On the Formation of Interdendritic Internal Cracks During Dendritic Solidification of Continuously Cast Steel Slabs

In this study of solidification during the continuous casting of steel slabs, the effect of the different spray cooling conditions on the interdendritic internal cracks formed between the columnar dendrites has been examined by a metallographic study of the slab samples and by performing a set of mathematical analyses. The metallographic study involved plant trials to measure the slab surface temperature within different secondary spray cooling conditions. Also, macro/microexaminations of the collected samples from plant trials, measurements of dendrite arm spacing, and interdendritic distance between the columnar dendrites, as well as a segment length of interdendritic crack, have been performed. The experimental results show that the morphology of the interdendritic cracks described by the segment width and length fluctuate with the distance from the slab surface based on the secondary spray cooling conditions. A one-dimensional mathematical model of the heat transfer, solidification, structure evolution, interdendritic strain, and elementary interdendritic area (EIA) has been developed. This model takes into account also calculating the width of interdendritic crack. The model predictions are in a good agreement with the measurements. The results pointed out also that this criterion can be considered as the most important tool to measure the inner quality of the continuously cast steel slabs. Therefore, it helps also to define the required mechanism and reduction level of hot working deformation to close these interdendritic internal cracks. The formation mechanism of these cracks during the dendritic solidification of continuously cast steel slabs has been discussed and the available solutions have been proposed.     

Observations of the columnar-to-equiaxed transition in stainless steels

Observations are reported for the columnar-to-equiaxed transition (CET) in stainless steel bars which have been solidified slowly and progressively in a horizontal configuration. For ferritic, austenitic, and ferritic/austenitic stainless steels containing more than 0.085 wt pct carbon, CETs occur at about the same distance from the start of solidification at a given growth rate. With increasing growth rates, the transition occurs closer to the start of solidification. At low carbon levels, near 0.02 wt pct carbon, the ferritic/austenitic steel is entirely columnar, in most cases. Adding nickel to the ferritic/austenitic steel, which makes the leading phase austenitic, produces a CET with small equiaxed grains. This suggests that different particles which are effective with austenitic growth become operative as nucleants. The transition from a columnar to an equiaxed structure occurs abruptly across the diameter of the sample. There is extensive fluid flow in the bulk melt, which produces shallow temperature gradients in the melt prior to the onset of solidification. The bulk melt flow does not appear to interact significantly with the melt in the interdendritic region or the melt immediately ahead of this region. The width of the solid/liquid region in the present experiments is observed to be between 10 and 20 mm, depending on the growth velocity and the distance from the start of solidification.     

热喷涂制备高熵合金涂层的研究现状与展望

首先介绍了高熵合金的理论基础。然后从不同的热喷涂工艺出发,综述了等离子喷涂、超音速火焰喷涂、高速电弧喷涂、冷喷涂四种技术在制备高熵合金涂层上的研究发展现状,重点从原料选用、制备工艺优化、性能研究、后处理工艺等方面对以上四种热喷涂技术制备高熵合金涂层的研究进行系统地归纳与总结。最后提出现有制备高熵合金涂层的热喷涂技术较少、热喷涂材料受限、高熵合金设计盲目这三个问题,针对性地提出了在优化已有技术的基础上开发新技术;开发高熵陶瓷、高熵非晶合金、高熵复合材料等新型热喷涂材料;沿用材料基因组理念建立高熵合金数据库这三点热喷涂制备高熵合金涂层在未来的发展趋势。      

Corrosion Resistance of Reactive Element Aluminide Diffusion Coatings on a Nickel‐based Superalloy and an Austenitic Stainless Steel

Continuously cast aluminium alloy wires containing active elements such as yttrium, cerium, lanthanum or silicon were used as evaporation sources for ion vapour deposition on nickel‐based Inconel IN738 and stainless steel 310S substrate materials. The samples were subsequently diffusion heat‐treated to form protective corrosion resistant nickel aluminide and iron aluminide coatings. The coated alloy materials were exposed for prolonged periods to high temperature cyclic oxidation, molten sulphate, and molten carbonate environments to evaluate their resistance to hot oxidation. For IN738, the lanthanum‐modified aluminide coating provided good thermal cyclic oxidation performance while the cerium‐modified coating exhibited effective hot corrosion resistance in a molten sulphate environment. For 310S stainless steel, all of the modified aluminide coatings provided equally good corrosion protection when exposed to molten carbonate attack under an oxidizing atmosphere.     

Segregation at halfway cracks in continuously cast steel

The mechanism of hot-short embrittlement in a continuously cast steel was explored through application of Auger electron spectroscopy (AES) and Secondary Ion Mass Spectrometry. The AES results demonstrated that the interdendritic fracture surface of previously cracked specimens were highly oxidized with a relatively high sulfur concentration. These results were confirmed with the ion microprobe. AES results showing sulfur segregation to intact interdendritic interfaces which were free of oxidation, demonstrated that the embrittlement was due to sulfur, not oxygen. A wide band of low level phosphorus segregation corresponding to the interdendritic regions was also detected with the ion microprobe.     

