Carlos Mario Garzón

New high temperature gas nitriding cycle that enhances the wear resistance of duplex stainless steels

Interstitially dissolved nitrogen improves the corrosion and wear resistance as well as the mechanical properties of stainless steels (SS) [1–5]. Production routes of High Nitrogen Stainless Steels (HNSS) by alloying, pressure metallurgy, powder metallurgy, and solid-state diffusion have been studied [6–10]. In the production route, which involves solid-state diffusion, the steel surface and near surface regions are alloyed with nitrogen through chemical, implantation, plasma, or laser techniques [9, 10]. Recently, a chemical solid-state nitrogen alloying technique was developed [10–14], consisting in annealing SS in a N2-containing gas atmosphere in the range 1273–1473 K. In this High Temperature Gas Nitriding treatment (HTGN), atomic nitrogen is absorbed at the surface of the steel and then diffuses into the near surface region. Case-depths from 0.5 to 2.0 mm and nitrogen contents in solid solution at the surface from 0.5–1.0 wt% can be obtained after 18 to 45 ks heat-treatments. HTGN has been successfully used to improve the surface properties of martensitic, austenitic, ferriticaustenitic and martensitic-ferritic SS [7, 10–13, 15]. Particularly, when ferritic-austenitic duplex stainless steels (DSS) are nitrided austenitic cases of higher wear and corrosion resistances are formed, on high strength DSS ferritic-austenitic cores. Due to both high temperatures and long nitriding times: (i) the austenitic cases grow forming coarse columnar grains [10–13], and (ii) the maximum attainable nitrogen content in precipitatefree cases corresponds to the nitrogen solubility limit at that temperature. The solubility limit in austenite, relative to nitride precipitation, increases with temperature; however the amount of ferrite in the dual-phase non-nitrided core increases with temperature too. Thus the optimum nitriding temperature for DSS is between 1423 and 1448 K. In the present work, a novel nitriding cycle that avoids formation of coarse grains in the austenitic case, inhibits nitride precipitation and leads to sharp textures is proposed. It consists on cycling the specimen between two different N2 partial pressures, PN2, (Fig. 1): a high-pressure stage (sorption stage) and a vacuum one (desorption stage). The high nitrogen pressure stage is a long term one where nitrogen is introduced in the specimen. It is followed by a short vacuum period (PN2 ∼ 0) where nitrogen desorption occurs and ferrite

Growth kinetics of martensitic layers during high temperature gas nitriding of a ferritic – martensitic stainless steel

Abstract A dual phase ferritic – martensitic AISI 410S stainless steel was nitrided in high purity N2 atmosphere between 1373 and 1473 K. After this treatment a high nitrogen martensitic case, free of precipitates, was formed. The growth kinetics of these fully martensitic cases during high temperature gas nitriding is studied with the aim of predicting martensitic case depths as functions of nitriding times, temperatures and N2 partial pressures. Thermocalc and Dictra software were used to calculate the equilibrium conditions and to solve the diffusion equations respectively. The results showed that the growth rate of the martensitic case is diffusion controlled and it can be calculated assuming local equilibrium. For the nitriding conditions used in this paper, the martensitic case depth is proportional to the square root of the nitriding time t1/2 proportional to the nitriding temperature and roughly proportional to the logarithm of the N2 partial pressure.

Modelamento termodinâmico e cinético por meio do método Calphad do processamento térmico e termoquímico de aços

In 1970s, Kaufman and Bernstein summarized the general features of computer numerical calculation of multicomponent phase equilibrium thermodynamics, by means of polynomial expressions of the Gibbs free energy, and they created the CALPHAD group (Coupling of Phase Diagrams and Thermochemistry). CALPHAD group aims to promote computational thermodynamics through development of models to (i) critical assessment of experimental and computed data and their incorporation into self-consistent databases, (ii) represent thermodynamic properties for various phases which permit prediction of properties of multicomponent systems, and (iii) development of software to improve understanding of various industrial and technological processes. Besides software for performing thermodynamic modeling, there are today several programs to modeling kinetics of diffusion controlled phase transformations. These kinetic programs are interfaced with programs for performing thermodynamic simulations as well as with kinetic databases. In this work, several case studies about numerical modeling of thermal as well as thermochemical processing of steels are reported. The analyzed case studies are brief descriptions of theoretical and experimental research works which are being carried out in the Metallurgical and Materials Engineering Department, University of Sao Paulo. It is shown the processing parameter optimization during: high-alloy steel nitriding, TRIP steel thermal processing, CrN and Cr2N thermal synthesis from chromium powder, solution annealing of stainless steels, and primary-carbide decomposition during heat treatment of tool steels.

Efeito do nitrogênio na usinabilidade do aço inoxidável austenítico: uma avaliação utilizando a técnica da esclerometria pendular instrumentada

High nitrogen stainless steels (HNSS) are being considered a new promising class of engineering materials. When nitrogen is added to austenitic steels it can simultaneously improve fatigue life, strength and wear and localized corrosion resistance. In this work, a single pass pendulum scratch test was used to study the effect of nitrogen on the scratch resistance and on the machinability of an UNS S30403 austenitic stainless steel. Samples with increasing nitrogen contents at the surface were obtained through high temperature gas nitriding. The thermochemical treatments were performed at 1473 K in (N2+Ar) gas atmospheres for 36.0 ks, obtaining fully austenitic cases (surface nitrogen contents up to 0.5 wt%) ca. 1.5 mm in depth. The scratch tests were performed in a single-pass pendulum, equipped with strain gages to measure normal and tangential forces during scratching. The specific absorbed energy was calculated as the ratio between the measured absorbed energy and the amount of mass removed from the specimen. An increase of the specific absorbed energy with increasing nitrogen content was observed. The results of the scratch tests were analyzed taking into account the stress-strain behavior during depth sensing indentation tests and the energy absorbed during Charpy impact tests. The improvement in scratch resistance due to nitrogen alloying was attributed to the strong hardening effect of nitrogen in solid solution, which does not affect significantly foundry hardening and toughness. A comparison between the scratch resistance and the pitting-erosion resistance, measured in previous work, was made too.

Avaliação da resistência ao desgaste erosivo gerado por cavitação em aços inoxidáveis austeníticos com alto teor de nitrogênio: estudo dos mecanismos de desgaste

Specimens of a UNS S31803 steel were submitted to high temperature gas nitriding and then to vibratory pitting wear tests. Nitrided samples displayed fully austenitic microstructures and 0.9 wt. % nitrogen contents. Prior to pitting tests, sample texture was characterized by electron backscattering diffraction, EBSD. Later on, the samples were tested in a vibratory pit testing equipment using distilled water. Pitting tests were periodically interrupted to evaluate mass loss and to characterize the surface wear by SEM observations. At earlier pit erosion, stages intense and highly heterogeneous plastic deformation inside individual grains was observed. Later on, after the incubation period, mass loss by debris detachment was observed. Initial debris micro fracturing was addressed to low cycle fatigue. Damage started at both sites, inside the grains and grain boundaries. The twin boundaries were the most prone to mass-loss incubation. Grains with (101) planes oriented near parallel to the sample surface displayed higher wear resistance than grains with other textures. This was attributed to lower resolved stresses for plastic deformation inside the grains with (101) || surface texture.