Quantitative aspects of precipitation at grain boundaries in an austenitic stainless steel

This paper shows, in a quantitative manner, how the precipitation of niobium carbide in an austenitic stainless steel is affected by varying the amount of deformation prior to ageing. In particular, the extrinsic dislocation content of grain boundaries is shown to govern the overall size distribution of grain-boundary precipitates developed during ageing.     

Modelling precipitation of niobium carbide in austenite: multicomponent diffusion,capillarity, and coarsening

Abstract

The growth of niobium carbide in austenite involves the diffusion of both niobium and carbon. These elements diffuse at very different rates. A model is presented for the overall transformation kinetics of niobium carbide precipitation in austenite that takes into account the multicomponent nature of the diffusion process while at the same time allows for the curvature of the transformation front. The inclusion of the curvature (capillarity) effect has, in a numerical scheme, permitted the precipitation and coarsening reactions to be treated in a single model. The model is compared with published experimental data.     

Kinetics of niobium carbide precipitation in a low carbon austenitic steel

An examination has been made of the kinetics of niobium carbide precipitation in a 18-10-1 austenitic stainless steel in the temperature range 650 to 750° C. Electrical resistance-time plots, thin film electron microscopy and hardness measurements have been employed to follow the ageing sequence. In these alloys the carbides precipitate on undissociated dislocations and in association with stacking faults; these processes are diffusion controlled and have an activation energy of ～318 kJ mol^?1. Prior to the reaction beginning a clear incubation period existed, e.g. ～30 h at 650° C and 20 min at 750° C. During the first 10% transformation the carbide nucleation rate increases and the associated faults nucleate and grow rapidly. The carbide nucleation rate appears to peak around this level and then falls away gradually to zero around 70% transformation. At this latter stage fault growth ceases, and transformation continues by a carbide growth process. The age-hardening peak occurs much beyond the end of the reaction by which time precipitate coarsening is in evidence. The precise effect mechanical deformation has upon stacking fault formation depends to a major extent on the niobium supersaturation in the quenched alloy.     

Effects of Si content on the microstructure of modified-HP austenitic steels

The microstructures of two centrifugally cast modified-HP steels with 1.97 wt.% Nb and two different Si contents, 1.84 and 2.62 wt.%, were investigated using light and electron microscopy and x-ray diffraction. As noted in a previous study, the instability of the niobium carbide at elevated temperature is a particular feature of these alloys, exhibiting a partial transformation to a nickel-niobium silicide, G phase. The time-temperature-transformation diagram shows that the time to reach the nose of the transformation curve decreases with increase in silicon content. After aging at 900°C for 1000 h, the alloy with higher silicon content showed extensive secondary precipitation, and at teh same time the niobium carbide particles were almost completely transformed to G phase.     

Indirect detection of grain-boundary segregation of carbon in iron-chromium alloys (Cr 10%) by electrochemical study

The electrochemical behaviour of ferritic iron-chromium alloys is strongly dependent on the quantity (even in trace amounts), the localization in the microstructure and the chemical state of impurities. By comparing the direct and reverse anodic curves of alloys with different microstructures, it is possible to distinguish the respective influences of carbon segregation and carbide precipitation on the formation and the destruction of passive film. Reciprocally, in some cases, the anodic curves can be used to detect micro segregations at grain boundaries whereas no microanalysis can yet achieve this. This possibility is very interesting because many properties of alloys are largely determined by the presence of grain-boundary segregation without precipitation.     

Structure of centrifugally cast austenitic stainless steels: Part 2 Effects of Nb,Ti, and Zr

Abstract

The microstructures of several centrifugally cast stainless steels containing strong carbide formers (Nb, Ti, and Zr) have been examined as cast and after prolonged creep in the range 800–1000°C. These additions refine the eutectic carbide, changing the morphology and composition as illustrated by the behaviour of IN 519 (Fe–Ni–Cr–Nb) and IN 519 TZ (Fe–Ni–Cr–Nb–Ti–Zr). The carbides present have been identified by electron diffraction and by energy-dispersive X-ray analysis. During creep, the precipitation of both MC and M₂₃C₆ carbides was observed, the former being very much finer and very resistant to coarsening. The role of both the grain-boundary carbide networks and the matrix precipitation in determining the creep properties is discussed.

