Austenite-forming Elements Research Articles

Effect of TIG welding parameters on 316 L stainless steel joints using taguchi L27 approach

The AISI 316 L stainless steel was welded using Tungsten Inert Gas (TIG) welding, utilizing ternary shielding gases Argon (Ar), Helium (He), and Nitrogen (N2). This study aimed to assess the effects of these ternary shielding gases on the microstructure, bead profile, and bead appearance. It provides a comprehensive grasp of welding parameters’ interplay with shielding gas compositions, enabling engineers to make informed choices that significantly influence the excellence, productivity, and lastingness of the welding process. The Taguchi L-27 approach was employed, incorporating different contents of N2 (2.5 vol. % to 7.5 vol. %) and He (10 vol. % to 30 vol. %) within the Ar shielding gas composition. Additionally, welding current intensities, ranging from 120 A to 180 A, were also used in the experiment. The results demonstrated that higher content of He and N2 resulted in elevated levels of austenite-forming elements. Therefore, for TIG welding at the arc current intensity of 150 A, it is recommended to utilize the shielding gas mixtures (2.5 vol. % N2 + 10 vol. % He + 87.5 vol. % Ar). Furthermore, by augmenting the content of both N2 and He within the Ar shielding gas mixture, in addition to adjusting the arc current, a notable expansion in both the width and depth of the weld profile was achieved. This achievement, in turn, played a pivotal role in securing comprehensive fusion throughout the welding process.

Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics.

Structure and mechanical properties of high strength austenitic Mn–Ni–Cu–V–C dispersionally hardened steel

The paper presents the results of a study of the structure and mechanical properties of high-strength austenitic dispersionally hardened Mn–Ni–V–C steel with a yield strength of at least 700 MPa. Its composition and the hardening method were selected so that the steel meets the requirements for high strength and nonmagnetic properties. It is shown that the introduction of 1–2% Cu into Mn–Ni–V–C steel expands the region of existence of the γ-phase in the Fe–Ni–Mn phase diagram, narrows the two-phase γ+α-region and shifts it towards lower Mn contents, increasing stability of austenite to martensitic transformation during cold deformation. A numerical assessment of the influence of alloying austenite-forming elements Ni, Mn, Cu on the critical degree of cold plastic deformation, leading to the formation of deformation martensite in steel, is proposed. The temperature range of the reverse transformation of this martensite into austenite during annealing is established, depending on the nickel content in the steel. For precipitation hardened steel with a composition of 10%Mn; 10%Ni; 2%Cu; 0.3–0.4%C; ~1.4%V the regularities of dissolution upon heating for quenching and precipitation during aging of particles of the strengthening carbide phase VC were studied. It has been shown that the maximum strength is achieved after quenching from 1150°C and aging at 650°C for 15 hours. Taking into account the studies carried out on the stability of austenite, static and cyclic strength and durability, the optimal alloying range of steel with nickel, manganese and copper was substantiated, and the optimal mode of heat treatment was revealed, which provides a combination of high strength with good ductility and toughness of steel.

ОПТИМИЗАЦИЯ ХИМИЧЕСКОГО СОСТАВА СУПЕРМАРТЕНСИТНОЙ НЕРЖАВЕЮЩЕЙ СТАЛИ С ИСПОЛЬЗОВАНИЕМ ТЕРМОДИНАМИЧЕСКИХ РАСЧЕТОВ

Supermartensitic steels have a unique complex of chemical and mechanical properties, so pipes made of these steels have found their application in the oil and gas industry. However, these steels tend to form a banding structure. The banding structure is the cause of the products properties anisotropy and in addition can lead to the semi-finished products destruction at the production stage. In this work, the steel of the supermartensitic class 10Kh13N3MFB was investigated for its tendency to form a defect “banding” in the structure. For this, thermodynamic calculations and mathematical modeling were carried out using the Thermo-Calc-3.01 software. The following program functions were used: calculation of phase equilibria for the average chemical composition, as well as for compositions with a maximum of austeniteforming elements with a minimum of ferrite-forming elements and with a maximum of ferrite-forming ones with a minimum of austenite-forming elements. An constructed thermodynamic equilibria analysis showed that steel 10Kh13N3MFB tends to form a banded structure. To reduce the likelihood of banding, it is necessary to carry out crystallization and hot rolling of steel only in single-phase states (δ-ferrite and austenite, respectively). Therefore, further calculations in Thermo-Calc were aimed at determining the steel chemical composition, at which the alloy crystallizes in the single-phase δ-region, excluding the appearance of phase segregation, and establishing the temperature range of hot deformation. After analyzing the data obtained, the optimal chemical composition of the steel melting was proposed and the temperature ranges of hot rolling were determined for the average chemical composition with different contents of Ni – 2, 3, 4 wt. %.

