Austenitic Substrate Research Articles

Revealing the critical failure factor and sub-surface damage mechanism of 316 stainless steel during fretting corrosion under the molten lead-bismuth eutectic

The critical failure factor and damage mechanism of 316 stainless steel during fretting corrosion under the molten lead-bismuth eutectic (LBE) are specially investigated. Representative Fτ-D-cycle curves are shown to reflect the changing process and characteristics of different cycles for each parameter during fretting corrosion tests. The fretting operation regime is the mixed fretting regime (MFR) with minor stroke. The increase in stroke and temperature convert the fretting operation regime from MFR to the gross sliding regime (GSR). According to the dissipated energy and wear volume among all the samples, the results demonstrate more information as follows: the change of dissipated energy and wear volume is positively correlated with stroke, cycle number and temperature. The most serious damage occurred at 450 ℃− 80 µm-2 × 106 cycles. Multivariate linear regression model was established for 18 groups of data, and it was concluded that temperature was the key failure factor within the parameter range of this experiment. The first effect of temperature increase is to increase the rate of diffusion of Ni element into LBE, and the inevitable dissolution corrosion will occur. The problem is that a large number of vacancy defects are generated. The second effect of temperature increase is that the austenite substrate is transformed into ferrite, which cannot resist damage caused by fretting, due to the dissolution of large amounts of Ni.

Microstructure of reactive magnetron sputtered S-phase coatings with a diffusion sub-layer

The main objective of the work was to investigate the microstructure of S-phase coatings obtained by reactive magnetron sputtering at different temperatures and with various nitrogen concentrations in the working atmosphere. Particular attention was paid to the formation of a diffusion sub-layer in the austenitic substrate. The coatings were deposited on austenitic stainless steel substrates in an argon–nitrogen gas mixture. Three different nitrogen concentrations in the working gas were used in the experiments (15, 33 and 50 vol.%), and five substrate temperatures (200, 250, 300, 350 and 400 °C). The phase composition was evaluated using X-ray diffraction (XRD). X-ray microanalysis (WDS) and glow discharge emission spectroscopy (GDOES) techniques were used to study the nitrogen content and distribution in the coatings. The microstructure was investigated using a scanning electron microscope (SEM). Formation of the S-phase diffusion sub-layers was observed in the atmospheres containing 50 and 33 vol.% N2 respectively at temperatures of 300 and 350 °C or higher. Nitrogen diffusion into the substrate also occurred with the 15 vol% N2 atmosphere at 350 °C or higher although no supersaturated solution was formed in these conditions.

EFFECT OF NITROGEN GAS RATIO ON THE PROPERTIES OF TI-ZR-N COATINGS DEPOSITED BY PULSED MAGNETRON SPUTTERING

The aim of this paper was to study the effectiveness of N2/Ar gas ratio on the mechanical and tribological properties of Ti-Zr-N films coated on (AISI 304 stainless steel) substrate. The thin films were produced at 150 W pulsed magnetron sputtering (PMS) technique operated for 90 min. The structure and tribo-mechanical properties of the Ti-Zr-N coatings were investigated at different N2 /Ar gas pressure ratio. X-ray diffraction (XRD), microhardness, surface roughness, wear and friction, corrosion performance and surface energy were investigated. The analysis of X-ray diffraction (XRD) indicated a formation of solid solution phase of (Ti-Zr)N with (FCC) structure and a chemical compound phase of TiN. The solid solution phase (Ti-Zr)N has the orientation (111). The microhardness of the thin films has high values compared to associated value of (AISI 304 stainless steel) substrate; up to 4-folds. Furthermore, the wear performance was improved from 73.06 mm3/m for AISI substrate to 5.01 mm3/m after depositing Ti-Zr-N. The coefficient of friction was decreased from nearly 0.69 for the austenitic substrate to nearly 0.12 after coating with Ti-Zr-N. Over and above, the corrosion resistance substantially improved (factor of 1000 times) after the coating.

