Elemental partitioning and microstructural development in duplex stainless steel weld metal

A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.     

Corrosion Behavior of High Nitrogen Nickel-Free Fe-16Cr-Mn-Mo-N Stainless Steels

The purpose of the current study is to develop austenitic nickel-free stainless steels with lower chromium content and higher manganese and nitrogen contents. In order to prevent nickel-induced skin allergy, cobalt, manganese, and nitrogen were used to substitute nickel in the designed steel. Our results demonstrated that manganese content greater than 14 wt pct results in a structure that is in full austenite phase. The manganese content appears to increase the solubility of nitrogen; however, a lower corrosion potential was found in steel with high manganese content. Molybdenum appears to be able to increase the pitting potential. The effects of Cr, Mn, Mo, and N on corrosion behavior of Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were evaluated with potentiodynamic tests and XPS surface analysis. The results reveal that anodic current and pits formation of the Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were smaller than those of lower manganese and nitrogen content stainless steel.     

Effect of Carbon Content on Stacking Fault Energy of Fe-20Mn-3Cu TWIP Steel

The influence of carbon content on the stacking fault energy(SFE)of Fe-20Mn-3Cu twinning-induced plasticity(TWIP)steel was investigated by means of X-ray diffraction peak-shift method and thermodynamic modeling.The experimental result indicated that the stacking fault probability decreases with increasing carbon addition, the SFE increases linearly when the carbon content in mass percent is between 0.23% and 1.41%.The thermodynamic calculation results showed that the SFE varied from 22.40to 29.64mJ·m-2 when the carbon content in mass percent changes from 0.23%to 1.41%.The XRD analysis revealed that all steels were fully austenitic before and after deformation,which suggested that TWIP effect is the predominant mechanism during the tensile deformation process of Fe-20Mn-3Cu-XC steels.     

Carbon diffusion and phase transformations during gas carburizing of high-alloyed stainless steels: Experimental study and theoretical modeling

Gas carburizing of high-alloyed stainless steels increases surface hardness, as well as the overall mechanical characteristics of the surface. The growth of chromium-rich carbides during carbon transfer into the steel causes precipitation hardening in the surface, but decreases the chromium content in solid solution. In order to maintain a good corrosion resistance in the carburized layer, the stainless steel composition and the carburizing process need to be optimized. To limit the experimental work, a methodology using software for modeling the thermodynamic and kinetic properties in order to simulate carbon diffusion and phase transformations during gas carburizing is presented. Thermodynamic calculations are initially used to find the optimum parameters (T, carbon wt pct, etc.) in order to maintain the highest Cr and Mo contents in the austenitic solid solution. In a second step, kinetic calculations using the diffusion-controlled transformations (DICTRA) software are used to predict how the amount of the different phases varies and how the carbon profile in the steel changes as a function of time during the process. Experimental carbon profiles were determined using a wavelength-dispersive spectrometer for electron-probe microanalysis (WDS-EPMA), while carbide compositions were measured by energy-dispersive spectroscopy_X (EDS_X) analyses. A good agreement between calculated and experimental values was observed for the Fe-13Cr-5Co-3Ni-2Mo-0.07C and the Fe-12Cr-2Ni-2Mo-0.12C (wt pct) martensitic stainless steels at 955 °C and 980 °C.     

基于相图和组织控制的先进材料开发

Recent progress on research activities of phase diagrams in our laboratory has been presented. Experimental studies and thermodynamic calculations based on CALPHAD (Calculation of Phase Diagrams) method have been conducted in the following alloy systems. 1. Database on microalloying steels including carbide, nitride and sulfide is now being constructed. 2. ADAMIS (Alloy Database for Micro-Solders) containing 8 elements of Ag, Bi, Cu, In, Sb, Sn, Zn and Pb hasbeen constructed, which can handle all combinations of these elements and all composition ranges. 3. A thermodynamic database of Cu-base alloys including Cu-X binary system and Cu-Fe, Cu-Ni, Cu-Cr base ternarysystems has been constructed. 4. Experimental and thermodynamic calculations on Fe, Ni, Co and Ti aluminides have been conducted. 5. Experimental and thermodynamic calculations on Co base magnetic recording media have been conducted. 6. Thermodynamic analysis of interaction between magnetic and chemical orderings has been performed. By utilizing the information on phase diagrams, the following advanced materials have been developed. (A) New type of high speed steel with high hardness about Hv≈1000 by carbide dispersion carburizing method, (B) New Pb-free machinable stainless steel using titanium carbosulphide. (C) New Pb-free solder for Die-attaching use. (D) Shape memory alloys; Cu-base, Ferromagnetic Ni-base and Fe-base. (E) Invar alloys. (F) Egg-type powder. Typical examples of phase diagrams, phase stability, database and its application for the development of advanced materials will be presented.     

