Production of nano/ultrafine grained AISI 201L stainless steel through advanced thermo-mechanical treatment

In recent years, great attention has been focused on the development of nanostructured stainless steels to improve their mechanical properties. This work reports the formation of nano/ultrafine grain structure in the AISI 201L stainless steel using advanced thermo-mechanical treatment. The cast specimens were first homogenized and then hot forged to provide a suitable microstructure prior to the treatment. Cold rolling was carried out with the reductions of 10-95% followed by annealing at temperature of 850 °C for 15-1800 s. X-ray diffraction, optical and scanning electron microscopy, and tensile and hardness tests were used to characterize the processed specimens. The results showed that the nanocrystalline austenitic structure with a grain size of about 65 nm was obtained by annealing at 850 °C for 30 s after the cold reduction of 95%. The yield strength, total elongation and hardness of this specimen were measured as 1485 MPa, 33% and 386 Vickers, respectively.     

Influence of silicon content on the microstructure and hardness of CrN coatings deposited by reactive magnetron sputtering

CrN and CrSiN films were deposited on the stainless steel and silicon substrates by DC magnetron sputtering and their microstructural features were investigated by X-ray diffraction (XRD), scanning electron microscope (FE-SEM/EDS), and atomic force microscopy (AFM). The influence of Si content along with process parameters such as power on the microstructural characteristics of Cr–Si–N and CrN films were investigated and compared between each other. The power and increasing Si contents strongly influence the microstructural and hardness of the deposited films. XRD analysis of the coatings indicates a grain refinement with increase in Si content during deposition of coatings, which is tandem with AFM and SEM results. Also, the surface roughness and particle size are decreasing with addition of Si in the films. The hardness of CrN and CrSiN was measured by microhardness tester and found that introduction of Si content in the CrN system increases its hardness from 1839 Hv to 2570 Hv.     

Chromizing of plasma nitrided AISI 1045 steel

In this study AISI 1045 steel specimens were plasma nitrided at 803 K for 5 h, in a gas mixture of 75% N₂ + 25% H₂. The specimens were then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test were used as characterizing techniques. The thickness of white nitrided layer was around 5 μm, which was mainly composed of iron nitrides and its hardness was around 740 HV. Chromizing of nitride layer resulted in formation of Cr₂N chromium nitride and Fe₃N iron nitrides. A significant increase was observed in hardness after chromizing of the nitrided layer. Despite its higher hardness, the post chromised specimen showed higher wear rate than single plasma nitrided specimen.     

Tribological and microstructural evaluation of friction stir processed Al2024 alloy

In this study, a new processing technique, friction stir processing (FSP) was applied to Al2024-T4 as a means to enhance the near-surface material properties. Samples were subjected to FSP using a constant tool rotating rate of 800 rpm and travel speed of 25 mm/min with a tool tilt angle of 3°. Microstructural evolution and tribological behavior of friction stir processed (FSP) Al2024-T4 were investigated. Microstructural characteristics of the samples were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). Evaluations of mechanical properties include micro-hardness and wear resistance. Dry sliding wear tests were applied using a reciprocating wear test. The results showed that FSP was beneficial concerning improvement of hardness and wear resistance. FSP reduced friction coefficient by approximately 30% and wear rate by an order of magnitude.     

Low temperature nitriding of medium carbon steel

The authors present the results of nitriding of medium carbon steel at a low temperature of 573 K by using the surface-alternating current nanocrystalline treatment (SACNT), which is much lower than conventional nitriding temperature (about 773 K). The SACNT induces electrovalent bond to part by means of the iron ion transgressing, and the surface evolves to ultrafine grain layer. The nanostructured surface layer enhances the nitriding kinetics of pre-treated medium carbon steel. The samples were characterized by metallographic testing (scanning electron microscopy (SEM) and optical microscope (OM)), microhardness tester and X-ray diffraction.     

