Austenitic Heat-resistant Steel Research Articles

Constructing high-density dislocations by primary (Nb,Ti)(C,N) to induce massive secondary precipitations in austenitic heat-resistant cast steel

The microstructural evolution and the high-temperature tensile properties of a novel Nb-Ti-N alloyed austenitic heat-resistant cast steel were investigated during the isothermal aging at 900 °C for up to 1000 h. The mechanisms of enhancing strength-ductility synergy and the high-temperature tensile fracture behaviors after the aging process were analyzed in detail through tensile tests at 900 °C. Obtained results show that the high-density dislocation regions and the Cr segregation, formed around the intragranular primary (Nb,Ti)(C,N), provided numerous nucleation sites for submicron-sized secondary M23C6 precipitates. The density of secondary M23C6 significantly increased, while the coarsening rate was notably reduced during the aging process. Simultaneously, the uniform dispersed precipitation of nano-sized secondary (Nb,Ti)C was promoted by massive dislocations, which exhibited high resistance to coarsening during long-term aging. The improvement of high-temperature strength was attributed to the precipitation strengthening effects induced by the submicron-sized M23C6 and nano-sized (Nb,Ti)C particles. Additionally, the enhancement of ductility during aging was mainly due to the more effective stress transmission and the ameliorated interface relationship between primary (Nb,Ti)(C,N) and austenitic matrix, which was facilitated by the abundant precipitation of secondary phases. The current findings indicate that high-density dislocations constructed by the intragranular primary (Nb,Ti)(C,N) induce the extensive dispersion of secondary strengthening phases, which enhanced both ultimate tensile strength of 158 MPa and elongation of 45.3% at 900 °C after isothermal aging for 1000 h.

Calculation of the elastic modulus of steel during tensile

The rational use of resources in the design of vessels and devices and ensuring their growing needs requires the performance of strength and stability calculations, which require the use of appropriate values of the modulus of elasticity during tension E. The purpose of the article is to improve the definition of the modulus of elasticity of steels during tension, the value of which is normalized by the current in Ukraine by regulatory documents. On the basis of standard tables, dot graphs of changes in modulus of elasticity of carbon, low-alloy, heat-resistant, corrosion-resistant chromium, heat-resistant and heat-resistant austenitic steels and iron-nickel-based alloys during tension E depending on temperature t were constructed and analyzed. It is proposed to approximate these dependencies with simple mathematical equations. As a result, it was determined that the dependence E = f (t) for carbon and low-alloy steels in the range of changing t from 20 oС to 300 oС is described by a simple linear equation E = – 100t + 2.01.105. When changing t from 300 oС to 450 oС inclusive, it is proposed to apply cubic regression E = – 5.33.10–3t3 + 5.2t2 – 1.827.103t + 3.95.105. In the above and further proposed formulas, the temperature t is substituted in °C, and the result is obtained in MPa. Heat-resistant and corrosion-resistant chromium steels in the temperature range t from 20 oС to 100 oС inclusive are characterized by the same modulus of elasticity during tension E = 2.15.105 MPa. The dependence of E = f (t) in the range of changing t from 100 oС to 250 oС inclusive can be described by the cubic regression E = 1.33.10–3t3 – 2.63.102t + 2.4.105, and when changing t from 250 oС to 600 oС inclusive – by cubic regression E = – 7.5.10–4t3 + 0.722t2 – 3.426.102t + 2.4748.105. The value of the modulus of elasticity during tension for heat-resistant and heat-resistant austenitic steels and iron-nickel-based alloys E = 2.15.105 MPa in the temperature range t from 20 oС to 100 oС. The dependence of E = f (t) in the range of changing t from 100 oС to 700 oС inclusive is described by the quadratic regression E = – 0.152t2 + 9.101t + 2.009.105. The obtained formulas make it possible to abandon the use of standard tables and additional interpolation of intermediate values of the modulus of elasticity of steels during tension when performing calculations, which in turn simplifies both the calculation itself and the development of appropriate computer programs. The average error value of the performed approximations is in the range from 0 % to 0.218 %, which indicates a high level of coincidence of the regression equations with the actual values. The use of the proposed formulas for calculating the modulus of elasticity of steels during tension makes it possible to simplify the calculation of vessels and devices for strength and stability.

