Nanostructured Steel Research Articles

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Mechanical properties of friction induced nanocrystalline pearlitic steel

Nanocrystalline structured variants of commercially available alloys have shown potential for boosting the mechanical properties of these materials, leading to a reduction in waste and thereby retaining feasible supply chains. One approach towards achieving these nanostructures resides in frictional treatments on manufactured parts, leading to differential refinement of the surface structure as compared to the bulk material. In this work the machining method is considered to be a testing platform for the formation and study of frictional nanostructured steel, assembly of which is stabilized by fast cooling of the produced chip. Analysis of the mechanical properties has shown extraordinary results at the surface, over 2000 MPa of strength on AISI1045 steel, more than three times the strength of the base material, demonstrating at the same time a reduction of 15% in the elastic modulus. The microscopic analysis suggests a reassembly of the elements in a new lattice of carbon supersaturated nano-ferrite.

Review of passivity and electrochemical properties of nanostructured stainless steels obtained by surface mechanical attrition treatment (SMAT): trend and progress

Abstract Surface nanocrystallization provides the opportunity to produce gradient-structured metallic materials with improved properties. Several attempts have been made to produce nanostructured stainless steel (SS), along with the study of the resultant corrosion resistance. However, the current knowledge is insufficient to address the corrosion mechanism and the possible ways of enhancing the corrosion resistance after surface treatment. The present work reviews the past significant works on the effect of surface treatment by surface mechanical attrition treatment (SMAT) method as well as its processing parameters on the corrosion properties of SS. The corrosion resistance of nanostructured SS is influenced by the extent of grain refinement, compactness, and homogeneity of the passive film, Cr content, grain boundary structure, composition, and alloying elements. In addition, the resulting corrosion properties can be controlled by choosing the right processing parameters during treatment. Progress on the corrosion behavior of nanostructured steels was summarized and new avenues for further research and developments are proposed.

Nanostructured steels for advanced structural applications

Abstract This vision summarizes the recent advancement of nanostructured steels for advanced structural applications, foresees possible challenges and pinpoints future directions as well as opportunities in this new era of industrial revolution 4.0.

Development of technology for the production of a lightweight observation and protective container (LOOK) made of nanostructured ultra-strength steels

The article contains results of research and analyses concerning application of nanostructured bainitic steel in the form of plates for manufacturing of armour components. The presented results of examination of microstructure and properties include a wide range of laboratory experiments and industrial tests, which resulted in the achievement of the assumed functional properties. In the period of 2017-2021, a scientific and industrial consortium consisting of Łukasiewicz – Institute of Ferrous Metallurgy (leader); WITPiS, Tarnów Mechanical Works, Alchemia and Heatmasters Poland carried out a project funded by the POIR 04.01.04 programme aimed to develop the design and to manufacture an observation and protective container with a specified resistance to penetration by armour-piercing projectiles and with a lower mass of steel armouring in relation to that currently produced. The aim of the project was achieved by using armour plates made of nanostructured bainitic steel (nanobainitic), which are characterised by high resistance to high-energy impact concentrated in a small area. The technological tests carried out in the project mainly concerned the development of a new container and industrial technology of armour plates production and their application in the armour of this container. Based on the results of investigation of the semi-industrial scale material, the optimum chemical composition for industrial scale melting and casting was determined. An industrial technology for the production of plates of nano-structured bainitic steel was developed, which includes the following processes: smelting and casting, preliminary heat treatment and ingot hot processing, as well as hot rolling, final heat treatment, and surface treatment. A test batch of the material in the form of 1500×2470 mm armoured plates was fabricated under industrial conditions. The final result of the project is a container armoured with bainitic nanostructured steel plates with implementation documentation and a technology for producing armoured plates from this steel under the technical and technological conditions of domestic steel manufacturers.

