Precipitation Of Carbonitrides Research Articles

Carbonitride Precipitation Kinetics Model During Continuous Casting of Ti Microalloyed Steel

Carbonitride Precipitation Kinetics Model During Continuous Casting of Ti Microalloyed Steel

Effect of nitrogen content on the static recrystallization and precipitation behaviors of vanadium–titanium microalloyed steels

Abstract Double compression tests were performed on vanadium–titanium microalloyed steels with different nitrogen contents by using a Gleeble-3800 thermo-mechanical simulator to study the softening behaviors of the deformed austenite during different time intervals between the two passes. The static recrystallization fractions were calculated by the stress offset method and static recrystallization diagrams for the tested steels were obtained. The effects of deformation temperature and interval time on the softening behaviors were analyzed. Especially, the effect of nitrogen on the softening behaviors of the tested steels is discussed in detail. The results showed that the softening behaviors of the tested steels with various nitrogen contents are different. As far as the steel with low nitrogen content is concerned, the softening fraction increases monotonically with increasing time interval, and higher temperature can promote the static recrystallization. However, with more nitrogen added into vanadium–titanium microalloyed steel, precipitated particles of vanadium titanium carbonitride can be observed in the tested steel at the temperature of 850 °C or 800 °C, which leads to the formation of plateaus on the softening curves. An increase in nitrogen content in the steel is favorable for vanadium titanium carbonitride precipitation, which leads to a stronger prohibition of static recrystallization and a longer plateau on the softening curves. Moreover, the precipitated particles in the tested steel will not play an inhibition role in static recrystallization until the nitrogen content in the steel reaches a critical value.

Structure and properties of high-speed R2M9YuAT alloy formed by plasma surfacing in nitrogen with flux-cored wire

The structure and properties of the high-speed R2M9YuAT alloy formed by plasma surfacing in nitrogen are studied. The samples are surfaced by a plasma arc at reverse polarity with the supply of non-current-carrying filler flux-cored PP-R2M9YuAT wire, additionally containing nitrided ferrochrome and titanium, into the weld pool. Surfacing of the samples is carried out using a plasma surfacing installation using UD-417D flux-cored wires. After surfacing is completed, the workpiece is cooled in air and subjected to high-temperature tempering at a temperature 560...580 °C, holding time 1 hour, number of temperings 4. The microhardness distribution of the surface layer of the deposited R2M9YuAT alloy in the state after plasma surfacing and after surfacing and high-temperature tempering is studied. Microhardness values vary from a minimum value of 4.68 to 5.93 GPa, and the distribution of microhardness of the deposited layer in the state after plasma surfacing is characterized by significant unevenness, which is explained by the comрlex thermal effect of multilayer plasma surfacing. Quadruple hightemperature tempering leads to the transformation of retained austenite and the precipitation of carbides and carbonitrides. The microhardness of the deposited metal after tempering increases from 6.89 to 7.48 GPa, and its distribution has become more uniform. It is established that the main phases in the deposited metal after high-temperature tempering are a solid solution of α-iron, carbides and carbonitrides based on iron, tungsten, molybdenum, chromium, aluminum and titanium.

Thermomechanical Processing of V-N Structural Plate Steels in Low Temperature Austenite

Austenite restoration during thermomechanical (TM) rolling of typical vanadium-microalloyed structural steels was studied to optimize strength in the as-rolled and air-cooled condition. Multi-pass plate rolling simulations were performed on V-N microalloyed and CMn steels to compare recrystallisation behaviour in various temperature regions. Included were a conventional schedule ending at high temperature and two TM schedules with mill exit temperatures in the intermediate and low austenite regions. Increasing delay periods after roughing enhance the suppression of recrystallisation after the start of finishing thereby increasing both nucleation site density and nucleation rate for ferrite formation and refinement in grain size. Good agreement was found between microstructures after industrial TM rolling and those obtained from laboratory simulations. Although precipitation of vanadium carbonitrides is an effective strengthening mechanism, appreciable gains in yield strength due to grain refinement can be achieved by rolling in the lower austenite region. Low nitrogen contents in V steels produce coarser final ferrite grain sizes and lower strengths probably due to a larger precipitate size. V-N steels display similar flow behaviour to CMn grades down to approximately 825°C at low to intermediate strain rates but may experience alternate regions of work hardening and dynamic softening at lower temperatures in austenite.

