Precipitation Of AlN Research Articles

Unusual Microstructural Development upon Gaseous Nitriding of Fe-4.65at% Al Alloy

Employing NH3/H2 gas mixtures, Fe-4.65at% Al alloy specimens were nitrided to assess how the presence of Al, originally dissolved in the ferrite matrix, influences the development of γ-Fe4N1-x phase in the surface adjacent region. The nitrided specimens were characterized by light microscopy, X-ray diffraction, Electron Backscatter Diffraction and Electron Probe Micro Analysis. Surprisingly, formation of ε-Fe2N1-x was observed, although, for the applied nitriding parameters (nitriding potential and temperature), only the formation of γ-Fe4N1-x would be expected in case of nitriding pure ferrite. An unusual plate-type morphology of γ-Fe4N1-x was observed, contrasting with the usual continuous layer-type growth observed upon nitriding iron, Fe-Cr and Fe-V alloys. These unexpected phenomena may be explained as consequences of the need to realize a very high nitrogen supersaturation in the ferrite matrix in order to initiate the precipitation of AlN.

In situ synchrotron X-ray diffraction experiments on Al–15%BN mechanically alloyed powder: Observation of AlN nanoparticles precipitation and enhanced thermal stability of nanostructured Al matrix

Two different in situ experiments using high energy X-ray diffraction from synchrotron source were performed in order to understand carefully the phase transformation in nanostructured Al–15%BN mechanically alloyed powder. After milling at room temperature for 10 h, a solid solution of Al, B and N was achieved. During the heating, the formation and the evolution of the metastable trigonal Al 2B 3 and a very fine precipitation of hexagonal AlN ( d < 8 nm) within Al grains were detected quantitatively. We found a stabilization of Al 2B 3 between 225 °C and 550 °C and, only around 600 °C the hexagonal AlB 2 starts to form. A detectable decreasing of the Al crystallite size between 420 °C and 470 °C was attributed to the precipitation of AlN nanoparticles that reduce the Al volume fraction and, at the same time, hinder the grain boundary propagation. The powder loses the nanostructure above 600 °C exhibiting an exceptional thermal stability at temperatures close to 0.9 T m .

Microstructural Evolution of Slurry Fe Aluminide Coatings during High Temperature Steam Oxidation

Slurry iron aluminide coatings are very resistant to steam oxidation at 600-650º C. These coatings can be used to protect new generation Ultra Super Critical (USC) steam power plant ferritic/martensitic steel components. The microstructure of the initially deposited coating changes as a function of time, mainly due to coating-substrate interdiffusion, going from mostly Fe2Al5 to FeAl, causing the precipitation of AlN in those substrates containing a minimum content of N and moreover, developing Kirkendall porosity at the coating-substrate interface. Steam oxidation at 650º C causes the formation of a protective thin layer of hexagonal χ-Al2O3 phase along with some α- and γ-Al2O3 after the first few hours of exposure. However, despite the relatively low temperature, and after several thousands hours the protective layer was mostly composed of α-Al2O3. A study of the evolution of the microstructure of slurry aluminide coatings deposited on P92 and exposed to steam at 650º C has been carried out by scanning and transmission electron microscopy and X ray diffraction.

The precipitation and effect of nano nitrides in HSLC steel

The existing forms of N and Al in HSLC (high strength low carbon) steel produced by TSCR (thin slab casting and rolling process), the precipitation thermodynamics and kinetics of AlN, and its effects on structure and mechanical property are studied. The experimental results show that only a small quantity of nitrogen is combined into AlN in HSLC steel produced by TSCR and most of the nitrogen in steel is still free nitrogen. Aluminum-nitride is mainly precipitated during the period of slow air cooling after coiling, but not during rolling and water cooling. The acid-soluble aluminum has no obvious effect on the grain size and mechanical property of HSLC steel produced by TSCR whose acid-soluble aluminum content is 0.005%–0.043%. The precipitation of AlN is not the main cause of grain refinement of HSLC steel produced by TSCR, nor is AlN the dominating precipitate that has precipitation strengthening effect. The nano nitrides are not pure AlN, but have complex compositions.