Simplified Thermodynamic Model for Pro-Eutectoid Ferrite Formation in Multicomponent Structural Steel

AnumberofthermodynamicmodelsforFe Cbinarysystemhavebeendeveloped ,namelytheKRC′streatment[1] whichbelongstogeometryre pulsivemodel ,theLFG′streatment[2 ] andtheMD′streatment[3 ] whichareofatominteractivemodel .Afterwards,ShifletGJetal[4 ] correctedandim provedthesemodels .Inordertoinvestigatethephasetransformationinmulticomponentsteels ,AaronsonHIetal[5] firstproposedthesuperelementmodelfollowingtheZenerC′sworks[6 ] .Atpresent ,thesuperelementmodelhasbeenwidelyappliedtothermodynamic…     

奥氏体不锈钢中氮溶解度的热力学计算和实验研究

采用热力学分析方法,对固态不锈钢304、304L、301S和301L(γ-相)以及奥氏体不锈钢熔体中氮溶解度进行了计算,得出了氮溶解度的计算模型;同时通过1 kg MoSi电阻炉对4种奥氏体不锈钢在1520～1580℃和33～100 kPa压力下的渗氮行为进行了实验研究。结果表明,氮在固态奥氏体不锈钢的γ-相中的溶解度最高;在常压冷却、凝固过程中存在的液相、δ-相至γ-相的转变;当不锈钢熔体中相对于δ-相过饱和的氮在钢中以气泡形式析出,则降低了奥氏体钢的氮含量,所以采用常压快速冷却或加压浇注有利于冶炼高氮奥氏体不锈钢。     

Mechanical properties of high purity Fe-26 Cr-1 Mo ferritic stainless steel

The physical metallurgical characteristics of high purity Fe-26 Cr and Fe-26 Cr-1 Mo ferritic stainless steels, containing low carbon and nitrogen contents, were studied. The studies demonstrate that a commercially produced Fe-26 Cr-1 Mo high purity alloy containing a maximum of 50 to 150 ppm of carbon and nitrogen is capable of good mechanical properties and fabricability. The alloy was previously demonstrated to exhibit good corrosion resistance. Because of these properties, the high purity Fe-26 Cr-1 Mo alloy is a viable alternate to 300-series stainless steels as well as more highly alloyed materials for many applications. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponsorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.     

An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition

The influence of silicon on κ-carbide precipitation in lightweight austenitic Fe-30Mn-9Al-(0.59-1.56)Si-0.9C-0.5Mo cast steels was investigated utilizing transmission electron microscopy, 3D atom-probe tomography, X-ray diffraction, ab initio calculations, and thermodynamic modeling. Increasing the amount of silicon from 0.59 to 1.56 pct Si accelerated formation of the κ-carbide precipitates but did not increase the volume fraction. Silicon was shown to increase the activity of carbon in austenite and stabilize the κ-carbide at higher temperatures. Increasing the silicon from 0.59 to 1.56 pct increased the partitioning coefficient of carbon from 2.1 to 2.9 for steels aged 60 hours at 803 K (530 °C). The increase in strength during aging of Fe-Mn-Al-C steels was found to be a direct function of the increase in the concentration amplitude of carbon during spinodal decomposition. The predicted increase in the yield strength, as determined using a spinodal hardening mechanism, was calculated to be 120 MPa/wt pct Si for specimens aged at 803 K (530 °C) for 60 hours and this is in agreement with experimental results. Silicon was shown to partition to the austenite during aging and to slightly reduce the austenite lattice parameter. First-principles calculations show that the Si-C interaction is repulsive and this is the reason for enhanced carbon activity in austenite. The lattice parameter and thermodynamic stability of κ-carbide depend on the carbon stoichiometry and on which sublattice the silicon substitutes. Silicon was shown to favor vacancy ordering in κ-carbide due to a strong attractive Si-vacancy interaction. It was predicted that Si occupies the Fe sites in nonstoichiometric κ-carbide and the formation of Si-vacancy complexes increases the stability as well as the lattice parameter of κ-carbide. A comparison of how Si affects the enthalpy of formation for austenite and κ-carbide shows that the most energetically favorable position for silicon is in austenite, in agreement with the experimentally measured partitioning ratios.     

