Ferritic-martensitic Stainless Steel Research Articles

Numerical predictions of low cyclic fatigue and creep-fatigue behavior of P92 steel under non-isothermal-mechanical loading

Low-Cycle Fatigue (LCF) and creep-fatigue-interaction (CFI) behavior of a ferritic-martensitic stainless steel P92 is studied under strain-controlled non-isothermal -mechanical fatigue loading in the temperature range of 300°C∼620°C and 500°C∼620°C, respectively. The strain-controlled tests with and without the tensile and compression holding times are simulated for different mechanical strains amplitudes. An improved Unified Viscoplastic constitutive Model (UVM) is proposed based on the framework of modified Chaboche non-linear kinematic hardening rule. The improved UVM incorporates strain rate sensitivity, temperature rate dependence, static recovery associated with plastic strain, mean stress evolution and cyclic softening of strain range dependence, and so on. Furthermore, the expressions of the implicit stress integration algorithm and the consistent tangential modulus are deduced on the basis of radial return mapping and backward Euler integration methods, respectively, and the improved UVM is implemented into the ABAQUS software using the user material subroutine UMAT. The simulation results of the UVM correlate well with the experimental data.

Erosion-corrosion behaviour of high manganese steel used in slurry pipelines

The regular replacement and maintenance of steel pipes used to transport slurry in oil sands mining and extraction continue to represent a major expense for oil sands operations. Pipeline steels such as API 5 L X65 and X70 are the main materials for the slurry transport system. In our previous work, dual-phase stainless steel showed excellent performance in aeriated (high dissolved oxygen) slurries operated at low to moderate velocities involving erosion-corrosion. However, the benefits of using stainless steel diminish as the oxygen in slurry is depleted or at higher slurry velocities at which mechanical properties, particularly deformation energy or strain-hardening capability, of the steel become more dominant. This article reports our recent studies on erosion-corrosion of high manganese steel at different slurry velocities (3.5 m/s and 5.5 m/s) and different dissolved oxygen levels (0.6 ppm and 3.8 ppm). The erosion-corrosion tests were performed at 60 °C in sand-containing slurry (20% vol. silica) with 500 ppm chloride, which simulated common field situation. Characterization of the steel was carried out with optical and scanning electron microscopy, energy dispersive x-ray spectrometry, x-ray diffraction technique, micro-mechanical probe, and electrochemical testing. The performance of the high manganese steel was compared to those of API 5 L pipeline steels commonly used in slurry transport, ASTM A1053 Gr.50 dual-phase ferritic-martensitic stainless steel, and AR400 martensitic hard plate. Results of this investigation demonstrate how the high strain-hardening capability of the high manganese steel produces an outstanding erosion resistance, which is more than two times as high as those of ferritic steels. The results are compared to those obtained through field trials performed in an oil sands coarse tailings pipeline.

Investigation of Stress Corrosion Cracking Resistance of Irradiated 12Cr Ferritic-Martensitic Stainless Steel in Supercritical Water Environment

The supercritical water-cooled reactors (SWCR) belong to Generation IV of reactors. These reactors have a number of advantages over currently operating WWERs and PWRs. These advantages include higher thermal efficiency, a more simplified unit design, and the possibility of incorporating it into a closed fuel cycle. It is therefore necessary to identify candidate materials for the SWCR and validate the safety and effectiveness of their use. 12Cr ferritic-martensitic (F/M) stainless steel is considered a candidate material for SWCR internals. Radiation embrittlement and corrosion cracking in the primary circuit coolant environment are the main mechanisms of F/M steels degradation during SWCR operation. Here, the stress corrosion cracking (SCC) in supercritical water at 390 and 550 °C of 12Cr F/M steel irradiated by neutrons to 12 dpa is investigated. Autoclave tests of specially designed disk specimens in supercritical water were performed. The tests were carried out under different constant load (CL), temperature 450 °C, and pressure in autoclave 25 MPa. The threshold stress, below which the SCC initiation of irradiated 12Cr F/M steel does not occur, was determined.