Low‐Pressure Nitrocarburizing in Standard Vacuum Furnaces—Preliminary Studies

For the first time, low‐pressure nitrocarburizing process is performed in a minorly modified standard vacuum furnace on 16MnCr5 alloy steel samples. Ammonia and acetylene are used as a source of nitrogen and carbon, respectively. Lowering the pressure of the process ensures longer and more efficient ammonia dissociation. To prove the safety of the process, exhaust gases are investigated online using residual gas analyzer mass spectrometer, whereas reaction products deposited on the sample's surface are studied by means of time‐of‐flight secondary ion mass spectrometer. Possible reactions taking place in the atmosphere of the chamber are written down. In the results of conducted examinations, it is confirmed that no toxic gases are generated, and no cyanides and polycyclic aromatic hydrocarbons are formed during the process. Additionally, microstructure analysis of the cross sections of nitrocarburized sample is performed using scanning electron microscopy (SEM) and optical microscopy. It confirms the diffusion of nitrogen and carbon into the near surface layer. SEM/energy‐dispersive X‐ray spectroscopy analysis demonstrates precipitation of carbide and carbonitride, what is proved by X‐ray diffractometer and microhardness profile analysis. The presented low‐pressure nitrocarburizing results in an increase in hardness from about 200 to over 500 HV and the formation of an effective case depth of about 20 μm, after only 20 min of the process.

Grain Growth during Mechanical Processing of Austenitic Stainless Steel AISI 321

The kinetics of austenite grain growth during thermomechanical treatment of AISI 321 steel with a relatively high content of carbon (0.07 wt. %) and titanium (0.50 wt. %) were studied. Hot deformation was carried out by the uniaxial compression of cylindrical specimens on a Gleeble 3800 thermomechanical simulator. A dependence is obtained for calculating the kinetics of austenite grain growth for a temperature range of 1150–1250 °C. The proposed dependence makes it possible to evaluate grain growth under non-isothermal conditions. The verification of the adequacy of the proposed dependence and the method for calculating the grain size at cooling rates 0.2, 1 and 5 °C/s showed its high convergence. The difference between the calculated and experimental grain size did not exceed 8%. The suppression of grain growth is due to the precipitation of titanium carbides and carbonitrides. Using the developed grain growth model, an analysis was made of the reasons for the formation of large grains in the shell after the elongating in the production process.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

On the role of carbonitrides in formation of austenite grains during continuous hot rolling of pipe steel

A study of the formation of austenite grain size during rolling of a thick strip on a continuous mill by simulating multi-stage rolling on a Gleeble complex has been performed. It is shown that at low degrees of deformation in the stands of the finishing group of a rolling mill, the formation of a different-grained austenite structure can occur due to selective grain growth without the formation of recrystallization nuclei, despite the precipitation of carbonitrides of microalloying elements. The value of the difference between the deformation strengthening of neighboring grains, sufficient for the grain boundary to overcome the retardation caused by the carbonitride particles, has been determined. It is possible to avoid this negative phenomenon by increasing the degree of deformation in each pass and ensuring the conditions for the formation of recrystallization nuclei. An example of a similar phenomenon observed when rolling beryllium is given.The research was carried out with the financial support of the Ministry of Education and Science of Russia as part of the implementation of the program of the world-class Scientific Center in the direction of "Advanced Digital Technologies" of SPbPU (agreement dated April 20, 2022 No. 075-15-2022-311).

Development of RAFM steels for high temperature applications guided by thermodynamic modelling

• Thermodynamic modelling helps to identify perspective compositions and heat treatments. • Thermodynamic modelling predicted unexpected composition with increased Ta and V content. • Theoretically predicted composition resulted in a higher yield strength of RAFM steel and comparable to EU97 toughness. Mechanical performance of reduced activation ferritic/martensitic (RAFM) steels for fission and fusion applications at high temperature (i.e. above the current limit of 550 °C) requires further improvement. In this contribution, we present the work aimed to improve the high temperature strength and creep resistance of RAFM steels. Two RAFM steel grades were developed based on thermodynamic modelling targeted to optimize the carbonitride precipitation distribution. A high content of carbonitride formers was considered to ensure high fraction of finely dispersed MX-type precipitates. Following the chemical tuning guided by thermodynamic modelling, the screening of different tempering conditions was performed in a high-throughput mode to find the optimal heat treatment for each model alloy. The model alloys were investigated in detail to assess the microstructure, tensile and impact properties. The results are used to validate the improvement of the new grades compared to the reference material EUROFER97. Furthermore, the results are compared with recent developments in high temperature RAFM steels obtained by other research groups.

