Cast Duplex Stainless Steel Research Articles

Effects of Nitrogen Content and Solution Temperature on Microstructure and Age Hardening Properties of Cast Duplex Stainless Steel

Effects of Nitrogen Content and Solution Temperature on Microstructure and Age Hardening Properties of Cast Duplex Stainless Steel

217 低ニッケル二相ステンレス鋳鋼の組織・耐食性・機械的性質の評価

217 低ニッケル二相ステンレス鋳鋼の組織・耐食性・機械的性質の評価

A Study on Phase Stress of Centrifugally Cast Duplex Stainless Steel by Neutron Diffraction

A Study on Phase Stress of Centrifugally Cast Duplex Stainless Steel by Neutron Diffraction

Accurate modeling and reconstruction of three-dimensional percolating filamentary microstructures from two-dimensional micrographs via dilation-erosion method

Heterogeneous materials are ubiquitous in nature and synthetic situations and have a wide range of important engineering applications. Accurate modeling and reconstructing three-dimensional (3D) microstructure of topologically complex materials from limited morphological information such as a two-dimensional (2D) micrograph is crucial to the assessment and prediction of effective material properties and performance under extreme conditions. Here, we extend a recently developed dilation–erosion method and employ the Yeong–Torquato stochastic reconstruction procedure to model and generate 3D austenitic–ferritic cast duplex stainless steel microstructure containing percolating filamentary ferrite phase from 2D optical micrographs of the material sample. Specifically, the ferrite phase is dilated to produce a modified target 2D microstructure and the resulting 3D reconstruction is eroded to recover the percolating ferrite filaments. The dilation–erosion reconstruction is compared with the actual 3D microstructure, obtained from serial sectioning (polishing), as well as the standard stochastic reconstructions incorporating topological connectedness information. The fact that the former can achieve the same level of accuracy as the latter suggests that the dilation–erosion procedure is tantamount to incorporating appreciably more topological and geometrical information into the reconstruction while being much more computationally efficient.

Mechanical and corrosion behaviors of 25Cr–5.3Ni–2.8Mo–0.15N duplex stainless steel castings affected by annealing process

Stress relief annealing treatment is generally applied to keep the dimension stability and tolerance during machining process for casting. However, this process might depress properties of duplex stainless steel (DSS) castings used for sea water circulating pumps in power plants. In this paper, the microstructure and composition analysis of the 25Cr–5.3Ni–2.8Mo–0.15N DSS cast samples affected by stress relief annealing treatment were investigated by SEM and EDS. Meanwhile, uniaxial tensile test, Charpy impact test, and Brinell hardness test were also applied to research the strength, impact toughness, and hardness. Furthermore, the corrosion resistance of 25Cr–5.3Ni–2.8Mo–0.15N cast DSS was also determined by potentiodynamics polarization curves and cyclic voltammety. Brittle sigma phase precipitation has been observed in stress relief annealed samples by SEM and domain elements distribution, especially for samples annealed at 650 °C, comparing to the solidification solution treated sample. The strength and hardness of DSS sample stress relief annealed increased significantly, however, the elongation and toughness is depressed comparing to the solidification solution treated samples. Furthermore, the results of electrochemical measurements showed that annealing treatment at 650 °C significantly depresses the corrosion resistance of DSS. Therefore, stress relief annealing process of casting samples shall be considered especially due to its brittle transformation and depression of corrosion resistance.

Temperature-dependent mechanical properties of an austenitic–ferritic stainless steel studied by in situ tensile loading in a scanning electron microscope (SEM)

In situ tensile tests at various temperatures, ranging from 25 to 750°C, were conducted on an austenitic–ferritic cast duplex stainless steel (CDSS) to investigate both the plastic deformation mechanisms and the effect of temperature on mechanical properties. A continual reduction in the mechanical properties, such as ultimate tensile strength (UTS) and yield strength (0.2% proof stress, σ0.2), was found as the temperature increased. Fractographic analysis demonstrated that tearing topography surface (TTS) was more likely to occur at elevated temperatures. In situ observations revealed that the plastic deformation occurred within the soft austenite matrix at first and was followed by slip gliding in the ferrite phase as the load increased. Voids tended to form at the ferrite–austenite interphase boundaries or around the inclusions and then merge and propagate in the austenite matrix. The present study also shows that the clustered distribution of the ferrite phase in the matrix can cause crack initiation easily at early stages of deformation.

