Precipitation Kinetics in a Nb-stabilized Ferritic Stainless Steel

The precipitation occurring in a Nb-stabilized ferritic stainless steel, containing initially Nb(C, N) carbonitrides and Fe₃Nb₃X precipitates, was investigated during aging treatments performed between 923 K and 1163 K (650 °C and 890 °C) by combining different techniques, (thermoelectric power (TEP), scanning/transmission electron microscopy (SEM/TEM), and atom probe tomography (APT)), in order to determine the precipitation kinetics, the nature and morphology of the newly formed precipitates as well as the chemistry of the initial Fe₃Nb₃X precipitates, where X stands for C or N. The following composition was proposed for these precipitates: (Fe_0.81 Cr_0.19)₃ (Nb_0.85 Si_0.08 Mo_0.07)₃ (N_0.8 C_0.2), highlighting the simultaneous presence of N and C in the precipitates. With regard to the precipitation in the investigated temperature range, two main phenomena, associated with a hardness decrease, were clearly identified: (i) the precipitation of Fe₂Nb precipitates from the niobium initially present in solution or coming from the progressive dissolution of the Fe₃Nb₃X precipitates and (ii) the precipitation of the χ-phase at grain boundaries for longer aging times. From the TEP kinetics, a time–temperature–precipitation diagram has been proposed.     

The Evolution of Oxygen-Based Inclusions in an Additively Manufactured Super-Duplex Stainless Steel

Super-duplex stainless steel powder feedstocks specified for use in directed energy deposition additive manufacturing processes can have an oxygen composition nearly five times higher than that present in comparable wrought forms. A combination of computational thermodynamic calculations and experimental validation showed that high levels of oxygen promoted the formation of oxygen-rich inclusions during directed energy deposition additive manufacturing. These inclusions play an important role in microstructural evolution during the rapid heating and cooling cycles prevalent in additive manufacturing and impact mechanical and corrosion properties. Inclusions observed across the powder feedstock and additively manufactured and post-processed materials exhibited complex structures with a combination of amorphous, metastable, and stable phases. The powder feedstock, which experiences rapid cooling rates during the gas atomization process, yielded amorphous inclusions that were rich in manganese, chromium, silicon, and oxygen surrounded by small crystalline MnS particles. After additive manufacturing, inclusions transformed to a combination of rhodonite (MnSiO₃) and spinel (MnCr₂O₄) with amorphous regions around the exterior. Post-process hot isostatic pressing treatments, which replicate conditions most similar to equilibrium, resulted in the formation of a stable spinel oxide with MnS particles around the exterior, matching the results predicted by thermodynamic equilibrium calculations.

     

Effect of Precipitation on Intergranular Corrosion Resistance of 430 Ferritic Stainless Steel

With Nb-Ti-stabilized 430 ferritic stainless steel(NTS430FSS) and SUS 430 ferritic stainless steel(SUS430FSS) as experimental materials, the influence of precipitation on intergranular corrosion resistance was investigated. A series of aging treatment were carried out. The free-exposure corrosion test and double loop electrochemical potentiokinetic reactivation(DL-EPR) test with a scan rate of 1.67 m V/s at 26 °C were applied to evaluate the intergranular corrosion(IGC) resistance. Metallographic observation, scanning electron microscope(SEM), transmission electron microscope(TEM) with energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD) analysis were conducted. The results show that IGC occurred in SUS430 FSS aged above 700 °C, while it occurred in NTS430 FSS as the temperature was improved to 1 050 °C. The critical degree of sensitization Ir/Ia reaches 0.305 in SUS430 FSS, which is higher than that of NTS430 FSS, i.e. 0.010, aged at 950 °C for 2 h. The TEM, EDS and XRD results show that a large amount of Cr23C6 precipitates with size of 60 nm×22 nm are located at the SUS430 FSS grain boundaries as chains. With the addition of Nb and Ti and reduction of C, the amount of precipitates reduces significantly in NTS430 FSS. A majority of Cr23C6 were replaced by Ti C and Nb C. Only a small amount of spherical Ti C(R=186 nm) and square Ti N(312 nm×192 nm) with Nb and Cr adsorbed are left along grain boundaries. Due to the dual stabilization of Nb and Ti, the precipitation of Cr23C6 is restrained, the chromium depleted region is avoided and accordingly the resistance to the intergranular corrosion is improved.     