Bainitic Ferrite Plate Thickness Evolution in Two Nanostructured Steels

Bainitic ferrite plate thickness evolution during isothermal transformation was followed at the same holding temperatures in two nanostructured steels containing (in wt.%) 1C-2Si and 0.4C-3Si. A dynamic picture of how the bainitic transformation evolves was obtained from the characterization of the microstructure present at room temperature after full and partial transformation at 300 and 350 °C. The continuous change during transformation of relevant parameters influencing the final scale of the microstructure, YS of austenite, driving force of the transformation and evolution of the transformation rate has been tracked, and these variations have been correlated to the evolution of the bainitic ferrite plate. Instead of the expected refinement of the plate predicted by existing theory and models, this study revealed a thickening of the bainitic ferrite plate thickness as the transformation progresses, which is partially explained by changes in the transformation rate through the whole decomposition of austenite into bainitic ferrite.

Low-carbon advanced nanostructured steels: Microstructure, mechanical properties, and applications

Low-carbon advanced nanostructured steels have been developed for various structural engineering applications, including bridges, automobiles, and other strength-critical applications such as the reactor pressure vessels in nuclear power stations. The mechanical performances and applications of these steels are strongly dependent on their microstructural features. By controlling the size, number density, distribution, and types of precipitates, it is possible to produce nanostructured steels with a tensile strength reaching as high as 2 GPa while keeping a decent tensile elongation above 10% and a reduction of area as high as 40%. Besides, through a careful control of strength contributions from multiple strengthening mechanisms, the nanostructured steels with superior strengths and low-temperature impact toughness can be obtained by avoiding the temper embrittlement regime. With appropriate Mn additions, these nanostructured steels can achieve a triple enhancement in ductility (total tensile elongation, TE of ~30%) at no expense of strengths (yield strength, YS of ~1100 to 1300 MPa, ultimate tensile strength, UTS of ~1300 to 1400 MPa). More importantly, these steels demonstrate good fabricability and weldability. In this paper, the microstructure-property relationships of these advanced nanostructured steels are comprehensively reviewed. In addition, the current limitations and future development of these nanostructured steels are carefully discussed and outlined.

Stability of nanograins and nanoparticles in La-doped nanocrystalline steel irradiated with Fe ions

Abstract Nanostructured materials are potential candidates for future structural materials in advanced nuclear reactors. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) is an advanced nanostructured steel, in which excellent small grain size of ~45 nm is achieved and stabilized by the doped La element. We carried out irradiation using 3.3 MeV Fe ions to 108 dpa at room temperature and 500 °C, and then characterized the microstructural change in NC304-La by transmission electron microscopy, scanning transmission electron microscopy and atom probe tomography. We studied the detailed microstructural evolution and elemental behaviors in irradiated NC304-La, focusing on the effects of La element on the stability of nanograins and nanoparticles in NC304-La.

Effect of external applied magnetic field on microstructures and mechanical properties of laser welding joint of medium-Mn nanostructured steel

Medium-Mn nanostructured steel developed recently demonstrates attractive mechanical property and high potential for structural application. In this study, a new welding method with external magnetic field assisted laser was applied to investigate the weldability of this steel. The effects of external applied magnetic field on the microstructure and tensile behavior of the joint were studied. Our results show that the external magnetic field significantly enhances the ultimate strength by 43.9% and dramatically turns the brittle fracture into ductile with a more than tripled elongation rate during uniaxial tension tests. The prior-austenite grain size in weld with external magnetic field is more than twice finer than those in the laser weld, together with the elimination of the Mn segregation at prior austenite grain boundary (PAGB), significantly improve the stability of the prior austenite and lead to an increased volume fraction of retained austenite in weld. Our analyses manifest that the thermoelectric magnetic convection (TEMC) generating from the coaction between the external magnetic field and the internal thermoelectric current is the main reason for the alteration of the microstructures and mechanical properties. Promotion of the heat and mass transfer driven by the interdendritic TEMC reduces the segregation at PAGB and decreases the grain size. The improvement of the strength originates from the refined grain-size and promoted mechanical-induced martensite transformation, while the reduction of the segregation and increased volume fraction of retained austenite contribute to the enhanced ductility.