Effect of N2- and CO2-containing shielding gases on composition modification and carbonitride precipitation in wire arc additive manufactured grade 91 steel

Additive manufacturing (AM) represents a promising technique to fabricate metallic alloys with greater control of the resulting material features as compared to traditional manufacturing routes. Recently, there is greater interest in AM research on 9 wt% Cr ferritic/martensitic (F/M) steels, which are commonly studied for use in the nuclear energy industry. This research aims to prove that wire arc AM can manufacture F/M steels with adequate mechanical properties in multiple processing atmospheres and aims to study how shielding gas composition can be leveraged during fabrication to induce specific precipitation pathways. The effect of shielding gas composition on MX (M=Nb and/or V, XC and/or N) carbonitride precipitation in a 9 wt% Cr ferritic/martensitic (F/M) steel alloy known as Grade 91 was studied using N2 and CO2 gas additions to an inert Ar shielding gas atmosphere during wire arc AM. The N and C atoms present in the processing atmospheres were absorbed into the melt pools during fabrication. Due to their differing affinities for precipitate-forming reactions, the varying levels of C and N between the samples contributed to differences in final carbonitride composition and morphologies. Such precipitate behavior is of interest as carbonitrides have been shown to contribute to increased mechanical performance. This increased performance was studied via electron microscopy and tested for strength, ductility, and fracture properties.

Interaction of Carbon, Titanium, and Boron in Micro-Alloy Steels and Its Effect on Hot Ductility

Varying contents of carbon, titanium and boron were used in the base steel composition of 0.30 wt% Si, 2.0 wt% Mn, 0.006 wt% S, 0.03 wt% Nb, and 30–35 ppm N. Hot ductility tests were performed with Gleeble-3800, after the steel sample was in-situ melted, solidified, and cooled to the test temperature. Investigation was completed with thermodynamic and kinetic simulations. The best results were obtained for steels containing 58–100 ppm B and 35 ppm Ti. They showed very good hot ductility of 80–50% RA within the temperature range between 1250 °C and 800 °C. It was shown that titanium and boron were effective in improving the hot ductility. Titanium protected boron from binding into BN and was low enough to prevent excessive (Ti,Nb) carbonitride precipitation, which both could decrease hot ductility. Boron that precipitated along austenite grain boundaries as iron boride Fe2B was very beneficial for the hot ductility of steel.

Study on the Hot Deformation Behavior of Stainless Steel AISI 321

In this study, the hot deformation behavior of austenitic Ti-modified AISI 321 steel with a relatively high content of carbon (0.07 wt.%) and titanium (0.50 wt.%) was studied in the temperature range of 1000–1280 °C and strain rates in the range of 0.01–1 s−1. Hot deformation was carried out with uniaxial compression of cylindrical specimens on a Gleeble 3800 thermomechanical simulator. It is shown that the flow stress increased with a decrease in the deformation temperature and an increase in the strain rate. The shape of the stress-strain curves indicates that, at high temperatures and low strain rates, the hot deformation of AISI 321 steel was accompanied by dynamic recrystallization. The passage of dynamic recrystallization was confirmed by microstructural studies. Hyperbolic sine type of constitutive equation with deformation activation energy Q = 444.2 kJ·mol−1 was established by analyzing the experimental flow stresses. The power-law dependences of the critical strain necessary for the onset of dynamic recrystallization and the size of recrystallized grains on the Zener–Hollomon parameter were established. The value of the parameter Z = 5.6 × 1015 was determined, above which the dynamic recrystallization was abruptly suppressed in the steel under study. It is speculated that the suppression of dynamic recrystallization occurs due to dispersed precipitates of titanium carbonitrides.

Grain growth behavior of niobium microalloyed 20MnCr5 steel and the effect of boron

AbstractGrain growth behaviors of niobium free, 0.021 wt. % niobium microalloyed and 0.024 wt. % niobium‐0.0022 wt. % boron microalloyed 20MnCr5 steels are investigated in this paper. Optical microscopy and field‐emission scanning electron microscopy were used to analyze the prior austenite grain. Transmission electron microscopy and energy dispersive spectroscopy were used to investigate the morphologies and compositions of the precipitates, respectively. The results show that adding niobium reduces the size of prior austenite grain. The addition of boron to niobium microalloyed steel, however, causes grain coarsening. Thermodynamic and kinetic calculations were performed to explain the mechanism of grain growth behavior. The results indicate that the addition of niobium and boron causes the difference in solution temperatures of aluminum nitride and niobium carbonitride precipitates. The addition of boron inhibits the precipitation of niobium carbonitride and aluminum nitride and causes precipitates coarsening. The precipitation observation backs up the calculation results.

