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High‐nitrogen stainless bearing steel (HNSBS) is the most representative third‐generation aviation bearing steel, and the cleanliness and microstructure characteristics of bearing steel are crucial to its high performance and long service life. In this work, the effect of cerium addition on the cleanliness and microstructure characteristics of as‐cast HNSBS was systematically studied by combining microstructural characterization and thermodynamic analysis methods. The results show that with increasing Ce content, the cleanliness of steel first improved and then decreased, and Ce2O2S, Ce2O3 and CeN inclusions were generated successively. The Ce content should be controlled to ≈0.012 wt% to ensure that all Al2O3, MgO · Al2O3, and MnS inclusions were modified, and minimize the formation of large‐size Ce‐bearing inclusions. Furthermore, cerium addition refined the dendrite structure due to more nucleation sites for γ‐Fe and the enrichment of Ce in the liquid phase at solidifying front. The finer and more dispersed distribution in precipitates was ascribed to the reduction of growth space and Cr segregation, as well as the increase in effective nucleation sites. These results can offer theoretical guidance for inhibiting the formation of harmful inclusions in high‐nitrogen alloy systems and refining the microstructure of alloy systems containing M23(C, N)6 and M2(C, N) precipitates. [ABSTRACT FROM AUTHOR]

By comparing the advantages and disadvantages of the existing single‐step pressurized metallurgical manufacturing technologies of high nitrogen stainless steel (HNS), a promising pressurized duplex manufacturing route, i.e., pressurized induction melting (PIM) and pressurized electroslag remelting (PESR), is proposed. The PIM stage mainly carries on the nitrogen alloying, deoxidation and desulfurization, and the PESR process mainly takes responsibility for further desulfurization, removal of large‐size inclusion, and elevating solidification quality. During PIM, the deoxidation is achieved by vacuum carbon deoxidation combined with magnesium and rare‐earth compound treatment. Subsequently, the PESR is conducted under high nitrogen pressure without adding nitrided ferroalloys. To precisely control the nitrogen content and avoid segregation and escape of nitrogen, the three‐stage nitrogen pressure collocation is developed. The high nitrogen stainless bearing steel 30Cr15Mo1N (DIN 1.4108) is manufactured as an example by the duplex route, and a PIM–PESR ingot with high nitrogen content, high cleanliness, fine and dispersive inclusions, and compact solidification structure is obtained. Moreover, the steel possesses a homogeneous microstructure, slightly better mechanical properties, and corrosion resistance compared with that manufactured by the single‐step PESR process. Finally, a pressurized ladle refining and PESR duplex route are proposed for large‐scale industrial production of HNSs. [ABSTRACT FROM AUTHOR]

High cleanliness is an important guarantee for the long-service of bearing steel. In this study, the effect of crucible materials on cleanliness and inclusion characteristics of high-nitrogen stainless bearing steel (HNSBS) during vacuum carbon deoxidation was comprehensively investigated by microstructure characterization and thermodynamic analysis. The results showed that the ultimate O contents using MgO·Al2O3, MgO, and ZrO2 (MA, M, and Z) crucibles could be decreased from about 0.0060 to 0.0028, 0.0012, and 0.0022 wt pct, respectively. The ranking of crucible thermodynamic stability during vacuum carbon deoxidation was M < MA < Z, representing the decreasing of oxygen transfer rate due to crucible decomposition. The deoxidation rate of carbon–oxygen reaction increased with the decreasing of CO partial pressure, and the ranking of deoxidation rate using various crucibles was Z < MA < M. Meanwhile, the main oxide inclusions in steel using MA, M, and Z crucibles transformed from Al2O3 to MgO·Al2O3, MgO, and ZrO2, respectively. The area and average size of inclusions in steel using M crucible were smaller than the others owing to the decreasing of large-size Al2O3 inclusions and the increasing of percentage of low-density Mg-containing inclusions, while the existence of high-density ZrO2 inclusions in steel using Z crucible restricted the floating and removal of inclusions. Therefore, MgO crucible was more appropriate to melt high-cleanliness HNSBS with lower O content and fewer deleterious inclusions. [ABSTRACT FROM AUTHOR]

