Polarization characteristics of magnetoelectropolishing stainless steels

The aim of the study is to present the polarization characteristics of electropolishing process carried out in a magnetic field (MEP—magnetoelectropolishing), in comparison with the ones obtained under standard/conventional process of electropolishing (EP) conditions. The polarization characteristics have been considered as the most important factor in establishing effective treatment conditions. With this paper we would like to show the results of the studies concerning the effects of a magnetic field strength and electropolishing conditions (convection, temperature) on the electrochemical polarization characteristics. Specifically the occurrence of electropolishing EP plateau has been observed in view of optimization of magnetoelectropolishing MEP process. It has been found that, depending on the magnetic field strength, under higher potentials the current density has been growing with the increase of potential to a certain value only. Afterwards the current density remains unchanged. This behavior in the course of polarization curves obtained under high potentials has been revealed to be the effect of cyclically disclosure of sample surface and re-submerge, this resulting in the “second limiting current”.     

Simulations of precipitation in ferritic steels

Abstract

A new technique based on Monte Carlo random sampling has been proposed to simulate the precipitation kinetics in alloys. The new approach employs time dependent nucleation and diffusion laws, considers both intergranular and intra-granular precipitation, and also combines precipitation kinetics with intergranular segregation. The simulation can be used not only to predict the average size of precipitate phase particles, but also to predict particle size distributions, volume fraction, and interparticle spacing. The new approach overcomes the shortcomings of earlier model calculations where only the average size of the precipitate phase is considered. In addition, the proposed simulation overcomes the difficulty of connecting Monte Carlo steps to real time using the Metropolis algorithm. The approach has been used to simulate M₂₃C₆ precipitation kinetics in a creep resistant steel, P92: the results are in good agreement with published experimental measurements, and the model is believed to be applicable to other types of precipitates in different alloys.     

Novel high-entropy and medium-entropy stainless steels with enhanced mechanical and anti-corrosion properties

ABSTRACT

Novel high-entropy and medium-entropy stainless steels (MESS), containing a high amount of alloying elements, were designed and prepared by arc melting. These high-entropy and MESS possess a simple phase constitution, mainly solid solution phases, and noticeably exhibit excellent anti-corrosion properties in sulphuric acid. In particular, an austenitic Fe₆₅Cr₁₃Co_4.75Mn_4.75Mo_4.75Ni_4.75Cu₃ (at.-%) alloy has a higher hardness of 182 HV and better corrosion resistance than those of the 00Cr19Ni14Mn2 austenitic stainless steel prepared under the same condition; a ferritic precipitation hardening Fe₆₅Cr₁₃Al_3.167Co_3.167Mn_3.167Mo_3.167Ni_3.167Ti_3.167Cu₃ (at.-%) alloy was also developed, with a higher hardness of 584 HV and even better corrosion resistance.     

Wetting and brazing of stainless steels by copper–silver eutectic

The wetting of stainless steels by Cu–Ag-based alloys was studied by the sessile drop and dispensed drop techniques in high vacuum at 800–900 °C. Experiments were performed by varying the steel type, composition of the Cu–Ag-based alloy and furnace atmosphere. The results were used to determine the processes controlling the wetting and brazing of these important technological materials.     

Future developments and applications of nitrogen-bearing steels and stainless steels

After considerations related with the global frame of the demand of society in the field of materials and some recalling of basic properties and principles of nitrogen alloying, possible future developments are listed and discussed.     

Effect of heating mode and temperature on sintering of YAG dispersed 434L ferritic stainless steel

This study examines the effect of heating mode, sintering temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt%) on the densification and properties of ferritic (434L) stainless steel. The straight 434L stainless steel and 434L–YAG composites were sintered in a conventional and a 2.45 GHz microwave furnace. The composites were sintered to solid-state as well as supersolidus sintering temperature at 1200 and 1400 °C, respectively. Both 434L and 434L–YAG compacts coupled with microwaves and underwent rapid heating (∼45 °C/min). This resulted in about 85% reduction in the processing time. For all compositions microwave sintering results in greater densification. As compared to conventional sintering, microwave sintered compacts exhibit a more refined microstructure, thereby, resulting in higher bulk hardness. The mechanical properties and sliding wear resistance of 434L stainless steel is shown to be sensitive both to the sintering condition as well as YAG addition and has been correlated to the effect of heating mode on the pore morphology.     