Influence of Al on strength and impact behaviour of hot rolled plain C–Mn steels

The influence of Al on the strength and impact properties of hot rolled plain C–Mn low S steels has been examined at two C levels, 0˙02 and 0˙1% and two N levels, 0˙001 and 0˙004–0˙005% (where percentage is to be taken as wt-%). It has been found that Al is an excellent element to add to hot rolled steels, an addition of 0˙2% to a plain 0˙1C–1˙4Mn steel with 0˙004–0˙005%N resulting in an impact transition temperature of −95°C at a strength level of ∼300 MPa. At this Al level, Al removes the N from solution but also importantly refines the grain boundary carbides and to a lesser degree the grain size resulting in a 40°C lowering of the impact transition temperature in a coarse grained steel (15–20 μm; d− 1/2, 7–8 mm−1/2). Strength is not affected by this Al addition. The decrease in strength due to the removal of nitrogen from solution as AlN is balanced by the solid solution hardening from the Al in solution in the iron lattice, (a 1%Al addition increasing the strength by ∼70 MPa), and the small grain refinement that occurs from AlN precipitation. Increasing the Al level to 1% in the 0˙1%C steels, results in the formation of martensite with consequent deterioration in impact behaviour. When the N level is reduced to very low levels, 0˙001%N, no grain refinement in the γ occurs in the 0˙02% and 0˙1%C steels at the 0˙2%Al level. The solid solution hardening effect of Al now dominates the properties with the impact transition temperature remaining constant due to Al refining the grain boundary carbides.

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300°C, cooled at rates of 0.3, 1.2 and 3.0°Cs−1 to temperatures in the range 750–1050°C, and then strained to failure at initial strain rates of 1×10−4 or 1×10−3s−1. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0°Cs−1 and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from ∼900°C (near the Ae3 temperature) to ∼1000 or 1050°C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Evaluation of Annealing Texture in IF and EDD Steel Sheets

The textural and microstructural studies of cold-rolled-batch annealed (CRBA) interstitial free-Ti (IF-Ti) stabilized steel and extra deep drawing steel (EDD) produced at Tata Steel have been investigated. In the IF steels, α-fiber has been found to be strong even after batch annealing, which is not desirable for deep drawing applications. The presence of a stronger recrystallization texture in the surface region rather than in the central region accounts for the deformation and orientation path history of cold-rolling texture components and the texture inherited from hot band. The hot band of IF steel has been observed to have mixture of grain structure along with inhomogeneous texture throughout the thickness. While the inhomogeneity in EDD steel, pancake grain structure in the through-thickness direction, milder pancake grains in the transverse section, and polygonal grain structure in the longitudinal section have led to the inference that the undesirable α -fiber is present. This observation has been explained by the early precipitation of AlN during coiling. The results of all the steels have been explained with the help of the orientation distribution functions, volume fraction of alpha and gamma fiber along with their components, volume fraction ratio (γ /α), and the microstructure. The analysis of the present data suggests that the deep drawability of both steels can be improved by modifying the hot-rolling and annealing parameters.

Inconsistent flow stress in low carbon boron steels during finishing

The influence of deformation parameters and composition on flow stress behaviour of boron-containing steels is described. High boron steels displayed a consistent flow stress during finishing due to the boron concentration at austenite grain boundaries having achieved steady state followed by the precipitation of BN. Higher flow stresses than those found in boron-free steel were due to solute boron at grain boundaries, which delayed the onset and the rate of dynamic recrystallisation (DRX) and consequently enhanced the solute drag effect. In low B steels, inconsistent flow stress behaviour at finishing temperatures above the BN solubility temperature was attributed to deformation at or near the maximum non-equilibrium grain boundary segregation (NGS) of boron, which occurred rapidly. The flow stress in low B steels was consistent when DRX occurred during roughing and early finishing and was promoted by weak NGS and a finer grain size when the reheat temperature was decreased. Flow stress consistency was also promoted by avoiding a NGS peak by deforming at temperatures below the BN solubility temperature. A high driving force for AlN precipitation in austenite increased flow stress inconsistency through the protection of solute boron for NGS and increased solute drag.