Effect of Pressures on the Corrosion Behaviours of Materials at 625°C

The corrosion behaviors of austenitic stainless steels (SS) 310, 304 and Ni- and Fe-based A-286 exposed to 0.1 MPa, 8 MPa and 29 MPa at 625°C for 1000 h were investigated. These represent exposure to superheated steam, subcritical and supercritical water (SCW) at 625°C, respectively. As SS 310 showed the smallest weight change, the oxide cross-sections made from 310 samples were examined by transmission electron microscopy. The results revealed a single-layer oxide at 0.1 MPa and dual-layer oxides at 8 MPa and 29 MPa, followed by a Cr-depleted region into the austenite substrate. The compositions of the inner oxides at 8 MPa and 29 MPa are Cr-rich and largely similar to those of the single-layer oxides at 0.1 MPa exposure. These results suggest that corrosion testing in superheated steam may be a suitable surrogate for scoping tests of materials under SCW conditions at >650°C.

Dependence of the nitriding rate of ferritic and austenitic substrates on the crystallographic orientation of surface grains; gaseous nitriding of Fe-Cr and Ni-Ti alloys

Gaseous nitriding of ferritic Fe–Cr and austenitic Ni–Ti solid solutions reveals that the extent of the uptake of dissolved nitrogen depends on the crystallographic orientation of the surface grains of the substrate. In both ferritic and austenitic substrates, the surface nitrogen concentration and the nitriding depth decrease upon increasing the smallest angle between the surface normal and the normal of a {1 0 0} plane of the surface grain considered. This phenomenon could be ascribed to the residual compressive macrostress developed during nitriding which varies as a function of crystallographic orientation of the (surface) grains due to the elastically anisotropic nature of ferrite and austenite solid solutions investigated in this study.

Nitrogen-induced selective high-temperature internal oxidation behavior in duplex stainless steels 19Cr–10Mn–0.3Ni–xN

Isothermal high-temperature oxidation of duplex stainless steels (DSSs) 19Cr–10Mn–0.3Ni–xN (x=0.211–0.315wt.%) at 1100°C in air has been investigated mainly by scanning electronic microscopy and Raman spectroscopy. High-temperature oxidation resistance of DSSs became worse with the increase of nitrogen content. Self-repairing behavior of local breakaway regions locating in ferrite diffusion layer occurred in DSSs with lower nitrogen content. High-temperature internal oxidation phenomenon preferred to occur in DSS with higher nitrogen content and penetrate into local breakaway regions in ferrite diffusion layer upon the austenite substrate regions as the increased outward diffusion of nitrogen ions.

Adhesion of Cr/CrN Multilayers to Steel Substrates

Cr/CrN multilayers with a bilayer period λ = 62 ÷ 1000nm, were investigated. They were deposited by PLD technique on austenitic and ferritic stainless steel substrates. Coating hardness and adhesion to substrates were measured by nanoindentation and scratch testing. Multilayer properties were compared with single Cr and CrN coatings. The 2xCr/CrN and 4xCr/CrN multilayers exhibited hardness slightly lower than the hardness of a single CrN coating, while the critical load of the 4xCr/CrN multilayer, measured in the scratch test, was two times higher than for CrN. Furthermore, the character of coating failure also indicates the higher fracture toughness of multilayers than ceramic ones. The highest scratch resistance of the hardest multilayer with a bilayer period λ = 250nm is extremely interesting. The measured values of the critical loads LC1and LC2of all coatings were higher for harder ferritic than austenitic substrates. However, analysis of scratch track geometry indicated that coating failure occurred under the same deformation of the coating-substrate system, while the higher values of critical load for coatings on ferrite derived from the higher hardness of the substrate.

DIFUSION POWDER Ti – Al-SILICONIZING OF STEEL 12Cr18Ni10Ti UNDER REDUCED PRESSURE

The phase and chemical composition, structure and microhardness, diffusion of multilayer coatings containing titanium, aluminum and silicon were examined at samples of steel 12Cr18Ni10Ti. It was established that the coating was formed on the basis of intermetallic compounds, carbides and titanium nitrides, as well as a solid solutions of titanium, aluminum, silicon, nickel and chromium in the austenite substrate.