Effect of Tempering Temperature on Microstructure and Properties of a New Type of Nitrogen‐Containing Hot‐Work Die Steel 3Cr7Mo2NiSiVN

Herein, the effect of tempering temperature on the microstructure evolution and mechanical properties of a new nitrogen‐containing hot‐work die steel 3Cr7Mo2NiSiVN is investigated. The new type of hot‐work die steel is quenched at 1080 °C and then conducts a double tempering from 200 to 700 °C. The quenching microstructure contains martensite, film‐retained austenite, and undissolved precipitates, and these precipitates play a Zener pinning effect role in prior austenite grains growing. Based on the Thermo‐Calc thermodynamic calculation and experimental results, the die steel contains V‐rich M2N nitride, Cr‐rich M23C6 carbide, Mo‐rich M6C carbide, and (Nb,V)‐rich M(C,N) precipitates. A secondary hardening phenomenon occurs and the brittle fracture turns to ductile fracture under the scanning electron microscope (SEM) field when the tempering temperature is 500 °C. Room strength decreases and the plasticity improves with the continual increase of tempering temperature. Many nano‐size spherical M2N nitride and strip‐like M(C,N) carbonitride precipitate when the tempered temperature exceeds 550 °C with transmission electron microscope (TEM) observation. Finely dispersed M2N nitride and M(C,N) carbonitride are beneficial for stabilizing microstructure at high temperatures and improving high‐temperature strength. Therefore, the high‐temperature strength of the new type of hot‐work die steel is much higher than that of the traditional hot‐work die steel.

High-resolution characterization of precipitates in multi-layer submerged-arc welded 2.25Cr-1Mo-0.25V steel

Creep-resistant 2.25Cr-1Mo-0.25V steel is commonly applied in the petrochemical industry, e.g., for hydrocracking reactors with large dimensions and high wall thicknesses. For this purpose, submerged-arc welding is the preferred joining process. As these reactors are operated under creep conditions, the sheet material, as well as the weld seams, require a balanced relation between creep strength and toughness in order to guarantee a long service life. In CrMoV steel, the stability and size of the carbides and especially the fine carbonitrides are of major importance for its mechanical properties at room and at elevated temperatures. This study uses high-resolution methods combined with thermo-kinetic precipitation calculation to investigate the impact of multi-layer welding on the carbides and carbonitrides in the bainitic microstructure of 2.25Cr-1Mo-0.25V steel. Atom probe tomography and transmission electron microscopy revealed the presence of solely M3C carbides in the last non-reheated weld bead, whereas multiple reheating by the deposition of overlying weld beads causes the precipitation of nano-sized V- and Nb-rich carbonitrides. Apart from these fine carbonitrides also M3C and Cr-, Mo- and Mn-rich M7C3 carbides were detected in the reheated area of the multi-layer weld metal. In contrast to results from the thermo-kinetic calculations, MX carbonitrides precipitate during cooling after reheating but not directly during cooling after welding.

High-throughput nitride and interstitial nitrogen analysis in ferritic/martensitic steels via time-of-flight secondary ion mass spectrometry

The following work focuses on characterizing nitrogen in several modified alloys of the precipitation strengthened ferritic/martensitic steel HT9 (12Cr-1MoWV (wt%)). Alloy HT9 is a candidate material for fuel rod cladding and ductwork in the core of the Versatile Test Reactor and other next-generation nuclear fast reactors. Nitrogen content has been shown to have a significant effect on irradiated properties in alloy HT9. To explore that theory, a more comprehensive assessment of the location of nitrogen must be performed; however, locating nano-precipitates, such as nitrides and light interstitial elements like nitrogen in steel, has proven difficult. To begin answering questions surrounding the location of nitrogen, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was employed as a primary analysis technique, together with extensive transmission electron microscopy, internal friction testing, and thermodynamic simulations, providing verification and context to trends found in the ToF-SIMS data. For the first time, ToF-SIMS was demonstrated to reliably measure relative differences in the vanadium carbonitride precipitate volume fraction across several chemistries and heat treatments of ferritic/martensitic steels. Additionally, associating matrix elements (iron and chromium) to nitrogen-containing ion clusters significantly reduced the ‘matrix effect that biases nitrogen detection towards nitride precipitates, providing a potential, high-throughput method for the measurement of relative interstitial nitrogen content in steels with ToF-SIMS. • ToF-SIMS provided a new high-throughput analysis method for microalloy precipitate and microstructure design. • ToF-SIMS was employed for semi-quantitative, bulk detection and measurement of nitrides and interstitial nitrogen in steel. • S/TEM provided a quantitative comparison of nitride content, while internal friction testing provided a semi-quantitative comparison of interstitial N.