Thermal stability of Al–Cr–N hard coatings

Heat treatment of arc-evaporated cubic Al0.7Cr0.3N hard coatings in Ar up to 1450 °C causes precipitation of AlN. The Cr-enriched matrix transforms into Cr via Cr2N under N2 release. These reactions are investigated by simultaneous thermal analysis, mass spectrometry, X-ray diffraction, and analytical transmission electron microscopy.

Effects of Nitrogen on the Mechanical Properties of Cold Rolled TRIP-aided Steel Sheets

Tensile tests were conducted in order to study the effects of nitrogen on the mechanical properties of the 0.2C–1.5Mn–1.5Si–0.04Al–(0.003–0.015)N steels often annealing at 800–830°C, followed by the austempering at 400–450°C. The results show that both tensile strength and elongation increase and the balance of tensile strength×elongation are improved upon nitrogen addition to the TRIP steel. It was found that the density of AlN precipitates increases with addition of nitrogen. The volume fraction of retained austenite increases due to AlN precipitation, because the precipitates retard the transformation of austenite during cooling and austempering. As a larger volume fraction of austenite remained after annealing has been transformed to martensite during deformation, elongation as well as strength has increased in the nitrogen added steel. It is also observed that the average grain size of ferrite and bainite decreases because of AlN precipitation that hinders grain growth. The refinement of ferrite and bainite by AlN precipitates also contributes to the increase in the strength of nitrogen added steels.

Influence of undercooling thermal cycle on hot ductility of C–Mn–Al–Ti and C–Mn–Al–Nb–Ti steels

The influence of an undercooling thermal cycle during cooling after melting on the hot ductility of C–Mn–Al and C–Mn–Al–Nb steels with titanium additions in the range 0–0·015% has been examined. Tensile specimens were melted and cooled at 100 K min−1 to test temperatures in the range 1000–800°C, and strained to failure using a strain rate of 3 × 10−3 s−1. The hot ductility curves obtained from this procedure were then compared with curves obtained when a 100 K undercooling step was introduced before tensile testing; this cooling programme simulated more accurately the cooling conditions during continuous casting. Titanium additions were found to impair the hot ductility, with niobium containing steels giving the worse ductility. When undercooling was applied to the titanium free, C–Mn–Al containing steels, greater amounts of AlN could form, and hence ductility was worse. Increasing the titanium level in these C–Mn–Al steels reduced the adverse influence of undercooling on hot ductility, as more of the nitrogen was taken out of solution as TiN. When only one thermal cycle was introduced, TiN precipitation was shown to have a more detrimental effect on the hot ductility than AlN precipitation. However, whereas increasing the number of thermal cycles would not lead to any further TiN precipitation, this was not so for AlN, since precipitation was sluggish, and, provided that free nitrogen was available, further precipitation could occur giving rise to worse ductility. For the niobium containing titanium free steel, undercooling gave worse ductility because the lower temperature allowed a finer precipitation of Nb(CN) to occur. Increasing the titanium level in these niobium containing steels again caused the ductility to deteriorate, but this time there was a small improvement in ductility on undercooling for the lower temperature range. Although titanium has commercially been found to be a beneficial element to add to reduce transverse cracking and improve surface quality on continuous casting, laboratory work has again shown that ductility is impaired even with an undercooling step. This difference in behaviour can be accounted for by more intense segregation occurring on solidification in commercial casts compared with laboratory casts, thus favouring coarser precipitation of TiN and hence better ductility.

Precipitation of single crystalline AlN from Cu-Al-Ti solution under nitrogen atmosphere

Precipitation and growth behavior of AlN from Cu-Al-Ti-N solution under the atmospheric pressure of nitrogen has been studied. The solubility of AlN in Cu flux ranges from 5 mol% to 20 mol% at the temperatures between 1700 ∘C and 2000 ∘C. The addition of the proper amount of Ti drastically enhances the solubility of AlN. Acicular crystallites and platelets of AlN precipitants, which were all water white colored, were obtained by super-cooling the Cu-Al-Ti-N solution. The largest pencil type AlN reached more than 3 mm in its length and 0.2 mm in its diameter. The axis of the pencil was c-axis of wurtzite AlN and it contained no dislocation, no diffractive defects. A platelet of AlN of which diameter exceeded 1.5 mm was also obtained by regrowth technique. Thermodynamic calculation reveals the validity of using Cu-Al-Ti-N solution for the growth of AlN.