Corrosion Resistance of Low-Temperature and Conventional Plasma-Nitrided 410S Ferritic-Martensitic Stainless Steels

Abstract Low-temperature plasma nitriding (LTPN) of 410S ferritic-martensitic stainless steel was carried out in a 75 % N2 – 25 % H2 gas mixture, at 400°C, for 20 h. Conventional plasma nitriding (CPN) was also carried out in the same atmosphere but at 530°C. The corrosion resistance was assessed by potentiodynamic polarization (PP) testing in 3.5-wt. % NaCl. Measuring of the mass loss was carried out after immersion of the specimens in a 3 % FeCl3 solution during 88 h. To define the sensitivity to intergranular corrosion, the degree of sensitization (DOS) was measured using the double-loop electrochemical PP technique (DL-EPR). DL-EPR results indicated that the potentiodynamic-polarization measurements showed that pitting corrosion resistance of the nitrided samples was enhanced, whereas the weight loss experiments showed that LTPN samples presented a corrosion resistance equal to the untreated samples. The DOS results displayed that the untreated samples were less resistant to intergranular corrosion correlated with the LTPN specimens. On the contrary, the CPN specimens exhibited typical behavior of general corrosion.

A machine learning aided interpretable model for rupture strength prediction in Fe-based martensitic and austenitic alloys

The class of 9–12% Cr ferritic-martensitic alloys (FMA) and austenitic stainless steels have received considerable attention due to their numerous applications in high temperature power generation industries. To design high strength steels with prolonged service life requires a thorough understanding of the long-term properties, e.g., creep rupture strength, rupture life, etc., as a function of the chemical composition and processing parameters that govern the microstructural characteristics. In this article, the creep rupture strength of both 9–12% Cr FMA and austenitic stainless steel has been parameterized using curated experimental datasets with a gradient boosting machine. The trained model has been cross validated against unseen test data and achieved high predictive performance in terms of correlation coefficient (R^{2} > 0.98 for 9–12% Cr FMA and R^{2} > 0.95 for austenitic stainless steel) thus bypassing the need for additional comprehensive tensile test campaigns or physical theoretical calculations. Furthermore, the feature importance has been computed using the Shapley value analysis to understand the complex interplay of different features.

Assessment of tribological properties of plasma nitrided 410S ferritic-martensitic stainless steels

Low Temperature Plasma Nitriding (LTPN) is a surface treatment that promotes surface hardening without harming the corrosion resistance of martensitic stainless steels. LTPN and conventional plasma nitriding (CPN) treatments were carried out to compare the influence of the obtained nitrided layers on the abrasive wear resistance of AISI 410S ferritic-martensitic stainless steel. The samples were plasma nitrided for 20 h in a gas mixture of 75% N2: 25% H2, at 250 Pa. LTPN was carried out at 400 °C, obtaining a 50 µm thick expanded martensite layer. CPN done at 530 °C, resulted in a 170 µm nitrided layer, with an outermost compound layer 15 µm in thick. Nanoindentation tests to assess the mechanical properties and the energy dissipation coefficient were carried out. Scratch and micro-abrasion tests were conducted to evaluate the friction coefficients, failure modes, critical loads and wear resistance. Tensile cracking was the prevalent mechanical failure mode of the nitrided layers. Micro-abrasion results showed that LTPN and CPN samples exhibited similar wear volume losses. A change in the wear volume loss rate with sliding distance was observed for the nitrided samples. A change from grooving to rolling abrasion explains the change in the wear rate.

Low Cycle Fatigue and Relaxation Performance of Ferritic–Martensitic Grade P92 Steel

Due to their excellent creep resistance and good oxidation resistance, 9–12% Cr ferritic–martensitic stainless steels are widely used as high temperature construction materials in power plants. However, the mutual combination of different loadings (e.g., creep and fatigue), due to a “flexible” operation of power plants, may seriously reduce the lifetimes of the respective components. In the present study, low cycle fatigue (LCF) and relaxation fatigue (RF) tests performed on grade P92 helped to understand the behavior of ferritic–martensitic steels under a combined loading. The softening and lifetime behavior strongly depend on the temperature and total strain range. Especially at small strain amplitudes, the lifetime is seriously reduced when adding a hold time which indicates the importance of considering technically relevant small strains.

