Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy

Metallurgical and Materials Transactions A - The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth...     

Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries

Pure iron and a series of iron-based Fe-Me alloys (with Me = Al, Si, Cr, Co, Ni, and Ge) were nitrided in a NH₃/H₂ gas mixture at 923 K (650 °C). Different nitriding potentials were applied to investigate the development of pores under ferrite and austenite stabilizing conditions. In all cases, pores developed in the nitrided microstructure, i.e., also and strikingly pure ferritic iron exhibited pore development. The pore development is shown to be caused by the decomposition of (homogeneous) nitrogen-rich Fe(-Me)-N phase into nitrogen-depleted Fe(-Me)-N phase and molecular N₂ gas. The latter, gas phase can be associated with such high pressure that the surrounding iron-based matrix can yield. Thermodynamic assessments indicate that continued decomposition, i.e., beyond the state where yielding is initiated, is possible. Precipitating alloying-element nitrides, i.e., AlN, CrN, or Si₃N₄, in the diffusion zone below the surface, hinder the formation of pores due to the competition of alloying-element nitride (Me _x N _y ) precipitation and pore (N₂) development; alloying elements reducing the solubility of nitrogen enhance pore formation. No pore formation was observed upon nitriding a single crystalline pure iron specimen, nitrided under ferrite stabilizing conditions, thereby exhibiting the essential function of grain boundaries for nucleation of pores.     

Synergistic Effect of Nitrogen and Refractory Material on TiN Formation and Equiaxed Grain Structure of Ferritic Stainless Steel

The effect of nitrogen content on the formation of an equiaxed solidification structure of Fe-16Cr steel was investigated. Moreover, two different kinds of refractory materials, i.e., alumina and magnesia, were employed to control the type of oxide inclusion. The characteristics of TiN(-oxide) inclusions were quantitatively analyzed in both molten steel and solidified samples. When the melting was carried out in the alumina refractory, the grain size continuously decreased with increasing nitrogen content. However, a minimum grain size was observed at a specific nitrogen content (approx. 150 ppm) when the steel was melted in the magnesia refractory. Most of the single TiN particles had a cuboidal shape and fine irregularly shaped particles were located along the grain boundary due to the microsegregation of Ti at the grain boundary during solidification. The type of TiN-oxide hybrid inclusion was strongly affected by the refractory material where Al₂O₃-TiN and MgAl₂O₄-TiN hybrid-type inclusions were obtained in the alumina and magnesia refractory experiments, respectively. The formation of oxide inclusions was well predicted by thermochemical computations and it was commonly found that oxide particles were initially formed, followed by the nucleation and growth of TiN. When the nitrogen content increased, the number density of TiN linearly increased in the alumina refractory experiments. However, the number of TiN exhibits a maximum at about [N] = 150 ppm, at which a minimum grain size was obtained in the magnesia refractory experiments. Therefore, the larger the number density of TiN, the smaller the primary grain size after solidification. The number density of TiN in the steel melted in the magnesia refractory was greater than that in the steel melted in the alumina refractory at given Ti and N contents, which was due to the lower planar lattice disregistry of MgAl₂O₄-TiN interface rather than that of Al₂O₃-TiN interface. When ?T_TiN (= difference between the TiN precipitation temperature and the liquidus of the steel) was 20 K to 40 K, the number density of effective TiN was maximized and thus, the grain size was minimized after solidification. Finally, although most of the TiN particles were smaller than 1 μm in the molten steel samples irrespective of the nitrogen content, TiN particles larger than 10 μm were observed in the solidified samples when the nitrogen content was greater than 150 ppm. The growth of TiN particles during melting and solidification was well predicted by the combinatorial simulation of the ‘Ostwald ripening model’ based on the Lifshitz–Slyozov–Wagner theory in conjunction with the ‘Diffusion controlled model’ using Ohnaka’s microsegregation equation.     

Phase and Formation/Dissolution of Precipitates in Biomedical Co-Cr-Mo Alloys with Nitrogen Addition

The precipitates in as-cast and heat-treated biomedical Co-28Cr-6Mo-(0 to 0.35)C-(0.15 to 0.25)N alloys (mass pct) were investigated. Heat treatment was carried out at temperatures of 1473?K to 1573?K (1200???C to 1300???C) for holding periods of 0 to 43.2?ks. In the as-cast and heat-treated Co-Cr-Mo-N alloys, no precipitates were detected; nitrogen effectively inhibited the formation of the ??-phase and stabilized the face-centered cubic (fcc) metallic ??-phase. The precipitates observed in the as-cast and heat-treated Co-Cr-Mo-C-N alloys were of the M₂₃X₆ type, M₂X type, ??-phase (M₂T₃X type with a ??-Mn structure), and ??-phase (M₆X-M₁₂X type). Complete precipitate dissolution was detected in the alloys with carbon contents of less than 0.3 mass pct regardless of the nitrogen content. The main precipitates were of the M₂X and M₂₃X₆ types after heat treatment for 1.8 to 43.2?ks. The ??-phase precipitate was detected in the early stage of heat treatment at high temperatures. The formation of the M₂X-type precipitate was enhanced by the addition of nitrogen, although the constitution of the precipitates depended on the balance between the nitrogen and carbon contents and the heat-treatment conditions.     