Effect of The Sigma Phase on the Mechanical Properties of a Cast Duplex Stainless Steel during the Ageing Treatment at 850°C

With about equal amount of austenite and ferrite in volume fraction, duplex stainless steel (DSS) is in advantage of mechanical properties and corrosive behaviors. Hence it is widely applied to the heavy castings for nuclear power plants inshore, such as impellers, pumps and valves. However, lots of cracks usually occur in these castings during manufacturing processes, because it is susceptible to precipitate the brittle intermetallic compound of sigma phase when the castings are exposed from 600 to 1000oC. In this work, the precipitation of sigma phase was observed by optical microscope (OM) and scanning electron microscope (SEM) in a cast DSS named as MAS/6001, which aged at 850oC from 5 to 300 minutes. The effect of sigma phase on the mechanical properties was analyzed by the tensile at room temperature and impact tests at -10°C. The results show that sigma phase in the MAS/6001 steel precipitated simultaneously with the secondary austenite, which obeyed the eutectoid reaction. The interfaces between austenite or secondary austenite and sigma phase were the locations where cracks generated from the void aggregation. Cracks are susceptible to propagate along or cross these interfaces, and to promote the sigma phase breaking-off, which severely deteriorated the mechanical properties.

Effect of sigma phase precipitation on the mechanical and wear properties of Z3CN20.09M cast duplex stainless steel

The effect of sigma phase on the mechanical and wear properties of a Z3CN20.09M cast duplex stainless steel (CDSS) used in primary coolant pipe of nuclear power plants has been investigated. The experimental results showed that the sigma phase precipitated from ferrite at a temperature range of 600–900°C. The tensile and yield strengths of the specimens aged at 700°C decreased first and then increased with the increase of aging time. With increasing aging time, the impact energy of specimens decreased slightly before 4h and then dropped drastically when aged a longer time more than 10h. Fracture surface analysis showed that the hard and brittle sigma phase degraded the toughness of the aged steel. The hardness and wear resistance of the specimens aged for 1–4h were lower than those of un-aged ones. However, the higher values of hardness and wear resistance were got in the specimens aged more than 10h.

Effect of Precipitated Phases on the Pitting Corrosion of Z3CN20.09M Cast Duplex Stainless Steel

The effect of precipitated phases on the pitting corrosion of a Z3CN20.09M cast duplex stainless steel (CDSS) which has been widely used in primary coolant pipes of nuclear power plants was investigated by using isothermal aging treatment and potentiodynamic anodic polarization methods. It was found that M23C6 carbide and · phase precipitated at the ferrite/austenite boundaries and in ferrite of the aged steel. The pitting potentials of the specimens aged at 700°C decreased with increasing of the precipitates content. The experimental results indicated that even a few percent of precipitates (about 1.0 vol%) in this steel could also worsen its pitting resistance. The pitting of the aged Z3CN20.09M specimens developed at the ferrite/austenite phase boundaries where precipitates formed often. These effects could be directly attributed to the presence of secondary austenite at the ferrite/austenite phase boundaries, which was found to be poor in Cr by energy dispersive X-ray (EDX). [doi:10.2320/matertrans.M2012410]

Effects of aging temperature on G-phase precipitation and ferrite-phase decomposition in duplex stainless steel

G-phase precipitation and ferrite-phase decomposition in a cast duplex stainless steel (DSS) aged at 623–723 K for up to 8000 h were investigated using atom probe tomography (APT). Large sample volume was observed in every APT experiment, which yielded significantly statistical results. The number density of G-phase precipitates tended to be high and their sizes were small at lower aging temperatures. G-phase precipitates grew during prolonged isothermal aging. The concentrations of nickel, silicon, manganese and molybdenum in G-phase precipitates tended to increase as the precipitates grew. Heterogeneous distributions of alloying elements within G-phase precipitates were observed. An interesting positional relationship of G-phase precipitates with dislocations was revealed. Regarding the ferrite-phase decomposition, local chromium concentrations in the ferrite phase varied fast at higher aging temperatures. Good correlation between the variation of local chromium concentrations and aging conditions was revealed, which indicates that the variation can be estimated for arbitrary aging conditions. Representative distances between chromium-enriched and chromium-diluted regions were long at higher aging temperatures. Time exponent of the representative distances of ferrite-phase decomposition as well as the size of G-phase precipitates increased with aging temperatures.

Effect of rolling deformation and solution treatment on microstructure and mechanical properties of a cast duplex stainless steel

The present study deals with the effect of rolling deformation and solution treatment on the microstructure and mechanical properties of a cast duplex stainless steel. Cast steel reveals acicular/Widmanstätten morphology as well as island of austenite within the \(\boldsymbol\delta \)-ferrite matrix. Hot rolled samples exhibit the presence of lower volume percent of elongated band of \(\boldsymbol\delta \)-ferrite (\(\boldsymbol\sim \)40%) and austenite phase which convert into finer and fragmented microstructural constituents after 30% cold deformation. By the solution treatment, the elongated and broken crystalline grains recrystallize which leads to the formation of finer grains (<10 \(\boldsymbol\mu \)m) of austenite. X-ray diffraction analysis has corroborated well with the above-mentioned microstructural investigation. Enhancement in hardness, yield strength and tensile strength values as well as drop in percent elongation with cold deformation increases its suitability for use in thinner sections. 30% cold rolled and solution treated sample reveals attractive combination of strength and ductility (25·22 GPa%). The examination of fracture surface also substantiates the tensile results. The sub-surface micrographs provide the potential sites for initiation of microvoids.