Theoretical treatment of the solidification of undercooled Fe-Cr-Ni melts

The solidification behavior of undercooled Fe-Cr-Ni melts is analyzed with respect to the competitive formation of body-centered cubic (bcc) phase (ferrite) and face-centered cubic (fcc) phase (austenite). The activation energies of homogeneous nucleation and growth velocities for both phases as functions of undercooling of the melt are calculated on the basis of current theories of nucleation and dendrite growth using data of thermodynamic properties available in the literature. As model systems for numerical calculations, the alloys Fe-18.5Cr-11Ni forming primary ferrite and Fe-18.5Cr-12.5Ni forming primary austenite under near-equilibrium solid-ification conditions are considered. Nucleation of the bcc phase is always promoted in the under-cooled primary ferrite alloy, whereas the barrier for bcc nucleation falls below that for fcc nucleation for large undercooling in primary austenite alloys. With rising undercooling, tran-sitions of the fastest growth mode were found from bcc to fcc and subsequently from fcc to bcc for the primary ferrite forming alloy and from fcc to bcc for the primary austenite forming alloy. The results of the calculations provide a basis for understanding contradictory experi-mental findings reported in the literature concerning phase selection in rapidly solidified stainless steel melts for different process conditions. Formerly Visiting Scientist at the Institut fur Raumsimulation     

氮含量对无镍奥氏体不锈钢平衡相转变的影响

高氮铬锰奥氏体不锈钢有着极为广泛的应用前景,然而氮含量对其相转变的影响尚不十分清晰。设计并冶炼了氮质量分数为0.02%～1.20%的试验钢,对各钢的平衡相转变进行了热力学计算,对δ铁素体和Cr₂N的形貌进行了观察。结果表明,钢中δ铁素体的最大析出量随着氮含量的升高而降低,当氮质量分数超过1.05%后,无δ铁素体析出。获得了试验钢加热时δ铁素体的析出温度与氮含量的关系式。随着氮含量的升高,试验钢在冷却时Cr₂N的析出温度逐渐升高,并获得了其定量关系式。在GN04钢中,1 200 ℃等温2 h后的δ铁素体主要沿三叉晶界分布。Cr₂N析出优先在晶界形成,然后朝着晶内发展。在相同等温条件下,试验钢中Cr₂N的析出量随着氮含量的升高而增大,且层片间距随之减小。     

Microstructural characterization of 5 to 9 pct Cr-2 pct W-V-Ta martensitic steels

The microstructure of 9Cr-2W-0.25V-0.1C (9Cr-2WV), 9Cr-2W-0.25V-0.07Ta-0.1C (9Cr-2WVTa), 7Cr-2W-0.25V-0.07Ta-0.1C (7Cr-2WVTa), and 5Cr-2W-0.25V-0.07Ta-0.1C (5Cr-2WVTa) steels (all compositions are in wt pct) have been characterized by analytical electron microscopy (AEM) and atom probe field ion microscopy (APFIM). These alloys have potential applications in fusion reactors because they exhibit reduced neutron activation in comparison to the conventional Cr-Mo steels. The matrix in all four alloys was 100 pct martensite. The precipitate type in the steels depended primarily on the chromium level in the alloy. In the two 9Cr steels, the stable phases were blocky M₂₃C₆ and small spherical precipitates previously identified as MC. The two lower-chromium steels contained blocky M₇C₃ and small needle-shaped carbonitrides in addition to M₂₃C₆. The AEM and APFIM analyses revealed that, in the steels containing tantalum, the majority of the tantalum was in solid solution. With the exception of a few of the small spherical precipitates in low-number densities in the 9Cr-2WVTa, none of the other precipitates contained measurable tantalum. The experimentally observed phases were in agreement with those predicted by phase equilibria calculations using the ThermoCalc software. However, a similar match between the experimental and predicted values of the phase compositions did not occur in some instances. Atom probe analyses directly confirmed the crucial role of trace amounts of nitrogen in the formation of vanadium-rich carbonitrides as predicted by thermodynamic equilibrium calculations.     

含锡C--Mn钢中锡的析出相

利用扫描电镜/能谱仪和透射电镜表征了残余元素Sn在C-Mn钢中的存在形式,同时考察了热处理温度对含Sn析出相类型的影响.结果表明:Fe-5% Sn中Sn以第二相形式在晶界与晶内析出;Fe-1.5% Sn-0.2% S中Sn以第二相形式在直径2~4 μm的球形MnS夹杂物上异质析出.透射电镜分析和热处理实验结果表明:在Fe-5% Sn及Fe-1.5% Sn-0.2% S中MnS夹杂上析出的含Sn第二相结构为四方晶系的FeSn₂相.      