用氮化铬、氮化锰冶炼高氮钢

 为了研究钢液增氮的新工艺,以钢液增氮的冶金热力学和动力学为基础,在10 kg感应炉上添加氮化铁合金以过饱和氮的方式来提高钢液最终氮含量,并对Cr18Mn18N和Cr23Mn17N两种实验材料的理论氮含量饱和值进行了计算。实验表明,采用氮化铬和氮化锰冶炼,钢液最终氮含量大幅度超过钢液与气相平衡时的氮含量,且氮化铬的增氮效果明显优于氮化锰。采用氮化铬冶炼,氮的收得率和氮的质量分数最高可达89%和098%。     

B443超低碳氮铁素体不锈钢的研究

为开发价格昂贵的SUS304奥氏体不锈钢的替代产品,试验研究了不同含量C N和Nb,Ti稳定化对B443系列铁素体不锈钢再结晶组织、力学性能、点蚀电位的影响.试验结果表明,Nb,Ti双稳定化提高了B443铁素体不锈钢的再结晶温度和点蚀电位.在试验室研究基础上,宝钢不锈钢分公司试制了B443NT超低碳氮铁素体不锈钢.该产品不仅具有与SUS304相当的耐腐蚀性能,而且具有优良的深拉伸性能和满意的焊接性能,可用于运输、建筑、厨具等众多行业.     

铁素体区热轧高温卷取条件下IF钢的织构特征

 研究了铁素体区热轧高温卷取条件下，2种不同成分的IF钢的织构特征。结果表明，高温卷取后，普通Ti IF钢发生了完全再结晶，中心面和1/4面上形成了很强的<111>∥ND再结晶织构；高强Ti IF钢大部分晶粒仍处于轧制状态，只有极少部分晶粒发生了再结晶，中心面和1/4面上形成的织构组分以<110>∥RD为主，<111>∥ND再结晶织构较弱。织构分析表明，2种钢的表面织构均较弱，普通Ti IF钢表面上<001>∥ND组分占优，高强Ti IF钢表面上<110>∥ND组分强度最高。     

Sintering Densification and Microstructure Formation Kinetics in Silicon Nitride Ceramics

Research has been done on how the shrinkage rate and microstructure in the Si₃N₄ Al₂O₃ Y₂O₃ are dependent on temperature and nitrogen pressure during sintering. The shrinkage rate curves alter as the activator content varies. Additional densification may occur above 1800°C because of diffusion through the increased amount of liquid.     

Flux Entrapment and Titanium Nitride Defects in Electroslag Remelting of INCOLOY Alloys 800 and 825

Electroslag remelted (ESR) ingots of INCOLOY alloys 800 and 825 are particularly prone to macroscale slag inclusions and microscale cleanliness issues. Formation of these structures near the ingot surface can cause significant production yield losses (~10 pct) due to the necessity of extensive surface grinding. Slag inclusions from near the outer radius of the toe end of alloy 800 and 825 ingots were found to be approximately 1 to 3 mm in size and have a multiphase microstructure consisting of CaF₂, CaTiO₃, MgAl₂O₄, MgO, and some combination of Ca₁₂Al₁₄O₃₂F₂ and/or Ca₁₂Al₁₄O₃₃. These inclusions were often surrounded by fields of 1- to 10-μm cuboidal TiN particles. A large number of TiN cuboids were observed in the ESR electrode with similar size and morphology to those observed surrounding slag inclusions in the ESR ingots, suggesting that the TiN particles are relics from the ESR electrode production process. Samples taken sequentially throughout the AOD processes showed that the TiN cuboidals that are found in ESR ingots form between tapping the AOD vessel into the AOD ladle and the casting of ESR electrodes.     

熔渣的氮容量及氮在渣气和渣钢中的分配

用铂坩埚测定了熔渣氮容量。在大气条件下,测得(质量分数/%)10CaO-33BaO-49Al₂O₃-8TiO₂和54CaO-21Al₂O₃-6MgO-6SiO₂-13CaF₂2种渣系的氮容量分别为10^-6和10^-5次方数量级。采用氧化镁坩埚测定了氮在钢与炉外精炼常用渣之间的分配系数。     

Experimental and Thermokinetic Simulation Studies on the Formation of Deleterious Zones in Dissimilar Ferritic Steel Weldments

The methods to predict and prevent the formation of hard and soft zones in dissimilar weldments of 9Cr-1Mo and 2¼Cr-1Mo ferritic steels during high-temperature exposure are examined in this article. The computational studies have been carried out using multicomponent diffusion model incorporated in Dictra and validated by experimental methods using EPMA and TEM. Carbon concentration profiles across the interface of the weld joint between the two ferritic steels were simulated in the temperatures ranging from 823 K to 1023 K (from 550 °C to 750 °C) for various time durations using “diffusion in dispersed phase model” in Dictra. When precipitation and diffusion were incorporated into the calculations simultaneously, the agreement was better between the calculated and the experimentally measured values of carbon concentration profiles, type, and volume fractions of carbides in the hard zone and diffusion zone, width, and the activation energy. Calculation results of thermodynamic potentials of carbon in 2¼Cr-1Mo and 9Cr-1Mo steels suggested that the diffusion is driven by the activity gradient of carbon across the joint. The effectiveness of nickel-based diffusion barrier in suppressing the formation of hard and soft zones is demonstrated using calculations based on the cell model incorporated in Dictra.     