Effects of Mechanical Vibration on Microstructure of Laser Welded Joint

The welding quality of welded joints can be improved effectively with vibratory welding process. To study the application of vibratory welding process on laser welding, 316 stainless steel is selected for the laser-vibration welding experiments. According to changing the vibration parameters and welding speeds, the microstructures of welded joints are observed with optical and scanning electron microscope, and are analyzed by contrast in different welding speeds and vibration frequencies. The results show that the columnar dendrites formed after welding are refined. The equiaxed dendrites in the center of weld bead and between the crystal axes are increasing. Meanwhile, the net-like σ-Fe and the dot-like carbides on the grain boundary of austenitic substrate are reduced and more diffuse with the mechanical vibration. In addition, microhardness test of the welded joints is conducted. The hardness of weld bead is higher with vibratory welding than that without vibration. It can also be found that the hardness is dramatically increased at a high resonance frequency.

Crystallography and morphology of a lathy ferrite in Fe–Cr–Ni alloys during directional solidification

Abstract

Electrical and thermal transport properties of vanadium oxide thin films on metallic bipolar plates for fuel cell applications

We have focused on the in-depth comparative evaluation of the suitability of electrically-induced thermal transport characteristics of highly disordered vanadium oxide thin films deposited onto metallic bipolar plates as an expeditious self-heating source for the successful cold-start of fuel cells in a subfreezing environment. To achieve this, sol–gel derived vanadium oxide thin films on the non-polished surface of 316L austenitic and 446M ferritic substrates have been fabricated by a dip-coating process. The effects of electrical properties on thermal energy dissipation rate of the as-synthesized thin films deposited onto 316L and 446M stainless steel plates were firstly investigated and compared with each other. Subsequently, a series of physical, chemical, and structural analyses of the thin films have been performed using several analytical techniques such as the ASTM D3359, the ASTM D5946, XPS, and FE-SEM. The most important finding of this study was that the electrical resistivity of the thin films on 446M ferritic substrate was extremely low on a level of 4.8% of the 316L sample at −20 °C, and then the surface temperature rise of the thin film on 316L austenitic substrates was approximately 21.8 times greater than that of 446M ferritic substrates under simulated cold starting conditions (i.e., at a current density of 0.1 A·cm−2 at −20 °C). Therefore, we concluded that vanadium oxide thin films on 316L austenitic stainless steel plates appears to be more applicable than those of 446M ferritic substrates for the cold-start enhancement of fuel cells from the practical point of view.

Non-Destructive Characterization of Nickel-Base Hardface Deposit on Austenitic Stainless Steel Through Eddy Current and Magnetic Barkhausen Techniques

Nickel-base Colmonoy (AWS ER NiCr) alloys find extensive application for hardfacing of austenitic stainless steel (SS) components in Fast Breeder Reactors. Gas Tungsten Arc deposited Colmonoy alloys suffer from significant loss in hardness and wear properties due to dilution from the austenitic SS substrate. In a multilayer deposit, dilution in first layer is highest and dilution decreases progressively in the subsequent layers. Although, both Colmonoy alloys and austenitic SS are non-magnetic, the deposit of Colmonoy on austenitic SS is ferromagnetic. The susceptibility to magnetic attraction in the hardface deposits is highest for maximum level of dilution and reduces with decreasing dilution. In the present study, Colmonoy-6 alloy co-deposited with austenitic SS filler wire, to obtain deposits with different levels of dilution, have been non-destructively evaluated by eddy current (EC) and magnetic Barkhausen (MB) techniques. The deposits were also characterized by microstructural and hardness studies. The EC parameters viz. magnitude and phase angle of the induced voltage showed an increasing trend with increasing dilution of the deposits. MB root mean square (RMS) voltage and peak height also indicated similar trend with dilution. The results produced from nondestructive tests could be correlated with hardness and microstructure of the deposits. Thus, it is possible to develop a non-destructive technique to predict hardness of the Colmonoy hardfacing alloy, which can serve as quality control tool in estimating the hardness of the hardfaced coating to ensure that dilution of the deposit by the substrate does not bring down the hardness of the deposit below the acceptable minimum values.