Effect of Mn addition on microstructure and mechanical properties of GX40CrNiSi25-12 austenitic heat resistant steel

Effect of Mn addition on microstructure and mechanical properties of GX40CrNiSi25-12 austenitic heat resistant steel

Synergetic Kinetics of High-Temperature Surface Degradation Processes in Austenitic Heat-Resistant Cast Steels

Synergetic Kinetics of High-Temperature Surface Degradation Processes in Austenitic Heat-Resistant Cast Steels

Developing the optimal chemical composition of heat-resistant Cr-Ni steel for aerospace equipment

The object of research in this work is the processes of forming the microstructure and mechanical properties of heat-resistant steels depending on their chemical composition. The microstructure and high-temperature mechanical properties of austenitic heat-resistant chrome-nickel steels of the proposed chemical composition were investigated in the work. The microstructure study determined the grain size and topography of inclusions in steels. Based on the results of mechanical tests, a multifactorial experiment was planned, which made it possible to establish the relationship between the mechanical properties of steels with alloying elements selected as factors. The obtained regression equations were used to estimate the quantitative influence of each alloying element on the corresponding mechanical properties. Analysis of the results of the experiment made it possible to determine the optimal chemical composition of steel for gas turbine engines used in the aerospace industry, as well as in the metallurgy of titanium production. It is shown that the state of solid solution and heat-resistant niobium and molybdenum carbides (chromium carbides dissolve at a temperature of 950 °C) are an important factor that significantly affects the structure, mechanical, and service properties of heat-resistant steel. The austenite structure is provided by the required amount of nickel. The resulting indicators of heat resistance of steels of different compositions, tested at temperatures of 850 °C, 950 °C, and 1050 °C, proved the superiority of steel with a higher content of carbon and chromium and a lower content of nickel. The mechanism of steel strengthening and the influence of alloying with carbon, chromium, and nickel on it have been determined. The optimal chemical composition of heat-resistant steel on an iron-nickel basis for operation at a temperature of 950 °C has been established

Segregation behavior of alloying elements at the fcc-Fe/TiC interface by first principles exploration

In this paper, the effects of rare earth elements on the bonding strength and stability of TiC/fcc-Fe interface are explored by using the first-principles method based on density functional theory. The results show that the Ti terminal is more stable than the C terminal in the process of forming the interface. The alloying elements tend to segregate at position 2 on the side of fcc-Fe. The segregation of Mo, Nb, Cr and Ce alloying elements increases the interatomic electron cloud enrichment and consumption between the interfaces and enhances the Fe–Ti interactions. The d orbitals of Mo, Nb, Cr and Ce and f orbitals of Ce have strong hybridization with Fe-d orbitals and Ti-d orbitals electrons near the Fermi energy level, indicating an increase in bonding strength and stability of the interfaces. When Fe atoms are replaced by W, Ni and Al atoms, the covalent bond strength between interfacial atoms is reduced, thus weakening the interfacial bonding strength. This provides solid theoretical foundation with regard to further application in austenitic heat-resistant steel fields.

Long-term creep behaviours and structural stabilities of austenitic heat-resistant stainless steels

ABSTRACT For heat resistant alloys, long-term structural stability at high temperatures is a critical issue for alloy design and applications. In this paper, the long-term creep behaviours and structural stabilities of six heat resistant high Ni alloys and austenitic stainless steels have been studied. The longest creep rupture life is up to 359 283 hours. High Ni and Cr alloys show a good combination of high creep and oxidation resistances. Precipitation of nano MX particles with a very low growth rate improves long-term creep resistance at high temperatures. Long-term stable multiple nanoprecipitates of MX, Cu-rich, Laves and M23C6 phases can greatly contribute to the creep strength. Low Ni austenitic stainless steels show comparatively low oxidation and creep resistances. It was first found that at 800°C, Cr2N could form in the low Ni steel with a long-term crept by the absorption of nitrogen from the air into the matrix.

Preferential distribution behavior of B and its enhancement on creep strength of alumina-forming austenitic heat-resistant steel

The addition of a trace amount of 20 ppm B significantly enhanced the creep performance of the 2.5Al-AFA alloy at 650 °C, demonstrating sustained improvement even after prolonged thermal aging. Boron (B) preferentially distributed in the Laves phase within the matrix, rather than in the NiAl or Cr - rich phases. APT analysis revealed M23C6 may serve as a reservoir of B, gradually allowing B diffusing into the surrounding Laves phase at grain boundaries (GBs). Creep cracks propagated mainly along GBs, with the σ phase serving as initiation sites. The average volume fraction of σ phase exhibited a linear correlation with the reciprocal of creep rupture time. B accelerated the precipitation of Laves with a larger size and higher volume fraction, which played a dual role of improved GB strengthening and more effective suppression of σ phase growth.