Features of chemical composition and structural-phase state decreasing corrosion resistance of parts from 18Cr-10Ni steel

Features of the chemical composition and structural-phase state of samples of steel 18Cr-10Ni (AISI 304) were investigated, which could contribute to the occurrence of general corrosion damage and the pittings formation on parts made of this steel under the influence of an aggressive environment. It has been established that the sulfur content in steel is almost 10 times higher than the level established by the standard for this steel (0.03 % S), therefore, it contains about 3 vol. % of manganese sulfides, 1 – ~ 50 μm in size, forming stitches and accumulations along direction of rolling. According to the literature, it is the particles of manganese sulfide (MnS) that are most corrosive in corrosion-resistant steels and alloys. They significantly reduce the ability of Fe – Cr – Ni steels to passivate in a corrosive environment. For the formation of FeSH + ions, a high concentration of S 2– ions is required. The larger the inclusions of sulfide particles are, the higher is their ability to reduce the corrosion resistance of steel. Therefore, the large size of MnS particles found in steel plays an important negative role. It is shown that an additional factor contributing to a decrease in the corrosion resistance of the studied steel is the presence of deformation martensite in the surface layer of the steel, which was formed in the process of machining during manufacturing by cutting and grinding parts from a billet. The appearance of this martensite is due to the low concentration of austenite-forming elements (0.01 – 0.04 % C, 7.96 – 8.23 % Ni). The steel on the modified Scheffler-Delong diagram is in the region where martensite can form; the calculated value of М d (30/50) for it was 28 °С. According to literature data, deformation martensite in steels of 18-10 type causes a decrease in their resistance to pitting corrosion in solutions of acids and salts. It is shown that the presence of an electric potential activates the corrosive effect on 18Cr-10Ni steel samples in an acidic environment. It is concluded that the corrosion damage of parts made of the studied steel was facilitated by the presence of accumulations of sulfide particles in individual areas of the metal, combined with the presence of deformation martensite in these areas.

ОСОБЕННОСТИ ГРАДИЕНТНОГО МАТЕРИАЛА НА ОСНОВЕ НЕРЖАВЕЮЩЕЙ ХРОМОНИКЕЛЕВОЙ СТАЛИ И СПЛАВА Х20Н80, ИЗГОТОВЛЕННОГО МЕТОДОМ ЭЛЕКТРОННО-ЛУЧЕВОЙ 3D-ПЕЧАТИ

The main problem of additively manufactured chromium-nickel austenitic stainless steels is the formation of a two-phase γ-austenite/δ-ferrite dendritic microstructure, which complicates their use and distinguishes them from cast single-phase analogs. The reasons for the formation of a two-phase structure are nonequilibrium solidification conditions, complex thermal history, and melt depletion by austenite-forming elements (nickel and manganese). Therefore, additional nickel alloying under the additive manufacturing of steels can stabilize the austenitic structure in them. In this work, the authors used electron-beam additive production with simultaneous feeding of two wires from austenitic stainless steel Fe-18.2Cr-9.5Ni-1.1Mn-0.7Ti-0.5Si-0.08C wt.% (SS, Cr18Ni10Ti) and alloy 77.7Ni-19.6Cr-1.8Si-0.5Fe-0.4Zr wt.% (Ni-Cr alloy, Cr20Ni80) to obtain two gradient billets. The authors used two wire-feeding strategies (the first one is four layers of SS/one layer of Cr20Ni80; the second one is one layer of SS/one layer of a mixture 80 % SS + 20 % Cr20Ni80). The study identified that the Ni-Cr alloying in the process of electron-beam additive production of SS billets suppressed δ-ferrite formation and contributes to the stabilization of the austenite phase. The deposition of Ni-Cr alloy next to the four layers of SS leads to inhomogeneity of the structure and chemical composition in the billet, low plasticity, and premature failure of these specimens during tensile tests. The sequential alternation of pure SS layers with those of a mixture of wires (80 % SS + 20 % Cr20Ni80) promotes the uniform mixing of two wires components and the formation of a more homogeneous structure in the gradient billet, which leads to an increase in the ductility of the specimens during mechanical tests.