The Correlation Between the Distribution/Size of Carbides and Electrochemical Behavior of 17Cr-1Ni Ferritic-Martensitic Stainless Steel

The correlation between the distribution/size of carbides and corrosion resistance of 17Cr-1Ni ferritic-martensitic stainless steel after different heat treatment temperatures was investigated by transmission electron microscope, electrochemical tests, and corrosion morphology observations. The results showed that the size of the precipitated phase decreased and corrosion resistance increased with an increase in the annealing temperature. When the tempering temperature was low (290 °C), carbides precipitated mainly at the phase boundaries due to a low degree of atomic matching and higher grain boundary energy. In this case, the polarization curve had a passivation interval and the pits were mainly initiated at the phase boundary. When the tempering temperature was higher than 400 °C, the carbides gradually precipitated in the martensite laths because the accelerated diffusion of Cr healed the Cr depletion zone at phase boundary. This outcome resulted in a polarization curve that had no passivation range and uniform corrosion occurred in martensitic region.

Investigation on joint characteristics of laser beam welded press hardenable ultra-high strength steels with ferritic-martensitic and martensitic microstructure

The introduction of stainless ultra-high strength steels with ferritic-martensitic and martensitic microstructure into sheet metal manufacturing depends on proper welding processes for these steel grades. The chromium and carbon content as well as the lack of appropriate process information for thermal joining of these steel grades in press hardened condition with UTS of up to 2.000 MPa at 15 % fracture strain classifies the weldability of these materials as limited. Efficient heat control and minimum heat input enable the welding of ferritic-martensitic and martensitic stainless steels by laser beam. This paper presents the results of strength evaluations of laser welded joints for the steel grades 1.4003, 1.4021, and 1.4034 in similar and dissimilar combinations with already established steels. By maintaining optimal welding conditions with pre- and postheating, and with respect to the testing angle and speed, press hardenable steels with ferritic-martensitic and martensitic microstructure overtop joining strength of already established 1.5528 by approx. 20 %. The paper gives an overview of the press hardening conditions, the welding parameterization, the welding seam hardness profiles for different steel-combination-arrangements, and the selected force-displacement-behavior. The paper finalize with a comparison of joint properties for considered material combinations for simple one-joint-specimen and complex structure specimen.

Degradation modes of austenitic and ferritic–martensitic stainless steels in He–CO–CO2 and liquid sodium environments of equivalent oxygen and carbon chemical potentials

The objective of this work is to explore possible thermodynamic correlations between the degradation modes of austenitic and ferritic–martensitic alloys observed in high temperature He–CO–CO2 environments with oxygen and carbon chemical potentials equivalent to that in a liquid sodium environment containing 2–5molppm oxygen and 0.02–0.2molppm carbon at temperatures 500–700°C. Two He–CO–CO2 environments (Pco/Pco2=1320, Pco=1980molppm, and Pco/Pco2=9, Pco=13.5molppm) were selected to test alloys NF616 and 316L at 700 and 850°C. Upon exposure to He environments at 850°C, 316L samples exhibited thick surface Cr2O3 scales and substantial internal oxidation; however at 700°C no significant internal oxidation was observed. NF616 samples exhibited relatively thinner surface Cr2O3 scales compared to 316L samples at both temperatures. NF616 samples exposed to liquid sodium at 700°C and He–Pco/Pco2=9 at 850°C showed decarburization. No surface oxide formation was observed on the sample exposed to the Na environment. Results obtained from He exposure experiments provide insight into what may occur during long exposure times in a sodium environment.

The role of radiation damage on retention and temperature intervals of helium and hydrogen detrapping in structural materials

An experimental study of hydrogen/deuterium behavior in ferritic–martensitic stainless steels EP-450 (Cr13Mo2NbVB), EP-852 (Cr13Mo2VS), and RUSFER-EK-181 (Fe12Cr2WVTaB) is presented. The effect of displacement damage (dpa) resulting from irradiation with helium, hydrogen, and argon ions on features of deuterium detrapping and retention in steels was studied using ion implantation, nuclear reaction depth profiling, and thermal desorption spectrometry techniques. Numerical simulation on the basis of the continuum rate theory was applied for obtaining thermodynamic parameters of deuterium trapping and desorption in steels.

