Influence of nitrogen on corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy

Effect of nitrogen on microstructure and corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy was investigated by microscopy, electrochemical and spectroscopy analyses. Results indicated that increasing nitrogen content significantly enhanced the corrosion properties of martensitic stainless steels, while excess nitrogen deteriorated the corrosion resistance. The impacts of increased nitrogen content could be summarized as three aspects: the change of precipitation content and conversion of main precipitates from MC to MN; the enhanced protection performance of passive film by enrichment of Cr, especially CrO and CrN; the improved repassivation ability by increased nitrogen content in solid solution

Relationship between Microstructure and Corrosion Behavior of Martensitic High Nitrogen Stainless Steel 30Cr15Mo1N at Different Austenitizing Temperatures

The relationship between microstructure and corrosion behavior of martensitic high nitrogen stainless steel 30Cr15Mo1N at different austenitizing temperatures was investigated by microscopy observation, electrochemical measurement, X-ray photoelectron spectroscopy analysis and immersion testing. The results indicated that finer Cr-rich M2N dispersed more homogeneously than coarse M23C6, and the fractions of M23C6 and M2N both decreased with increasing austenitizing temperature. The Cr-depleted zone around M23C6 was wider and its minimum Cr concentration was lower than M2N. The metastable pits initiated preferentially around coarse M23C6 which induced severer Cr-depletion, and the pit growth followed the power law. The increasing of austenitizing temperature induced fewer metastable pit initiation sites, more uniform element distribution and higher contents of Cr, Mo and N in the matrix. In addition, the passive film thickened and Cr2O3, Cr3+ and CrN enriched with increasing austenitizing temperature, which enhanced the stability of the passive film and repassivation ability of pits. Therefore, as austenitizing temperature increased, the metastable and stable pitting potentials increased and pit growth rate decreased, revealing less susceptible metastable pit initiation, larger repassivation tendency and higher corrosion resistance. The determining factor of pitting potentials could be divided into three stages: dissolution of M23C6 (below 1000 °C), dissolution of M2N (from 1000 to 1050 °C) and existence of a few undissolved precipitates and non-metallic inclusions (above 1050 °C)

Design of ESR Slag for Remelting 9CrMoCoB Steel under Simple Protective Ar Gas

Thermodynamic calculations by using Factsage 7.3 and simple protective gas electroslag remelting (ESR) experiments were conducted to design the appropriate ESR slag for remelting the qualified 9CrMoCoB ingot. First, the proper basic slag was determined based on the phase diagram of CaF2–CaO–Al2O3–x%MgO calculated using Factsage 7.3. Second, equilibrium reactions between 9CrMoCoB and the basic slag containing varied SiO2, and B2O3 contents were calculated to study the effects of B2O3 and SiO2 on B, Si, and Al contents in steel. Then, equilibrium reaction experiments were conducted to validate the calculated results. Finally, the appropriate slags were attained and the simple protective Ar gas ESR experiments were conducted to make a verification. Results showed that the liquid phase region of the slag of CaF2–CaO–Al2O3–x%MgO at 1300–1400 °C increased first, then decreased with the MgO addition. With the B2O3 content increasing, the boron content increased, while the Si, Al contents decreased, and at a given B2O3 addition, the B and Al contents decreased with the SiO2 content increasing, whereas the Si content increased. The appropriate slags for simple protective gas ESR remelting 9CrMoCoB were 55%CaF2–20%CaO–3%MgO–22%Al2O3–2%SiO2–1.3%B2O3 and 55%CaF2–20%CaO–3%MgO–22%Al2O3–3%SiO2–1.7%B2O3

Dissolution Behavior of Alumina-Based Inclusions in CaF2-Al2O3-CaO-MgO-SiO2 Slag Used for the Electroslag Metallurgy Process