Effect of Lattice Defects on Tribological Behavior for High Friction Coefficient under TCP Added PAO Lubrication in Nanostructured Steels

Effect of Lattice Defects on Tribological Behavior for High Friction Coefficient under TCP Added PAO Lubrication in Nanostructured Steels

Design of high-manganese nanostructured austenitic steel with particle swarm optimization

ABSTRACT Austenitic steels with high manganese content exhibit various deformation mechanisms depending on their stacking fault energies. Here we present a novel approach of combining the algorithm of particle swarm optimization with thermodynamic modeling, to design high-manganese steels with desired stacking fault energies. Implementation of this method is demonstrated for nanostructured austenitic steels, where composition, grain size, and temperature are used as the tunable parameters. The results show that for a given value of fault energy, the proposed method yields multiple solutions, which may offer valuable guidelines for designing the alloy. In contrast with the coarse-grained steels, the design parameters of the nanostructured alloys are shown to depend on the austenitic grain size.

Optimisation of parameters for the production of perforated armour panels made of nanobainitic steel using dynamic hardness measurement methodology

The study is a continuation of the development of material characteristics in order to expand the range of products for the production of which nanostructured bainitic steels can be used. The tests included measurement of dynamic properties important in the material qualification process for firing tests and for other applications requiring dynamic wear resistance. The novelty of the implemented development of the innovative grade of nanostructured steel and the technology of manufacturing products – including armour systems containing perforated panels made of this grade of steel, consisted in developing the basics of dynamic hardness measurement methods and dynamic indentation tests using a Gleeble simulator.

OPTIMISATION OF BALLISTIC PROPERTIES OF NANOSTRUCTURED BAINITIC STEEL PLATES FOR CONTAINER ARMOUR SYSTEM

Abstract: An innovative armour system containing plates made of nanostructured bainite steel is under development to improve operating properties of a light armoured observation-protective container (LAOC) providing the 2nd protection level according to STANAG 4569. Armour system solution of the container has been modified because the new plates have different mechanical and technological properties than currently used armour plates. The paper presents results of investigation of material characteristics and firing tests of plates made of nanostructured steel in the semi-industrial scale. To optimise parameters of heat treatment of the plates the kinetics of phase transformation was examined, and mechanical properties were measured, and changes in microstructure in the area of projectile interaction at firing tests were analysed for different variants of treatment. The reported investigations precede the industrial scale production process for plates made of the new grade steel designed for the armour system of LAOC. Optimisation of mechanical properties of the plates has been carried out by selection of such production parameters as chemical composition of the steel, hot working, interprocess heat treatment and temperature and time of final heat treatment (isothermal annealing). Effect of primary segregation of alloying and residual elements on protective properties of the plates is indicated. As a result of applying the annealing temperature in the range of 210-225°C (Ms temperature is ca. 200°C at cooling rate 1-2°C/s) and time in the range of 120-70 hours respectively, the following properties have been achieved: YS0.2 (yield strength) 1400-1500 MPa; UTS (ultimate tensile strength) 2000-2150 MPa; TE (total elongation) 10-13%, impact toughness KV at room temperature 10-16 J and hardness 590-610 HV (53-54 HRC). Microstructure of the plates consists of carbide free lathy nanobainite and 10-21% (volume fraction) of retained austenite. Plates of thickness 6.3 and 7.5 mm and ammunition type of 5.56x45 mm M193 and 7.62x51 mm API BZ have been used in the firing tests. Based on results of firing tests and microstructure examination in the area of projectile interaction the phenomena have been identified indicating high efficiency of ballistic protection of investigated plates, manifested by high ability to absorption and dissipation of projectile energy without susceptibility to cracking. Based on the results of mechanical properties measurements and firing tests a chemical composition of the nanostructured bainitic steel adjusted for industrial production of plates of thickness in the range of 6-8 mm has been developed. An analysis of container armour system mass reduction possibility, basing on results of investigation of the nano-structured bainite steel plates made in the semi-industrial scale, has been carried out. Basing on the analysis of the container construction and operational conditions some sensitive areas of ballistic protection have been specified to be subjected to firing tests. Ballistic examination will be carried out using the container segments representing the selected sensitive areas.

