Duplex Alloy Research Articles

Determination of Nitrogen in Duplex Stainless Steels by EPMA

We describe the quantitative determination of low nitrogen contents (< 1 %wt) in highly-alloyed, duplex stainless steels by EPMA using a calibration curve. Five monophasic stainless steels with various nitrogen concentrations were used for the calibration curve, namely, three highly alloyed steels prepared in the laboratory, with nitrogen contents 0.066, 0.565 and 1.016 %wt and two industrial alloys with nitrogen concentrations 0.174 %wt and 0.023 %wt. For these samples, the plot of nitrogen X-ray intensities versus nitrogen contents exhibited an acceptable linear relationship. Average nitrogen contents of the analysed duplex alloys were found to vary in the range < 0.045–0.07 %wt in ferrite and 0.52–0.77 %wt in austenite. These results suggest that the nitrogen solubility limit in ferrite has been reached; on the other hand, nitrogen solubility in austenite increases due to the high manganese and low nickel contents.

Evaluation of a threshold-based model of the elevated-temperature fatigue of impact-damaged γ-TiAl

Step-loading fatigue tests have been conducted on two γ-TiAl alloys with differing microstructures following quasi-static indentations intended to simulate assembly-related impact damage to low-pressure turbine blades. Fatigue tests were conducted at 600 °C using computer-controlled servohydraulic loading at a frequency of 20 Hz. Reasonably good agreement was achieved between the fatigue data and calculated fatigue strength based on the fatigue threshold and measured impact severity. In certain cases, the fatigue threshold model fails to completely describe the data. These discrepancies may be related to residual stresses, variations in crack-shape morphology, and small-crack effects. Residual stresses could not be directly measured, given the small size of the damage zones. However, a comparison of fatigue threshold approximations based on a through-thickness crack geometry and a corner-crack geometry suggests that these two models may represent the upper and lower bounds of the actual fatigue behavior. In addition, the behavior of small cracks was examined by modeling the stress-lifetime response of lightly damaged specimens of the duplex alloy. This effort indicates the need for small-crack fatigue threshold values when designing fatigue-critical γ-TiAl components.

Effect of Aging and Cold Rolling on the Microstructure and Mechanical Properties of a New Fe-Mn-Al-Cr-C Duplex Alloy

The microstructural changes that occur during aging and cold rolling of a new Fe-Mn-Al-Cr-C duplex alloy have been investigated. Two treatments were developed to produce either a good combination of tensile strength and ductility (σu=800 MPa, σy=525 MPa, and A=46%) or a high strength (σu=1340 MPa, σy=1200 MPa, and A=15%) with a ductile type of fracture after aging at 320 °C. Aging between 550 °C and 700 °C led to a significant decrease in strength and ductility due to the precipitation of the brittle βMn phase. However, aging above 750 °C showed a considerable increase in strength and ductility due to the precipitation of very fine grains of ferrite within the austenite phase.

Effects of carbon and chromium on the solidification structure and properties of ferrite-austenite duplex heat-resistant alloy

In order to design a new kind of low-cost high-temperature ferrite-austenite duplex alloy, the effects of carbon and chromium on the alloy solidification structure and properties have been investigated with orthogonal experiments. The addition of carbon promotes strongly the formation of austenite and that of carbides in the alloy solidification structure and refines the alloy grains. With the increase of carbon content, the alloy high temperature strength and oxidation resistance at 1250°C improves at first, but then begins to deteriorate greatly when the carbon content exceeds 0.15%. The addition of chromium facilitates the formation of ferrite in the alloy solidification structure. As the chromium content increases, the alloy rupture strength at 1250°C initially is enhanced, but then reduces rapidly, while the alloy oxidation resistance improves continuously.