Investigations on the mechanical properties and microstructure of dissimilar cp-titanium and AISI 316L austenitic stainless steel continuous friction welds

Friction welding process is a solid state joining process that produces a weld under the compressive force contact of one rotating and one stationary work piece. In this study, the friction welding of dissimilar joints of AISI 316L stainless steel and cp-titanium is considered. The optical, scanning electron microscopy studies of the weld were carried out. Moreover, the X-ray diffraction analysis was performed. The integrity of welds was achieved by the micro hardness and tensile tests. The fracture surface was examined by the scanning electron microscopy. The study showed that the magnitude of tensile strength of the dissimilar welded specimen was below that of the titanium base material if preheating was not applied at the interface. The high weld tensile strength was achieved by preheating the 316L stainless steel material to 700 °C, smoothing and cleaning of the contact surfaces. Results illustrated that in dissimilar joints, different phases and intermetallic compounds such as FeTi, Fe₂Ti, Fe₂Ti₄O, Cr₂Ti and sigma titanium phase were produced at the interface. The presence of brittle intermetallic compounds at the interface resulted in degradation of mechanical strength which in turn led to premature failure of joint interface in the service condition. Preheating caused to produce oxide layer at the interface which was harmful for bonding. The oxide layer could be eliminated by applying pressure and smoothing the surface. Results of hardness tests illustrated that the high hardness was occurred in the titanium side adjacent to the joint interface. Moreover, the optimum operational parameters were obtained in order to achieve the weld tensile strength greater than the weak titanium material.     

Fabrication and characterization of functionally graded Al–SiC nanocomposite by using a novel multistep friction stir processing

Functionally graded materials are one of the most promising candidates among advanced materials. However, some challenges still exist in its fabrication methods. The current study aims to produce functionally-graded bulk Al–SiC nanocomposites by a novel multistep friction stir processing (FSP) for the first time. The SiC nanoparticles were packed into a groove on the 6061 aluminum plate and FSP was performed by using a tool with pin length of 6 mm. Subsequently, FSP was reapplied on another groove by using a tool with a shorter pin length of 3.2 mm. The desirable distribution of SiC nanoparticles in the matrix was confirmed by scanning electron and atomic force microscopes. The composition of graded sample was changed continuously from 18 to 0 wt% SiC along the thickness. Accordingly, the microhardness profile showed a maximum of 160 Hv in the enriched zone which is 3.2 times higher than the hardness of the particle-depleted zone. However, a constant hardness value of 135 Hv was obtained along the thickness of homogenous sample which is 15% lower than that of superficial layer in graded sample. Moreover, the hardness values were linearly correlated with the inverse of interparticle spacing.     

Nanoporous cluster-assembled WOx films prepared by radio-frequency assisted laser ablation

The influence of substrate temperature and ambient gas pressure-composition on the characteristics of WO_x films synthesized by radio-frequency assisted pulsed laser deposition (RF-PLD) are studied with the aim to obtain nanostructured films with large surface area that appear promising for gas sensing applications. A tungsten target was ablated both in chemically reactive molecular oxygen at 5 Pa and in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at 473, 673 and 873 K. Other deposition parameters such as laser fluence (4.5 J/cm²), laser wavelength (355 nm), radio-frequency power (150 W), and target to substrate distance (4 cm) were kept fixed. The sensitivity on the deposition parameters of roughness, morphology, nanostructure and bond coordination of the deposited films were analysed by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy. The role of the investigated process parameters to nanoparticle formation and to the development of an extended nanostructure is discussed.     

Mechanical alloying behavior of Ti6Al4V residual scraps with addition of Al2O3 to produce nanostructured powder

The present investigation has been based on production and subsequent comparison of different physical, mechanical and thermal properties of nanostructured Ti6Al4V and Ti6Al4V/Al₂O₃ powders by means of high energy ball milling. In this regard, the structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy and microhardness measurements. The results revealed that ball milling process reduced the grain size of Ti6Al4V and Ti6Al4V + 10 wt% Al₂O₃ to approximately 20 and 15 nm, respectively. For both compositions also a remarkable change in morphology and particle size occurred during ball milling of powders with different compositions. Moreover, phase evolution during milling and heat treatment was taken into consideration. The as-milled Ti6Al4V + 10 wt% Al₂O₃ powder exhibited higher microhardness (∼900 Hv) comparing to as-milled Ti6Al4V (∼536 Hv) and as-received samples (∼400 Hv).     