Removal of non-metallic inclusions to CaF2-based slag is one of the most important functions of electroslag remelting. In this work, the dissolution behavior for alumina-based inclusions in CaF2-Al2O3-CaO-MgO-SiO2 slag has been investigated. Results indicate that the diffusion or permeability capacity of slag components into alumina particles is F−, Ca2+, Si4+, Mg2+, from strongest to weakest, for CaF2-Al2O3-CaO-MgO-SiO2 slag. Alumina inclusions react with F− in liquid slag at first and then react with CaO to form xCaO-yAl2O3 system. Subsequently, MgO substitutes for CaO to form a MgO-Al2O3 system layer surrounding the other product and reactant, and then enters the liquid slag. CaF2 can improve the dissolution capacity of slag to alumina inclusions. A complex region was formed between alumina-based particles and the slag, with different areas dominated by CaF2, CaO-Al2O3, CaO-SiO2 and MgO-Al2O3. The dissolution process of alumina particles in slag is different from the formation of compound inclusions originated from the Al-O deoxidization reaction

Critical evaluation and thermodynamic optimization of the Si-N, Si-C, C-N and Si-C-N systems and its applications to high purity SiC production

© 2022 Elsevier LtdThermodynamic optimization of the Si-N, Si-C, C-N and Si-C-N systems were performed using the CALculation of PHAse Diagrams (CALPHAD) method based on critical evaluation of all available experimental data. The liquid and solid solutions within the Si-C-N system were described using the Modified Quasichemical Model and Compound Energy Formalism, respectively. The Gibbs energies of β-Si3N4 and β-SiC were carefully optimized to resolve inconsistencies among existing thermodynamic properties and phase equilibria data. The solubilities of C and N in both solid and liquid Si solutions were critically optimized. Besides, the solubility of N in β-SiC to form a narrow homogeneity range of β-Si(C,N) solid solution is considered for the first time in the present thermodynamic modeling. Based on the present thermodynamic database, experimentally unexplored phase diagrams and thermodynamic properties within the Si-C-N alloys were predicted. As an application of the database, the influences of various operation conditions on high purity SiC production using sublimation process were calculated.N

Effects of Ce on Microstructure and Mechanical Properties of LDX2101 Duplex Stainless Steel

In the present study, the effects of rare earth metal (REM) Ce on the number, size distribution and type of inclusions, as well as the grain size, tensile and impact properties of LDX2101 duplex stainless steel, were investigated using thermodynamic analysis, optical microscope, image software, scanning electron microscope, tensile and impact testing machines. The results indicate that when the Ce content in steel is 0%, the main inclusions are irregular and large size Al2O3 and Al2O3-MnS. When the Ce content in steel is 0.005% or 0.02%, the inclusions are mainly spherical and small size inclusions CeAlO3, Ce2O2S and Ce2O3. With the increase in Ce content, the proportion of small size inclusions gradually increases; the number of inclusions per unit area decreases from 122 to 58 and the average grain size reduces from 16 ± 0.7 μm to 12 ± 0.3 μm. Moreover, the mechanical properties of steels are improved, and the tensile strength, yield strength and elongation are elevated by 4.69%, 2.83% and 4.9%, respectively. The impact fracture mode of steel without Ce is cleavage fracture, however, the fracture mode is transformed into a mixed fracture of cleavage and dimple after adding 0.005% and 0.02% Ce

Effect of Mold Flux Containing Ce2O3 on the Contents of Aluminum, Silicon, and Titanium in Incoloy825 Super Alloy

The effect of mold flux containing Ce2O3 on the contents of aluminum, silicon, and titanium in Incoloy825 super alloy was investigated based on the slag-steel interfacial chemical reaction experiment between mold flux and alloy. Firstly, the activity model of the CaO-SiO2-Al2O3-Na2O-MgO-CaF2-Ce2O3 slag system was established according to the ion and molecule coexistence theory (IMCT), and the calculation results show that with the increase of Ce2O3 content in the mold flux, the activity of Al2O3 decreases significantly and the activity of SiO2 decreases and gradually tends to 0. Secondly, thermodynamic calculations of the slag-steel interfacial chemical reaction revealed that the main chemical reaction in this study system is [Ti] + (SiO2) = [Si] + (TiO2). With the increase of Ce2O3 content in the mold flux, the slag-steel interfacial chemical reaction is weakened and the oxidation of Al and Ti in steel is inhibited. Finally, the results of slag-steel reaction experiment show that the increase rate of Al content increases from 1.03% to 10.31%, the increase rate of Si content decreases from 55.95% to 31.25%, and the oxidation rate of Ti content decreases from 33.27% to 20.00% when Ce2O3 content in the mold flux increases from 0% to 15% and the slag-steel reaction for 40 mins