Effect of different mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mo contents of 0, 2, and 4 pct, respectively, were examined in the present study. The polarization and the in-situ slow-strain-rate (SSR) tests were conducted in a 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking (SCC) with different Mo contents. The addition of Mo, which is a strong ferrite stabilizer, affected the microstructure of the present alloy and, eventually, the tensile and corrosion behaviors in a complex manner. The tensile properties of CD4MCU cast duplex stainless steel, for example, were found to be determined by the volume fraction of hard ferritic phase, the presence of the second precipitates of soft austenitic phase in the ferrite matrix, and the shape of the austenitic phase. The addition of 2 pct Mo was detrimental to the corrosion properties of CD4MCU cast duplex stainless steel due to the significant increase in the volume fraction of ferritic phase. With the addition of 4 pct Mo, however, the resistances to pitting corrosion and SCC recovered to those of the specimen without Mo. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the micrographic and fractographic observations.     

等离子合金化AlCoCrCuFe_xMnNi高熵合金涂层的组织与性能

在45#钢基体上采用等离子束合金化法制备AlCoCrCuFe_xMnNi高熵合金涂层。采用SEM,EDS,XRD等研究高熵合金涂层的组织,利用显微硬度计测试涂层的显微硬度分布。结果表明:采用等离子束合金化Al,Co,Cr,Cu,Mn和Ni等摩尔单质金属粉,在等离子束作用下45#钢基材中的Fe元素参与表面合金化,形成了厚度约为1 mm的AlCoCrCuFe_xMnNi七主元高熵合金涂层,涂层主要由BCC结构的枝晶和FCC结构的枝晶间组织组成。另外,还有σ相主要分布在枝晶间,涂层从表面到基材,体系的混合熵呈高熵-中熵-低熵梯度变化。涂层的维氏显微硬度(HV0.2)达到670~400的梯度分布。     

Stellite12和Stellite20涂层的耐磨性和耐腐蚀性能对比研究

利用超音速火焰喷涂技术(High-velocity-oxy-fuel,HVOF)在F316不锈钢表面制备Stellite 12和Stellite 20两种钴基合金涂层,对比研究了涂层的摩擦磨损性能以及在不同浓度硫酸溶液中耐腐蚀性.采用HT2101销盘磨损试验仪,进行了摩擦磨损试验.利用电子探针仪(EPMA)观察涂层的微...     

The Role of Carbon in Grain Refinement of Cast CrFeCoNi High-Entropy Alloys

As a promising engineering material, high-entropy alloys (HEAs) CrFeCoNi system has attracted extensive attention worldwide. Their cast alloys are of great importance because of their great formability of complex components, which can be further improved through the transition of the columnar to equiaxed grains and grain refinement. In the current work, the influence of C contents on the grain structures and mechanical properties of the as-cast high-entropy alloy CrFeCoNi was chosen as the target and systematically studied via a hybrid approach of the experiments and thermodynamic calculations. The alloys with various C additions were prepared by arc melting and drop cast. The as-cast macrostructure and microstructure were characterized using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The cast HEAs transform from coarse columnar grains into equiaxed grains with the C level increased to ≥ 2 at. pct and the size of equiaxed grains is further decreased with the increasing C addition. It is revealed that the interdendritic segregation of Cr and C results in grain boundary precipitation of M₂₃C₆ carbides. The grain refinement is attributed to the additional constitutional supercoiling from the C addition. The yield stress and tensile strength at room temperature are improved due to the transition of columnar to equiaxed grains and grain refinement.     

Effect of dendrite coarsening on the solidification of interdendritic liquid in iron alloys

The solidification of the interdendritic liquid in austenitic 110G13L steel and white cast iron is studied. In the absence of dendrite coarsening, the solidification mechanism of the interdendritic liquid in the manganese steel is shown to change and solidification occurs in the form of polycrystalline aggregates around dendrites from different centers. The relation between the standard solidification of the interdendritic liquid and the dendrite coarsening in iron alloys is grounded.     

氧化膜对14Cr12Ni2WMoVNb钢QPQ渗层的室温摩擦磨损和腐蚀性能的影响

为了研究14Cr12Ni2WMoVNb钢QPQ(淬火-抛光-淬火)处理后的氧化膜对渗层室温摩擦磨损和腐蚀性能的影响,利用金相、X射线衍射分析、扫描电镜、能谱分析、划痕仪、摩擦磨损试验机和电化学工作站对试样进行了表征.结果表明:氧化膜对渗层室温摩擦学性能的影响与载荷大小有关.在摩擦时间均为4 min情况下,载荷较小(50 N)时,氧化膜可以降低摩擦系数和体积磨损率;载荷较大(100 N)时,氧化膜被破坏无法降低体积磨损率.氧化膜可明显提高渗层的耐腐蚀性能.含氧化膜试样的极化曲线有明显的钝化区,点蚀电位为-13 mV,去除氧化膜试样在盐雾腐蚀12 h后表面有大范围的腐蚀区域,而含氧化膜试样盐雾腐蚀48 h后才有大区域腐蚀发生.      