Carburization and wear behavior of self-lubricating Ti(C,N)-based cermets with various secondary carbides

Self-lubricating Ti(C,N)-based cermets were manufactured using vacuum sintering and solid carburizing in this study, and the effects of various carbides on the carburization and wear behavior of the cermets were investigated. The findings demonstrate that solid carburizing resulted in strong graphite precipitation in all the Ti(C,N)-based matrices. Refractory carbides significantly affect the morphological characteristics of graphite in cermets. The activated carbon atoms in the cermet matrix are consumed in two ways: rearrangement followed by precipitation in the form of graphite and combination with metal atoms followed by precipitation in the form of carbides or carbonitrides. As the amount of refractory carbide dissolved in the binder phase increases, more carbon atoms are consumed by bonding with metal atoms, and less graphite is precipitated. The precipitation of carbides and carbonitrides in the ceramic phase increases the rim phase thickness. After carburization, the mechanical properties of the cermets were noticeably degraded owing to the introduction of graphite and the coarsening of ceramic particles. However, the presence of graphite significantly reduced the friction coefficient of the cermet material owing to its good lubrication effect. The decreased mass loss and better wear morphology suggest that the wear deterioration of all the cermets is remarkably mitigated by carburization. The wear properties of cermets are determined by the mechanical properties and the lubrication conditions of the contact surfaces, and good lubrication conditions can partially compensate for the degradation of the mechanical properties of cermets in the sliding wear process.

Evolution of strengthening factors during long-term aging at 650 °C in advanced 10% Cr heat-resistant steel

The effect of long-term aging for 1000…∼40,000 h at 650 °C on the tensile strength at ambient temperature was studied in a low-nitrogen and high-boron 10%Cr martensitic steel. In order to establish the reason for increasing the yield stress and ultimate tensile strength after 10,000 h of aging, the evolution of strengthening factors was analyzed. A decrease in the substructure and solid solution strengthening during long-term aging is compensated by an increase in the dispersion strengthening due to the precipitation of V-rich MX carbonitrides.

Influence of microalloying elements (Ti, Nb) and nitrogen concentrations on precipitation of pipeline steels—A thermodynamic approach

AbstractA CALculation of PHase Diagrams (CALPHAD) approach was used to study the precipitation of nitrides and carbonitrides in pipeline steels, aligned with new developments of complex chemical compositions and thermomechanical processing of High Strength Low Alloyed (HSLA) Steels. This is in response to growing demand for improved mechanical and chemical properties, manufacturing flexibility and reduced production cost. The calculated results indicated that the precipitation temperatures of nitrides in Ti‐Nb microalloyed steels increased by titanium concentration, while the niobium concentration significantly increased the precipitation temperature of niobium carbonitrides. Carbonitride precipitates formed at much lower temperatures (∼100 K) in low carbon steels (<0.03 wt%) than the high carbon steels (>0.1 wt%), suggesting precipitates larger in size. This is in good agreement with independent experimental data from the literatures, where austenite grain growth was studied in similar steel compositions. Although the dissolution and growth of precipitates are controlled kinetically, these results proved that the thermodynamic calculation can efficiently predict compositions and sequence of precipitation in chemically complex systems, guiding more accurate designs of experiments to identify critical temperatures of grain coarsening during reheating, recrystallisation during hot rolling, and transformation during cooling. This can minimize the number of tests required to obtain optimum chemical compositions and heat treatment procedures.

The X80 Pipeline Steel produced by a Novel Ultra Fast Cooling Process

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels produced by ultra fast cooling process was investigated using energy dispersive spectrometer, scanning and transmission electron microscopy. Results showed that ultra fast cooling kept hot rolled austenitic work hardened, induced a large number of carbonitride precipitation, limited the growth of cementite. Therefore ultra fast cooling was beneficial to increase the steel strength and improve the steel ductility and toughness.