Sintering of 17-4PH stainless steel feedstock for metal injection molding

The sintering behavior of 17-4PH stainless steel feedstock for metal injection molding was investigated in the temperature range of 650-1050 °C. Effects of sintering conditions, such as sintering temperature and sintering atmosphere, were examined. Results showed that when sintered in the hydrogen/nitrogen atmosphere, the 17-4PH feedstock was oxidized over the temperature range of investigation. The degree of oxidization increased with the sintering temperature. The main oxidization product was Cr₂O₃ as revealed by X-ray diffraction and composition analysis. The oxidation can be avoided by sintering in vacuum or argon atmosphere.     

In-situ synthesis of TiC/Fe alloy composites with high strength and hardness by reactive sintering

Fe alloy composites reinforced with in-situ titanium carbide(Ti C) particles were fabricated by reactive sintering using different reactant C/Ti ratios of 0.8,0.9,1 and 1.1 to investigate the microstructure and mechanical properties of in-situ Ti C/Fe alloy composites.The microstructure showed that the in-situ synthesized Ti C particles were spherical with a size of 1–3 μm,irrespective of C/Ti ratio.The stoichiometry of in-situ Ti C increased from 0.85 to 0.88 with increasing C/Ti ratio from 0.8 to 0.9,but remained almost unchanged for C/Ti ratios between 0.9 and 1.1 due to the same driving force for carbon diffusion in Ti Cxat the common sintering temperature.The in-situ Ti C/Fe alloy composite with C/Ti = 0.9 showed improved mechanical properties compared with other C/Ti ratios because the presence of excess carbon(C/Ti = 1 and 1.1) resulted in unreacted carbon within the Fe alloy matrix,while insufficient carbon(C/Ti = 0.8)caused the depletion of carbon from the Fe alloy matrix,leading to a significant decrease in hardness.This study presents that the maximized hardness and superior strength of in-situ Ti C/Fe alloy composites can be achieved by microstructure control and stoichiometric analysis of the in-situ synthesized Ti C particles,while maintaining the ductility of the composites,compared to those of the unreinforced Fe alloy.Therefore,we anticipate that the in-situ synthesized Ti C/Fe alloy composites with enhanced mechanical properties have great potential in cutting tool,mold and roller material applications.     

The effect of niobium on the microstructure of ferritic stainless steel

The effect of different amounts of Nb and of homogenization on the ferritic stainless steels containing 17–18 wt.% Cr was investigated with scanning electron microscopy (SEM), optical microscopy, energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). It was observed that M₂₃C₆, NbC and sigma phase formed in these steels. In addition, the formation of Nb₂C was observed in the sample containing 3.0 wt.% Nb. While the amount of Nb increased from 0.5 to 3.0 wt.% Nb, the microhardness of the matrix and the amount of M₂₃C₆ decreased and the toughness of the samples increased. After homogenization, the increase in the toughness of the samples containing 1.5–3.0 wt.%Nb was considerable and impressive.     

Sintering behaviour of Copper/carbon nanotube composites and their characterization

The potential usage of Copper (Cu) is very limited, where combined mechanical and thermal properties are desirable, which can be overcome by using carbon nanotube (CNT) as a reinforcement. An attempt was made to synthesize Cu/CNT composites by varying CNT diameter and its concentration through a molecular level mixing technique followed by uniaxial compaction and conventional sintering. The sintering behaviour of Cu and Cu/CNT composites was studied to understand the influence of different parameters. The sintering duration of Copper was decreased with an increase of CNT diameter. The maximum enhancement of properties was achieved at 0.25 wt.% CNT irrespective of its diameter, where the thermal conductivity and hardness were obtained as 328 W/mK at 20–40 nm diameter CNT composites and 81.2 ± 2.9 VHN at 40–60 nm diameter CNT composites, respectively. The conventional method of synthesize can generate the desired characteristics of composites at par with high end techniques, such as SPS.     