The cleanliness and inclusion characteristics are crucial to the service life of bearing steels. In this work, the effects of addition order and contents of Mg and Ce on cleanliness and inclusion characteristics in high-nitrogen stainless bearing steels were systematically investigated by microstructure characterization and thermodynamic analysis. The results showed that the O and S contents could be decreased from about 0.0040 wt pct and 0.0028 wt pct to 0.0012 wt pct and 0.0014 wt pct, respectively. The inclusion evolution processes by Mg pretreatment and Ce addition (MgCe) were: Al2O3 → MgO and MnS → MgS + Ce–O–S, and those of Ce pretreatment and Mg addition (CeMg) were: Al2O3 → Ce–O–S and MnS → Ce–O–S + MgS, which were consistent with thermodynamic calculation. For MgCe treatment, owing to the high Ce content (0.018 wt pct), the number density (NA), total area (S) and average size (d) of inclusions were large, and those would decreased by reducing the Ce content to 0.012 wt pct. For CeMg treatment, a large number of Mg-containing inclusions (especially deleterious Mg3N2) formed because of the high Mg content (0.0066 wt pct). As the Mg content decreased to 0.0043 wt pct, the NA and S of inclusions increased instead. In this study, the optimum addition order was Mg pretreatment and Ce addition, and the optimized contents were about 0.0044 wt pct and 0.012 wt pct, respectively. [ABSTRACT FROM AUTHOR]

The AlN inclusion induced by high aluminum content is a serious threat for the high quality of high nitrogen stainless bearing steels produced by pressurized electroslag remelting (PESR). In this study, based on the lab-scale experiments and thermodynamic analyses, the reaction mechanism of aluminum increase during PESR was revealed, and the effects of atmosphere, slag and steel compositions on aluminum content were investigated to restrain aluminum increase. The results show that aluminum pickup was accompanied with noticeable silicon loss in steel and SiO2 increase in slag. The aluminum content in the ingots was dominated by the thermodynamic equilibrium of [Si] + (Al2O3) reaction between slag and steel. The high-temperature condition in electroslag metallurgy process promoted aluminum increase, and the reaction temperature was determined as approximately 1973 K. Due to the severer silicon loss that induced by the electrode oxidation in air, the aluminum content in ESR ingot was relatively lower than that in PESR. Increasing the SiO2 content in slags to 4.5 to 6 mass pct was an effective way to achieve low-aluminum content, but excessive SiO2 led to the high oxygen content and burning loss of alloying elements in ingots. Moreover, reducing the initial silicon content in electrodes from 0.45 to 0.18 mass pct further decreased the aluminum content in ingots. Through the process optimization, the restriction on aluminum increment in HNSBSs during PESR was well realized and the AlN inclusions almost disappeared. [ABSTRACT FROM AUTHOR]

The controls of cleanliness and inclusion characteristics are important issues for the manufacturing of bearing steels. In this work, the method of Mg pretreatment and Ce addition was firstly proposed, and its effect on cleanliness and inclusion evolution in high-nitrogen stainless bearing steels was systematically investigated by microstructure observation and thermodynamic calculation. The results showed that Mg pretreatment and Ce addition decreased O and S contents to 0.0004 and 0.0012 wt pct, respectively. The main inclusions before Mg pretreatment were large-size Al2O3 and elongated MnS. With the increase in Mg content to 0.0079 wt pct, the inclusion evolution processes were Al2O3 → MgO·Al2O3 → MgO, and MnS → MgS. After adding Ce to about 0.015 wt pct, the inclusion evolution processes were Al2O3 → CeAlO3 → Ce–O–S, MgO·Al2O3 → MgO, and MnS → Ce–O–S. Specially, Mg3N2 inclusion formed when Mg content was higher than 0.0040 wt pct, which would induce formation of deleterious AlN. Furthermore, large-size Ce–O–S attached by low-density MgO inclusion tended to float upward because of the decreasing density, thus reducing size and number density of inclusions. However, as Mg content increased to 0.0079 wt pct with same Ce addition, the size and number density of Mg-containing inclusions (MgO, MgS, and Mg3N2) increased. Finally, the optimum Mg content of Mg pretreatment with 0.015 wt pct Ce addition was 0.0026~0.0040 wt pct. [ABSTRACT FROM AUTHOR]