Effect of top and bottom filling on reliability of investment castings in Al, Fe, and Ni based alloys

The effect of bottom and top filling running systems on the properties of four investment cast alloys susceptible to contamination by oxide films during filling was studied. The alloys were air cast 2L99 Al–Si–Mg alloy and 254-SMO super duplex stainless steel and vacuum cast IN939 and IN738LC nickel based superalloys. The Weibull moduli for the tensile strength of investment cast bars produced using top and bottom filling were compared as indicators of casting reliability and of oxide damage produced by the running systems. A Weibull modulus of 18 was obtained for top filled 2L99 castings; this was increased to 34 when a correctly designed bottom filling system with a filter was used, thus reflecting the decreased scatter in properties. However, a similar effect was not found for the stainless steel. The use of improved running system designs led to minor increases in the Weibull modulus of the IN738LC and IN939 Ni alloys.

Control of omega phase volume fraction precipitated in a beta titanium alloy : Development of an experimental method

Because of their unique properties of high strength, excellent hardenability and good corrosion resistance beta titanium alloys have been the subject of much attention. Recently, since cost is an increasingly important parameter, a LCB (low cost beta) titanium alloy(Ti6.8Mo-4.5Fe-1.5A1) has been developed to be used in non-aerospace new technology as high performance substitutes for more classical material. Metastable β Ti alloys are typically known to precipitate additional phases (α andω phases) during thermomechanical treatments. The morphology, size and distribution of these precipitates determine in large part the mechanical properties of the alloy [1]. It is known thatω precipitates can be formed during quenching by a diffusionless martensitic mechanism (ωath) and during aging by a diffusion controlled process (ωiso) [2]. Nevertheless, the parameters allowing the control of ωiso phase crystals density are still unclear [3]. Therefore, it seems necessary to control the appearance of ω phase particles in β titanium alloys and particularly to understand the nucleation mechanism. The direct interest of such a knowledge is crucial since it was recently reported that the ωiso precipitates could behave as nucleation sites of an extremely fine and dispersed α phase at intermediate aging temperatures leading to higher yield strength (β + α) duplex alloys [4–6]. In this present paper a method based on an experimental new approach of the β→ω transformation is described. This method could allow an experimental control of ωiso dispersion. The β phase decomposition during heating was studied on Ti-6.8Mo-4.5Fe-1.A1 water quenched from the β field (1123 K). This method was developed by studying the electrical resistivity changes of samples during subsequent heat treatments. The electrical resistance measurements were made by a computer driven four-point technique. R/Ri (Ri is the initial resistance) is plotted as a function of time and temperature. In this way, microstructural evolutions and transformations kinetics can be investigated [7, 8]. It was reported that the metastable β titanium alloys had after quenching a resistivity increasing with cooling and decreasing with heating. This effect named NTD (negative dependence temperature) was ascribed to the ωath precipitation during quenching and reversed in an entirely reversible way and without hysteresis during the subsequent heat treatment [8, 9]. In addition, it is known that an ωiso phase also hexagonal could precipitate during aging, implying diffusional phenomena. We choose to consider the TCR (temperature coefficient of resistivity= d(R/Ri)/dT ) between 343 and 383 K as a parameter for monitoring microstructural changes during heating and cooling of Ti-6.8Mo4.5Fe-1.A1 samples. A first series of resistometric experiments (Fig. 1) allowed us to notice that cycles carried out up to 423 K (v= 10 K/min) display a total recovery of TCR (TCR recovery ratio= 100× TCRcooling/TCRheating) showing that the reaction ωath⇔β is completely reversible in good agreement with previous results [8]. No hysteresis was noticed during these thermal cycles below 423 K. Above 423 K, the recovery of our coefficient is only partial which can be connected to irreversible microstructural changes. The assumption that the precipitation of the ωiso prevents the reprecipitation of ωath during cooling is quite well established now [9]. Nevertheless, the involved mechanisms binding the two types of particles remains to be clarified. In order to investigate the state of irreversible microstructural changes, resistometric experiments were carried out. Quenched samples on one hand, and subsequent cold rolled samples (at different reduction rates) on the other hand were aged for 18 ks at 473 K and 523 K. TCR (measured during heating and cooling under conditions quoted above) and TCR recovery ratio were then graphically evaluated. During the course of this work, extensive characterization using transmission electron microscopy and X-ray analysis confirmed that irreversible microstructural changes above 423 K could be associated with ωiso precipitation (with ellipsoidal morphology) in the β matrix. Figs 2 and 3 suggest that rate of rolling has little influence on the TCR recovery ratio evolution. Thus, although the applied stress is thought to create lattice defects such as vacancy clusters and misfit lattices acting as potential nucleation sites, the ωiso

