtained. Compression tests at temperatures ranging from room temperature up to 800 °C reveal a significant improvement of plastic deformability at room temperature as compared to the formerly investigated equiatomic MoCrTiAl with its B2 crystal structure.

Dissimilar weld joints of P91 and 316LN for power plants Applications-A review

Austenitic stainless steel 316LN and P91 (modified 9Cr 1Mo) ferritic-martensitic steel are being preferred as a structural material in fast breeder reactors, steam generators and prototype fast breeder reactor, These material possess good high temperature mechanical properties, such as tensile strength, creep resistance, toughness, and resistance to oxidation. The welds between these materials were usually welded by using various arc welding processes. Also, when these weld joints operated at high temperatures are mainly subjected to creep phenomenon or creep failures. Both these parameters (i.e. welding and high operating temperatures) lead to creating a lot of heterogeneity in its microstructure and results in degradation of strength in its weld joint over period of time. Therefore, a study is carried out in this paper to highlight the effects of creep behaviour of this dissimilar weld joints at various test conditions and parameters involved.

Assessment of Impact Strength of Welds Produced by Cold Wire and Hot Wire Gas Tungsten Arc Welding (GTAW) Processes

Modified 9Cr 1Mo material is used as principal material of construction of Steam Generators (SG) for 500MWe Prototype Fast Breeder Nuclear Reactor (PFBR), at Kalpakkam, India. One of the major problems associated with welding of modified 9Cr 1Mo steel is poor impact strength of weld metal produced from the processes that involves the use of flux. Due to this reason, 100% Gas Tungsten Arc Welding (GTAW) process is chosen for fabrication of PFBR Steam Generators.The major limitation of conventional cold wire GTAW process is low deposition rate which makes it highly uneconomical especially for welding component with large section thickness. Therefore, hot wire GTAW process is selected for welding of PFBR Steam Generators wherever possible in addition to the conventional cold wire GTAW process. The impact properties of modified 9Cr 1Mo welds produced by hot wire GTAW process are compared with impact properties of cold wire GTAW welds. The impact strength of welds produced by hot wire GTAW process is found to be inferior compared to that of welds produced by cold wire GTAW process. Nevertheless, hot wire GTAW system has the benefit of high weld deposition rate with excellent profile of weld with reduced dilution and porosities

Effect of friction stir and activated-GTA welding processes on the 9Cr–1Mo steel to 316LN stainless steel dissimilar weld joints

ABSTRACTThe consequence of friction stir welding (FSW) and activated-gas tungsten arc welding (A-GTAW) processes on the evolution of microstructure and mechanical properties of 9Cr–1Mo (P9) steel to 316LN stainless steel dissimilar weld joint is investigated. The FSW specimen shows considerably higher tensile strength (∼652 MPa) compared to A-GTAW specimen (∼595 MPa) as well as its base metal of P9 (∼642 MPa) and 316LN (∼608 MPa) owing to the formation of tempered martensite and refined austenite in P9 and 316LN weld portion, respectively. The cross-weld tensile test revealed that the specimens failed in the base metal of 316LN SS for both FSW and A-GTAW process with ductile mode fracture. This study proves that FSW could be an alternate joining technique.

Development of a Sulfamic Acid-Based Chemical Formulation for Effective Cleaning of Modified 9Cr–1Mo Steel Steam Generator Tubes

A new sulfamic acid-based formulation is developed for the effective chemical cleaning of modified 9Cr–1Mo steel as a replacement for the conventional corrosive nitric acid (HNO3) and hydrofluoric acid (HF) mixture. The effect of sulfamic acid concentrations and inhibitor (2-mercaptobenzimidazole, MBI) on the metal loss during cleaning of modified 9Cr–1Mo steel was studied using weight loss and electrochemical impedance spectroscopic methods. The metal loss was found to increase with increase in acid concentration, but it was significantly lower with increasing inhibitor concentration, with an efficiency of > 90%. The corrosion rates of modified 9Cr–1Mo steel with 10% sulfamic acid + 2 mM MBI, 10% sulfamic acid without inhibitor and HNO3 + HF mixture were 1290, 6426, and 303,515 μm year−1 respectively. The optimal composition for efficient cleaning, with least base metal loss, was found to be 10% sulfamic acid + 2 mM MBI. Laser Raman spectroscopic (LRS) analysis of the corrosion products obtained during chemical cleaning process revealed that a protective chromium oxide film was formed during the cleaning with sulfamic acid + inhibitor as compared to iron oxide-based films with HNO3 + HF mixture and 10% sulfamic acid solutions.