Microstructure and Mechanical Properties of High-Alloyed 23Cr-5Mn-2Ni-3Mo Cast Steel / Mikrostruktura I Właściwości Mechaniczne Wysokostopowego Staliwa 23Cr-5Mn-2Ni-3Mo

The article presents the microstructure and mechanical properties of cast duplex stainless steel type 23Cr-5Mn-2Ni-3Mo. It has been shown that the structure of the tested cast steel is composed of ferrite enriched in Cr, Mo and Si, and austenite enriched in Mn and Ni. In the initial state, at the interface, precipitates rich in Cr and Mo were present. A high carbon content (0.08%C) in this cast steel indicates that probably those were complex carbides of the M23C6type and/or σ phase. Studies have proved that the solution annealing conducted at 1060°C was not sufficient for their full dissolution, while at the solutioning temperature of 1150°C, the structure of the tested material was composed of ferrite and austenite.Partial replacement of Ni by two other austenite-forming elements, which are Mn and N, has ensured obtaining mechanical properties comparable to cast duplex 24Cr-5Ni-3Mo steel of the second generation. Basing on the results of static tensile test, a twice higher yield strength was proved to be obtained, compared to the cast austenitic 18Cr-9Ni and 19Cr-11Ni-2Mo steel commonly used in the foundry industry. In addition to the high yield strength (YS = 547 ÷ 572 MPa), the tested cast steel was characterized by the following mechanical properties: UTS = 731 ÷ 750 MPa, EL = 21 ÷ 29.5%, R.A. = 43 ÷ 52%, hardness 256 ÷ 266 HB. Fractures formed in mechanical tests showed ductile-brittle character.

A Study of the Process of Mechanical Alloying of Iron with Austenite-Forming Elements

Results of experimental studies of the effect of treatment time on the processes of phase formation and dissolution of alloying elements during mechanical alloying of iron with austenite-forming elements in a nitrogen-containing atmosphere are presented. The special features of the nanocrystalline structure of a Fe – 18% Cr – 8% Ni – 12% Mn – xN powder alloy obtained by mechanical alloying and the effect of the method of compaction on the mechanical properties of articles are determined.

Investigation of the Process of Mechanical Alloying of Iron by Austenite Forming Elements

In this study the effect of the treatment duration on the processes of phase formation and dissolution of alloying elements in the process of mechanical alloying (MA) of iron by austenite forming elements in the nitrogen-containing atmosphere was investigated. Investigating the influence of MA parameters on the phase composition of the alloy showed that the first alloying elements dissolved in the lattice of iron are nickel, chrome and manganese. According to experimental data, the dissolution proceeds through the formation of a layered composite.

To selection of technological scheme of softening heat treatment for high chromium cast iron

Purpose. High chromium cast irons with austenitic matrix have low machinability. The aim of work is search of new energy-saving modes of preliminary softening heat treatment enhancing the machinability of castings by forming an optimum microstructure. Methodology. Metallographic analysis, hardness testing and machinability testing are applied. Findings. It was found out that high temperature annealing with continuous cooling yields to martensite-austenite matrix in cast iron 270Х15Г2Н1MPhT, which abruptly affects the machinability of cast iron. Significant improvement of machinability is achieved by forming of structure "ferrite + granular carbides" and by decline of hardness to 37-39 HRC in the case of two-stage isothermal annealing in the subcritical temperature range or by the use of quenching and tempering (two-step or cyclic). Originality. It was found that the formation of the optimal structure of the matrix and achievement of desired hardness level needed for improving machinability of high chromium cast iron containing 3 % austenite-forming elements, can be obtained: 1) due to pearlite original austenite followed by spherodization eutectoid carbides, and 2) by getting predominantly martensite structure followed by the decay of martensite and carbides coagulation at high-temperature tempering. Practical value. The new energy-saving schemes of softening heat treatment to ensure the growth of machinability of high chromium cast iron, alloyed by higher quantity of austenite forming elements, are proposed.

Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels

11-12Cr oxide dispersion strengthened (ODS) tempered martensitic steels underwent manufacturing tests and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and metallography. It was demonstrated that excessive formation of residual-α ferrite, due to increasing Cr content, could be suppressed by appropriately controlling the concentration of the ferrite-forming and austenite-forming elements on the basis of the parameter “chemical driving force of α to γ reverse transformation. 11Cr-ODS steel containing a small portion of residual-α ferrite was successfully manufactured. In the as-received condition, this 11Cr-ODS steel was shown to have satisfactory creep strength and ductility, both as high as those of the 9Cr-ODS steel, while its 0.2% proof strength at 973K was lower than in the 9Cr-ODS steel.

The effect of the chemical composition of a nitrogen austenitic steel on its forgeability

Nitrogen-bearing corrosion-resistant austenitic steel is a new product of some metallurgical works; it has a high strength, forgeability, impact toughness, corrosion resistance, and a low nickel content [1]. The properties of nitrogen-bearing corrosion-resistant steels were studied in [2], and the thermodynamics and kinetics of nitrogen dissolution in a metallic melt were considered in [3, 4]. At present, the methods for the optimization of the production of high-nitrogen corrosion-resistant steel and the problem of its quality control are being analyzed. Alloying elements in steels of this class exhibit a strong tendency toward micro- and macrosegregation, which causes nonuniform mechanical properties across an ingot and several types of defects. 12Cr18Mn18N steel is used to produce retaining rings and has a significant potential for use in other fields of engineering. The OOO OMZ-Spetsstal company has a great deal of experience in making steel of this grade. The steel under study was melted in a 50-t arc furnace; its chemical composition was finished in an outof-furnace unit; and two 22-t and one 9-t ingots were poured. After cutting the sinkheads of the 22-t ingots, their face ends were fettled, and they were then remelted to produce 18-t ingots in an electroslagremelting setup. The 9-t ingot was forged to produce an electrode and it was then remelted in the electroslagremelting setup at the OAO Mechel company to form a 5-t ingot. After electroslag remelting the ingots were heat-treated and forged to yield sleeve-tube workpieces. When retaining rings were produced, the forgeability of the 12Cr18Mn18N steel was found to be insufficient. To improve the forgeability of the ingots, we will study the causes affecting their forgeability and determine the possibilities for controlling their quality. Forgeability is known to be related to the presence of different phases, which serve as stress concentrators, in the bulk of the metal. These phases for the steel under study are nonmetallic inclusions and a ferrite phase forming along grain boundaries [5]. These defects form due to various technical factors, one of which is the chemical composition of the metal. To determine the effect of the chemical composition on the plastic properties of the steel, we used group and regression analyses to process experimental data. These results indicate the causes of the low forgeability of the steel. To estimate the effect of the chemical composition of the 12Cr18Mn18N steel on ingot forgeability, we compared the averages and dispersions of two statistical samples at different ductilities (see table). The first statistical sample consists of the chemical compositions of twelve ingots whose forgeability during forging was insufficient. The second sample consists of the chemical compositions of eleven ingots whose forgeability during forging was sufficient. The averages were compared using Student’s test; the dispersions were compared using Fishsher’s variance ratio; and the confidence level in both cases was 95%. In the ingots with a sufficient forgeability, the aluminum concentration and the Mn/Cr ratio are higher than those in the ingots with an insufficient forgeability. Our results indicate that the cause of the low forgeability of the steel is a nonoptimal composition of nonmetallic inclusions, which is determined by the aluminum content in the metal. A second factor is the presence of a ferrite phase in the steel, which depends on the ratio of the concentration of austenite-forming elements to the concentration of ferrite-forming elements [6].

Phase and structural transformations in high-nitrogen austenitic Fe−18% Cr alloys

When a “superequilibrium” amount of nitrogen (0.9–1.3%) is introduced into corrosion-resistant low-carbon steel with about 18% chromium, the structure of the steel becomes completely austenitic after cooling to room temperature without alloying with austenite-forming elements (Ni, Mn, Cu, Co). The structural transformations in cooling and heating such alloys, and hence their properties, can be controlled by changing the nitrogen content.

Structure and properties of low-carbon nitrogen-containing augstenite-ferrite corrosion-resistant steels

The concentration of elements in existing austenite-ferrite steels varies in a rather narrow range. This is associated with the necessity of ensuring an optimum austenite/ferrite proportion in the structure (∼ 40–60%), which is attained by adding ferrite- and austenite-forming elements in specified amounts. The article concerns the structure, mechanical properties, and corrosion resistance of low-carbon austenite-ferrite steels with 18–25% Cr, 2–6% Mo at −7% Ni and 0.15% N and the choice of the most suitable limiting concentrations of chromium and molybdenum.