激光熔覆高熵合金涂层组织结构及强化机理研究进展

激光熔覆作为一种绿色、高效的表面处理技术,能够快速制备组织致密、晶粒细小,与基体呈高强度冶金 结合的涂层,是近年来高熵合金领域的研究热点之一。概述了现有高熵合金涂层材料体系和制备方法,重点讨论 了激光熔覆CoCrFeNi-M 典型过渡族高熵合金涂层的组织结构,及其耐磨、耐蚀、抗高温氧化等性能,并归纳了 涂层的强化机制和方法。CoCrFeNi-M 系合金涂层主要呈现FCC 固溶体结构,综合力学性能普遍较好,通过合金 体系调控,在细晶强化、固溶强化、第二相强化等作用下,能够获得硬度、耐磨性、耐蚀性等性能的进一步提升。 同时,概述了激光熔覆难熔高熵合金涂层的组织结构,耐磨、耐蚀、抗高温氧化性能及性能强化机制,该体系合 金涂层主要呈现BCC 固溶体结构,硬度较高但室温韧性普遍不足,具有较好的高温强度,在高温领域具有较好 的应用前景,但抗高温氧化性能普遍不足,仍需通过合金体系优化进一步提升。此外,总结了基于激光熔覆技术 开展的高熵合金涂层制备及研究中存在的问题和不足,并展望了未来的发展方向。     

Columnar to equiaxed transition of metals during solidification by using electric current pulse

 A new approach of applying the Electric Current Pulse (ECP) with parallel electrodes to promotion of the transition from columnar to equiaxed crystals and improvement of macrosegregation was introduced. And the experiments of application of ECP in the different stage of solidification have been carried out. The results show that applying ECP in both the initial and the late stage of solidification can promote the columnar to equiaxed transition (CET). Analysis shows that during solidification application of ECP induce a large number of nuclei around the upper surface, which subsequently showers down into the melt and impinges growth front of the columnar crystal. Therefore the CET occurs. In addition, this method is also employed to influence the solidification processing of bearing steel, the results show that the structrue is changed from columnar to equaixed, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.     

热老化对核电主管道Z3CN20.09M不锈钢点蚀性能的影响

采用极化曲线和电化学交流阻抗法研究了400℃长期热老化对压水堆核电站一回路主管道用Z3CN20.09M铸造奥氏体不锈钢点蚀性能的影响.结果显示,材料的点蚀击破电位和电荷转移电阻随着热老化时间的延长而降低.热老化材料经550℃退火处理1 h后的点蚀击破电位和电荷转移电阻值均恢复到未老化材料的水平.研究表明此时α'相已经回溶,G相仍然存在,导致热老化试样点蚀性能下降的主要原因是α'相的析出.      

3Cr钢在油水两相层流工况下的腐蚀行为

油水两相是海底管道和集输管线常见的腐蚀工况之一。以3Cr钢为代表的低Cr合金钢是目前具有良好耐蚀性能的重要材料,但是,在油水两相层流工况下,特别是加注了一定缓蚀剂的条件下,3Cr钢的适用性尚不明确。通过高温高压反应釜模拟了油水两相层流工况的腐蚀环境,结合扫描电子显微镜（SEM）、X射线衍射谱（XRD）、激光共聚焦拉曼光谱、电化学交流阻抗等测试表征方法,研究了3Cr钢的腐蚀行为及缓蚀剂对其耐蚀性能的影响。结果表明,在油水分层工况下,3Cr钢的腐蚀产物膜为明显的双层膜结构,其内层腐蚀产物膜为结构致密的富Cr层,表现出良好的抗CO₂腐蚀性能,但加入100 mg·L?1十七烯基胺乙基咪唑啉季铵盐缓蚀剂后,3Cr钢并未得到有效的缓蚀保护。腐蚀产物分析和电化学研究表明,烷烃分子、缓蚀剂分子及富Cr层间存在竞争关系,烷烃分子干扰了缓蚀剂分子的有序排列,影响了3Cr钢的耐蚀性。      

Columnar to Equiaxed Transition During Solidification of Small Ingot by Using Electric Current Pulse

A new approach to applying the electric current pulse (ECP)with parallel electrodes to the promotion of the transition from columnar crystal to equiaxed crystal and the improvement of macrosegregation was introduced. The ECP was applied to different stages of the solidification. The results showed that the application of the ECP in both the initial stage (the thickness of solidified shell reached 2 mm approximately)and the late stage (the thickness of solidified shell reached 14 mm approximately)of solidification can promote the columnar to equiaxed transition (CET). The analysis showed that during solidification, a large number of nuclei around the upper surface fell off due to ECP, which subsequently showered on the melt and impinged the growth front of the columnar crystal. There-fore, the CET occurred. In addition, this method was also employed to influence the solidification process of bearing steel, and the results showed that the structure was changed from columnar crystal to equiaxed crystal, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.     