411 2 相合金の硬さ測定による脆化評価

411 2 相合金の硬さ測定による脆化評価

A new model for diffusion bonding and its application to duplex alloys

Abstract Diffusion bonding is an advanced bonding process in which two materials, similar or dissimilar, can be bonded in solid state. This provides to bond materials in a wide range from low carbon steels to ceramics and composites which cannot be bonded with conventional welding methods. One of the major advantages of this method is to produce new bimetal or dissimilar material couples. The process is diffusion-based and occurs in solid state and because of its increasing use as a commercial process. The estimation of final bonding time is very important but difficult without experiments for many materials. In this study, therefore, a new mathematical model is presented to predict the final bonding time for a sound bonding interface prior to bonding practice. Being different from the previous models, this model assumes a new surface morphology as a sine wave and a new creep mechanism for duplex alloys. The mechanisms operating during diffusion bonding are based on those in pressure sintering studies but here mass transfer by evaporation has been ignored. The driving forces and rate terms for those mechanisms have been altered to reflect the difference of the geometries of the two processes. Also the effect of grain size has been included in the model in case of joining fine-grained materials. In determining diffusion coefficients for duplex alloys, Darken’s equation for binary alloys has been used. Depending on this new approach, it was shown that a more realistic final bonding time could be predicted for duplex alloys by comparing the results from this new model with those from the previous ones. As a result, it was determined that the new model could be used in order to estimate the final bonding time of the duplex alloys for a sound bond interface and the relationships between its parameters safely. The predictions from this new model show a very good agreement between practice and theory.

Metastable pitting in 25 Cr duplex stainless steel

Evidence of metastable pitting in conventional austenitic stainless steels at room temperature is now common. Because of the small currents associated with individual events, most studies have been carried out on micro electrodes (typically 50 mm diameter). Here in contrast, with high chromium duplex stainless steels (DSS), metastable pitting is clearly observed on larger electrodes in tests at elevated temperatures in neutral NaCl solutions. Below the Critical Pitting Temperature (CPT), large metastable current peaks are observed across a range of low potentials, with less activity at higher potentials. Above the CPT, stable pits initiate at the same low potentials where metastability occurs below the CPT, usually preceded by one or two metastable events. There are evident differences between the growth of metastable pits—and the transition from metastable to stable pitting—in high-chromium DSS from that in single phase austenitic steels. A proposed model to explain the metastability in these highly alloyed duplex steels is described.

Investigation of Hydrogen Permeability through Copper Alloy CUCR1ZR0.1 and Duplex Structure Be-Cu

Be-copper alloy duplex structures are planed to be used as the structural material of the first wall for ITER. CuCr1Zr0.1 copper alloy is one of the prospective components of such structures. To ensure safety and to solve ecological problems concerning fusion reactor operation it is necessary to have estimations of tritium inventory in the copper alloy CuCr1Zr0.1 and in Be-copper alloy duplex structures.In this work the results of the following experiments on determination of hydrogen permeation parameters are presented:• Investigation of hydrogen permeation for beryllium-copper duplex structure. Experiments were carried out in the temperature range from 873 K to 1023 K, at input hydrogen pressures between 102 Pa and 103 Pa.• Investigation of hydrogen permeation for copper alloy CuCr1Zr0.1 in the temperature range from 723 K to 973 K, at input pressures of hydrogen between 102 and 103 Pa and for samples with different thickness.