Prediction of long-term creep life of 9Cr–1Mo–V–Nb steel using artificial neural network

In this work, the method of artificial neural network was employed to predict the long-term creep rupture time of 9Cr–1Mo–V–Nb steel using the NIMS datasheet. In order to verify the performance of this method, the long-term creep rupture times of 23 000–41000 h were predicted using the data lower than 17 000 h. Meanwhile, the detailed analyses were carried out by comparison with the traditional time-temperature parametric (TTP) methods, such as Larson-Miller, Manson-Harferd, and Orr-Sherby-Dorn method. The results showed that by the artificial neural network method, the predicted creep rupture times above had an average relative error of 17%, which was significantly lower than those of TTP methods. It further demonstrated that the artificial neural network offers a convenient tool to predict the accurate creep rupture time of 9Cr–1Mo–V–Nb steel due to its robust ability in law learning and extrapolation generalization.

Face centered cubic substructure and improved tensile property in a novel β titanium alloy Ti–5Al–4Zr–10Mo–3Cr

A novel metastable β-Ti alloy based on Ti–Al–Zr–Mo–Cr system was successfully designed and fabricated. Superior combination of high strength (UTS = 1534-1198 MPa) and ductility (El = 7.0%–20.9%) is achieved in this alloy. After high reduction β transus forging and subsequent heat treatment, a multi-scale distribution of α phase is displayed which contains micro-scale elongated primary α (αp), sub-micro α rod (αr) and nano-sized α platelets (αs). Detailed selected area electron diffraction (SAED) pattern and high resolution-TEM analysis show the production of FCC substructure inside αp and αr with an orientation relationship of <0001>hcp//<001>fcc, {112‾0}hcp//{2‾20}fcc, {101‾0}hcp//{220}fcc. Two variants of FCC lamella are discovered which is related by a 120° rotation around <001> axis. The effect of microstructure on the improved strength-ductility combination is briefly discussed. Both FCC lamella and multi-scale distribution of α phase could create an effective strain partition during plastic deformation and increase the ductility. High strength originates from the distribution of high fraction nano-scaled αs which could effectively hinder dislocation slip in β matrix.

Molten salt corrosion (FLiNaK) of a Ni–Mo–Cr alloy and its welds for application in energy-generation and energy-storage systems

The molten salt corrosion performance of a Ni–Mo–Cr (GH3535) alloy weldment, produced using matching filler metal, was assessed. Corrosion testing was performed in FLiNaK molten salt at 750 °C for 500 h. Present results reveal that, despite a similar chemical composition, the weld metal has somewhat superior corrosion resistance to the parent metal. The difference is primarily attributed to the significantly lower density of high-angle grain boundaries (HAGBs) in the weld metal. Results further suggest that large M6C carbides present in the parent metal may contribute to corrosion attack of the alloy matrix via galvanic corrosion.

Studies on microstructure and mechanical properties of modified 9Cr–1Mo (P91) steel in submerged arc welding with TiO2-enriched fluxes

In the present work is studied the influence of titanium dioxide on the microstructure and mechanical properties of submerged arc weld P91 steel weldment. To manipulate the chemical composition of the weldment, the titanium dioxide (TiO2) powder was added with fresh welding flux in various proportions (2.5–12.5%). The results of optical emission spectroscopy test showed that Ti wt% is systematically increased from 0.00 to 0.021, whereas, other alloying elements were fairly uniform. The micrographs of the scanning electron microscope reveal the presence of finely dispersed precipitates in the weld metals, and the percentage of precipitation increases with respect to the enrichment of titanium dioxide. The hardness profile of weld metal increases significantly with respect to the TiO2 addition. Formation of TiC and TiN could be the possible reason for the enhancement of hardness, which has a positive impact on the creep property of steel weldment. Enrichment of TiO2 reveals the appreciable improvement in the tensile strength properties. However, the highest percentage of TiO2 addition provides the adverse effect on the yield strength and ultimate tensile strength values. The presence of surplus quantity of oxide content into the weld metal could be the possible reason behind this. Toughness value of the weld samples showed the negative trend with the addition of TiO2 which can be increased by increasing the post-weld heating time.

Dissimilar Cladding of Ni–Cr–Mo Superalloy over 316L Austenitic Stainless Steel: Morphologies and Mechanical Properties

