Precipitation Of Carbonitrides Research Articles

Modelling Strain Induced Precipitation of Niobium Carbonitride during Multipass Deformation of Austenite

A new model of strain induced precipitation in niobium microalloyed austenite is proposed. This is based on the experimental observation of formation of microbands during hot deformation of iron – 30% nickel, which remains austenitic to room temperature. Precipitates are preferentially nucleated on nodes in the dislocation network in the microbands. This geometry enables features of earlier models to be simply explained, and facilitates extension of the model to multipass deformation. It is shown that the model captures all the essential features of previous experimental observations on microalloyed C – Mn steels. Currently, sensitivity analysis of the model and systematic experimental work are being undertaken to quantitatively validate the model.

Precipitation of Microalloy Carbo-Nitrides Prior, during and after γ/α Transformation

A brief summary is given of the desired effects of precipitation of microalloy nitrides and carbides in austenite and ferrite prior, during and after g−a transformation. Precipitation of microalloy compounds of Nb(C,N), TiC and V(C,N) is discussed in relation to several grain refinement and precipitation strengthening mechanisms. An improved understanding of the thermodynamics and kinetics of precipitation and phase transformations is presented using the approach based on the chemical driving force. Nucleation of intragranular polygonal ferrite on VN particles that grow in austenite and the formation of acicular ferrite inside the austenitic grains in Vsteels is described. The role of carbon in increasing the driving force for nucleation is elucidated and the benefits of using microalloy carbo-nitrides for precipitation strengthening of bainitic steels are reviewed. An expanded view on the role of microalloy carbo-nitrides in grain refinement and precipitation strengthening is presented.

Strain induced precipitation of complex carbonitrides in Nb–V and Ti–V microalloyed steels

Abstract Strain induced precipitation of complex carbonitrides in two microalloyed steels (Nb–V and Ti–V) has been studied using a stress relaxation technique. The results confirm the faster kinetics of precipitation in the case of Nb–V steel. The thermodynamic software Thermo-calc ™ has been used to determine the composition gradient of equilibrium precipitates for the two steels.

Kinetics of vanadium carbonitride precipitation in steel: A computer model

We have constructed a computer model of the precipitation kinetics of vanadium carbonitride in steel that takes into account the composition evolution of the precipitates with time. The model takes advantage of the fast diffusion of nitrogen and carbon compared to niobium to derive the composition, size and rate of formation of the precipitates during their nucleation. A local equilibrium condition is used at the precipitate–matrix interface to derive the growth rate of each precipitate as a function of its size and the current matrix composition. Coarsening occurs naturally on account of the Gibbs–Thomson capillarity effect. For isothermal heat treatments, the calculations show that the precipitates nucleate as almost pure vanadium nitrides. They subsequently grow at the expense of solute nitrogen. When nitrogen is exhausted, the solute carbon precipitates and progressively transforms the nitrides into carbonitrides. The coarsening stage leads to a steady-state size distribution of niobium carbonitrides of the equilibrium composition.

Effect of Quench and Interpass Time on Dynamic and Static Softening during Hot Rolling

The categories of interpass time encountered in hot rolling are characterized. These are shown to fall into three broad categories: short (in rod rolling); intermediate (in strip processing); and long (in plate schedules). The effects of interpass time on recrystallization and carbonitride precipitation are described, and particularly on the interaction between these two mechanisms. The interactions, in turn, determine the final microstructure produced as well as the mechanical properties.

Microalloyed V–Nb–Ti and V steels Part 2 – Precipitation behaviour during processing of structural beams