Stabilisation of ferritic stainless steels with Zr and Ti additions

Abstract

Traditional 11·5 wt-%Cr ferritic stainless steels are single stabilised with Ti or dual stabilised with Ti–Nb additions. A dual-stabilised ferritic stainless steel 409 with Zr–Ti additions was studied, which was selected through thermodynamic and kinetics analysis. The alloy was subjected to thermomechanical processing using both hot and cold rolling and annealing. The intergranular corrosion resistance and microstructure of this alloy was evaluated. Of particular interest was to study the stabilisation behaviour of this alloy under all processing conditions. The results showed that the precipitation of Cr_xC_y was effectively prevented; hence, the alloy used in this investigation had an excellent resistance to intergranular corrosion.     

Microstructure and mechanical behavior of laser additive manufactured AISI 316 stainless steel stringers

Laser additive manufacturing of stainless steels is a promising process for near net shape fabrication of parts requiring good mechanical and corrosion properties with a minimal waste generation. This work focuses on high aspect ratio AISI 316 steel structures made by superposition of sequential layers. A special nozzle for precise powder delivery together with a monomode fiber laser allowed producing high quality steel stringers on AISI 316 steel substrates. Although the stringers average compositions were inside the austenite plus ferrite range, only austenite phase was verified. The cladded structure presented some internal pores and cracks, responsible by the low Young’s moduli. However, the tensile properties were similar to the base material and other literature results. The three-point flexural tests also produced good results in terms of formability. The fabricated structures proved to be useful for use in mechanical construction.     

Investigation of Sintering Technology for Composite of Stainless Steel and Partially Stabilized Zirconia

The sintering technology for mixed powedered extrusion rods of different proportions of stainless steel to magnesia partially stabilized zirconia(PSZ) was investigated..The effects of some sintering parameters including holding time ,atmosphere and protective gas pressure on shrinkage,relative density,micostructure,micro-Vickers hardness and compression strength of sintered samples were mainly researched.The experimental results are as follows(1)The shrinkage and the relative density of the sintered samples decrease as increasing stainless steel content in the composite,except for the xase containing 90 percent of stainless steel;(2)The porosity in PSZ matrix rises as increasing the stainless steel content in the composite;(3)Longer sintering holding time ,higher sintering vacuum and gas-pressure sintering process not only enhance the relative density ,but also improve microstructure of composite;(4)Micre-Vickers hardness of PSZ matrix decreases as increasing stainless steel content,while that of stainless steel particles in sintered samples varies unnoticeably.     

Powder injection moulding of premixed ferritic and austenitic stainless steel powders

In this experimental work, powder injection moulding (PIM) of premixed 316L and 430L gas-atomized powders was developed to obtain duplex stainless steels. A multicomponent binder constituted of high density polyethylene (HDPE) and paraffin wax (PW) in a volume ratio 50/50 was selected for the process. Feedstocks with powder loadings of 50, 65, 68 and 70 vol.% were prepared. Mixing experiments were carried out at 170 °C according to differential scanning calorimetry (DSC) results. The rheological characterization of feedstocks allowed establishing different rheological parameters as power flow index (n) and activation energy (E_a) in order to know their suitability for injection moulding. Critical powder volume concentration (CPVC) was determined by means of oil absorption method and a rheological model. The feedstock was injected at 170 °C and three-point bending and tensile parts were obtained. Thermogravimetrical analysis (TGA) of binder allowed us to design the thermal debinding cycle and sintering was carried out in low vacuum at different temperatures. Finally, mechanical properties such as hardness and tensile strength were evaluated.     