Localized Corrosion of Thermally Aged Cast Duplex Stainless Steel for Primary Coolant Pipes of Nuclear Power Plant

Localized corrosion of cast duplex stainless steel Z3CN20.09M, which used widely in primary coolant pipes of nuclear power plants, has been investigated after thermally aged at 400°C for 100, 300, 1000 and 3000h by making use of electrochemical method. Double loop electrochemical potentiodynamic reactivation (DL-EPR) and potentiodynamic polarization tests were used for assessing intergranular corrosion (IGC), pitting and general corrosion resistance, respectively. The experimental results showed that the general corrosion resistance of Z3CN20.09M did not change almost with increase of thermal aging time, however, IGC and pitting corrosion resistance decreased. In the polarization curves, the peak anodic current density of general corrosion was not found to change almost with increase of thermal aging time, but DL-EPR results showed that the value of Ir/Ia increased. The deterioration in localized corrosion resistance of thermally aged specimen was directly related to the precipitation of alpha prime phase in ferrite by spinodal decomposition.

611 2相ステンレス鋳鋼の低温脆性破壊(OS4-(3)オーガナイズドセッション《材料と組織》)

Ductile to brittle transition (DBT) behavior in two cast duplex stainless steels was investigated; one prepared from a real large -scaled product and the other from small-sized test sample. DBT temperature was higher in the latter specimen than the former specimen. SEM/EBSD observations of the fracture surface and the cross-sectioned plane reveal that microstructure difference brings different fraction of cleavage fracture of ferrite phase. That is, the small-sized test specimen fractures mostly along ferrite cleavage plane, while the specimen prepared from the real product fractures along both ferrite and austenite grains.

Mechanical Behaviour of SS 316 (N) Weld after Long Term Exposure to Service Temperatures

The composition of the welding consumable for the SS 316 L(N) components of prototype fast breeder reactor is adjusted such that the weld deposit contains 3 to 7% delta ferrite, to balance between resistance to hot cracking during welding, and embrittlement of the weld metal during service. The operating temperature range for these components is 623-823K, which is the range for the so-called “748K embrittlement”, noticed in cast duplex stainless steel. Therefore, a study was taken up to assess the degradation, if any, in the mechanical properties of this weld due to long term service exposure. Tensile, charpy-impact and fracture (J-R curves) toughness tests were done at ambient temperature, in the as-welded condition and after thermal ageing at 643, 748 and 823K for durations in the range 1000 to 20000hours. A decrease in toughness, both charpy energy and J-R curves, was observed after ageing at 643, 748 and 823K, after long ageing times. The variations in properties are discussed in the light of microstructural changes in the delta ferrite phase observed using optical and TEM methods and the fractographic observations on the fracture tested specimens.

Investigation of thermal aging damage mechanism of the Cast Duplex Stainless Steel

Besides the macro-mechanical properties for thermal aging effect published in “Thermal aging effect on Z3CN20.09M Cast Duplex Stainless Steel” (Nuclear Engineering and Design 239(2009) 2217-2223), the thermal aging damage mechanism is investigated in this paper through nano-indentation tests and micro-structures evolution examination. Numerical simulations were carried out with GTN continuum damage model to investigate the different crack propagation process for aging. The nano-indentation hardness values increase with aging time for both phases while the hardness values of the ferrite phase are much higher and increase much more. The nano-indentation energy indicating the toughness decreases for both phases with aging time. TEM results show that the Cr-enriched α′ phase precipitates in the ferrite phase which is considered as the critical reason making the dislocation slip difficult and causing the increase of the strength and reduction of the toughness. The crack initiates from the ferrite phase instead of the austenite phase from the SEM observation and FEA simulation results, which reflects the change of the fracture mechanism for thermal aging.

Quantitative analysis of a complex metal carbide formed during furnace cooling of cast duplex stainless steel using EELS and EDS in the TEM

In this work, a method to determine the atomic ratio of Mo and C within complex metal carbides using EELS in the TEM has been developed. The method is based on the determination of k-factors for given experimental conditions from the EEL spectra of Mo 2C and MoO 3 standards, which had been independently checked by XRD and EPMA. Factors affecting the k Mo/C value of the Mo 2C standard were also investigated and the value was shown to be insensitive to background subtraction window width but sensitive to prolonged irradiation and specimen thicknesses above a critical value. The method and k-factor obtained from the Mo 2C standard was applied to spectra from a complex metal carbide precipitate formed during furnace cooling of a cast duplex stainless steel. Using EELS and EDS in the TEM, the composition was estimated to be (Cr 1.52Fe 2.33Mo 1.25Ni 0.17Si 0.46)C, which is close to M 6C stoichiometry, and the structure was confirmed by electron diffraction.