439铁素体不锈钢的研制

439是在430不锈钢基础上降低C＋N含量,添加Nb、Ti等稳定化元素发展起来的新型不锈钢材料,已逐步代替304不锈钢应用于电梯面板等行业。为开发满足用户使用要求的439不锈钢,试验室研究了成分、热处理制度对439不锈钢性能的影响,结果表明：Ti、Nb稳定化元素的添加对改善439不锈钢综合性能比较有利、热轧439退火再冷轧可明显改进深冲性能。生产试制结果表明,宝钢生产的439冷轧产品完全能满足用户对材料的性能要求。     

Heterogeneity and Solidification Pathways in Additively Manufactured 316L Stainless Steels

Metallurgical and Materials Transactions A - A unique microstructural feature often referred to as “fish-scale” has been reported in 316L austenitic stainless steel parts made by laser...     

Effect of Build Height on Temperature Evolution and Thermally Induced Residual Stresses in Plasma Arc Additively Manufactured Stainless Steel

Metallurgical and Materials Transactions A - Plasma arc additive manufacturing (PAM) is receiving an increasing attention because of its efficiency of dimensional size and cost, as compared to...     

Bainite Formation Kinetics During Isothermal Holding in Modified High Cr Ferritic Steel

Microstructural observation and high-resolution dilatometry has been employed to investigate the course of isothermal holding at various temperatures during cooling after austenization in a modified high Cr ferritic steel. The formation of bainite during isothermal holding was identified. The amount of bainite increases as transformation temperature decreases. The phenomenon of thermal stabilization of austenite disappears after isothermal holding due to the formation of bainite. The kinetics of incomplete bainite transformation was described by a displacive model in view of autocatalytic nucleation. Kinetics analysis suggests that reduction of holding temperature promotes to bainite transformation by means of an increase in the number of embryos for autocatalytic nucleation and a decrease in activation energy.     

Effects of Static Recrystallization and Precipitation on Mechanical Properties of 00Cr12 Ferritic Stainless Steel

The 00Cr12 ferritic stainless steel samples were isothermally held at different temperatures in the range of 700 °C to 1000 °C to investigate the effect of static recrystallization and precipitation on mechanical properties, such as microhardness, tensile strength, and yield strength. The results show that the formation of the fine recrystallized grain, as well as precipitation, coarsening, and dissolution of the second-phase particles, influences the mechanical properties remarkably. The fine recrystallized grain can provide a positive grain boundary-strengthening effect in the sample under a relatively high holding temperature. Coarsening and dissolution of M₂₃C₆ result in partial depletion of precipitate hardening. In contrast, the size and number density of MX particles are almost constant, regardless of the holding temperature; therefore, it can provide a better precipitation-hardening effect.     

宝钢铁素体不锈钢产品的开发

简述了不锈钢的产线,针对不同用途铁素体不锈钢系列的开发是建立在Thermo-Calc模拟的基础之上的,因而该软件已成为研发工作的手段之一。根据铁素体不锈钢产品在实际应用过程中的技术要求,该软件可以进行合金体系的设计、析出相的分析以及加工过程工艺参数的优化。所开发的铁素体不锈钢产品系列在强度、延伸率、成形性、焊接性以及耐蚀性等方面具有优良的性能。宝钢已经形成了一个较齐全的铁素体不锈钢产品系列,该产品系列可以对公司的盈利以及产品的多样化起到一定的作用。     

Particle Stimulated Nucleation in Coarse-Grained Ferritic Stainless Steel

Particle-stimulated nucleation (PSN) is investigated in Nb-containing ferritic stainless steel. Coarse-grained sheets were cold rolled to 80 pct thickness reduction and annealed from 973 K to 998 K (700 °C to 725 °C) to obtain partially recrystallized microstructures. Electron backscatter diffraction was performed around coarse niobium carbonitride particles (larger than 1 ??m) within coarse grains (~1 mm), with different host orientations in both deformed and annealed states. In the deformed state, the deformation zones around both spherical and rectangular particles were investigated. The local lattice rotations about the transverse direction necessary to accommodate the particle-matrix strain incompatibility were observed in all grains investigated. After annealing, recrystallization occurs preferentially around coarse particles at the initial stages of recrystallization. Based on a total number of 130 grains nucleated via PSN, we observe both, randomly oriented and minor {111}??110?? oriented texture components. The results also reveal that PSN in this material is not associated with a specific host orientation.     

Transformation and Precipitation Kinetics in 30Cr10Ni Duplex Stainless Steel

To improve the microstructure during casting, hot forming, and heat treatment of 30Cr10Ni duplex stainless steel, accurate data on the precipitation and transformation processes at high temperatures are needed. In this article, the precipitation and transformation processes at various aging times in the temperature range 873 K to 1573 K (600 °C to 1300 °C) were studied. The 30Cr10Ni ferrous alloy contains a relatively large amount of Cr, Ni, and C, which results in a complex microstructure. In addition to the ferrite, austenite, and sigma phase, the M₂₃C₆ and MC carbides were also observed in the microstructure. The precipitation of the sigma phase was observed after just 3 minutes of aging, and after 30 minutes of aging at approximately 1053 K (780 °C), its fraction exceeded 40 pct. An intensive austenite-to-ferrite transformation was observed above 1423 K (1150 °C). Optical microscopy, energy-dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD), as well as micro-indentation hardness, hardness, impact toughness, and tensile tests, were carried out to evaluate the obtained microstructures of aged samples.     