Effects of initial precursor and microwave irradiation on step-by-step synthesis of zinc oxide nano-architectures

ZnO nano-architectures were produced with the aid of a fast, simple and low cost microwave-assisted synthesis method. Solid semispherical ZnO nanoparticles on the order of 600 nm in diameter along with rice-like ZnO nanorods 95 nm thick were produced from butanol, triethanolamine (TEA), and zinc acetate dihydrate. Solid spherical ZnO nano-architectures with an average diameter of 250 nm were produced from the same starting materials in addition to NaOH. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used to characterize the ZnO nano-architectures as well as the precursor. This method is cheap, fast and simple; capable of producing large quantities of each ZnO nanostructure. Investigation of the step-by-step formation mechanism for each ZnO nanostructure was conducted.     

Wear assessment of plasma nitrided AISI H11 steel

Plasma nitriding is one of the effective methods for improvement of the hardness, wear and corrosion resistance of steels. In this research AISI H11 hot working tool steel was plasma nitrided in various gas mixtures for different times and temperatures. The morphology, size and composition of nitride nanoparticles formed on the surface of the specimens were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD). The wear behavior of plasma nitrided samples was studied by means of unlubricated pin-on-disc method under constant load of 80 N, sliding speed of 1 m/s, sliding distance of 2000 m at room temperature. The results showed plasma nitriding process improved the wear behavior of H11 steel. The increase in time and temperature of plasma nitriding decreased the hardness and increased the wear weigh loss of the specimens.     

Duplex surface treatment of AISI 1045 steel via plasma nitriding of chromized layer

In this work AISI 1045 steel were duplex treated via plasma nitriding of chromized layer. Samples were pack chromized by using a powder mixture consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. The samples were then plasma-nitrided for 5 h at 803 K and 823 K, in a gas mixture of 75%N₂ + 25%H₂. The treated specimens were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test. The thickness of chromized layer before nitriding was about 8 μm and it was increased after plasma nitriding. According to XRD analysis, the chromized layer was composed of chromium and iron carbides. Plasma nitriding of chromized layer resulted in the formation of chromium and iron nitrides and carbides. The hardness of the duplex layers was significantly higher than the hardness of the base material or chromized layer. The main cause of the large improvement in surface hardness was due to the formation of Cr_xN and Fe_xN phases in the duplex treated layers. Increasing of nitriding temperature from 803 to 823 K enhanced the formation of CrN in the duplex treated layer and increased the thickness of the nitrided layer.     

Ball milling of stainless steel scrap chips to produce nanocrystalline powder

Nanocrystalline stainless steel powder was produced by ball milling of austenitic stainless steel scrap chips. The structural and morphological changes of samples during ball milling and after subsequent heat treatment were investigated by X-ray diffractometery, scanning electron microscopy and microhardness measurements. During ball milling the austenite in as-received chips partially transformed to the martensite phase with nanoscale size grains of ∼15 nm. This structure exhibited high microhardness value of about 850 Hv which is much higher than that for original samples. The deformation-induced martensite partially transformed to austenite after annealing at 700 °C for 1 h reducing the hardness of powder particles.     

Niobium boride coating on AISI M2 steel by boro-niobizing treatment

In this study, niobium boride coating was applied on pre-boronized AISI M2 steel by the thermo-reactive deposition technique in a powder mixture consisting of ferro-niobium, ammonium chloride and alumina at 950 °C for 1-4 h. The coated samples were characterized by X-ray diffraction, scanning electron microscope and micro-hardness tests. Niobium boride layer formed on the pre-boronized AISI M2 steel was smooth, compact and homogeneous. X-ray studies showed that the phases formed on the steel surfaces are NbB, Nb₃B₂, FeB and Fe₂B. The depth of the niobium boride layer ranged from 0.97 μm to 3.25 μm, depending on treatment time. The higher the treatment time the thicker the niobium boride layer observed. The hardness of the niobium boride layer was 2738 ± 353 HV_0.01.     