High cleanliness is a crucial factor determining the service performance of bearing steel. Considering the strong deoxidation ability of carbon under vacuum, vacuum carbon deoxidation was adopted in the cleanliness control of high nitrogen stainless bearing steel. In this paper, a mathematical model for vacuum carbon deoxidation during pressure/vacuum induction melting (PVIM) was proposed considering the occurrence of carbon–oxygen reaction at the free surface of molten pool and at the interface between CO bubbles and molten steel as well as crucible decomposition. The effect of electromagnetic stirring on the mass transfer of oxygen was analyzed using the finite element numerical method and the effect of CO bubbles on the deoxidation process was also considered. The influence of pressure, temperature and initial carbon content on deoxidization was studied and experimentally verified. The results indicated that the predicted variation of oxygen content in molten steel with time agreed well with the experimental results. Electromagnetic stirring effectively promoted the mass transfer of oxygen in molten steel and the carbon–oxygen reaction at free surface of molten pool was the main pathway for deoxidization. The pressure, temperature and initial carbon content influenced the vacuum carbon deoxidation process by affecting the carbon–oxygen reaction rate, crucible decomposition and mass transfer coefficient of oxygen. Finally, optimum melting parameters were recommended. [ABSTRACT FROM AUTHOR]

The existence of angular and hard AlN inclusions would seriously deteriorate the service life of high-nitrogen stainless bearing steels (HNSBSs). In this work, the formation mechanism of AlN inclusion in HNSBSs under as-cast, annealing and austenitizing states was systematically investigated by microstructure observation and thermodynamic, kinetic analyses. The results showed that the concentration product of Al and N could exceed the critical solubility of AlN inclusion at liquidus temperature with the Al content higher than 0.050 wt pct, which led to the formation of AlN inclusions about 1 to 5 μm (equivalent diameter) in liquid steel. Based on the 'Clyne-Kurz' model, AlN inclusion could form at the solidifying front due to the enrichment of N in the residual liquid steel with the Al content higher than 0.030 wt pct. Besides, the precipitation of Cr2N and the extremely low diffusion coefficient of Al in α phase restrained the precipitation of AlN during annealing at 1023 K. However, AlN and AlN-MnS composite inclusions less than 0.6 μm could precipitate during austenitizing at 1323 K with the Al content higher than 0.006 wt pct, which was the critical Al content to avoid AlN formation in HNSBSs after melting, solidification, and heat treatment processes. [ABSTRACT FROM AUTHOR]

We elucidated the significance of partial substitution of C by N in three high nitrogen Fe-15Cr-1Mo-C-N martensitic stainless steels (MSSs), and particularly its influence on microstructural features and the strength-toughness balance. The results show that partial substitution of C by N avoided the formation of coarse intergranular carbides and caused significant changes in the type of carbonitrides from M23C6 to M23C6+M2N and finally to M2N. Meanwhile, partially replacing C by N first increased and then decreased the effective grain size, the fraction of retained austenite, and the plate martensite, while the variations of the amount of carbonitrides and dislocation density were just the opposite. The microstructural evolutions with varying (C+N) contents played a crucial role in determining the overall properties: the 0.35C-0.37N steel exhibited an improved impact toughness (~ 86.1 J), which was 2 or 3 times higher than those of 0.50C-0.16N and 0.20C-0.54N steels, even at a strength level of over 2 GPa. The contributions of partial substitution of C by N on the strengthening mechanisms of MSSs were also systematically revealed by combining the experimental and theoretical data. [ABSTRACT FROM AUTHOR]

Packing plays an important role on the mass transfer and mixing in a rotating packed bed (RPB) which is regarded as an important gas-liquid reactor or contactor for process intensification. Stainless steel wire mesh packing exhibits good mass transfer performance when applied in a RPB. However, its construction repeatability and mechanical strength should be improved. Therefore, we introduced and designed a novel structured stainless steel wire mesh packing, aiming to overcome the above problems. In this work, the effective interfacial area ( a e ) in the packing and cavity zones of a RPB with four different types of structured stainless steel wire mesh packings was investigated at various rotational speeds, gas flow rates, and liquid flow rates by using a NaOH-CO 2 mass transfer system. A brief analysis of the gas-liquid mass transfer interface area ( A ) and utilization ratio of packing’s specific surface area ( w p ) is presented. A modified correlation for the effective interfacial area in the packing zone of the RPB with structured stainless steel wire mesh packing was also proposed, and the predicted values were found to be in agreement with the experimental values with deviations generally within ±15%. [ABSTRACT FROM AUTHOR]