Influence of Slag System Ingredients on Hydrogen Content in 316H Stainless Steel for Electroslag Remelting in Nuclear Power Applications

During the process of electric arc remelting， the content of gas components （such as H， O， N， etc.） in the steel is significantly influenced by the gas permeability of the slag system used. In order to explore a slag system with low hydrogen permeability suitable for nuclear-grade 316H stainless steel in electric arc remelting， the hydrogen permeability of five slag systems used in electric arc remelting was determined. The results showed that the newly developed 63%CaF2-30%Al2O3-7%MgO slag system had the lowest hydrogen permeability.meanwhile the hydrogen permeability of slag system， 65%CaF2-30%Al2O3-5%MgO， was similar to that of the 63%CaF2-30%Al2O3-7%MgO slag system， and hydrogen permeability of both slag systems exhibited a significant reduction compared to the original slag system used in the steel plant's electric arc production，declined from 6.58×10-6 mol/（cm·min） to 1.89×10-6 mol/（cm·min） and 2.18×10-6 mol/（cm·min） respectively.The higher the optical basicity of the slag was， the higher the hydrogen permeability was. CaO in the slag had a high affinity for water and can easily increase the hydrogen content in the steel ingot during electric arc remelting， while the addition of MgO in the slag system can significantly reduce hydrogen permeability. Under the research conditions， the influence of the slag system's constituents on its hydrogen permeability was more significant than the optical basicity

The Latest Progress and Prospect of Special Metallurgical Products， Technology and Equipment in China

The high-end equipment industries in the fields of aerospace， energy， petrochemical， shipbuilding， rail transportation， new energy vehicles， energy conservation and environmental protection， and electronic information have developed strongly， putting forward higher requirements for the quality and performance of special steel and special alloy materials， and the demand has surged. Therefore， in recent ten years， China's special metallurgy industry has been rapidly development. This paper first analyzes and summarizes the new requirements of ultra-high strength steel， supper alloy， corrosion resistant alloy， heat resistant steel， special stainless steel， high performance bearing steel， tool and die steel and precision alloy for the above-mentioned high-end equipment manufacturing. Secondly， the development status and trend of traditional special metallurgical processes and several new special metallurgical processes are analyzed. It is emphasized that the combination with basic oxygen furnace/electric arc furnace steelmaking process can provide high clean consumable electrode for electroslag remelting and vacuum arc remelting， and can also provide pure raw material for vacuum induction furnace. The short process of the electroslag remelting with continuous casting billet as the consumable electrode can significantly improve the production efficiency and reduce the production cost. At the same time， the duplex process of high nitrogen stainless steel smelting and the process flow of powder metallurgy and spray forming of tool and die steel are also briefly introduced. Third， China's special metallurgical industry development status， as well as the progress of new technology and new equipment have been summarized. Finally， suggestions and prospects for the technical development of special metallurgy in China in the next ten years are put forward

High Purity Smelting Technology for Ultra-high Strength Steels

Ultra-high strength steel with high tensile strength, good toughness, high specific strength, modulus and other characteristics are widely used in aviation, aerospace and national defense and other fields. Ultra-high strength steel is preferred material for aircraft and aero-engines and other aviation equipments. The application of ultra-high strength steel represents a country's highest level of steel research and production, and it is also an important symbol of the development of national science and technology and national defense industry. The development and application of high purity smelting technology for manufacture of ultra-high strength steels at domestic and overseas is briefly reviewed in the paper, and then the control ability about the impurity elements such as S, P, O and N in typical ultra-high strength steels, and the research status and development trend of non-metallic inclusions control are discussed. The progress in research work of high purity smelting technology for ultra-high strength steels carried out by the authors in recent years has been introduced, it shows that the control level of impurity elements and non-metallic inclusion has been greatly improved, and it also creates a new route for China to manufacture the ultra-high strength steel with high alloy, especially with high purity for ultra-high strength stainless steel, bearing steel and gear steel. Finally, the development direction of high purity smelting technology of ultra-high strength steel in China is pointed out