Investigation of dry sliding wear properties of boron doped powder metallurgy 316L stainless steel

This paper describes wear behavior of powder metallurgy (PM) 316L stainless steel with additions of elemental boron with the aim of producing superior mechanical properties. The wear test of the samples is conducted using a pin specimen of PM 316L stainless steel doped with elemental boron and a steel disc specimen with hardness of 180 HV10. Densification achieved with boron addition due to the liquid phase formation up to 95% of theoretical density with 0.6 wt.% boron addition. Most mechanical properties such as hardness, tensile strength and yield strength were improved with boron addition. The wear rate of PM 316L stainless steel decreased from 4.3 × 10⁻⁶ to 0.8 × 10⁻⁶ mm³/Nm with the addition of 0.6 wt.% elemental boron. The abrasive-induced delamination wear dominated at the samples. Scanning electron microscopy observations of the worn surfaces revealed that plastic deformation occurred with delamination of surface layers in the sintered conditions.     

Effects of boronizing on mechanical and dry-sliding wear properties of CoCrMo alloy

In this study, CoCrMo alloy was boronized at 950 °C for 2, 4, 6 and 8 h, respectively. The boronized samples were characterized by scanning electron microscopy, X-ray diffraction, microhardness tester and ring-on-block wear tester. X-ray diffraction studies showed the boride layer formed at 950 °C for 2–8 h consisted of the phases Co₂B and CrB. A large number of pores formed in diffusion zone were probably attributed to the Kirkendall effect. Depending on boronizing time, the thickness of boride layer ranged from 4 to 11 μm. The excellent wear resistance of the boronized CoCrMo alloy was attributed to the high surface hardness of the Co₂B and CrB under dry-sliding conditions when compared to the as-received state.     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

The growth kinetics of borides formed on boronized AISI 4140 steel

The growth kinetics of boride layer on boronized AISI 4140 steel is reported. Steel samples were boronized in molten borax, boric acid and ferro-silicon bath at 1123, 1173 and 1223 K for 2, 4, 6 and 8 h, respectively. The morphology and types of borides formed on the surface of AISI 4140 steel substrate were analyzed by means of optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (XRD). The boride layer thickness ranged from 38.4 to 225 μm. Iso-thickness diagrams for pre-determined thickness according to treatment time and temperature, were graphed by MATLAB 6.0 software. The hardness of borides formed on the samples changed between 1446 and 1739 HV_0.1, according to treatment time and temperature. Layer growth kinetics way analyzed by measuring the extent of penetration of FeB and Fe₂B sublayers as a function of treatment time and temperature in the range of 1123-1223 K. For practical use, an iso hardness diagram was established as a function of treatment time, temperature and boride layer thickness. The depth of the tips of the most deeply penetrated FeB and Fe₂B needles were taken as measures for diffusion in the growth directions. The kinetics of the reaction, were also determined by varying the treatment temperature and time. The results show that K increased with boronizing temperature. The activation energy (Q) was formed to be 215 kJ mol⁻¹. The growth rate constant (K) ranged from 3×10⁻⁹ to 2×10⁻⁸ cm²s⁻¹.     

Investigation of the microstructure and corrosion performance of a nanostructured titania-containing hybrid silicate film on mild steel

A pre-treatment system consisting of a nanostructured titania interlayer loaded with an inhibitor and a hybrid silicate film deposited on the TiO₂ layer is shown to provide protection against active corrosion of mild steel. A nanostructured TiO₂ interlayer was prepared on the mild steel surface via controllable hydrolysis of titanium alkoxide. To further improve this pre-treatment, the hybrid silicate film was synthesized from tetraethylorthosilicate and 3-glycidoxypropyltrimethoxysilane precursors. The morphology and structure of the titania interlayer and hybrid silicate film were characterized with atomic force microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques. The corrosion performance of the coatings was examined using electrochemical techniques, including potentiodynamic scanning and electrochemical impedance spectroscopy. The TiO₂ nanostructure calcinated/inhibitor/hybrid silicate system shows enhanced corrosion performance, as confirmed by impedance and polarization measurements.     

Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al-Bi-Sb) and Ca-Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm⁻², despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.