MST/136     

Effect of strain rate on fracture of U-5.5Nb alloy

The microstructure and tensile properties of an aged U-5.5?wt-% Nb alloy have been experimentally investigated with the aim of obtaining the influence of strain rates on fracture behaviour. The result shows that strain to failure is sensitive to strain rate and decreases with an increase in the strain rate. Fracture surface analysis indicates that the alloy exhibits a typical ductile fracture. Two types of carbides (niobium carbide and uranium carbide), mainly distributed at the grain boundary, are confirmed, which participate in the process of fracture and are involved in different void nucleation mechanisms during the final ductility fracture. Namely, niobium carbide tends to generate voids by debonding with the matrix, while uranium carbide is more likely to experience cracking.     

Preparation of Niobium Carbide-Based High-Temperature Ceramics by Direct Niobium Carburization

Inorganic Materials - Stoichiometric niobium carbide (NbC) samples with a tailored shape have been synthesized by direct carburization of rolled metallic niobium in an atmosphere of an argon +...     

An investigation into microwave bonding mechanisms via a study of silicon carbide and zirconia

It is known that the glassy grain-boundary phase present in low-purity aluminas has two primary functions during direct microwave bonding. Firstly, it increases the dielectric loss of the host ceramic, allowing heating to occur; secondly, the bonding mechanism itself has been found to be based on viscous flow of the glassy grain-boundary phase. However, some evidence has also been found for the bonding of individual grains where they come into direct contact across the join line. To investigate the role of grain-boundary phases further, the microwave bonding of two different grades of silicon carbide and one grain of zirconia has been studied. A single-mode resonant cavity operating at 2450 MHz was used for both studies. The temperature and axial pressure were varied and the bonding time was kept to a minimum. Analysis of the resultant bonds indicated that both reaction-bonded silicon carbide and partially stabilized zirconia could be successfully joined using microwave energy with bonding times typically 10 min or less. For reaction-bonded silicon carbide ceramics, the silicon grain-boundary phase softened at the bonding temperature, allowing the butting faces to be "glued" together. Unlike the glassy grain-boundary phase for alumina ceramics, the silicon phase did not allow grain motion but always formed a discrete and continuous layer at the interface, even under optimum joining conditions. The work with zirconia confirmed that it is possible to join ceramics without the presence of a substantial grain-boundary phase. The mechanism is thought to be either solid-state diffusion and/or grain-boundary sliding. © 1998 Kluwer Academic Publishers     

注浆成型SiC多孔陶瓷的工艺和性能研究

选用SiC颗粒作为多孔陶瓷的骨料材料,长石、石英、粘土组成的低共熔混合物形成晶界玻璃相结合剂,活性炭作为成孔剂,采用注浆成型工艺,对多孔陶瓷的性能进行了研究.SiC骨料颗粒的Zeta电位等电点对应的pH值为5.2,注浆浆料的pH值在8～12的范围内具有很好的流动性和稳定性;烧成温度的提高,使SiC多孔陶瓷的气孔尺寸分布范围缩小,但基本孔径不变;晶界玻璃相的高温粘性流动在SiC晶粒之间形成“桥架”结构,提高了两者之间的粘结能力;高温下,SiC颗粒的氧化产物参与晶界反应,生成新的针状莫来石相,使SiC多孔陶瓷的强度出现异常提高.     

Route to transition metal carbide nanoparticles through cyanamide and metal oxides

We have designed an efficient route to the synthesis of transition metal carbide nanoparticles starting from an organic reagent cyanamide and transition metal oxides. Four technologically important metal carbide nanoparticles such as tungsten carbide, niobium carbide, tantalum carbide and vanadium carbide were synthesized successfully at moderate temperatures. It is found that cyanamide is an efficient carburization reagent and that the metal oxides are completely transmitted into the corresponding carbide nanoparticles. A possible mechanism is proposed to explain the results of the reaction between cyanamide and the metal oxides.     

