CHARACTERIZATION OF CHROMIUM NITRIDE PRECIPITATION IN THE HEAT AFFECTED ZONE OF THE SUPERDUPLEX STAINLESS STEEL UNS S32750: AN EXPERIMENTAL AND NUMERICAL ANALYSIS

It is well known that pitting corrosion resistance of duplex and superduplex stainless steels strongly depends on microstructural characteristics such as ferrite/austenite proportion, presence of intermetallic phases and elemental partitioning between the austenite and ferrite phases. In particular, during the welding operation, very fine chromium nitrides may precipitate within ferrite grains of the heat affected zone drastically reducing the corrosion resistance of welded joints of duplex and super duplex stainless steels. However, due to their small size and low distribution, analyzing the chemical composition and crystallography of chromium nitrides is quite difficult and only a restricted number of advanced techniques of investigation may discriminate their signal from the surrounding matrix. This work is aimed at supporting the microstructural characterization of a welded joint of a superduplex stainless steel by means of a field-emission gun scanning electron microscope. Sub-micron chromium nitride precipitates, identified within the ferritic grains of the heat affected zone, are recognized to be the main reason for the reduced pitting corrosion resistance of the analyzed welded joints. The results are supported by a multi-pass welding process numerical simulation aimed at estimating the cooling rates promoting chromium nitride precipitation in the heat affected zone.

Pump elements recondition by electrolytic chromium plating

Improving the efficiency of maintenance production in agriculture closely connected to the creation and implementation of such methods of repair and recondition of elements that improve the physicomechanical and service properties of wear joint assemblies.The presence of a nitrided case on the surface of elements lays the groundwork for obtaining a complex covering based on chromium nitrides. Electrolytic chromium plating is one of the ways to recondition elements that increase the service life and machines reliability, and reduce maintenance cost. However, the widespread adoption of chromium plating for the recondition of worn elements is hold back by the low productivity of the process. Therefore, the intensification of chromium plating in reconditioning worn-out machine elements is an urgent problem. It is advisable to recondition critical parts subjecting to abrasive wear, for example, precision vapors of fuel injection equipment, with chromium plating. However, the low productivity and high-energy consumption of the process require the improvement of the electrolyte in order to increase the current output of chromium and the permissible cathode density. The article describes a method for improving the quality of an electrolyte, a method for increasing its versatility, namely the supplementation of various organic additives and complexing substances into it.

Modelamento computacional das transformações de fase durante os ciclos térmicos de processamento de um aço inoxidável superdúplex

This project aims to perform the computer simulation of the transformation’s kinetics and phase evolution during the thermal processing cycles of a superduplex stainless steel, considering the stages of heating, hot working and cooling, using DICTRA® software. The input data for the simulations were chemical composition and phase size, desired simulation temperature, and heating and cooling rates when necessary to describe the thermal cycle. TCFE9 thermodynamic database and MOBFE4 atomic mobility database were used in order to obtain results for different models, determining the one that best describes the phase transformation kinetics. Different rates were simulated during the heating of the material from 950 ° C, considering the initial microstructural condition of 50% of ferrite [alpha] and 50% austenite [gamma], up to 1250 ° C, typical forming temperature. In heating, a maximum fraction of 66.6% [alpha] was obtained at a rate of 0.30 ° C/s, [alpha] value close to the 70% expected by the equilibrium simulation in Thermocalc®. After 1000 s of plateau at 1250 ° C and cooling to the solubilization temperature, 1090 ° C, at the rate of 0.30 ° C/s, the fraction of [alpha] reduced to values of 58.7%. In the sequence, different cooling rates were also simulated with or without the presence of solubilizations plateau. Considering 3600 s of plateau at 1090 ° C, it was possible to recover the desired duplex condition, reaching 55.5% [alpha], but not reaching the 50% expected by the equilibrium balance, since there is still a composition gradient in [alpha] and [gamma] by DICTRA® simulations. Seeking the maintenance of the duplex microstructure, a cooling was performed from 1090 °C to 790 ° C at a critical rate of 3.0 ° C/s, obtaining volumetric fractions of 56% [alpha], 43% [gamma] and sigma fractions equal to or less than 1%. If the plateau at 1090 ° C was not considered, that is, promoting cooling from 1250 °C, where the condition of 58.7% [alpha] was reached, to 790 ° C at the rate of 3.0 ° C/s volumetric fractions of 59.6% alpha], 39.4% [gamma] and 0.9% [sigma] were obtained. DICTRA® was unable to simulate the precipitation of chromium nitrides (Cr2N) during cooling, either because there was no nitrogen supersaturation (N) in [alpha] or because it was unable to predict this supersaturation. From the results of kinetics and evolution of the phases’ volumetric fraction obtained in the thermal cycle of steel processing UNS S32750, it was possible to obtain the computational model that best describes the real behavior of the studied steel

Effect of Heat Treatment on the Microstructure of Duplex Stainless Steel Welds

Duplex stainless steels (DSS) are gaining in popularity due to their characteristic features, excellent mechanical properties, and corrosion resistance. The microstructure of DSSs consists of ferrite up to 50 %, and the rest is built up from austenite. The ferritic microstructure can cause chromium-nitride precipitation because the nitrogen solubility in the ferrite phase is very low below 700 °C. Our research showed that electrochemical etching is an acceptable process for revealing chromium-nitrides. Additionally, our research points out that chromium-nitride acts as a secondary austenite nucleation site.