应加快开发能适应不锈钢市场波动的高性能铁素体不锈钢

2005年以来,镍价经历了正常、上涨、暴涨、下跌波动、深跌五个阶段,并严重影响着不锈钢的生产和消费.传统镍不锈钢能较好适应正常的镍、铬、钼和不锈钢市场,但一旦镍价暴涨暴跌就有可能发生亏损;传统铬不锈钢在铬铁价格高涨时毛利下降;而高性能超纯铁素体不锈钢能经受住镍价的波动,在镍价高涨时期尤其能获得可观的效益.虽然目前镍、铬、钼价格已回落到较低水平,但应该不断克服困难,继续开发生产高性能超纯铁素体不锈钢,以适应今后镍价可能的上涨波动.     

Static Recrystallization Kinetics and Crystallographic Texture of Nb-Stabilized Ferritic Stainless Steel Based on Orientation Imaging Microscopy

In the present study, Nb-stabilized ferritic stainless steel was prepared with annealing (430-A) and without annealing (430-NA) annealing, and the microstructure of the resulting samples was examined. The steel was then subjected to cold rolling and isothermal annealing in order to analyze its recrystallization kinetics and texture evolution. Microstructural characterization was performed by scanning and transmission electron microscopies. Recrystallization kinetics were evaluated by measuring the microhardness of the samples, and analyzing their kernel average misorientation and grain orientation spread via electron backscatter diffraction. The Avrami exponent data revealed that one-dimensional grain growth occurred owing to the migration of high-angle grain boundaries. The mean activation energies for recrystallization for 430-NA and 430-A was found to be 365 and 419 kJ mol⁻¹, respectively. The recrystallization texture was influenced by oriented nucleation and selected growth mechanisms, as well as by the Nb carbonitride distribution and grain boundary energy. The recrystallized and growing grains with the {554}〈225〉 orientation showed a dimensional advantage over the other recrystallized components. The coincident site lattice boundaries were attributed to the progression of recrystallization since the CSL numeric fraction increased as the temperature increased. The {554}〈225〉 component was associated with the ∑19a boundary, which exerted a significant control on the selective growth during the recrystallization.

     

Phase Transformation Kinetics During Annealing of Cold-Rolled AISI 309Si Stainless Steel

Metallurgical and Materials Transactions A - The effects of cold rolling and the subsequent annealing were studied for the AISI 309Si stainless steel. During annealing of cold-rolled sheets, the...     

超级铁素体钢S44660脆性现象探讨

由于S44660方坯在轧制后堆冷至室温后发现部分脆断,紧急采用高温固溶也脆断,检验表明轧后有原始晶粒度、高温固溶晶粒度较粗大,晶界断裂,稳定化元素Ti高等,为此在50kg感应炉试验,采用Ti+Nb稳定化元素、降低加热及终锻温度、快冷等措施消除了脆性,细化了晶粒,提高了韧性.     

超纯铁素体不锈钢热轧生产控制方法

结合宝钢不锈钢事业部生产不锈钢产品经验,说明了超纯铁素体不锈钢在热轧生产过程中存在的主要质量问题,同时从轧制计划编排、关键工艺控制、轧辊材质使用、设备功能状态等多个角度详细阐述了热轧生产的具体控制要求。着重从解决超纯铁素体不锈钢轧制过程中存在的黏结现象着手,对关键工艺控制点提出了合理的控制范围,并将理论分析与大生产实践经验相结合对其控制原理进行了解释和说明。     

Precipitation Kinetics of Cr2N in High Nitrogen Austenitic Stainless Steel

The precipitation behavior of Cr2 N during isothermal aging in the temperature range from 700 ℃ to 950 ℃ in Fe-18Cr-12Mn-0.48N （in mass percent） high nitrogen austenitic stainless steel, including morphology and content of precipitate, was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The isothermal precipitation kinetics curve of Cr2 N and the corresponding precipitation activation energy were obtained. The results show that Cr2N phase precipitates in a cellular way and its morphology is transformed from initial granular precipitates to lamellar ones in the cell with increasing aging time. The nose temperature of Cr2 N precipitation is about 800 ℃, with a corresponding incubation period of 30 min, and the ceiling temperature of Cr2N precipitation is 950℃. The diffusionactivation energy of Cr2 N precipitation is 296 kJ/mol.