Experimental and theoretical studies have been carried out to investigate the effects of the slag on desulfurization during the electroslag remelting (ESR) process with a focus of developing a mass transfer model to understand the mechanism of desulfurization. Stainless steel 1Cr21Ni5Ti was used as the electrode and remelted with two different kinds of slags using a 50-kg ESR furnace. The contents of sulfur along the axial direction of product ingots were analyzed. It was found that the sulfur content of 350 ppm in the electrode is reduced to 71 to 95 ppm in the ingot by remelting with the slag containing 5 wt pct of CaO, and lowered more to 47 to 59 ppm with another slag having 20 wt pct CaO. On the basis of the penetration and film theories, the theoretical model developed in this work well elucidates the kinetics of desulfurization revealing the mechanism of sulfur transfer during the ESR process. The calculation results obtained from the model agree well with the experimental results. The model indicates that when sulfur content in electrode is given, there is a corresponding minimum value of sulfur content in the ingot due to the kinetics limit. This lowest sulfur content cannot be further reduced even with increasing L (sulfur distribution coefficient between metal and slag phases) or decreasing sulfur content in the slag. Constant addition of extra amount of CaO to the molten slag with the increase of sulfur content in the slag during the remelting process can improve the macrosegregation of sulfur distributed along the axial direction of ESR ingots. Since the rate-determining steps of the sulfur mass transfer lie in the metal phase, adding calcium as deoxidizer can change mass transfer of sulfur and thus promote desulfurization further during the ESR process. [ABSTRACT FROM AUTHOR]

TGA, isothermal and thermal-cycle tests were carried out to investigate the oxidation behaviour of duplex stainless steels at 1220 °C in various atmospheres. The weight gain in air is initially in parabolic relation to time. This is followed by a linear relationship for super alloy and a dramatic growth one for standard alloy that is subject to breakaway oxidation by forming nodules. The thickness of γ-layer increases with increasing annealing time and oxygen content in the atmosphere, and it is slightly higher in standard alloy than in super alloys. A single layer of (Fe,Cr)3O4 spinel is noticed in the external oxide scale after annealing for 2 h, and two layers are found after 4 h with an inner layer of the spinel and an outer layer of iron oxides. Nodules grow on the δ/γ packets where the oxidation of austenite is accelerated and that of ferrite is suppressed. Both chains and Si-enriched scales grow inside the γ-layer along grain and phase boundaries. [Copyright &y& Elsevier]

The impact of inclusion types on pitting corrosion behavior of high-nitrogen stainless bearing steels (HNSBSs) with different Ce contents was investigated combining electrochemical tests, microstructure characterization and thermodynamic analysis. The oxide (Al 2 O 3 /Ce 2 O 3) and sulfide (MnS/Ce 2 O 2 S) inclusions exhibited induction effect on pitting corrosion, while nitride (CeN) inclusion exhibited inhibition effect. Interestingly, CeN inclusion also acted as a protective armor wrapping around Ce 2 O 3 inclusion, alleviating its adverse effect. Moreover, the mechanism of inclusion evolution with Ce content was clarified, and the optimum Ce content in HNSBS was obtained (0.020 wt%). This work provides theoretical guidance for improving corrosion resistance of high-nitrogen steels. [Display omitted] • Cerium treatment improved the corrosion resistance of HNSBSs. • The optimum Ce content in HNSBS was approximately 0.020 wt%. • The oxide and sulfide inclusions exhibited induction effect on pitting corrosion. • The nitride (CeN) inclusion exhibited inhibition effect on pitting corrosion. • CeN inclusion acted as a protective armor wrapping around Ce 2 O 3 inclusion. [ABSTRACT FROM AUTHOR]

• Mo reduces lattice distortion and increases structural stability of high N MSSs. • Pressurized metallurgy and Mo-alloying increase N solubility in solid solution. • Mo-alloying reduces M 2 N content and alleviates detrimental effect of Cr-depletion. • Mo-alloying significantly improves corrosion resistance of high N MSSs. In this work, a new high corrosion resistant martensitic stainless steel (MSS) containing high nitrogen content (N > 0.33 wt.%) was designed based on density functional theory (DFT) calculation and pressurized metallurgy. The results indicate that Mo-N alloying significantly improve the corrosion resistance of MSS. This observation has been explained from point of view of crystal structural stability, wherein the reduced formation energy, bonding energy, density of state (DOS) and lattice distortion. It results in the increasing N content in solid solution and the decreasing fractions of Cr depletion induced by the precipitation of Cr 2 N. [ABSTRACT FROM AUTHOR]