A comparative evaluation of the precipitation behaviour in V–Nb–Ti and V steels using transmission electron microscopy was undertaken to study the effective cumulative role of the microalloying additions. While the mechanical properties were similar, there were significant differences in the precipitation behaviour of the two steels. The microstructure of the V–Nb–Ti and V steels consisted predominantly of polygonal ferrite and fine pearlite; in addition, the former contained a small amount of bainite. The V–Nb–Ti steel exhibited significantly greater precipitation of carbonitrides compared with the V steel. In the V–Nb–Ti steel the carbonitrides precipitated as compounds (triplex and duplex type) of Ti, Nb, and V, while in the V steel they were V(C,N). They were characterised by cuboid (45–70 nm), spherical/irregular (20–45 nm), and fine/needleshaped (10–20 nm) morphology. The carbonitride precipitates grew as multimicroalloying compounds, depending on the processing conditions, as the number of microalloying elements increased. Coarse carbonitrides tended to precipitate preferentially along the grain boundaries, whereas fine carbides were dispersed in the matrix. The stoichiometric ratio of triplex carbonitrides in the V–Nb–Ti steel was Ti0.55Nb0.35V0.10, while those of duplex type were Ti0.95V0.05, Nb0.70V0.30, and Ti0.73Nb0.27. Three microalloying elements (Ti, Nb, V) formed a coherent M4C3 type of carbide in the V–Nb–Ti steel and exhibited short range order with the ferrite matrix, displayed by the diffraction pattern as a 'chemically sensitive' or 'superlattice' reflection. However, Nb formed the finest carbides (< 2 nm) and exhibited a ring form of selected area diffraction pattern. The carbonitrides that precipitated in the ferrite followed Baker–Nutting orientation relationships, and their partitioning was not observed in the pearlite or bainitic ferrite. The bainite region was characterised by complete precipitation of cementite. In general, the microstructure had a low dislocation density.

Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of boron for creep cavitation

Composition of type 347 austenitic stainless steel was modified with the addition of boron and cerium. An improvement of creep strength coupled with creep ductility of the steel was observed with boron and cerium additions. The observation of enhanced precipitation of carbonitrides in boron-containing steel over that of boron-free steel may in part contribute to the increase in creep strength. Both grain boundary sliding and nucleation and growth of intergranular creep cavities were found to be suppressed in steel-containing boron. This results in an increase in creep strength and creep ductility. Auger electron spectroscopic analysis of the chemistry of creep cavity surfaces (exposed by breaking the creep-exposed steel specimen at liquid nitrogen temperature under impact loading) revealed the segregation of elemental boron on the creep cavity surface. Boron segregation, on the creep cavity surface in the absence of sulfur contamination, suppressed the cavity growth and provided the steel with a self-healing effect for creep cavitation. Cerium additions enabled boron to segregate on the cavity surface by effectively removing the traces of free sulfur in the matrix by the formation of ceriumoxysulfide (Ce2O2S).

The Aging Behavior for Low Carbon Bainitic Steel Bearing Cu-Nb

By the aid of optical microscope, Hardness measurement, SEM, TEM and the chemical quantitative phase analysis technique, the influence of copper, niobium, and chromium on the aging hardness has been investigated. The aging precipitation behavior and the interaction between various precipitates have also been discussed. The results indicate that since there are multi aging-hardening elements in the steel the aging hardening behaviors are complicated. During the aging the ε－copper， and carbides containing iron and chromium will precipitate. Also, new niobium carbonitride precipitation occurs. These precipitation processes cause marked hardening effect and various hardness peaks. The ε－copper can precipitate at dislocation and lath boundaries preferentially. During the aging the bainite lath united and became wider and then gradually vanished.

Precipitation of Carbonitrides and Their Strengthening upon Non-quench Aging for Micro-alloyed Acicular Ferrite Pipeline Steels

A non-quench aging (reheating hot-rolled plates to 600D C for isothermal aging), different from the conventional quench aging (quench followed by temper), was carried Out to improve the strength level for acicular ferrite (AF) pipeline steels, while their impact energies and ductilities did not decrease obviously, and the Charpy impact fracture appearances were similar between the as-rolled and the aged samples, showing the ductile dimple rupture over the whole fracture surface. The strengthening was ascribed to the further additional precipitation of microalloyed carbonitrides, and the little degradation of ductile and toughness is believed to relate to both the tempering of martensite in martensite/austenite (M/A) islands and the improvement of microstructural uniformity upon the non-quench aging. The nucleation and strengthening of the carbonitrides in the Current non-quench aging were hypothetically calculated with the present mathematical modes and hypothesis based on the thermodynamic consideration and Orowan looping mechanism, which was somewhat consistent with the present experiment, indicating that the strengthening mechanism of the present AF pipeline steels by the current non-quench aging evidently resulted from the additional precipitation of carbonitrides in ferrite region of approximately 600D C.