Prediction of High-Temperature Creep Life of Austenitic Heat-Resistant Steels Based on Data Fusion

The creep life prediction of austenitic heat-resistant steel is necessary to guarantee the safe operation of the high-temperature components in thermal power plants. This work presents a machine learning model that can be applied to predict the creep life of austenitic steels, offering a novel method and approach for such predictions. In this paper, creep life data from six typical austenitic heat-resistant steels are used to predict their creep life using various machine learning models. Moreover, the dissimilarities between the machine learning model and the conventional lifetime prediction method are compared. Finally, the influence of different input characteristics on creep life is discussed. The results demonstrate that the prediction accuracy of machine learning depends on both the model and the dataset used. The Gaussian model based on the second dataset achieves the highest level of prediction accuracy. Additionally, the accuracy and the generalization ability of the machine learning model prediction are significantly better than those of the traditional model. Lastly, the effect of the input characteristics on creep life is generally consistent with experimental observations and theoretical analyses.

Evolution of the oxide layers of an aluminum-forming austenitic heat-resistant steel with high-Mn substitution for Ni: Experiment and first-principle calculations

High Mn (8 wt%) was used to save Ni resources to replace 10 wt% Ni in Aluminum-forming austenitic (AFA) heat-resistant steel; the changes in oxidation resistance and morphology evolution of multiple oxide scales for the 8Mn-AFA steel were then studied. The 8Mn-AFA steel formed a three-layer scale on the surface oxidized at 750 °C for 100 h: inner dense Al2O3 sub-layer closed to the austenite matrix, intermediate Cr2O3 sub-layer with partial Cr substituted by Mn, and outer loose Mn2O3 + Mn3O4 sub-layer with FeMnO3 surface nodules. Mn from the matrix to the α-Al2O3 diffusion is spontaneous. However, Mn moving from an α-Al2O3 to α-Cr2O3 leads to partial Cr in the Cr2O3 sub-layer was substituted by Mn. When the 8Mn-AFA steel oxidated at 750 °C for 360 h, the Mn oxides, including Mn2O3, Mn3O4 and FeMnO3, increased significantly with the prolongation of the oxidation time. First-principle calculations based on density functional theory (DFT) also confirmed the diffusion of Mn did not destroy the dense Al2O3 scale. The oxidation weight gain of 8Mn-AFA steel oxidized at 750 °C in the air for 360 h follows a parabolic law, and the oxidation rate constant is 2.65 × 10−4.

Comparative study on microstructure and mechanical properties of a novel nano-composite strengthening heat-resistant steel and two typical heat-resistant steels

In this study, a new nano-composite strengthening austenitic heat-resistant steel designed for ultra-super critical (USC) thermal power plants was analyzed in comparison with the traditional H3RC and S30432 heat-resistant steels to assess its reliability. The microstructure, precipitates, hardness, tensile property and low cycle fatigue behaviors of the three steels at room temperature and service temperature were characterized. The results show that the SP2215 has finer grains, higher twin density, more primary MX, NbCrN and M23C6 precipitates compared with the HR3C and S30432 steels. The main precipitates in the steels after solution treatment are secondary (Nb, V)(C, N) phase (MX). The subsequent aging treatment of the SP2215 steel results in precipitation of the M23C6 carbides at the grain boundaries and Cu-rich phase inside the grains. As a result, the SP2215 steel shows higher Vickers hardness, tensile and fatigue strength (400 MPa) at room temperature than the other two steels. Transgranular fracture is the dominant tensile fracture mechanism of the three steels at room temperature, while intergranular cracking becomes the main fracture mode at high temperature due to precipitation of the continuous M23C6 at the grain boundaries. At the temperature of 700 ºC, fatigue strength of the SP2215 under a peak stress lower than 450 MPa decreases only a little.