Finite element analysis on rolling contact fatigue of surface nanostructured steel (Effect of friction coefficient on mean shear stress under cyclic loading)

Finite element analysis on rolling contact fatigue of surface nanostructured steel (Effect of friction coefficient on mean shear stress under cyclic loading)

Effects of La on Radiation Tolerance and Stability of La-Doped Nanocrystalline Steel

Nanostructured materials are potential candidates for structural materials in future nuclear reactors. An advanced nanostructured steel - La-doped nanocrystalline austenitic stainless steel (NC304-La) - is reported, in which excellent small grain size of ~45 nm is achieved and stabilized by the doped La element. Irradiations using 3.3 MeV Fe ions to 108 dpa (displacement per atom) at room temperature (RT) and 500℃ were carried out, and the microstructural evolution in NC304-La was characterized by transmission electron microscopy, scanning transmission electron microscopy and atom probe tomography focusing on the effects of La element on the radiation tolerance and stability of NC304-La.

Effect of Surface Nanocrystallization Pretreatment on the Tribological Properties of Plasma Nitrided AISI 316 L Stainless Steel Under Boundary Lubrication

It has been proved that surface nanocrstallization pretreatment is beneficial to plasma nitriding of steel by enhancing nitrogen diffusion, while the tribological properties of the nitrided nanostructured steel under boundary lubrication are not clear. In this work, AISI 316 L stainless steel with and without ultrasonic cold forging technology (UCFT) pretreatment was plasma nitrided at 500 °C for 4 h. The effects of UCFT pretreatment on the microstructure and properties of the nitrided layer and the tribochemical interactions between the nitrided layer and friction modifier molybdenum dithiocarbamate (MoDTC) and antiwear additive zinc dialkyldithio-phosphate (ZDDP) were investigated using SRV tribometer, scanning electron microscopy (SEM), vickers hardness tester, optical microscope, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Surface analyses confirm the formation of a 20 μm thick nitrided layer on the UCFT-pretreated sample and it had higher hardness than that on the unpretreated sample. Furthermore, the nitrided UCFT-pretreated sample presented better synergetic effect with MoDTC and ZDDP on tribological behaviors than the nitrided unpretreated sample. This is attributed to the higher contents of Mo, S, Zn, P, and MoS2/MoO3 ratio in the tribofilms on the nitrided UCFT-pretreated sample.

Effect of Lattice Defects on Tribological Behavior for High Friction Coefficient under TCP added PAO Lubrication in Nanostructured Steels

Effect of Lattice Defects on Tribological Behavior for High Friction Coefficient under TCP added PAO Lubrication in Nanostructured Steels

The Influence of Tensile Stresses on the Properties of Reverted Austenite in Nanostructured Steel at Overcooling

The resistometry method is used for building the diagrams of the isothermal decomposition for reverted austenite in cold-rolled steel of martensitic and austenitic grades in the range of the temperatures of overcooling of (0 ÷ -90) °C. The influence of tensile stresses on the character and kinetics of γ→α - transformation is established.

Joint inhomogeneity in dissimilar friction stir welded martensitic and nanostructured ferritic steels

ABSTRACTDissimilar welding between oxide dispersion strengthened ferritic (ODS) steel and reduced activation martensitic steel would be required for constructing the advanced blanket of progressive fusion reactors. In this study, we achieved dissimilar joints by friction stir welding, and aimed to characterise and ameliorate joint inhomogeneity. Main results reveal that the joint inhomogeneity is generated from discrepant microstructural evolutions within the martensitic and ODS ferritic steels. The ODS steel achieves evolution by the dynamic recrystallisation, while the martensitic steel undergoes phase transformation that drastically hardens the stir zone. By a proper post-weld heat treatment, the joint inhomogeneity can be effectively ameliorated due to carbide reprecipitation and stress relief in the joint.

Microstructural characterization and residual stress distribution in a nanostructured austenitic stainless steel

Abstract In this paper, residual stress distribution is investigated by a novel X-ray cosα diffraction technique in a nanostructured austenitic stainless steel after hydrostatic extrusion processes. Hydrostatic extrusion performed at 20°C and with a total true strain of 2.3 leads to the creation of a nanostructure consisting of nanotwins and shear bands. The results reveal that the greatest compressive residual stresses of −1 GPa are found 3 mm from the surface of the nanostructured austenitic stainless steel. These compressive residual stresses restrict crack growth into the material, thereby preventing catastrophic failure.