The study on the ultrafine mechanism of steels: strain-induced phase-transform ation form austenite to ferrite

The models of interphase boundaries of austenite/ferrite(γ/α) in the undeformde austenite and deformed austenite, and the model of 12[1 1 0] edge dislocation in austenite matrix were set up with computer programming. The interfacial energ y of γ/α interphase boundaries in the undeformde austenite and deformed austen ite, the environment sensitive embedding energies (EESEESE) of C, N a nd microalloying elements Nb, Ti or V (in the form of air mass) in the area of e dge dislocation in austenite were calculated by using recursion method. The refineme nt mechanism of steels in the deformation process was discussed. Interfacial ene rgy calculation results show that the most preferred ferrite nucleation sites ar e the area of high density dislocations such as deformation bands, the austenite grain boundaries and subbounaries, which leads to a considerable increase in th e nucleation rate. This is why ferrite grains formed in the deformed austenite a re far more smaller than those in undeformed austenite. The EESEESE calcu lation results show that C,N microalloying elements (Nb, Ti or V) in form of the air mass, C,N together with microalloying elements (Nb or Ti) in the form air m ass are all easy to aggregate to the area of edge dialocations, which results in the carbonitride precipitation in there (deformation band, grain boundary or gr ain subboundary). The growth of ferrite grains is considerably suppressed by def ormation due to impingement with carbonitride particles and other ferrite grains , which leads to the further refinement of ferrite grains.

Effect of the Coupling of Recovery and Precipitation on the Recrystallization Behavior of Ti-Stabilized Extra Low Carbon Steels

The effect of carbo-nitride precipitation on the recrystallization behavior of Ti-stabilized extra low carbon steels was investigated. The precipitation behavior of titanium carbo-nitride was analyzed using transmission electron microscopy (TEM), a chemical extraction method, and the small angle neutron scattering (SANS) method. The recrystallization temperature was varied from 590°C to 680°C depending on the alloy chemistry and hot rolling process. The total amount of precipitates in the hot bands did not significantly affect the recrystallization temperature of the alloys. Isothermal annealing treatment showed a plateau in the stress relaxation curves, i.e. a delay in the recovery, when precipitation occurred during the annealing treatment. A model developed to explain the dynamic interaction of the precipitation with the recovery in the microalloyed austenite was successfully adopted to simulate the delay of the recovery during the recrystallization treatment of the cold rolled Ti-stabilized extra low carbon steels.

Nitrogen pick-up during spray forming of high-alloyed steels and its influence on microstructure and properties of the final products

Spray forming is now getting established as a production route for high-alloyed steels. Tool steels for hot work, cold work and high-speed cutting applications and heat resistant steels generally contain high amounts of numerous elements including chromium, vanadium and aluminium, elements with high affinity for nitrogen. Within the spray forming process nitrogen is usually used as protective atmosphere as well as for gas atomization. The investigation considers nitrogen interaction with steel alloys during the spray forming in a closed vessel. Atmospheres with different nitrogen partial pressure are applied during the process. The microstructure of the spray formed, hot worked and finally heat treated steels is compared. Impact strength, wear resistance and strength at elevated temperature are analysed for the tool steels with respect to the nitrogen content. During melting intensive interaction of the melt with the atmosphere occurs according to the solubility of nitrogen in the alloy and high amounts of nitrogen are picked up. As a general result for the conventional alloys under investigation in spray formed condition this leads to reduction of toughness. This can be explained by additional precipitation of nitrides or carbonitrides. Nitrogen pick-up during spray forming can be avoided primarily by remelting in an argon atmosphere.

Effect of carbon concentration on precipitation behavior of M23C6 carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M23C6 carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M23C6 carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M23C6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above TNR and strain induced carbonitride precipitation. Deformation below TNR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.

Model for the industrial prediction of carbonitride precipitation in HSLA steels cast by a thin slab casting plant

In HSLA steels (high strength low alloys), the most important strengthening mechanisms are grain refinement and precipitation hardening by second phases. Therefore, the control of precipitation and chemical composition of the second phases is essential. A model based on thermodynamics alone does not predict the precipitation phenomena that are observed in thin slabs. This paper introduces a kinetic model that makes it possible to take into account the influence of the thin slab casting process on the precipitation and composition of carbonitrides.