The solid solution strengthened Inconel 625, a Ni-based alloy is known for its excellent strength and good corrosion resistance at extreme environments used in thermal plants, boiler tubes, petrochemical industry and power plant. The presence of Cr content (~ 20 wt%) along with Mo-rich, Nb and Fe makes Ni–Cr–Mo–Nb austenitic alloy called as Inconel 625 to achieve excellent corrosion resistance property. Using cold metal arc transfer (CMT) cladding, the metallurgical, mechanical and corrosion properties of Inconel 625 on 316L is evaluated. The process parameters selected includes welding current, torch angle and travel speed with a constant voltage. From the results of microstructural and EDS inferences, the formation of cellular dendritic structure with secondary phases like Laves phase, complex nitrides along with the interdendritic segregation of Mo and Nb as well as microsegregation of Cr, Ni and Fe. In case of Ni–Cr–Mo alloy, Ni and Cr contribute to resistance to corrosion in NaCl environments. The formation of Cr2O3 and the passivation action of the clad zone is due to the presence of Cr. The solid solution effect in Ni–Cr matrix is contributed by the presence of Nb and Mo. Apart from that the strengthening action happens due to the precipitation of Ni3 (Al, Ti, Nb) commonly known as γ′, γ″ and MC carbides confirmed through XRD. Uni-axial tensile tests and Vickers-micro hardness indentation tests were performed on Inconel 625 cladded over 316L. Based on the fractographic results fatigue striations, tear rigdes with river markings, dimples with fibrous structure and cleavages are observed. Unlike other studies, unique type of cuboidal precipitates are seen, which is due to the presence of Ti, which form carbonitrides containing Ti, which are further characterised as NbC. The potentiodynamic polarisation tests is performed on 3.5% NaCl solution. The results suggest that Ni–Cr–Mo alloy protects the substrate from corrosion.

Structure and properties of wear-resistant materials based on Co–Mo–Cr–Si–B alloying system

Structure and properties of wear-resistant materials based on Co–Mo–Cr–Si–B alloying system

Структура и свойства износостойких материалов системы легирования Co–Mo–Cr–Si–B

Структура и свойства износостойких материалов системы легирования Co–Mo–Cr–Si–B

Studies on chemical compatibility of steels with liquid lithium

Interaction of steel specimens such as SS 316 LN, D9, 9Cr–1Mo, oxide dispersed steel (ODS) and pure iron with liquid lithium has been studied at 823 and 973 K (550 and 700 °C) for various durations from 250 to 1000 h at 250 h intervals. The relevance of this study is to identify a suitable container material for liquid lithium which has been proposed as a liquid poison to control the reactivity in nuclear reactors. The metallic specimens were characterised before and after exposure to liquid lithium by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) and mechanical testing. The results indicate that except for pure iron, lithium interacts with all steel samples in varying degree. The depth of interaction varies from 20 to 60 μm depending on nature of specimen, temperature and time of exposure.

Creep Properties of Intercritical Heat Treated Boron Added Modified 9Cr–1Mo Steel

Creep properties of intercritically heat-treated (ICHT) boron free modified 9Cr–1Mo steel (P91-IC) and boron containing modified 9Cr–1Mo (P91B-IC) steel have been studied at 923 K over a stress range of 50–120 MPa. Minimum creep rate of the P91B-IC steel was significantly lower than the P91-IC steel. Stress exponent and threshold stress values in P91-IC and P91B-IC steels were found to be 6.5 and 28 MPa, and 8.7 and 38 MPa respectively. P91B-IC steel has exhibited higher creep rupture life than the P91-IC steel. Higher creep rupture strength in the P91B-IC steel as compared to P91-IC steel has been attributed to the presence of stable M23C6 precipitates at the lath boundaries and other high angle boundaries. The creep rupture strengths of both the ICHT steels were lower than the respective base metal.

Influence of Oxidation on Estimation of Long-Term Creep Rupture Strength of 2.25Cr–1Mo Steel by Larson–Miller Method

AbstractThe influence of oxidation on the estimation of long-term creep rupture strength is investigated for 2.25% chromium (Cr)–1% molybdenum (Mo) steel specified as JIS STBA 24, JIS SCMV 4 NT, and ASTM A542/A542M by the Larson–Miller method using creep rupture data in the National Institute for Materials Science (NIMS) Creep Data Sheets at 450–650 °C for up to 313,000 h. The creep rupture data exhibit a change in slope of the stress versus time to rupture curves due to oxidation in air during 600 °C creep tests at 15,000–40,000 h and 650 °C tests at 2000–3500 h for different size specimens, which indicates degradation in creep life by the oxidation. The estimated 100,000 h creep rupture strength using regression analysis is increased by the elimination of long-term data degraded by the oxidation. Several metallurgical factors, such as the initial strength represented by the 0.2% proof stress at the creep test temperature and the concentration of aluminum (Al) impurity, also affect the creep life of the tested steel.