On CTE of SPS consolidated SiCp/Al composites with various particle size distributions

Abstract

The coefficient of thermal expansion (CTE) of spark plasma sintering consolidated SiC_p/Al composites with various size distributions was investigated with the combination of experimental measurements and modelling analyses. The CTE of the composites decreased with increasing particle volume fraction, and large particles played a major role in the decline of CTE. The measured CTE lay between the predictions of Kerner model and Schapery lower bound, but the possible formation of percolating particle network and the influence of matrix plasticisation led to the slight deviation of the experimental values from model predictions. A CTE peak appeared for all the composites with increasing temperature to about 250–300°C due to the action of matrix plasticisation filling the microvoids in the composites. The composites with mixed particles of substantially different sizes were prone to concentrate thermal stresses on large particles, which induced an early appearance of matrix plastic deformation that can result in a comparably low CTE peak temperature.     

Statistical assessment of notch toughness against cleavage fracture of ferritic steels

The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.     

Sintering behaviour of spinel-alumina composites

Study of alumina-magnesia binary phase diagram reveals that around 40–50 wt% alumina dissolves in spinel (MgAl₂O₄) at 1600°C. Solid solubility of alumina in spinel decreases rapidly with decreasing temperature, which causes exsolution of alumina from spinel phase. Previous work of one of the authors revealed that the exsolution of alumina makes some interlocking structures in between alumina and spinel phases. In the present investigation, refractory grade calcined alumina and spinel powder were used to make different batch compositions. Green pellets, formed at a pressure of 1550 kg cm⁻² were fired at different temperatures of 1500°, 1550° and 1600°C for 2 h soaking time. Bulk density, percent apparent porosity, firing shrinkage etc were measured at each temperature. Sintering results were analysed to understand the mechanism of spinel-alumina interactions. SEM study of present samples does not reveal the distinct precipitation of needle shaped α-alumina from spinel, but has some effect on densification process of spinel-alumina composites. Microstructural differences between present and previous work suggest an ample scope of further work in spinel-alumina composites.     

Thermomechanical fatigue behavior and lifetime prediction of niobium-bearing ferritic stainless steels

In this study, the thermomechanical fatigue (TMF) and isothermal low-cycle-fatigue (LCF) behaviors of niobium-containing ferritic stainless steels are presented for the temperature range from 100 °C to the maximum temperatures between 500 and 800 °C; furthermore, we propose a new fatigue failure criterion to predict the fatigue lives of the components for different thermal cycle ranges using the TMF condition. Higher maximum temperatures during TMF cycle resulted in shorter TMF life. By modifying the Coffin–Manson equation using the temperature factor, we obtained a new parameter that was successfully correlated with the life under different maximum temperatures. The deformation responses during fatigue cycling and the fatigue microstructure were compared to elucidate the different fatigue behaviors under the TMF and LCF conditions.     

Comparative study of precipitation effects during aging in superaustenitic and superferritic stainless steels

Abstract

The concept of designing a steel that would match high corrosion resistance coupled with improved mechanical properties when exposed at elevated temperatures has always been a challenge for a metallurgist. The present paper relates the results of microstructural analysis of two commercial superaustenitic stainless steel grades and a novel experimental grade of superferritic stainless steel. A study of the microstructural stability and attendant mechanical property evolution has been carried out, employing a variety of aging experiments. Following prolonged aging up to 1000 h in the temperature range between 650 and 950°C, microstructure evolution was determined, while the mechanical properties were preliminary assessed via hardness testing. The present study helped clarify the effect of high temperature exposure on the precipitation sequences of both superaustenitic and novel superferritic stainless steels. The heat treatments performed indicate a number of precipitation sequences of intermetallic phases taking place, such as σ phase formation, clearly specifying a time–temperature regime where care must be taken during the fabrication and use of these materials.