Phase composition and residual stresses in thermal barrier coatings

The phase composition and the residual stresses in multilayer thermal barrier coatings, which consist of an external ZrO₂–8Y₂O₃ ceramic layer, an intermediate gradient (metal ceramic) layer, and a transient metallic NiCrAlY sublayer, are studied. It is shown that an increase in the specific volume of the metallic sublayer as a result of the formation of thermal growing oxide Al₂O₃ generates high compressive stresses in this sublayer. The ceramic layer undergoes tensile stresses in this case. A method is proposed to estimate the stresses in gradient coatings from X-ray diffraction results.     

Calculation of residual stresses in case-hardened SAE5115 steel cylinders

The development of stresses for five differently case-hardened SAE5115 steel cylinders with variable surface carbon mass contents c_S and carburization depths CD during single hardening are calculated. The carbon profiles and the retained austenite distributions in the near-surface material areas are approximated in a stepwise manner. Exemplarily, in the middle plane of cylinders with c_S = 0.5 % C (CD = 0.07 mm) and 1.1 % C (CD = 0.2 mm), the local axial stresses at the surface, at four near-surface points and in the core are described during the hardening process up to temperature balance. The influence of c_S and CD on the surface tangential residual stress values along the casing of the cylinders is outlined. Also the depths distributions of the tangential residual stresses in the middle plane are presented for differently carburized cylinders. At c_S = 1.1 % C = const. the distance of maximum compressive residual stresses beneath the surface as well as the thickness of the compressed surface and the distances with the largest tensile residual stresses increase with increasing CD.     

Phase transformations in a corrosion-resistant high-chromium nitrogen-bearing steel

The structure, phase composition, and mechanical properties of an austenitic corrosion-resistant high-chromium nitrogen-bearing (～0.5% N) steel are studied in the as-cast state and after homogenizing heat treatment (HT) followed by quenching. The main structural constituents of the as-cast steel are austenite and the σ phase (12%), which forms as an interdendritic metal during solidification, and δ ferrite and M ₂₃C₆-type chromium carbides are absent. Homogenizing HT at 1100–1200°C leads to the σ → γ transformation through the stage of the formation of intermediate δ ferrite via the restructuring of the tetragonal into the bcc lattice. Upon long-term homogenizing HT, the chromium concentration in ferrite decreases due to diffusion chromium redistribution and the δ → γ transformation takes place. The austenite in both the as-cast steel and the steel subjected to homogenizing HT followed by water quenching contains numerous (Cr, V)N nanoparticles. The twofold yield strength of this steel (～400 MPa) as compared to nitrogen-free 18Cr-10Ni-type steels can be explained by not only the solid-solution hardening of austenite by nitrogen but also by precipitation hardening.     

Controlling residual stresses in 52100 bearing steel by heat treatment

Compressive residual stresses can be induced in the surface of quenched and tempered 52100 steel (1 pct carbon, 1.5 pct chromium) by austenitizing in a carburizing atmosphere, even though the austenitizing temperature is well below that needed to dissolve all pri-mary carbides. The carburized surface layer contains a larger volume fraction of pri-mary carbides, more retained austenite and is slightly harder than the interior. Rolling contact fatigue tests show that carburizing can produce a 50 pct improvement in the fa-tigue life of 52100 steel.     

喷射沉积H13钢的组织和硬度

用传统铸造和喷射成形工艺制备了H13钢,然后再进行锻造加工,利用光学金相显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射分析(XRD)等分析方法及硬度测试对不同工艺制备的H13钢的显微组织和硬度进行分析.研究结果表明,喷射成形及其锻造加工的H13钢的组织及硬度明显的优于传统工艺制备的H13钢.     

Phase transformations at steel/IN626 clad interfaces

The microstructures of 4130 and 2.25Cr-1Mo steels clad to nickel base IN625 by welding and HIPing were examined by Analytical Electron Microscopy (AEM) and Secondary Ion Mass Spectroscopy (SIMS) to determine the interfacial microstructural characteristics which could affect their mechanical properties and corrosion resistance. The interface microstructures of the clads produced by the two methods were considerably different. The clad produced by welding was characterized by a low density of carbide precipitates confined to a very narrow region (∼1 μm) at the interface of ferrite and austenite. In addition, a thin region of untempered martensite was present at the interface which could affect its resistance to hydrogen embrittlement as well as other mechanical properties. The interface of the HIP clad composite contained several regions of distinct microstructural characteristics with widely varying densities of carbide precipitates. Relative to the clad produced by welding, extensive precipitation was observed both in the steel and in the IN625 at the interface, separated by a region free from precipitation. The extent of precipitation at the interface regions appears to be controlled essentially by the extent of carbon transport across the interface. The article describes the detailed analysis of the interface characteristics, and models are proposed to explain the microstructural evolution at the interface of the HIP and weld clad composites.     

FeCrAl不锈钢的平衡凝固相变与析出行为

借助Thermo-calc软件对FeCrAl不锈钢所属的Fe-（18~21） Cr-（3~5） Al-（0~0.03） C-（0~0.2） Si-（0~0.2） Mn多元体系在凝固过程中的相变及析出行为进行了研究.采用Thermo-calc中TCFE7数据库对该体系的垂直截面图进行计算,分析了不同组元对凝固和冷却过程中相变的影响,并得到FeCrAl不锈钢的平衡凝固相变路径图.结果表明FeCrAl不锈钢由1600℃平衡冷却至300℃的过程中完整的平衡相变路径为：L→AlN+αδFe→AlN+αδFe+Cr₇C₃→AlN+αδFe+Cr₇C₃+Cr₂₃C₆→AlN+αδFe+Cr₂₃C₆→AlN+αδFe+Cr₂₃C₆+σ→AlN+αδFe+Cr₂₃C₆+σ+α'→AlN+αδFe+Cr₂₃C₆+α'.凝固过程中Cr₇C₃与σ相是否析出分别取决于体系中C、Si含量;Al含量的提高可扩大αδFe+Cr₇C₃的稳定区,降低α'相的析出温度,抑制σ相的析出;Cr含量的提高可以减小αδFe+Cr₇C₃的稳定区,扩大σ相和α'相的稳定区.     

Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M₂₃C₆, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M₆C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M₂₃C₆ on the basis of their lattice parameters and chemistries, with M₆C containing less Cr and Fe, and more Ni, Mo, and Si than M₂₃C₆. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M₂₃C₆ and M₆C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M₇C₃ carbides, which nucleated at Z-phase/austenite interfaces; M₂₃C₆ carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.     

Influence of heat transfer on the development of residual stresses in quenched steel cylinders

In this paper results of systematic FE-calculations about the influence of characteristic points of the temperature dependent heat transfer coefficient, especially the Leidenfrost point and the point of maximum heat transfer coefficient on the development of residual stresses are discussed. The numerical investigations were carried out for SAE 1045 and 4140 steel cylinders with 10 and 20 mm 0 quenched in water and oil, respectively. In this work experimentally determined h, T-curves are linearly approximated in the successive stages of heat transfer. Changes of the Leidenfrost-temperature do not influence the middle plane residual stresses of the cylinders investigated. Increasing maximum heat transfer coefficients and low temperatures of maximum heat transfer coefficient, respectively, cause higher magnitudes of residual stress. The development of residual stresses is determined by the temperature dependent gradient of the heat flux density δq/δT in the temperature range of martensitic transformation. Increasing Leidenfrost-temperatures cause more homogeneous stress and residual stress states at the surface of quenched cylinders due to the symmetrical cooling of the sample in axial as well as in radial direction. In particular, it was shown that during immersion cooling of cylindrical parts the heat transfer is locally dependent. Simulating immersion cooling this dependence has to be considered using effective local heat transfer coefficients.     

Microstructure and forming characteristics of spray-formed M42 high speed steel

M42 high speed steel(HSS),a high grade alloy,was produced using the spray forming technology.Optical metallography(OM),X-ray diffraction(XRD) and scanning electron microscope(SEM) were used to investigate the microstructures of the as-sprayed M42 HSS and the as-atomized powders.It is found that as-sprayed M42 HSS is composed of martensite,retained austenite and carbides.The grain size,as well as the morphology,size and distribution of the carbides have been greatly improved compared with those of the as-cast M42 steel(a traditional process).The size of the powder obtained at a high cold speed was ranged from 50 μm to 100 μm in diameter.Its representative microstructures include dendrite,dendritic fragments,a mixture of dendrites,equiaxed grains and dendritic fragments,and equiaxed grains.The rapid solidification in the spray forming is a key factor to cause the refinement of the M42 HSS.     

Microstructure characteristics of spray-formed high vanadium and cobalt high speed steel

The microstructure of high vanadium and cobalt high speed steel(high-V/Co HSS) and the morphology of its carbides were analyzed by optical microscope(OM),scanning electron microscope(SEM),electron probe microanalysis(EPMA),X-ray power diffraction(XRD),atomic force microscopy(AFM) and single phase erosion(SPE).The results suggest that the as-sprayed high-V/Co HSS has fine equiaxed grains(about 20 μm in size),which were homogeneously distributed.The carbides have two classical morphologies:one is fine particles(about 2 μm in size) distributed along the grain boundaries and the other is needle-like one,distributed on the grain boundaries.There are MC carbides,M2C carbides,M6C carbides and Cr2WC2 carbides in the as-sprayed high-V/Co HSS samples,however,in the as-cast high-V/Co HSS,there are MC carbides and Cr2WC2 carbides only.The SPE results show that there are two types of MC carbides in the as-sprayed HSS:the sphere one and the particle-like one.The former is about 2 μm in size and the latter is less than 1 μm,dispersed inside the grains,quite different from the MC carbides in the as-cast HSS.According to the AFM results,the skeleton-like M6C carbides of the as-sprayed high-V/Co HSS are embedded in the matrix along the grain boundaries.It is found that there are sharp membrane pieces on the carbides.Some small bamboo-shoot-like MC carbides grow from the matrix and are dispersed inside the grain.Those larger MC carbides are spherical particles embedded at the grain boundary junctions.     

大规格调质材残余应力测试分析及稳定调质工艺

42CrMo系列≥Φ500mm大规格调质锻材回火后锯切时夹锯及锯切至心部产生炸裂,通过残余应力测试后,观察各向应力数值差值较大,且分布不均衡,经过系统分析明确锯切时开始夹锯、最后炸裂问题产生的根本原因、稳定调质工艺,包括水温、水冷时间、水冷后返热温度、回火冷却速度,回火快冷后补充回火。     

中厚板冷却过程中热残余应力的控制

确定了中厚板ACC冷却系统的换热边界条件,建立了钢板温度场和应力场有限元计算模型.利用现场实测数据对温度场计算结果进行验证,利用间接耦合方法对钢板的应力场进行计算.分析了不同集管开启方式、不同辊道速度和不同冷却介质温度对钢板热残余应力的影响规律.     

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

The solidified carbide morphology, the decomposition behavior of the M₂C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M₂C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M₂C carbides formed during solidification have been fully decomposed into MC and M₆C carbides after sufficient annealing at high temperatures. Initially, the M₆C carbides nucleate at M₂C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M₆C carbides or at the interfaces between M₆C carbides. With this increasing degree of decomposition of the M₂C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M₂C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.