Investigation of Centreline Segregation and Centreline Porosity in CC - Slabs

Improvements of solidification processing in conventional or near net shape casting depend on sophisticated methods of macroscopic examination of central unsoundness and inhomogeneity of the cast material. As long as remarkable deviations referring to the quality features exist between the slab centre and the bulk material such methods should be looked after. Two semi-macroscopic methods of segregation analysis are discussed in this paper: firstly, the potential of the emission spectral analysis combined with sectioning of a sample and secondly, the computer aided micro-probe analysis. Both methods are restricted to small local areas. The proneness of elements and steel grades with respect to micro- and macrosegregation as well as distribution- and segregation coefficients are determined. The effect of a diffusion anneal on homogenisation of segregation is studied. Last not least the precipitation of sulphides, phosphides or carbonitrides is estimated. Ultrasonic detection of the central unsoundness of conventional slabs or thin slabs is a new approach towards a quantitative macroscopic examination. The sample size is 400 mm x 300 mm x 30 mm. To achieve a low ultrasonic noise using a 5 MHz-transducer the samples are heat treated for grain refinement. Dog bone porosity, macroscopic topographical misfit between the upper and lower solidifying shells, periodical corner cracks or systematic transverse centre cracks are detected. Obviously, segregations do not reflect the ultrasonic beam. However, it can be presumed that an interrelation exists between porosity and segregation, although these might be locally apart from each other. This new ultrasonic test of the central unsoundness of continuous cast (c.c.) slabs gives valuable arguments to machine builders and maintenance people to decide on the right concepts for strand guidance, support rollers and secondary cooling. Soft reduction can be optimized and variations in casting speed counteracted by dynamic means. R&D service of this kind can help steelmakers when decisions are impending to build new casting machines or revamp old ones.

Evolution of Austenite Microstructure and Precipitation State during Hot Rolling of a Nb-Microalloyed Steel

Evolution of Austenite Microstructure and Precipitation State during Hot Rolling of a Nb-Microalloyed Steel

A Model for Niobium Carbonitride Precipitation in Ferrite

We have constructed a computer model for the precipitation kinetics of niobium carbonitrides in ferrite. The model describes the time evolution of each age-class of precipitates. This procedure allows for the full coupling of the physical laws for nucleation, growth and coarsening phenomena. The originality of this work is to take into account the composition evolution of the precipitates with time, thus allowing for the prediction of both effects of carbon and nitrogen in solid solution. Specific techniques such as chemical analysis after selective dissolution and transmission electron microscopy with electron energy loss measurement (TEM-EELS) have been used for a thorough investigation of the precipitation state. Time, temperature and composition dependence have been explored. The measured precipitate composition, size and number of particles compare satisfactorily with the calculated values.

TEM investigation of high-nitrogen chromium steels for power plant applications

High-nitrogen chromium steels for power plant application were investigated. The mechanical properties of these steels are determined by microstructural parameters such as volume fraction and dispersion of precipitates, grain size and dislocation density. The proper ausaging treatment followed by controlled cooling is an effective tool to control the particle/dislocation substructure. The modification of microstructure of the high-nitrogen chromium steels is achieved by the control of dissolution and reprecipitation of MN-type vanadium nitrides with a face-centered cubic crystal structure. Tempering after ausaging results in softening associated with pronounced recovery of dislocation structure and precipitation of carbonitrides.

Effect of Co Addition on Microstructure in High Cr Ferritic Steels

Co is one of the interesting alloying elements in advanced high Cr ferritic steels used to improve their creep properties at elevated temperatures. However, it has been reported that Co bearing tends toward rapid deterioration in creep property at long-term testing. In this study, microstructures were studied in detail by comparing two kinds of 9% Cr ferritic steels with 3% Co and Co free for better understanding difference in those creep behaviors.Creep property of the steel with 3% Co was superior to that of the steel without Co within this study, as many researchers pointed out. Adding Co certainly suppressed δ-ferrite formation and there was large difference in prior-austenite grain size between two steels after tempering. In addition to this macroscopic difference, there was also remarkable change in precipitation behavior between them. It was found that heterogeneous precipitation of inter-metallic compounds and MX type carbonitrides was observed in Co free steel, while any extreme localization of such precipitation were not seen in 3% Co steel.It was deduced that such microstructural difference in connection with δ-ferrite formation between two steels was an important factor to understand better high temperature creep properties in 3% Co steel rather than those in Co free steel.