Structure and Topography Modifications of Treated AISI 316LN Stainless Steel Surfaces after Friction in Dry Sliding Contact by Case Hardening Process

Austenitic stainless is having corrosion resistance property, but certain mechanical applications require improved resistance to wear, inferior cavitation, susceptibility to sensitization. These steels have numerous favourable circumstances for great cryogenic - properties, anti-corrosion, and bio-compatibility. So these steels have a broad application in low temperature innovation, saltwater applications, nourishment industry, bio-medicine, petro-chemical handling, and so forth when alloyed with nitrogen, austenitic treated steels has a progressively steady austenite structure, better mechanical properties and better wear opposition, which has animated extraordinary enthusiasm for this exploration work. Many surface hardening techniques are available, the best surface modification technique is chosen for improved service performance. Surface engineering is a technique to alter the surface of a material by mechanical or microchemical method without affecting the material properties. The alterations are done on the surfaces subjected to the liquid nitriding process to produce a hardened surface. Chosen for this research workbased on their wide application, the wear behaviour of AISI 316LN stainless steels was investigated. Of the various surface hardening techniques available, nitriding is chosen, so that these nitrogen gets penetrated into the material, in which hard iron chromium nitrides are formed at the surface level. AISI 316LN specimens were subjected to salt bath nitriding process. The specimens were nitrided to 60 minutes, 120 minutes and 180 minutes respectively. The specimens were undergone with wear tests by standardized tribiological wear machine and finally the metallographic studies were made.

Formation of Chromium Nitride and Intragranular Austenite in a Super Duplex Stainless Steel

Precipitation of chromium nitrides and formation of intragranular austenite were studied in detail for the super duplex stainless steel grade 2507 (UNS S32750). The situation of multipass welding was simulated by heat treatment at 1623 K (1350 °C) and quenching followed by short heat treatments at 1173 K (900 °C). The microstructural evolution was characterized using transmission and scanning electron microscopy, electron backscatter, and transmission Kikuchi diffraction, and it was observed that the interior of the ferrite grains contained chromium nitrides after quenching. The nitrides were predominantly of CrN with a cubic halite-type structure and clusters of CrN-Cr2N where rod-shaped trigonal Cr2N particles had nucleated on plates of CrN. After heat treatment for 10 seconds at 1173 K (900 °C), the nitride morphology was transformed into predominantly rod-shaped Cr2N, and finely dispersed intragranular secondary austenite idiomorphs had formed in the nitride-containing areas within the ferrite grains. After 60 seconds of heat treatment, both the Cr2N nitrides and the secondary austenite were coarsened. Analysis of electron diffraction data revealed an inherited crystallographic relationship between the metastable CrN and the intragranular austenite. The mechanism of chromium nitride formation and its relation to secondary austenite formation in duplex stainless steels are discussed.

Superconductivity in chromium nitrides Pr3Cr10-xN11 with strong electron correlations

Exploration of superconductivity in Cr-based compounds has attracted considerable interest because only a few Cr-based superconductors (CrAs, A2Cr3As3 and ACr3As3 (A = K, Rb, Cs, Na)) have been discovered so far and they show an unconventional pairing mechanism. We report the discovery of bulk superconductivity at 5.25 K in chromium nitride in Pr3Cr10-xN11 with a cubic lattice structure. A relatively large upper critical field of Hc2(0) ∼ 12.6 T is determined, which is larger than the estimated Pauli-paramagnetic pair-breaking magnetic field. The material has a large electronic specific-heat coefficient of 170 mJ K−2 mol−1—about 10 times larger than that estimated by the electronic structure calculation, which suggests that correlations between 3d electrons are very strong in Pr3Cr10-xN11, and thus quantum fluctuations might be involved. Electronic structure calculations show that the density of states at the Fermi energy are contributed predominantly by Cr 3d electrons, implying that the superconductivity results mainly from the condensation of Cr 3d electrons. Pr3Cr10-xN11 represents a rare example of possible unconventional superconductivity emerging in a 3D system with strong electron correlations. Nevertheless, clarification of the specific pairing symmetry needs more investigation.

Effects of TIG Reheating on Duplex Stainless Steel Weld Microstructure

Duplex stainless steels (DSS) gaining their excellent mechanical properties and corrosion resistance due to their austenitic-ferritic microstructure, ideally in the same amount. However, to keep this ideal phase ratio during arc welding is very difficult. Generally, the arc welding processes will result in more ferritic microstructure in the weld metal and in the heat affected zone, due to the rapid cooling. The ferritic microstructure can cause chromiumnitride precipitation, because the nitrogen solubility in ferrite phase is very low below 700 °C. These chromiumnitride precipitations can cause loss of corrosion resistance and mechanical properties. However, during subsequent reheating, the chromium-nitrides can dissolve and act as a secondary austenite nucleation site in the ferritic microstructure. In our research we welded DSS specimen autogenously, with tungsten inert gas welding using pure argon and 94 % argon + 6 % nitrogen as shielding gasses. In the first case the sub-sequent solid-state reheating caused 20 % increase in the austenite fraction of the weld metal but with the use of mixed shielding gas only 5 % increase.