Influence of Cutting Speed, Feed Rate and Bulk Texture on the Surface Finish of Nitrogen Alloyed Duplex Stainless Steels during Dry Turning

This paper presents the results of experimental work carried out in dry turning of cast duplex stainless steels (ASTM A 995 Grade4A and ASTM A 995 Grade5A) using TiC and TiCN coated cemented carbide cutting tools. The turning tests were conducted at five different cutting speeds (80, 100, 120, 140 and 160 m/min) and three different feed rates (0.04, 0.08 and 0.12 mm/rev) with a constant depth of cut (0.5 mm). The influence of cutting speed and feed rate on the machined surface roughness was investigated. Texture analysis (Bulk) was also carried out to study the impact of preferred orientation on the resulting surface roughness. The result reveals that the increasing cutting speed decreases the surface roughness till a particular point and then increases whereas; the surface roughness value decreases with the decreasing feed rate. Presence of alpha fiber (Bulk texture analysis) in the austenite phase of 4A work piece material leads to better surface finish. Among both the grades, surface finish of grade 4A is better than grade 5A work piece material.

Fracture Toughness of Heat-Treated Superaustenitic Stainless Steels

Cast duplex and superaustenitic stainless steels are attractive alloys for applications that desire high strength and high corrosion resistance. Due to the large amounts of alloying additions present in these systems, a proper solution heat-treatment schedule is critical in ensuring that the alloys are able to meet the desired specifications. While studies to determine the phase transformation behavior of duplex and superaustenitic alloys have shown that the precipitation kinetics are comparatively slow when considering large amounts of precipitation, studies involving Charpy impact specimens have noted significant decreases in impact strength in materials improperly heat treated for times as short as 10 min, well before any significant amount of precipitation (<1 vol.%) has occurred. The goal of this study was to investigate a particular superaustenitic alloy, CN3MN, in an effort to determine the reason for this large drop in impact strength. Charpy impact specimens were tested from polycrystalline and single crystal samples in the solution heat-treated condition and after heat treatment at 872 °C for times ranging from 5 to 60 min to embrittle the material. Fracture surfaces were characterized using optical and scanning electron microscopy (SEM), Auger spectroscopy, and energy dispersive x-ray spectroscopy. The fracture surfaces showed ductile and brittle characteristics for the standard and improperly treated samples, respectively. Close observation using SEM reveals precipitation takes place initially on grain boundaries, which may account for the observed embrittling effect. While difficult to see at very short times, measurements of linear coverage of precipitation along grain boundaries of polished samples show a definite relationship between drop in fracture toughness and grain boundary precipitation. The results of the Charpy tests and SEM observations, along with chemical analysis studies of the grain boundaries using x-rays and auger electrons, will be presented.

Thermal aging effect on Z3CN20.09M Cast Duplex Stainless Steel

Instrumented impact tests were performed on Z3CN20.09M Cast Duplex Stainless Steel (CDSS) aged at 400 °C for up to 3000 h. The material was cut from the primary pipe of a PWR to investigate the accelerated thermal aging embrittlement effect on the dynamic fracture properties. The load–deflection curves from the instrumented impact tests described both the loading and the fracture stages, from which the dynamic ultimate loading level and the dynamic ultimate strengths were calculated as a function of increased aging time. The increase in the dynamic ultimate strengths with aging time is much higher than those for the static ultimate strengths. With the increment of the aging time, both the crack initiation energy and the crack propagation energy decrease, which are separated form the total impact energy taking the maximum loading level as the crack initiation point. It is shown that the reduction tendency of the fracture propagation energy is the predominant reason for the reduction of the impact energy. With the increment of the aging time, the unloading slopes reflecting the crack propagation speed increase and the fracture surface patterns change from ductile fracture with shallow dimples to cleavage brittle fracture caused by dislocations piling up in the ferrite matrix. Mechanical properties of inner wall with fine equi-axed grains are much better than those of the outer wall with coarse columnar structures.

Characterization of functional gradient structures in duplex stainless steel castings

Titanium carbide particle reinforced metal-matrix composite coatings, formed on duplex (austenitic/ferritic) stainless steel castings, were studied. These coatings, developed for wear applications, develop a functional gradient structure through a self-propagating high-temperature synthesis (SHS) of titanium carbide (TiC). This SHS-reaction was ignited in reactive inserts by the heat of liquid steel. The reactive inserts were compressed from mixtures of titanium, carbon and molybdenum powders together with iron or duplex stainless steel powders. The coating micro- structure depends on the reactive insert composition. Molybdenum reduced the TiC particle diameter, and reducing the binder content reduced the areal fraction of TiC.