Influence of post-treatment on the microstructural evolution of thermally sprayed Fe-base MMC containing TiC and Cr3C2

In this study, ledeburitic cold-work tool steel X220CrVMo13-4 (AISI D7) was admixed with TiC and Cr3C2 and the resulting feedstock powder was deposited on an austenitic substrate X5CrNiMo17-12-2 (AISI 316L) by the High Velocity Oxy Fuel (HVOF) process. The coatings exhibited low porosity and good adhesion to the substrate material. Further improvement of the material and tribological properties of the deposited coatings was achieved by hot isostatic pressing (HIP) and heat treatments. HIP treatment, which was carried out without encapsulation, led to a fully dense MMC coating. Further heat treatment to secondary peak hardness of the ledeburitic cold-work tool steel increased the hardness and thus the tribological properties. Diffusion reactions and the embedding behavior of the hard particles in the steel matrix were investigated with respect to the post-treatment processes by means of diffusion calculations and microstructural examinations. The results indicated that diffusion processes at the hard particle/metal matrix interface exert a strong influence on the embedding behavior of TiC and Cr3C2 particles in the coating microstructure. During HIP and heat treatment, new phases with a larger amount of metallic bonding were formed, thus improving particle adhesion to the metal matrix.

Advanced chromium carbide-based hardfacings

Ceramic metal composite powders are widely used in thermal spray technologies; however, application of such hardphases in cladding systems has not been strongly developed yet. In the present study, two different hardfacings produced by the plasma transferred arc (PTA) process were analysed and compared to reveal differences between NiCrBSi coatings reinforced with standard chromium carbide and chromium-based cermet powders. The coatings were produced from a mixture of hardphases (Cr3C2 or Cr3C2–Ni) and nickel based powder with a ratio of 40/60vol.%. The coatings' thickness was set to 2–2.5mm on an austenite substrate. Hardfacings were characterised in terms of their microstructures, mechanical properties and impact–abrasion wear resistance at room and elevated temperatures. The manufactured Cr3C2–Ni reinforced hardfacing alloy has shown promising microstructural features with a low level of carbide dissolution and high temperature wear performance.

The influence of temperature and nitrogen pressure on S-phase coatings deposition by reactive magnetron sputtering

The paper presents the results of investigations on S-phase coatings obtained at various temperatures from the range 25–400° and for different nitrogen content in plasma. The influence of both parameters on layer formation is investigated. A special attention is paid to phase composition of the coating. The phase composition was evaluated using X-ray diffraction (XRD and GXRD). EMPA (EDS and EDS) and GDOS techniques were used to study element composition of the layers. It was observed that nitrogen content in plasma has an important effect on nitrogen concentration in the coatings and on the deposition rate, while temperature influence is negligible. During the deposition at high temperature and in the presence of high nitrogen content nitrogen diffusion into the austenitic substrate takes place and a formation of S-phase layer is observed.

Diffusion and Phase Transformation at the Interface between an Austenitic Substrate and a Thermally Sprayed Coating of Ledeburitic Cold‐Work Tool Steel

AbstractAn overlay coating against wear or corrosion on components is required for various technical applications. Thermal spraying is a well‐established and near‐net‐shape deposition method. In this work, high‐velocity oxy‐fuel spraying of two different ledeburitic cold‐work tool steels was employed to produce wear‐resistant Fe‐base coatings on a stainless steel substrate. This work focuses on the investigation of diffusion processes across the coating/substrate interface. Specimens were heat‐treated for different dwell times and then analyzed by means of EBSD, XRD, OM, as well as SEM. Results of phase formation and diffusion profiles were compared with equilibrium and diffusion calculations obtained with ThermoCALC® and DICTRA®. The influence of diffusion processes across the coating/substrate interface on the mechanical properties, such as adhesive bond strength and hardness, was investigated by shear tests and microhardness profiles.

Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour

Abstract The aim of the present study is to develop a Fe-based metal matrix composite (MMC) coating using high velocity oxy-fuel spraying (HVOF) process. A ledeburitic high alloyed cold work tool steel (X220CrVMo13-4) and NbC with an average size of 2 µm at different volume fractions have been considered as metal matrix and hard particles respectively. MMC coatings were deposited on austenitic stainless substrates and the coatings were subsequently densified by hot isostatic pressing (HIP) with and without encapsulation. Microstructural analysis of the as-sprayed and HIPed coatings were characterized by SEM and XRD methods. Results showed that the feedstock preparation involving fine NbC was an influencing factor on the coating deposition. A relatively homogeneous dispersion of fine NbC up to 30 vol.% in cold work tool steel matrix was possible using optimized HVOF spraying. Besides, HVOF spraying and its subsequent HIP treatment induced significant microstructural and phase changes in the MMC coatings. The study showed the potential of HVOF spraying for the development of steel based MMC coatings and its subsequent densification can be achieved by HIP process with and without encapsulation.

Formation of nitrided layers on duplex steel – influence of multiphase substrate

The present paper reports the results of gas nitriding of duplex stainless steel and study of the effect of substrate structure on the nitrided layers. The differences in the growth of the layers on austenitic and ferritic substrate were studied. The microstructure and phase composition of the layers were investigated using light microscopy analysis), X-ray diffraction (XRD and grazing angle XRD – GXRD) and electron microprobe spectrometry. Moreover, the microhardness of the layers was studied. A particular role of steel substrate in layer formation was observed at a low temperature (below 500°C) nitriding. The layers were mostly composed of the S phase and expanded martensite formed on austenite and ferrite grains respectively. The substrate influence was of minor importance at treatment temperature higher than 500°C.

Diffusion Aluminide Coatings Using Spherical Micro-Sized Aluminium Particles

Previous work on the oxidation of nano- and micro-sized Al particles revealed a particle size window, where no meta-stable alumina phases were observed. Depositing such particles on an austenitic substrate, diffusion layers with reduced Al contents were obtained. These findings opened new perspectives for investigating the potential impact of the Al particle size and shape on the formation of diffusion aluminide coatings. Spherical Al particles sized in the range of 2 to 5 µm were deposited with a binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). For the curing process, the samples were annealed in air at 400°C for 1h. The diffusion effect of Al into the base material was studied in isothermal experiments at 700°C and 900°C with exposure times up to 2000 h in air. The sample surfaces and the diffusion aluminide coatings in cross-section were analysed by field emission scanning electron microscopy (FE-SEM). The results show in the initial state the formation of a diffusion layer consisting of a less aluminium-rich Fe(Cr)-Al phase containing a Fe(Cr)-Al phase with higher content of Al in the region beneath the surface. On further exposure a double-layered structure is found with Kirkendall-pores between the two layers, which may lead to a complete separation of the outer layer. A thin adherent alumina scale is observed on the remaining diffusion layer after 1000 h and 2000 h at both temperatures, however overgrown by Cr2O3 at 900°C. The structure of the diffusion zone beneath agglomerates of Al particles reveals the influence of the particle size on the Al supply for the formation of the aluminide diffusion zone.

Effect of molybdenum and copper on S-phase layer thickness of low-temperature carburized austenitic stainless steel

Surface hardening of austenitic stainless steels without degradation of corrosion resistance has been a subject of continuing interest in the field of surface engineering technology. This paper presents a method, using low-temperature plasma-carburized processing, to produce a hard and corrosion-resistant layer for austenitic stainless steels. The process is based on the diffusion of carbon and/or nitrogen from the surface into the austenitic substrate without forming any carbides or nitrides. The resultant carburized and/or nitrided surface layer shows highly superior saturation of carbon or nitrogen. It is called expanded austenite or S-phase. Important details of the S-phase have not been revealed yet, e.g., the formation of super saturated solid solution at constant processing temperature. For this study, austenitic stainless steel with 3 mass% copper was selected for use as a substrate of low-temperature carburizing. Actually, Cu shows no tendency for formation of carbides in steel. Samples were plasma-carburized using DC plasma apparatus under 667 Pa of mixed gas flow of 5% CH4+45% H2+50% Ar at 673 K or 723 K for various durations. The treated S-phase of steel with copper was compared to those of 304 steel and 316, 317 steels containing molybdenum. Results show that copper addition, just as molybdenum addition, similarly enhances carbon super saturation and the surface-layer hardness. Furthermore, the carburized layer depth was enhanced with copper addition as well as Mo addition. The effects of copper and molybdenum are discussed with measurements of the lattice constants in consideration of the size effect.