A comparative study of the mechanical properties and the behavior of carbon and boron in stainless steel cladding tubes fabricated by PM HIP and traditional technologies

The ring tensile test method was optimized and successfully used to obtain precise data for specimens of the cladding tubes of AISI type 316 austenitic stainless steels and ferritic–martensitic stainless steel. The positive modifications in the tensile properties of the stainless steel cladding tubes fabricated by powder metallurgy and hot isostatic pressing of melt atomized powders (PM HIP) when compared with the cladding tubes produced by traditional technology were found. Presently, PM HIP is also used in the fabrication of oxide dispersion strengthened (ODS) ferritic–martensitic steels. The high degree of homogeneity of the distribution of carbon and boron as well the high dispersivity of the phase-structure elements in the specimens manufactured via PM HIP were determined by direct autoradiography methods. These results correlate well with the increase of the tensile properties of the specimens produced by PM HIP technology.

Reaction between a rare earth element and 9Cr-2W steel

We analyzed the reaction between a rare earth element and ferritic-martensitic stainless steel in order to evaluate the role of the rare earth element in the fuel cladding chemical interaction (FCCI). A diffusion couple test between Misch metal (70Ce-30La) and 9Cr-2W (Gr.92) steel at both 660 °C and 800 °C was conducted, and a microstructural analysis was carried out. The result showed that Fe in the Gr.92 material and Ce in the Misch metal mainly diffused and reacted to form intermetallic compounds, namely Fe2Ce and Fe17Ce phases, above 660 °C, whereas La hardly diffused or reacted in the Gr.92 material. In the specimen tested at 660 °C, the Fe diffused outside of the clad interface so that Cr-rich precipitated beneath the clad interface. In the specimen tested at 800 °C, a local reaction caused by the eutectic transformation of the (Fe,Cr)17Ce phase resulted in a local penetration across the clad.

Effect of Vapor Deposition on the Interdiffusion Behavior between the Metallic Fuel and Clad Material

This study aimed to evaluate the performance of diffusion barriers in order to prevent fuel-cladding chemical interaction (FCCI) between the metallic fuels and the cladding materials, a potential hazard for nuclear fuel in sodium-cooled fast reactors. In order to prevent FCCI, Zr or V metal is deposited on the ferriticmartensitic stainless steel surface by physical vapor deposition with a thickness up to 5μm. The diffusion couple tests using uranium alloy (U-10Zr) and a rare earth metal such as Ce-La alloy and Nd were performed at temperatures between 660~800°C. Microstructural analysis using SEM was carried out over the coupled specimen. The results show that significant interdiffusion and an associated eutectic reaction ocurred in the specimen without a diffusion barrier. However, with the exception of the local dissolution of the Zr layer in the Ce-La alloy, the specimens deposited with Zr and V exhibited superior eutectic resistance to the uranium alloy and rare earth metal. (Received January 17, 2011)

Fabrication of U-Pu-Zr Metallic Fuel Elements for the Irradiation Test at Experimental Fast Test Reactor Joyo

The Central Research Institute of Electric Power Industry and Japan Atomic Energy Agency have fabricated the first sodium-bonded metallic fuel elements in Japan as a collaborative research for the irradiation test at the experimental fast test reactor Joyo. The U-20 wt.%Pu-10 wt.%Zr fuel slugs of 200 mm length and approximately 5 mm diameter were fabricated in a small-scale injection casting furnace using U metal, U-Pu alloy and Zr metal. The fuel slugs met the specifications determined on the basis of the specifications of the Experimental Breeder Reactor II driver fuels in the United States and the results of preliminary casting tests. Each fuel slug was loaded into a ferritic-martensitic stainless steel (PNC-FMS) cladding tube with a few pieces of sodium rods as thermal bonds, the U-10 wt.%Zr thermal insulator and the PNC-FMS reflector in a helium gas atmosphere glove box. After top-end plug welding to the cladding tube and the heat treatment of the welding area, each fuel element was subjected to sodium bonding to fill the annular gap between the fuel slug and the cladding with the melted sodium. After inspection, six metallic fuel elements were manufactured successfully and transferred to the Joyo site.