A rotating packed bed (RPB) reactor plays an important role in process intensification of micromixing and mass transfer. However, the commonly used stainless steel wire mesh packing suffers from serious corrosion in various chemical processes. Polytetrafluoroethylene (PTFE) material, which is well-known for its excellent anticorrosion property, was employed as packing in the RPB reactor. Micromixing efficiency of the RPB reactor with PTFE packing was studied experimentally by adopting the iodide-iodate reaction system. Results showed that the micromixing efficiency of the RPB reactor with PTFE packing was better than that with the conventional stainless steel wire mesh packing. The micromixing time ( t m ) was calculated using an incorporation model based on the experimental data. Furthermore, the RPB reactor with PTFE packing was applied to a precipitation process for the preparation of magnesium hydroxide (Mg(OH) 2 ) nanoparticles. The obtained Mg(OH) 2 nanoparticles have small size and narrow particle size distribution, which indicates that the RPB reactor with PTFE packing is a promising reactor for preparing small size nanoparticles with its excellent micromixing efficiency. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) is a powerful tool used to investigate the hydrodynamics in various chemical devices. As one of the typical process intensification equipment, rotating packed beds (RPBs) have been widely used in various fields. However, it is still a challenge to obtain the detailed information by CFD analysis due to the complex packing structure in the rotor of a RPB. In this study, we firstly built a three dimensional (3D) physical model with the same structure and size as the physical RPB and structured stainless steel wire mesh packing. The realizable k - ε model is applied to investigate the gas pressure drop, pressure distribution, and gas flow at different rotational speeds and gas flow rates. Based on the breakdown of the overall gas pressure drop, the gas pressure drop in the inner cavity zone is the major contributor to the overall gas pressure drop under most operation conditions. The 3D physical model describing the actual RPB can give deep understanding of the gas flow in the entire RPB as well as the gas flow behavior around every fiber of the packing. Furthermore, one special phenomenon of strong turbulence, named as gas-side end effect, was revealed in the outer annular packing zone in the rotor. [ABSTRACT FROM AUTHOR]

The prediction models of pitting corrosion lifetime for martensitic stainless steels (MSSs) with different N contents were developed by statistical approach. The modelling and experimental results show that after N-alloying, the metastable pit initiation time was dramatically improved, and the transformation from metastable into stable pitting was retrained, which resulted in the increase of overall stable pit initiation time from 0 to 419.05 and 459.84 days with 0.15 and 0.41 wt% N addition, respectively. Afterwards, N significantly restrained the growth of stable pits. The present models reveal the mechanism of how N influences each stage of pitting corrosion. • Prediction models of pitting corrosion lifetime were established to reveal how N influences each stage of pitting corrosion. • N significantly prolonged the pitting corrosion lifetime of martensitic stainless steel. • N-alloying dramatically prolonged the pit initiation time and significantly restrained the growth of stable pits. [ABSTRACT FROM AUTHOR]

• Effect of N on corrosion of steel in S 2 O 3 2−-Cl- solution was firstly reported. • N significantly enhanced corrosion resistance of 316 L in S 2 O 3 2−-Cl- solution. • N restrained reduction of S 2 O 3 2− by consuming H+ and alleviated its harmful effect. • N reduced defects and raised fraction of Cr 2 O 3 and Fe 2 O 3 in passive film. Thiosulfate is a threat to corrosion resistance of steels in chloride solution. This work revealed the inhibitory effect of nitrogen (N) on thiosulfate-accelerated corrosion of stainless steel by electrochemical measurements, microscopy observation and X-ray photoelectron spectroscopy analyses. Results manifested that N-alloying significantly enhanced pitting corrosion resistance and repassivation ability of 316 L steel in thiosulfate-chloride solution. On the one hand, N consumed H+, then restrained reduction of S 2 O 3 2− and subsequent formation of S and MS 2. On the other hand, N reduced defect density and strengthened passive film by increasing fraction of Cr 2 O 3 and Fe 2 O 3 , thus improving protective ability of passive film. [ABSTRACT FROM AUTHOR]