High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel <Emphasis Type="Italic">In-Situ</Emphasis> Neutron Scattering Approach

Duplex stainless steels are designed to solidify with ferrite as the parent phase, with subsequent austenite formation occurring in the solid state, implying that, thermodynamically, a fully ferritic range should exist at high temperatures. However, computational thermodynamic tools appear currently to overestimate the austenite stability of these systems, and contradictory data exist in the literature. In the present work, the high-temperature phase equilibria of four commercial duplex stainless steel grades, denoted 2304, 2101, 2507, and 3207, with varying alloying levels were assessed by measurements of the austenite-to-ferrite transformation at temperatures approaching 1673 K (1400 °C) using a novel in-situ neutron scattering approach. All grades became fully ferritic at some point during progressive heating. Higher austenite dissolution temperatures were measured for the higher alloyed grades, and for 3207, the temperature range for a single-phase ferritic structure approached zero. The influence of temperatures in the region of austenite dissolution was further evaluated by microstructural characterization using electron backscattered diffraction of isothermally heat-treated and quenched samples. The new experimental data are compared to thermodynamic calculations, and the precision of databases is discussed.     

铝含量对TWIP钢中夹杂物特征及AlN析出行为的影响

采用扫描电镜、X射线能谱仪以及扫描电镜配置的夹杂物自动扫描统计软件(INCAFeature)表征了Fe-Mn-C(-Al)系TWIP钢中夹杂物的成分、形貌和数量,考察了Al质量分数在0.002%~1.590%的四种TWIP钢中夹杂物的特征和Al含量对AlN析出行为的影响.并在此基础上,采用了适合TWIP钢中高锰高铝特点的热力学参数对AlN夹杂物进行了系统的热力学分析.研究表明,在含有相似N质量分数(0.0078%~0.0100%)的TWIP钢中,当钢中Al质量分数升高至0.75%时,AlN夹杂物开始在钢中析出,并在MnS(Se)-Al₂ O₃上局部析出形成MnS(Se)-Al₂ O₃-AlN复合夹杂;当Al质量分数升高至1.07%时,热力学计算表明AlN已经可以在TWIP钢液相中形成,经不断长大后在MnS(Se)夹杂物表面局部析出形成MnS(Se)-AlN复合夹杂物;在Al质量分数为1.59%的TWIP钢中,AlN的平衡析出温度比其液相线温度高出42℃,在液相中形成的AlN可以作为异质核心,MnS(Se)夹杂在其表面包裹形成MnS(Se)-AlN复合夹杂物.另外,在Fe-18.21% Mn-0.64% C-1.59% Al体系的TWIP钢中,AlN在液相中析出所需的最低氮的质量分数仅为0.0043%.因此,在TWIP钢的冶炼过程中,应尽可能的降低钢中的氮含量,避免生成过量的AlN夹杂.      

Optimised sintering route for cold work tool steels

Abstract

Two different powder metallurgy cold work tool steels, D7 and a nickel alloyed D7 modification, were compacted to full density by utilisation of super solidus liquid phase sintering in vacuum and nitrogen atmospheres. The experiments were carried out with respect to the fabrication of wear resistant metal matrix composites. These materials exhibit a higher durability than cast materials and are increasingly used in the mineral and mining industry. In the present paper, sintering exhibits a more cost effective processing of these materials compared with hot isostatic pressing. The experiments were supported by thermodynamic calculations with Thermo-Calc. Significant differences of the sintering behaviour in vacuum and nitrogen atmospheres were detected. Full density could be obtained at a sintering temperature as low as 1210°C.     

ChemSage—A computer program for the calculation of complex chemical equilibria

An extensive computer program called ChemSage, based upon the SOLGASMIX Gibbs energy minimizer, is presented together with several examples which illustrate its use. ChemSage was designed to perform three types of thermochemical calculations in complex systems involving phases exhibiting nonideal mixing properties. These are the calculation of thermodynamic functions, heterogeneous phase equilibria, and steady-state conditions for the simulation of simple multistage reactors. The thermodynamic functions module calculates specific heat, enthalpy, entropy, and Gibbs energy with respect to a chosen reference state for a given phase and, if this phase is a mixture, the partial properties of its components. Chemical equilibrium calculations can be made for a system which has been uniquely defined with respect to temperature, pressure (or volume), and composition. One of these quantities may also be replaced by an extensive property or phase target,e.g., for the calculation of adiabatic and liquidus temperatures, respectively.     

Microstructural evolution in two variants of NF709 at 1023 and 1073 K

The microstructural state of two grades of the creep-resistant austenitic stainless steels NF709, a Fe-20Cr-25Ni (wt pct) based steel, has been studied, in the as-received state, after an additional solution treatment and after static aging at 1023 and 1073 K. Although the two variants are chemically similar, they exhibit different microstructures following identical heat treatment. In particular, the Z phase and σ are found in largely different quantities. The study suggests that a seldom observed variant of the η structure, Cr₃Ni₂SiX, is stabilized by nitrogen. Both steels exhibit a yet undocumented precipitation sequence when compared to more conventional austenitic steels (for which the precipitation sequences have been reviewed by Sourmail (Mater. Sci. Technol., 2001, vol. 17, pp. 1–14)).