Effect of cold rolling on microstructure and magnetic properties in a metastable lean duplex stainless steel

Microstructural and magnetic properties chan- ges of a metastable ferritic-austenitic stainless steel due to cold rolling were studied together with the possibility to develop a new ferritic-martensitic stainless steel. In order to reduce costs low-Ni content was maintained in the lean duplex stainless steel considered, making it more suscep- tible to strain-induced martensitic transformation. In this study a practically complete c ? a 0 transformation was found for 80% of thickness reduction, resulting a new two- phase ferritic-a 0 martensitic stainless steel. To investigate the structural evolution different values of thickness reduction were applied. Light optical and scanning electron microscopy were performed to characterize the morphol- ogy and grain refining of the structure after each rolling step. Martensitic transformation and work hardening were detected and analyzed by studying of magnetic properties (saturation magnetic polarization, relative magnetic per- meability, coercivity). Additionally, hardness tests were performed. The results highlighted a strong grain refining and increase in martensitic phase and hardness with increasing cold deformation. A direct relationship between microstructure and magnetic properties was revealed. In particular the reciprocal of relative magnetic permeability and the coercivity increased with martensite content and the amount of cold deformation. Therefore, the possible application of magnetic measurements as non-destructive tests to study microstructural evolution during cold rolling was shown for the steel considered.

Development of Manufacturing Process of PNC-FMS Wrapper Tube with SUS316 Short Joint

When a ferritic-martensitic stainless steel (PNC-FMS) wrapper tube having far greater swelling resistance against neutron irradiation is applied in the JOYO or MONJU reactor, it becomes necessary to weld it with SUS316 austenitic stainless steel (entrance nozzle and handling head). Such welding between PNC-FMS and SUS316 causes the delta (δ) ferrite formation at heat-affected zone, which leads to significant toughness degradation. In addition, bending of wrapper tube caused by their differential thermal expansion should be straightened. For preventing those problems, manufacturing process of the complex wrapper tube was developed. This process involves TIG-welding with SUS316 short pipe joints in 50mm length to both ends of a PNC-FMS round tube, and then performing the drawing and normalizing and tempering. Normalizing induces complete disappearance of the δ ferrite in the course of wrapper tube manufacturing. The mechanical properties of PNC-FMS/SUS316 welded zone were confirmed to be equivalent to those of the base metal even after thermal aging.

Microstructure and residual stresses of stainless steels case hardened with nitrogen

In the presence of 13–16 wt-%Cr, nitrogen enhances interstitial solubility during austenitising. Therefore, five different martensitic or ferritic–martensitic stainless steels were case hardened by solution nitriding instead of carburising. The highest compressive residualstress at the surface was obtained by a high content of δ-ferrite in the core and a low content of retained austenite in the martensitic case. The limits of alloying, the resulting microstructure and the implications for production and service are discussed.

Empirical Correlation of Specimen Size Effects in Charpy Impact Properties of 11Cr-0.5Mo-2W, V, Nb Ferritic-Martensitic Stainless Steel

An effect of specimen size on the Charpy impact property was evaluated in the ferritic-martensitic 11Cr-0.5Mo-2W, Nb, V stainless steel (PNC-FMS). The upper shelf energy (USE) is expressed by the power law equation as USE[J]=m(Bb) n where m is the material constant (m=0.277 in PNC-FMS), B[mm] is the specimen thickness and b[mm] is the ligament size. The normalizing exponent n was empirically derived using the USE of the full size specimen as n=1.38×10–3USEfull+1.20. The n value varies in the range of 1.2 to 1.7, which is affected by the fracture volume below the notch. The PNC-FMS wrapper takes n=1.42. The ductile-to-brittle transition temperature (DBTT) of PNC-FMS is correlated as DBTT=115(log10 BKt)—158 using an elastic stress concentration factor Kt that represents the effects of notch size, e.g. notch depth, notch root radius and notch angle.

Empirical Correlation of Specimen Size Effects in Charpy Impact Properties of 11Cr-0.5Mo-2W, V, Nb Ferritic-Martensitic Stainless Steel

Empirical Correlation of Specimen Size Effects in Charpy Impact Properties of 11Cr-0.5Mo-2W, V, Nb Ferritic-Martensitic Stainless Steel