The important role of titanium microalloying in refining austenite grain of heavy-duty H-beam steel during rough rolling produced by a new technology

A new technology of austenite grain refinement, fine austenite enhanced ferrite transformation, is proposed for heavy-duty hot-rolled H-beam steels in this work. Titanium microalloying is very important and necessary for the new technology. The effect of titanium on the prior austenite grain size of steels during simulated rough rolling was investigated. The results show that the prior austenite grain sizes of specimens with titanium and niobium elements are much finer than those of specimens with niobium but without titanium deformed at the same parameters. For the alloying composition of studied steels, titanium nitride particle maybe precipitated in specimens with titanium at above 1,200 °C, however, niobium carbide particles can't form in specimens without titanium at above 1,150 °C. The thermodynamically stable titanium nitride particles can impede the grain growth at high temperature for example furnace heating before rough rolling and bring the epitaxial growth of niobium carbide on pre-existing themselves which induces a large number of titanium nitride-niobium carbide composite precipitates. These fine precipitates can pin austenite grain boundaries effectively and ensure austenite grain refinement.     

Effect of impurities on the weldability of powder metallurgy,electron-beam melted and arc-melted molybdenum and its alloys

Welds of various molybdenum metals and alloys were examined by bend tests between 183 and 333 K, and the fracture surfaces were observed by scanning electron microscopy and Auger electron spectroscopy. In PM-Mo, blow holes were formed but weldability was improved by selecting high-purity powder, which contained low concentrations in oxides forming impurities, to reduce the total content of oxygen. Nitrogen segregated to grain boundaries promoted intergranular brittleness in PM-Mo. However, in EB-Mo, AM-Mo, Mo 0.56%Nb and TZM, nitrogen was hardly detected and carbon segregation was always present at grain boundaries. Carbon segregation and carbide precipitation were found to strengthen the grain-boundary cohesion and improve the ductility.     

A STEM study of grain-boundary segregation in Al-6.5 wt% Mg alloy

A STEM-EDX analysis study of grain-boundary segregation in an AI-6.5 wt% Mg alloy is presented. STEM-EDX analysis using an electron probe size of nm is shown to provide statistically significant compositional data on grain-boundary segregation in Al-Mg alloys. Solute profiles taken normal to grain boundaries show both non-equilibrium segregation and precipitation phenomena in Al-6.5 wt% Mg alloys water-quenched from 350, 400 and 570° C.     

Passivity breakdown due to discontinuous precipitation during ageing of 21Cr-10Mn-5Ni stainless steel

Discontinuous precipitation in a 21Cr-10Mn-5Ni steel was found to result in breakdown of passivity when tested in a 0.5 m H₂SO₄ + 0.01 m KSCN solution. The electrochemical response was similar to that due to sensitization of conventional stainless steels. The ageing time and temperature affected the resistance of the passive film, with lower ageing temperatures being more severe in terms of passivity breakdown. The effect of interlamellar spacing on the chromium depletion, the effect of volume diffusion on the healing of the matrix, and the solubility product reaction for precipitation, were considered to explain the observed different strengths of passive films formed on specimens aged at different temperatures and times. The observance of good corrosion protection in spite of the presence of profuse precipitates was in contrast to the effect of grain-boundary carbide proliferation on the corrosion resistance of conventional stainless steels.     

Effect of process parameters and niobium carbide addition on microstructure and wear resistance of laser cladding nickel-based alloy coatings

Based on the background of the engineering application of automobile mold repair and surface strengthening, the effects of process parameters on the formation and microstructure of laser cladding nickel(Ni)-based alloy coating were studied. The optimal parameters were: laser power 2000 W, powder feeding rate 15 g/min, scanning speed 4 mm/s. Under this process, the cladding layer and the substrate can exhibit good metallurgical bonding, and the cladding layer has fine crystal grains and a low dilution ratio. On this basis, different mass fractions of niobium carbide (NbC) powder were added to the nickel-based powder and laser cladding was carried out on the surface of die steel. The phase composition, microstructure, hardness and wear resistance of the coating were studied. The results show that with the increasing of niobium carbide addition, the hardness of the cladding layer decreases, and the wear loss of the cladding layer decreases first and then increases. When the niobium carbide addition reaches 6 wt.%, the wear loss of the cladding layer is the least, and the wear resistance is the best.     