Characterization of aluminide coatings formed by diffusion alloying on nitrogen-containing type 316L stainless steels

Diffusion alloying of pre-deposited aluminum film in nitrogen-bearing 316L stainless steels resulted in the formation of intermetallic phases like FeAl 2 and Al 13Fe 4. The high affinity of nitrogen for aluminum resulted in surface enrichment of nitrogen, as well as increased AlN precipitation, particularly in the high nitrogen steels.

Experiment investigation of deep-drawing sheet texture evolution

To ensure the deep-drawability of the deep-drawing sheet, the normal anisotropy and planar isotropy of the sheet is required. The drawing property of the sheet is controlled by its crystallographic texture, the type and orientation density of the texture directly influence the drawing-quality of deep-drawing sheet. {111}〈112〉 and {111}〈110〉 are the ideal textures for deep-drawing sheet, because the correct texture gives the proper orientation of slip system so that the strength in the thickness direction is greater than that in the plane of the sheet. During the production processing of the deep-drawing sheet, the development of crystallographic texture is continuous, and the final texture character of the product is determined by the texture evolution processing at the different technological process. In the present work, the whole production flow of a coiled sheet has been flowed in Benxi Iron and Steel (Group) Co., Ltd., and the samples were cut from this coiled sheet at the different production flow, such as after hot rolling, cold rolling and recrystallization annealing, respectively. The texture evolution processing has been investigated for the whole production flow of the deep-drawing sheet. Some factors that influence the evolution of the crystallographic texture have been analyzed, such as the grain size of hot-rolled sheet, the reduction ration in the cold rolling, the annealing process, the precipitation of AlN during the coiling and, etc. This paper centers on the research of the texture evolution in each different process. An optimum technique to improve the deep-drawability of the deep-drawing sheet has been put forward.

Optimization of the strain aging resistance in aluminum killed steels produced by continuous annealing

The influence of manganese, boron and carbon content and of the coiling temperature on the strain aging resistance of continuous annealed Al killed steels was investigated, using five different steel compositions and three coiling temperatures. The material was produced and processed in laboratory and the continuous annealing cycle was performed using a thermomechanical simulation machine. In boron added steels, it was found that the coiling temperature has no influence on the strain aging resistance. On the other hand, in boron free steels of similar composition, coiling temperatures equal to or higher than 720 °C were required to reduce the amount of nitrogen in solid solution at room temperature after the overaging treatment and thus to increase the aging resistance. However, an increase in the Mn content of the boron free steels produced the same results observed in boron added steels. This effect is discussed in terms of the influence of the Mn content on the kinetics of AlN precipitation in these steels.

Morphology and precipitation kinetics of AlN in hot strip of low carbon steel produced by compact strip production

Morphology and precipitation kinetics of AlN were investigated both experimentally and using a predictive model in hot strip of low carbon steel produced by compact strip production (CSP), a process for the production of hot strip from a continuously cast thin slab using direct charging. By an electrolytic dissolution technique and TEM analysis, the shapes of AlN were cube or plate, the precipitates were uniform and very fine, with average size less than 8 nm. The paper presents a model to describe the precipitation kinetics of AlN under thermomechanical process conditions of CSP, which predicted volume fraction evolution and the precipitate size of strain induced precipitation on the basis of classical nucleation theory. For the process conditions of this paper, about 3 s time was sufficient for appreciable strain induced precipitation occurred immediately and entirely on dislocations after the sixth finishing rolling pass, and the computing radius of precipitate was 2 nm, which was in good agreement with experimental data. The differences of AlN precipitation between CSP and conventional process are also discussed.