Carbide precipitation associated with carburisation of 9Cr–1Mo steel in hot CO2 gas

Experimental ferritic 9Cr–1Mo steels exposed at high temperature and pressure to CO2-based gas undergo oxidation and carburisation. Oxidation proceeds initially as a protective phase with parabolic kinetics (stage I). At high temperatures and longer times, more rapid “breakaway” oxidation occurs (stage II). Post-breakaway oxidation follows linear kinetics (stage III). A virgin sample and three oxidised samples were selected for analysis. Precipitates embedded in the matrix with four different morphologies were observed using focused ion beam imaging, X-ray diffraction and electron microscopy techniques. Coarse carbides (M23C6) were observed as the predominant precipitates in all four samples. Fine needles (M2C), large needles (MC) and cored coarse carbide precipitates (M23C6) were also present in stages (I), (II) and (III), respectively. TEM analysis was used to give the lattice types and orientations of the carbides found. Energy-dispersive X-ray analysis was used to obtain compositions. The precipitation is considered particularly with respect to the formation of cored M23C6 carbide precipitates. These data provide a threshold for the matrix composition that defines the onset of breakaway oxidation.

Effect of Fe–Mo–Cr pre-alloyed powder on the microstructure and mechanical properties of TiC–high-Mn-steel cermet

In the study, a TiC–high-Mn-steel cermet is fabricated using FeMo and Fe–Mo–Cr pre-alloyed powders as metallic binders by powder metallurgy techniques. The effect of Cr on the microstructure and mechanical properties of the cermet is studied and the cermet preparation process is optimized. The microstructure and fracture morphology of the cermets are observed with scanning electron microscopy, while phase identification and analysis are performed by X-ray diffractometry. The results show that the particles of Cr-free cermet are angular and polygonal, while those of cermet with added Cr are rounded and ellipsoidal. The grains of Cr-free cermet are larger than those of the Cr-added cermet, which is unlike similar conventional cermets. The grain size of the Cr-added cermet increases slightly with increased Cr content. In addition, the relative density of the cermet decreases slightly with increased Cr content. The hardness of cermet is maximized at HRC 64.8 with the Cr content of 1.0 wt%; with further increases in Cr, the hardness decreases gradually. The transverse rupture strength and impact toughness first increase and then decrease with increasing Cr content, reaching the maxima of 2355 MPa and 13.42 J/cm2, respectively, at the Cr content of 1.5 wt%. The strength and toughness of the cermet are improved greatly compared to those of conventional similar TiC–high-Mn-steel cermets.

Crystallographic ordering in a series of Al-containing refractory high entropy alloys Ta–Nb–Mo–Cr–Ti–Al

High entropy alloys based on the Ta–Nb–Mo–Cr–Ti–Al system are expected to possess high creep and oxidation resistance as well as outstanding specific mechanical properties due to presumed high melting points and low densities. However, we recently reported that arc-melted and subsequently homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall. Mater. Trans. A 49 (2018) 772–781 and J. Alloys Cmpd. 661 (2016) 206–215]. Thermodynamic calculations suggest the formation of a B2-type ordered phase below the homogenization temperature. In the present article, we provide results of a detailed microstructural characterization of a series of Ta–Nb–Mo–Cr–Ti–Al derivatives and evaluate if B2-type ordering could be the origin for the observed lack of ductility. Backscatter electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT) were used to verify uniform elemental distribution after homogenization. X-ray diffraction (XRD) patterns indicate both, A2 or B2-type crystal structure, whereas transmission electron microscopy (TEM) diffraction experiments unambiguously confirm B2-type order in the as-homogenized state of all investigated alloys. In MoCrTiAl, planar defects that show antiphase boundary contrast with a {100}-type habit plane were detected by TEM dark field (DF) imaging. They are wetted by a Cr-enriched and Ti-depleted layer as confirmed by scanning transmission electron microscopy (STEM)-EDX line scans as well as APT analyses. The planar defects arise from a disorder-order solid-state phase transformation during cooling, as indicated by differential scanning calorimetry (DSC).

Role of Heat Treatment on Grain Refinement and Microhardness of 9Cr–1Mo–V–Nb Steel

Effect of ‘double austenitizing’ (DN) on microstructure evolution and mechanical properties of martensitic 9Cr–1Mo–V–Nb (P91) steel were studied and compared with the ‘conventional normalizing’ (CN) process. In CN treatment, P91 steel is normalized at 1050 °C for 1 h, finally air cooled. In DN treatment, the first stage of normalizing was performed at 1050 °C/1 h/air cool. The second stage of normalizing was performed in the temperature range of 950–1150 °C for 1 h (950 °C-DN1, 1050 °C-DN2, 1150 °C-DN3), followed by water quenching. The grain size was measured 42 and 35 µm for CN and DN1 treatment, respectively. The double normalizing (DN) produced complete martensitic microstructure as a result of complete dissolution of precipitates. In DN-based heat treatment, optimized microstructure and mechanical properties were obtained for the sample that normalized at 950 °C, followed by water quenching.