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M₆C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M₂₃C₆ carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M₂₃C₆ carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.     

Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450°C: Part II 0.20 wt% Al-Zn baths

The effect of solute additions of titanium, titanium and niobium and phosphorus on interstitial-free steels on Fe-Zn phase formation after immersion in a 0.20 wt% Al-Zn bath was studied to determine the morphology and kinetics of the individual Fe-Zn phases formed. These results were contrasted to the previous study using a pure zinc (0.00 wt% Al) bath in Part I. It was found that in the 0.20 wt% Al-Zn bath, an iron-aluminide inhibition layer prevented uniform attack of the steel substrate. Instead, localized Fe-Zn phase growth occurred, termed outbursts, containing a two-phase layer morphology. Delta-phase formed first, followed by gamma-phase. Zeta-phase did not form in the 0.20 wt% Al-Zn bath, in contrast with zeta-phase formation in the pure zinc bath. As in the pure zinc bath, the growth kinetics of the total layer was controlled by the Fe-Zn phase in contact with the liquid zinc during galvanizing. For the 0.20 wt% Al-Zn bath, the Fe-Zn phase in contrast with the liquid zinc was the delta-phase, whereas the zeta-phase was in contact with liquid zinc in the pure zinc bath. The delta-phase followed t1/2 parabolic growth, while the gamma-phase showed essentially no growth after its initial formation. Titanium and titanium + niobium solute additions, which enhance grain-boundary reactivity, resulted in more rapid growth kinetics of the gamma- and delta-phases. Phosphorus additions, which decrease grain-boundary reactivity, generally increased the incubation time and retarded the growth rate of the gamma-phase. These results further confirm the concept that solute grain-boundary reactivity is primarily responsible for Fe-Zn phase growth during galvanizing in a liquid Zn-Al bath in which an iron aluminide inhibition layer forms prior to Fe-Zn phase formation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Properties and microstructure of stress-relieved submerged-arc weld metal containing niobium

A review of previous investigations on the mechanical properties and microstructure of niobium micro-alloyed steel weldments indicated that there is a loss of ductility after post-weld heat treatment (PWHT). This effect has been ascribed to the precipitation of finely dispersed niobium carbonitrides. However, it can be shown that such precipitation is only observable when the niobium content in the weld metal exceeds 0.025 wt%. This limit appears to be confirmed in the current work by TEM observations on samples containing 0.03 wt% niobium, annealed for various times at 625° C.     

Electrical properties of the grain boundaries of oxygen ion conductors: Acceptor-doped zirconia and ceria

This article reviews the current understanding of the electrical properties of the grain boundaries of acceptor-doped zirconia and ceria, however, with an emphasis on the grain-boundary defect structure. From an electrical point of view, a grain boundary consists of a grain-boundary core and two adjacent space-charge layers. The grain-boundary cores of acceptor-doped zirconia and ceria are positively charged, probably owing to the oxygen vacancy enrichment there. Oxygen vacancies are therefore depleted in the space-charge layer. The grain-boundary conductivities of acceptor-doped zirconia and ceria are at least two orders of magnitude lower than the corresponding bulk values, depending on temperature and dopant level. Such a phenomenon is due to the facts: (1) that oxygen vacancies are severely depleted in the space-charge layer, and (2) that the grain-boundary impurity phase blocks the ionic transport across the grain boundaries by decreasing the conduction path width and constricting current lines. In materials of high purity, the effect of the space-charge depletion layer is dominant; however, in materials of normal purity, the effect of the grain-boundary impurity phase is dominant. A Schottky barrier model satisfactorily explains all the phenomenological observations of the grain-boundary electrical properties of materials of high purity, and experimental evidence soundly supports the model. Various factors (alumina addition and grain size) influencing the grain-boundary electrical properties are discussed, and some special aspects of nanocrystalline materials are highlighted.