Development of Non-Oriented Electrical Steel Sheets for Energy Efficient Motor based on Ultra Low Sulfur Technology.

The effect of sulfur on the magnetic properties of non-oriented electrical steel sheets was studied. In an ultra low sulfur steel with annealing temperatures ranging from 850°C to 975°C, hysteresis loss is lower than that of a high sulfur steel. However, the hysteresis loss in the ultra low S steel increases with an increase in the final annealing temperature ranging from 975°C to 1050°C. This phenomenon is associated with the precipitation of AlN near the surface of the steel. The nitride forming reaction is accelerated with a decrease in S content. In the ultra low S non-oriented electrical steel sheets, the low core loss was obtained by the reduction of the surface nitriding and the high magnetic flux density was obtained by a reduction of Si and Al content. This steel shows; 1)low hysteresis loss, 2)high magnetic flux density, 3)excellent punching properties and 4)low production cost.

AlN Precipitation in Dual-phase 3% Si Electrical Steels.

The existing approaches to the detection of precipitation are largely based on room temperature measurements on quenched samples. However, the direct measurement of precipitation kinetics at high temperatures is also possible through analysis of the mechanical properties of the specimens. In this work, a creep method was developed and applied to the detection of aluminum nitride precipitation in a dual phase 3 % silicon steel containing 0.038 % C. Prior to loading, the specimens were solution treated for 20 min and then cooled to the test temperature. A constant stress was applied to each sample by means of a computerized MTS machine and the strain was recorded continuously during testing. Microstructural examination revealed that the austenite fraction and morphology and the microstructure of the ferrite matrix are quite different depending on whether samples are directly heated or heated and then cooled to the test temperature. The resulting creep rate is sensitive both to the occurrence of precipitation as well as to phase transformation; when the microstructure remains fixed, the slope of the true strain-log(time) curve decreases immediately after the initiation of precipitation. The precipitation-time-temperature diagrams determined in this way are of classical C shape.

Influence of nitrogen on hot ductility of steels and its relationship to problem of transverse cracking

The present review examines the influence of nitrogen on the hot ductility of steels, with particular relevance to the problem of transverse cracking during continuous casting. Nitrogen itself is not detrimental to hot ductility, but when it is present with aluminium or microalloying additions, ductility can be adversely affected through the formation of nitrides or carbonitrides. The addition of aluminium to low nitrogen C–Mn steels (0·005%N)impairs ductility during casting at an acid soluble level as low as 0·02%Al. This arises because segregation of aluminium to the grain boundaries occurs on solidification, and the temperature cycling that takes place when the strand is cooled encourages AlN precipitation. However, for low nitrogen, high strength low alloy (HSLA) steels with carbon levels in the peritectic range 0·08–0·17%C, transverse cracking is not generally encountered until the aluminium level is >0·04%. Higher nitrogen levels are likely to cause problems even at very low aluminium levels, as precipitation of AlN is controlled by the product of the aluminium and nitrogen contents. The microalloying additions vanadium and niobium are detrimental to ductility but, of the two elements, niobium is more damaging, as it gives finer precipitation. Increasing the nitrogen level has a more pronounced influence on ductility in vanadium containing steels, since vanadium forms a nitride while niobium forms Nb (CN), which is mainly carbon based. Nevertheless, the product of vanadium and nitrogen contents has to approach 1·2 × 10-3, for example 0·1%V and 0·012%N, before ductility deteriorates to that normally given by a niobium containing steel with 0·03%Nb and 0·005%N. When small titanium additions are made to low nitrogen C–Mn–Al steels (0·005%N), the best ductility is likely to be given by a high Ti/N ratio of 4–5 : 1; the excess titanium in solution encourages growth of the TiN particles. For high nitrogen steels (0·01%N), a low titanium level (0·01%)is recommended to limit the volume fraction of TiN particles. A low soluble aluminium level is also needed to prevent the excess nitrogen from combining to form AlN. For C–Mn–Nb–Al steels, similar recommendations can be made with regard to adding titanium. However, the presence of niobium and aluminium appears to have little influence on ductility, since these elements coarsen the titanium containing precipitates.