An ultrasonic prospecting of shape-memory alloy behaviour under thermal charges

Thermomechanical models may be produced to describe the macroscopic deformations of shape-memory alloys “educated” to be deformed with special shapes as a function of temperature. To be accurate, these models need to take into account evolution of the microstructure via homogenization theories. So, the aim of this work was to provide all available information about phase transformations occurring in the grain structures from an investigation close to the microscopic scale. In this work, we have visualized grain structures of Cu–Zn–Al duplex alloys using acoustic microscopy. Evolution of phase transformations as a function of temperature has also been followed on these acoustic images with a spatial accuracy up to few micrometres. This observation of sample surface has also enabled estimation of grain baring due to phase transformations. Using the same experimental device, “acoustic signatures” have been taken on samples in complete austenic or martensitic forms to measure the speed of Rayleigh surface waves. Despite the use of a wide ultrasonic frequency range from 15–600 MHz, it seems that wave attenuation due to viscosity is important and disables velocity measurements by this method. Finally, using an acoustic echographic technique, we have correlated attenuation and velocity of longitudinal waves to the global phase transformation of heated samples. © 1998 Kluwer Academic Publishers

Properties and microstructure of TiAl-based alloys

A range of TiAl-based alloys has been plasma melted and subsequently either isothermally forged, extruded or atomised and HIPped. The microstructure and properties of these alloys have been determined and some aspects of the relation between processing route, alloy composition and properties will be discussed. It is clear that room temperature strengths between 1000 and 300 Mpa can be obtained for different compositions and processing routes, but room temperature elongation is generally the least attractive of the properties with values of 2% being typical of the best observed. The relation between room temperature strengths and creep response is not straightforward and some typical results will be reported. On the basis of these observations and earlier work on the influence of high temperature exposure on the structure of lamellar and duplex alloys, it is suggested that there may be significant advantages of using processing routes which generate a fine grained duplex structure.

Investigation of the microstructures of iron based wrought CrNiMo duplex alloy

Samples of duplex stainless steel, consisting of iron-based CrNiMo alloy containing some other elements undergoing various heat treatments after having been wrought at 1150°C, have been investigated using X-ray diffraction, thermal analysis and metallographic techniques. It is determined that some intermetallic phases such as Laves, σ, G, μ, R, χ, and some carbide phases as such M 23C 6, M 7C 3, M 6C may be found in these samples and that the ratio of carbide phases decreased in samples annealed at 950°C. In samples cooled in liquid nitrogen, it is seen that some dissolving elements in the matrix have accumulated around the σ phases and some precipitation phases have occurred. It is concluded that the use of annealing of this material at 950°C affects the chemical, mechanical and other properties adversely.

Investigation of the casting microstructure of a Fe-base, CrNiMo duplex alloy

The microstructures of Fe-base cast duplex stainless steel specimens, which contain Cr, Ni, Mo and other elements and have been subjected to various heat treatments, were examined with X-ray diffraction, thermal analysis and optical microscopy and their hardnesses were also measured. From the metallographic examinations, it was seen that these specimens included austenite, ferrite, some inter-metallic phases, grain boundary carbides and oxide inclusions. It has been determined that the amount of grain-boundary carbides decreased, and second-phases precipitated in ferrite, in specimens annealed at 950°C for 2 h and then cooled in water and liquid nitrogen. In addition it has been found by X-ray diffraction that the heat treatments applied did not change the types of the phases, but caused lattice deformations in the phases. Furthermore, beside ferrite and austenite, Laves, σ, G, μ, R, χ, Fe 3C, Fe 3P, FeS 2, MnS phases and M 23C 6, M 7C 3, M 6C carbides were determined to be in the microstructure. DSC (differential scanning calorimeter) curves showed that some stoichiometry compounds of small amounts occurred between 370 and 470°C, and in specimens cooled in liquid nitrogen after annealed at 950°C for 2 h, a phase with a small percentage dissolving at about 310°C. DTA (differential thermal analysis) curves also showed that austenite phase transformed to ferrite phase completely between 1182 and 1220°C. In addition, it was found that the hardness of the specimens were different.