Solidification Characteristics and Eutectic Precipitates in 15Cr–22Ni–1Nb Austenitic Heat-Resistant Stainless Steel

Solidification Characteristics and Eutectic Precipitates in 15Cr–22Ni–1Nb Austenitic Heat-Resistant Stainless Steel

Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical properties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M23C6 carbides, Laves, σ and G phases occurred at the grain boundaries. However, Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound complexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M23C6 carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C.

Effect of Cr and Ni concentrations on resilience of cast Nb-alloyed heat resistant austenitic steels at extreme high temperatures

Austenitic Cr–Ni alloyed heat resistant steels with Nb additions are used for intensively thermo-mechanically loaded cast components working in extreme high temperature oxidizing environment. Their performance during static oxidation and transient thermo-mechanical loading was investigated to recommend an optimal cost-effective Cr/Ni composition of Nb-alloyed austenitic class steels. The static oxidation and transient thermo-mechanical behavior of three austenitic steels with different Cr/Ni alloying levels were investigated and compared for variety of working conditions. Static oxidation was performed between 900 °C and 1000 °C in air for 400 h. The critical temperature which increases spallation during static oxidation was determined for each of the steel alloying levels. In addition, thermal cycling of a constrained specimen was done with varying upper cycling temperatures between 850 °C and 1000 °C. SEM and TEM analyses were supported by thermodynamic simulation of the phases precipitated in the metal matrix and the structure of formed oxide layers. These studies were used to determine the mechanisms of degradation of thermo-mechanically loaded Cr/Ni austenitic steels at extreme high temperatures. A recommendation for a cost-effective Cr/Ni alloying level for different working conditions was determined.

Eliminating mixed grain structure of Fe–22Cr–25Ni austenitic heat–resistant steel via strain–induced M23C6 precipitation and re–dissolution

Austenitic stainless steels are prone to mixed grain structure (MGS) defect during solution treatment or hot work. We demonstrate that strain–induced continuous M23C6 precipitates at grain boundaries (GBs) enable obtaining uniform deformed structures through hot–rolling at 1000 °C and 1100 °C. After solution treatment of the hot–rolled specimens at 1200 °C for 10–60 min, static recrystallization occurs. After the solution time exceeds 10 min, the M23C6 precipitates at GBs dissolve completely. The recrystallized grains grow normally as the solution time increase, and there is no abnormal grain growth. Strain–reduced the M23C6 precipitation can pin GBs during the dissolution process, thereby eliminating MGS defect.

Effect of on-site service for 16,000 and 38,000 h on microstructure and mechanical properties of austenitic steel HR3C reheater tubes

The microstructure and mechanical properties of austenitic heat-resistant steel HR3C reheater steam tubes were extensively investigated after 16,000 and 38,000 h exposure at 623 °C/7.2 MPa. The surfaces of the tubes are mildly oxidized, and the oxidation layers grew slowly with service time. Metallographic observation shows that only M23C6 carbides formed at the grain boundaries (GBs) and fine NbCrN nitrides that precipitated dispersedly in the matrix after 16,000 h in service. When service time up to 38,000 h, σ phases, (Cr3Ni2Si)C carbides precipitated at GBs. The large σ phase precipitated preferentially at the triangular GBs. The (Cr3Ni2Si)C carbides precipitated by the M23C6 particles or formed inside the σ phases at the GBs. In addition, granular and needle-like M23C6 carbides formed in the matrix, and more of them precipitated close to GBs. With service time prolonged, the precipitates close to the tube walls became plainly denser and larger. The mechanical test results indicate that the strength and hardness of the tubes increased with service time, but plasticity and impact toughness decreased. The large σ phases precipitated at the triangular GBs and M23C6 carbides obvious coarsened at GBs led to the degradation trend, specially deteriorated the tube surface performance.

Effects of Mn content on austenite stability and mechanical properties of low Ni alumina-forming austenitic heat-resistant steel: a first-principles study