Simulation of Secondary Dendritic Arm Spacing of Free Cutting Steel 38MnVS

 The secondary dendritic arm spacing (SDAS) of free cutting steel 38MnVS during continuous casting process was simulated based on a Mixed Lagrangian and Eulerian Method (MiLE Method) and SDAS model. The simulation results are basically in agreement with measured ones. The effect of composition, superheat and casting speed on SDAS are studied, and the relationship between SDAS and interdendritic segregation is discussed. The results show that SDAS increases with increasing carbon and silicon content, and decreases with increasing manganese and sulfur content. The increase of superheat and casting speed also makes SDAS increase. The permeability of columnar mushy zone which is parallel to the primary dendritic arms is calculated based on Carman Kozeny relationship, and the increase of SDAS makes the permeability increase, which exacerbates interdendritic segregation of columnar crystal zone.     

多功能水性低表面处理涂料及涂层防护性能

基于锈蚀碳钢表面,针对一种多功能型的酸性水性低表面处理涂料中主要防锈组分和配制工艺对涂料和涂层性能的影响进行研究,考察涂层的防护性能,并与同类进口涂层进行对比.结果表明:在基材表面处理等级为St1时,涂层附着力随着防锈组分含量的增加均呈现先上升再下降的趋势,螯合剂所含活性基团稳定了基材表面锈蚀,带来涂层附着性能的提高.复合锈转化剂将基材表面锈蚀络合转化,进一步提高涂层附着力.最佳配制工艺为后加复合锈转化剂与低速搅拌相结合.所得涂层在基材表面处理等级分别为St1和St3时附着性能无明显差异,附着力达5 MPa.涂层耐蚀性能也无明显差异:耐盐水1200 h,耐盐雾1000 h,耐紫外老化2000 h,远高于进口同类水性带锈底漆,适合应用于大气环境中复杂钢铁构件的维护.      

Prediction of Columnar to Equiaxed Transition during Diffusion-Controlled Dendritic Alloy Solidification

A model is presented to predict the columnar to equiaxed transition (CET) in alloy castings. The model is based on a multiphase approach and accounts for heat and solute diffusion, as well as for grain nucleation, growth, and morphology. The model equations are applicable to both columnar and equiaxed dendritic solidification, thus offering an efficient single-domain formulation. A fixed grid, fully implicit finite-difference procedure is employed in the numerical solution, and a novel front tracking technique is incorporated that is also implicit in nature and readily applies to multidimensional situations. Calculations are performed for one-dimensional (1-D) and two-dimensional (2-D) castings of Al-Cu and Sn-Pb alloys. The calculated CET positions are compared with previous measurements in a (1-D) ingot cast under well-controlled conditions, and good agreement is found. The effects of various casting parameters on the CET are numerically explored.     

低压冷喷涂铸造缺陷修复涂层性能研究

低压冷喷涂(LPCS)是一种拥有便携式冷喷涂系统的涂层技术。例如DYMET304K系统就应用于这项涂层技术中。通常情况下,压缩空气作为冷喷涂工艺中的运载气体。低压冷喷涂适用于喷涂金属基陶瓷复合粉末,如Cu基、Ni基、Zn基、Al基添加Al2O3粉的复合粉。硬质陶瓷相主要起到清洁喷嘴、增加表面活性和喷丸强化的作用,该方法在尺寸修复领域中具有优势。在这个领域里,修复铸造加工中的缺陷和气孔是一个很热门的应用。例如,Zn基复合材料就适用于防止电化学腐蚀和修复机械损伤造成的尺寸差异。本文对Zn+Al+Al2O3,Zn+Cu+Al2O3和Zn+Ni+Al2O3等复合材料做了实验研究。Zn和Al在腐蚀环境中起到阴极保护的作用,而Cu和Ni也有助于提高材料的机械性能。经过对微观孔蚀电位反应和力学性能(硬度和结合强度)的研究发现,涂层具有相对致密的结构和耐蚀性能。Zn在复合涂层中对其它金属起到阴极保护的作用。此外,在Fe52型铁基材料上的涂层有着足够的力学性能,硬度和结合强度较高。这一类涂层在修复宏观的铸造缺陷上具有很高的潜在应用价值。