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

AbstractLaboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic‐austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits‐crevices were found under the corrosion crust deposits on the duplex stainless steel.

SANDVIK SAFUREX

Abstract Sandvik Safurex is a high alloy duplex (austenitic-ferritic) stainless steel primarily developed to withstand the corrosive environments present in the urea industry. This datasheet provides information on composition, physical properties, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming, machining, and joining. Filing Code: SS-671. Producer or source: Sandvik.

II. Deformed microstructures during creep of TiAl alloys: role of mechanical twinning

Abstract The deformation mechanisms responsible for the creep behaviour of two TiAl alloys with duplex and lamellar structures have been analysed after carrying out microstructural observations as a function of creep strain and applied stress at 700 °C. The duplex alloy exhibits extensive mechanical twinning and the new twin interfaces subdivide the γ grains throughout the primary stage of creep. At the onset of the minimum creep rate, the twin interfaces in the duplex alloy behave in the same way as the γ γ or the α 2 γ interfaces in the lamellar alloy. Thus, dislocation accumulation and emission from the interfaces occur under the action of the high local stresses. The high stress exponent, n = 19, measured in both alloys at the minimum strain rate has been interpreted in terms of a threshold stress representing the high local stresses required to liberate dislocations from the interfaces and initiate their propagation within the γ matrix.

ARMCO NITRONIC 19D

Abstract Nitronic 19D is an alloy with a mixed ferrite and austenite microstructure. The alloy provides a good compromise of strength, formability, and economy with slightly less corrosion resistance than the reference duplex alloy 2205 (see Alloy Digest SS-462, August 1985). This datasheet provides information on composition, physical properties, hardness, tensile properties, and bend strength as well as deformation and fatigue. It also includes information on high temperature performance as well as forming, machining, and joining. Filing Code: SS-661. Producer or source: Armco International Sales.

High-resolution EM investigation about effect of stress on formation of ω-phase crystals in β-tttanium alloys

ω-phase particles formed in β-titanium alloys (bcc structure) act important roles to their mechanical properties such as ductility and hardness. About the ductility, fine ω-phase particles in β–titanium alloys improve the ductility, because ω-phase crystals becomes nucleation sites of α-phase and it is well known that (β+α) duplex alloys have higher ductility. In the present study, the formation sites and the formation mechanism of ω-phase crystals due to external stress and aging are investigated using the conventional and high resolution electron microscopy.A β-titanium alloy (Til5Mo5Zr) was supplied by Kobe Steel Co., and a single crystal was prepared by a zone refining method. Plates with {110} surface were cut from the crystal and were pressured hydrostatically, and stressed by rolling and tensile testing. Specimens for aging with tensile stress were also prepared from Ti20Mo polycrystals. TEM specimens from these specimens were prepared by a twin-jet electron-polishing machine. A JEM 4000EX electron microscope operated at 400k V was used for taking dark field and HREM images.

Investigation of Superplastic Behavior of NiAI and Ni3Al Duplex Alloy

ABSTRACTThe superplastic behavior of a NiAI and Ni3Al duplex alloy was investigated. It was found that the alloy exhibits superplastic behavior over a narrow temperature range, from 975 °C to 1025°C at the strain rate of 1.52 × 10-4 s-1. A maximum tensile elongation of 149% was obtained at 1000°C with the strain rate sensitivity up to 0.375. The superplastic deformation of the duplex alloy can be approximately described by an empirical equation of the form: ε = Ao2.67 exp(-303,000 / RT). Optical microstructure and TEM observation show that the superplastic behavior mechanism of the investigated alloy is a process of continuous recovery and recrystallization during deformation.