Low Ni alumina-forming austenitic (AFA) heat-resistant steel is an advanced high-temperature stainless steel with reduced cost, good machinability, high-temperature creep strength, and high-temperature corrosion resistance. Using the First-principles approach, this study examined the effect of Mn content on austenite stability and mechanical properties at the atomic level. Adding Mn to low Ni-AFA steel increases the unit cell volume with an accompanying increase in the absolute value of formation energy; the austenite formed more easily. The austenitic matrix binding energy decreases and remains negative, indicating austenite stability. As the Mn content increases from 3.2 to 12.8 wt%, the system's bulk modulus (B) rises significantly, and the shear modulus (G) falls. In addition, the system's strength and hardness decrease, and the Poisson ratio of the austenite matrix increases with improved elasticity; the system has excellent plasticity with an increase in the B/G. For the Fe22–Cr5–Ni3–Al2 system, with the increase of Mn content, the electron density distribution between the atoms is relatively uniform, and the electrons around the Mn atoms are slightly sparse, which will slightly reduce the structural stability of the matrix. The experiment demonstrated the matrix maintains the austenitic structure when adding 3.2–12.8 wt% Mn elements to low Ni-AFA steel. At an Mn content of 8 wt%, the overall mechanical properties of the high-Mn AFA steel are optimal, with a tensile strength of 581.64 MPa, a hardness of 186.17 HV, and an elongation of 39%.

Synergistic effect of grain size and second-phase particle on the oxidation behaviour of a high-manganese austenitic heat-resistant steel

The effect of grain size and second-phase particles on the oxidation behaviour of a high-manganese austenitic (HMA) heat-resistant steel are investigated. Four HMA steels with controlled grain size and volume fraction of second-phase particles are prepared and then exposed at 1023 K in air for 400 h. The results show that the particle-reinforced HMA heat-resistant steels exhibit excellent oxidation resistance. A physical model was proposed to investigate the synergistic effect of grain size and second-phase particle on the oxide scale evolution in HMA steels. The presence of second-phase particles can significantly reduce the effective grain size for the oxide scale formation by providing diffusion channels. Therefore, the formation of nodular oxide and the occurrence of internal oxidation are effectively inhibited in the particle-reinforced heat-resistant steels.

Hot Corrosion Behavior of Fe–Cr–Ni-Based Austenitic Heat-Resistant Steel Weld Metal in Na2SO4–NaCl Molten Salts at Different Temperatures

The hot corrosion behavior of the austenitic heat-resistant steel weld metal produced by multi-pass gas tungsten arc welding was investigated. A bilayer oxide scale formed on the surface of the weld metal by the selective oxidation of the element Cr and Fe, and the dissolution of the oxide film occurred at elevated temperatures due to the existence of the molten salt. Morphologies and structures of the oxide scale were characterized by electron microscopy and X-ray diffraction techniques, and then the underlying reason for the hot corrosion of the nonequilibrium weld metal was identified. The interdendritic region could act as the shortcut path for the oxygen, which promotes the internal oxide distribution in chains, and a large internal oxidation zone appears under the bilayer oxide scale.

Evolution and Formation of Non-Metallic Inclusions during Electroslag Remelting of Ce-Bearing 15Cr-22Ni-1Nb Austenitic Heat-Resistant Steel

The evolution of inclusions in austenitic heat-resistant steel with different Ce content during protective argon gas atmosphere electroslag remelting (ESR) was studied. All oxide inclusions in the Ce-free consumable electrode are MgO·Al2O3. A part of these MgO·Al2O3 inclusions was removed before metal droplets entered the liquid metal pool during the ESR. The soluble oxygen (arising from the reoxidation) reacted with soluble aluminum, calcium, and magnesium in liquid steel to form MgO·Al2O3 and CaO–Al2O3 inclusions in liquid steel. All oxide inclusions in the electrode with 0.016 mass% Ce are Ce2O2S. A portion of these Ce2O2S inclusions was dissociated into soluble oxygen, cerium, and sulfur in liquid steel during the ESR process, whereas the others were removed by absorbing them into molten slag. The oxide inclusions in the liquid metal pool and remelted ingot were Ce2O3, CeAlO3, and Ce2O2S. The CeAlO3 and Ce2O3 inclusions were reoxidation products formed by the chemical reaction between the soluble oxygen, soluble aluminum, and cerium. The oxide inclusions in the electrode with 0.300 mass% Ce are CeS and Ce2O2S. These CeS inclusions were removed by molten slag adsorption during the ESR. A part of these Ce2O2S inclusions was removed by slag adsorption, and the remaining entered into the liquid metal pool. The oxide inclusions in the liquid metal pool and the ingot were Ce2O3 and Ce2O2S. The Ce2O3 inclusions were formed through the chemical reaction between the soluble oxygen and cerium in the liquid metal pool. The Ce2O2S inclusions in the liquid pool originate from reoxidation products during the ESR process and the relics from the electrode.