邢钢低碳400系铁素体不锈钢AOD控氮工艺实践

在低碳400系不锈钢中,氮能恶化晶间腐蚀、低温冲击韧性、缺口敏感性和焊接等性能。因此,在低耗、高效前提下降低其氮含量成为AOD冶炼中的重要课题。结合生产试验,通过探讨回归分析法在其吹氩脱氮工艺控制中的应用,结合脱氮的热力学计算对AOD生产实践数据的分析,由拟合方程确定的AOD冶炼410S不锈钢和低碳430不锈钢时氮氩切换点为吹氮450 m³时切换氩气,410S不锈钢正常冶炼时一般吹氩量控制在1 550 m³,低碳430不锈钢AOD吹氩量一般控制在2 050 m³时可以将AOD终点氮含量控制在0.009 0%以下,并降低氩气消耗。     

Formation of Structure and Properties of Aluminum Alloys During Roll Casting

Metallurgist - Physical and technological parameters for preparing aluminum alloy thin sheet billets in a two-roll casting and rolling unit are developed. For each stage of the production chain of...     

Effects of Mg Addition on Inclusions Formation and Resultant Solidification Structure Changes of Ti-stabilized Ultra-pure Ferritic Stainless Steel

The effects of Mg addition on the formation of nonmetallic inclusions and solidification structure of Ti-stabilized ultra-pure ferritic stainless steels were investigated by experimentally casting ingots with different compositions.Thermodynamic analyses on the formation of complex inclusions after adding Mg into steels were carried out combined with the scanning electron microscopy-energy dispersive spectrometry(SEM-EDS)analysis.And the EDS analysis showed that in steel samples with Mg addition,a new spinel crystal phase combined with Al2O3-TiOx formed.It was also found that after Mg addition,the proportions of equiaxed grain zone of 409L,4003,439 and 443NT steels increased from 10.2%,21.8%,13.4% and 18.6% to 84.3%,92.3%,91.1% and 100.0%,respectively.Since the planar disregistry between spinel and TiN is 5.1%,spinel could promote the precipitation of TiN and increase the number density of TiN inclusions in steel melts.The mechanism of solidification structure refinement after adding Mg into steels supposed that the complex inclusions of spinel and TiN in high number density enhanced columnar-to-equiaxed transition,since the planer disregistry betweenδphase and spinel is 1.4%.     

Formation,Structure, and Crystallization Behavior of Cu-Based Bulk Glass-Forming Alloys

We studied Cu-Zr–based alloys having exceptionally high glass-forming ability (GFA) and investigated the influence of Ag and Al addition on their structure and crystallization behavior. Most of the bulk glassy alloys (BGAs) do not contain any crystals, while some samples studied by high-resolution transmission electron microscopy (HRTEM) were found to contain well-developed medium-range order zones and nanoparticles in a bulk form. The crystallization kinetics of Cu₅₅Zr₄₅, Cu₅₀Zr₅₀, Cu_55–x Zr₄₅Ag _x (x = 0, 10, 20), Cu₄₅Zr₄₅Al₅Ag₅, Cu₄₄Ag₁₅Zr₃₆Ti₅, and Cu₃₆Zr₄₈Al₈Ag₈ glassy alloys was analyzed. An influence of the cooling rate on the formation of glassy phase and thermal stability of the Cu-based glassy alloys on heating was also studied. The crystallization kinetics and phase composition of the ribbon-shape and bulk glassy samples of Cu₃₆Zr₄₈Al₈Ag₈ alloys were also analyzed. The results also indicate that the best glass-forming compositions are possibly located at slightly off-eutectic area, owing to the shift of the eutectic point due to the nonequilibrium processing conditions.     

Hydrogen Transport and Rationalization of Porosity Formation during Welding of Titanium Alloys

The transport of hydrogen during fusion welding of the titanium alloy Ti-6Al4V is analyzed. A coupled thermodynamic/kinetic treatment is proposed for the mass transport within and around the weld pool. The modeling indicates that hydrogen accumulates in the weld pool as a consequence of the thermodynamic driving forces that arise; a region of hydrogen depletion exists in cooler, surrounding regions in the heat-affected zone and beyond. Coupling with a hydrogen diffusion-controlled bubble growth model is used to simulate bubble growth in the melt and, thus, to make predictions of the hydrogen concentration barrier needed for pore formation. The effects of surface tension of liquid metal and the radius of preexisting microbubble size on the barrier are discussed. The work provides insights into the mechanism of porosity formation in titanium alloys.