Effect of cobalt, copper, and manganese with combined alloying on the properties of 12% chromium steels of the martensitic class

1. On alloying chromium martensitic steel 12Kh12N2M with austenite-forming elements (cobalt, copper, manganese) the best properties are achieved with manganese additions. 2. On introducing 3.9% Mn into steel 12Kh12N2M its strength properties are almost unchanged; there is an increased ductility by 30% and impact strength (under 475-degree brittleness conditions) by 30–60% compared with these characteristics for steel alloyed with cobalt alone.

Effect of nitrogen on the phase composition and physical and mechanical properties of stainless austenitic-pearlitic steels

1. Being an austenite-forming element, nitrogen (up to 0.36%) hardens austenite into austenitic-ferritic steels, but significantly reduces the amount of ferrite — from 65 to 35%. The overall effect of this influence of nitrogen is to reduce the strength properties of the steels under investigation. 2. A reduction in the amount of α-phase in the structure of dual-phase steels with the introduction of nitrogen is accompanied by pronounced enrichment of the ferrite with ferrite-forming elements, primarily with chromium and molybdenum, and its impoverishment with austenite-forming elements. This contributes to the formation of σ-phase, which leads to embrittlement of the steel.

The nature of cracks in castings of steel 27KhGSNL

1. Cracks in castings of steel 27KhGSNL result from breaking along the boundaries of dendrite colonies or primary austenite grains, brittle fracture along ferrite located in primary austenite grain boundaries, intercrystalline fracture along definite planes, and transcrystalline fracture. 2. Fine fractographic analysis of the oxidized surfaces of cracks is possible with the scanning electron microscope. 3. To reduce the tendency of castings to crack it is necessary to increase the quantity of austenite-forming elements and reduce the amount of ferrite-forming elements and increase the disperisity of the dendritic structure by increasing the cooling rate of the liquid steel.

Effect of chemical composition on the properties of heat-resisting steel with 12% Cr

1. The steel of the following chemical composition has the best combination of properties: 0.16%C, 12% Cr, 1% Ni, 0.060% N, 0.07% Mo, 0.40% V, 0.30% Nb. 2. The addition of ferrite-forming elements increases the stability during tempering; to counteract the ferritizing effect of these elements it is necessary to increase the concentration of austenite-forming elements, Ni and N. 3. Nickel lowers the Ac1 temperature; its influence is reduced considerably by the addition of 0.060% N. The steel with 0.06% N has a constant value of Ac1 (780°C), i.e., 180° above the highest operating temperature for parts manufactured from this steel. 4. The steel with 12% Cr and nitrogen has the best mechanical properties at 670°C.

Low-nickel austenitic KH14G14N3T steel

1. Kh14G14N3T steel has an austenitic structure with a small amount of the α-phase; the amount of this phase is determined by the ratio between the ferrite and austenite-forming elements. 2. At room temperature Kh14G14N3T steel has a higher strength than Kh18N9T steel. 3. Kh14G14N3T steel can easily be welded by different methods and is recommended as a substitute for Kh18N9T steel in manufacturing welded apparatus subject to high pressures at temperatures as low as −196°C. 4. Preliminary cold working (up to 20%) increases the strength of the steel without impairing its ductility at low temperatures in any significant way. 5. The use of Kh14G14N3T steel in place of Kh18N9T steel makes it possible to decrease the use of nickel considerably and decrease the cost of the equipment.

The structure and stability of austenite in Cr?Mn?Ni steels containing nitrogen

1. We have defined the concept of steel "austeniticity." This term indicates the degree to which the austenite structure is stable and to what extent it is possible to change the composition of the steel (increase the concentration of ferrite-forming elements or decrease the austenite-forming elements) or subject the steel to deformation without inducing transformation of the austenite. 2. We investigated the effect of the variation of the chemical composition of Cr17G9ANi4 steel and showed that the standard composition of this steel is strongly austenitic. The concentration of the austenite-forming elements (N, Mn) can be decreased without inducing the occurrence of δ-ferrite and without rendering the steel susceptible to the formation of martensite under the effect of deformation.