André Paulo Tschiptschin

Correlations between microstructure and surface properties in a high nitrogen martensitic stainless steel

Nitrided and tempered AISI 410S stainless steel was tested under corrosion–erosion conditions and compared to conventional AISI 420 martensitic stainless steel. The corrosion–erosion resistance of the nitrided specimens was higher than that of the AISI 420 steel when tempered at 200 °C, but it decreased with tempering temperature in the range between 200 and 600 °C. The higher corrosion–erosion resistance of the high-nitrogen steel was credited to a more homogeneous distribution of chromium in martensite and a lower number of coarse second-phase particles, especially for tempering temperatures below 550 °C. The hexagonal -nitride was identified in specimens tempered at 200 °C, while finely distributed cubic CrN nitrides were observed in specimens tempered between 400 and 600 °C. Hexagonal Cr2N nitrides were observed at 550 and 600 °C. These coarse, high-chromium precipitates were responsible for the drop in corrosion resistance of the nitrided specimens.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The effect of testing temperature on corrosion-erosion resistance of martensitic stainless steels

Conventional AISI 420 and high-nitrogen martensitic stainless steels were tested under corrosion–erosion conditions in slurry composed by substitute ocean water and quartz particles. The tests were performed at 0, 25, and 70 ◦ C, with mean impact angles of 20 and 90 ◦ . Polarization tests in H2SO4 solution containing chloride ions were also carried out at the same temperatures. Both conventional and high-nitrogen specimens were tempered at 200 and 450 ◦ C before the tests. The high-nitrogen specimens were produced through gas nitriding of AISI 410S (13%Cr–0.03%C) and AISI 410 (13%Cr–0.15%C) stainless steels at 1100 ◦ C. These treatments allowed obtaining interstitial contents (nitrogen + carbon) at the surface of the specimens equivalent to the carbon content of conventional AISI 420 stainless steel. The best corrosion–erosion resistance was obtained in the nitrided AISI 410S samples tempered at 200 ◦ C and tested at 0 ◦ C under 20 ◦ -impact angle. Increasing testing temperature led to higher mass losses and wear rates due to the intensification of intergranular and pitting corrosion mechanisms, especially in the conventional AISI 420 stainless steels. In tests performed at 0 and 25 ◦ C, a reduction in the wear rate for longer testing times was observed, which was mainly associated to fragmentation and roughness changes of the abrasive particles. The mass losses under normal impact conditions were systematically higher than under oblique incidence, and some evidences of mass removal by brittle fracture were found after SEM examination of the worn surfaces.

Corrosion–erosion of nitrogen bearing martensitic stainless steels in seawater–quartz slurry

AISI 410S stainless steel was nitrided at 1473 K in N2 atmosphere, direct quenched and tempered at temperatures between 473 and 873 K. Martensitic cases with circa 0.52 wt.% N at the surface were obtained. Corrosion–erosion tests were carried out in slurries composed by quartz particles and tap or substitute ocean water. The concentration of solids, the impact angle and the pH of solution were fixed, while the temperature, surface changes and mass losses were monitored during the tests. Quenched and tempered AISI 410 and 420 stainless steels were used as comparison materials. The results showed that the erosion resistance and the corrosion–erosion resistance of the nitrided steel tempered at 473 K were higher than those of the AISI 410 and 420 steels tempered at the same temperature. This behavior was due to the higher hardness and better intergranular, pitting and generalized corrosion resistance of the nitrided alloy. The synergism between corrosion and wear was more important in the AISI 410 and 420 samples.

The particle size effect on abrasive wear of high-chromium white cast iron mill balls

Granite grinding tests, under dry and wet conditions, were performed to assess the influence of abrasive particle size to the wear behavior of martensitic high-chromium white cast iron mill balls. The tests were performed, at first, using raw granite particle sizes between 0.074 and 19.1 mm, and then with coarse and fine granite fractions obtained after screening the raw granite in a 3.36 mm sieve. It is demonstrated that the relative particle/ball size relationship is the determining parameter to ball wear. The highest ball wear rates were observed for fine granite grinding under dry (120 mg/cycle) and wet (129 mg/cycle) conditions. The lowest wear rate (ca. 50 mg/cycle) was observed for coarse granite grinding (dry and wet). These different results were attributed to the different size relationships between grinding body diameter and granite particles size. For wet-grinding of raw granite, the mineral components may influence significantly the wear behavior. Feldspar can act as a bonding agent, gluing fine quartz particles to the coarse granite and to the balls surface and turning the dependence of the relationship between the relative sizes of ball and granite particle less important to the wear process. This explains why wet-grinding of raw granite results in a ball wear two times greater (106 mg/cycle) than dry-grinding (51 mg/cycle).

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.

Structural and Mechanical Characterization of Duplex Multilayer Coatings Deposited onto H13 Tool Steel

Quenched and tempered H13 tool steel was plasma nitrided and Physical Vapour Deposition (PVD) coated in a hybrid reactor aiming to obtain a TiN/TiC multilayer coating deposited on a plasma nitrided substrate, with a more gentle transition of elastic-plastic properties between the outermost layer of the coating and the substrate. Duplex treatment (plasma nitriding and PVD coating) was carried out in a hybrid reactor. Plasma nitriding preceded the DC triode magnetron sputtering PVD process, conducted inside the same chamber, using CH 4 and N 2 as reactive gases. Multilayer TiN/TiC coatings deposited on a nitrided H13 substrate were obtained. The multilayer coating was composed by a first Ti interlayer to grant adhesion, followed by a second 18.4 at% C TiC layer with a cF8 NaCl type unit cell, then a 41.9 at% N TiN layer and finally an outermost 32.3 at% C TiC layer with the same cF8 NaCl type unit cell. The multilayer coating showed a gentle transition of elastic-plastic properties assessed by the H/E * and the H 3 /E *2 ratios and the elastic recovery as a function of the distance from the surface of the specimen. The adhesion of the multilayered coating to the substrate was greater in the case of the duplex coated specimen as compared to the non duplex treated H13 steel.

Predicting Microstructure Development During HighTemperature Nitriding of Martensitic Stainless SteelsUsing Thermodynamic Modeling

Thermodynamic calculations of the Fe-Cr-N System in the region of the Gas Phase Equilibria have been compared with experimental results of maximum nitrogen absorption during nitriding of two Martensitic Stainless Steels (a 6 mm thick sheet of AISI 410S steel and green powder compacts of AISI 434L steel) under N2 atmospheres. The calculations have been performed combining the Fe-Cr-N System description contained in the SGTE Solid Solution Database and the gas phase for the N System contained in the SGTE Substances Database. Results show a rather good agreement for total nitrogen absorption in the steel and nitrogen solubility in austenite in the range of temperatures between 1273 K and 1473 K and in the range of pressures between 0.1 and 0.36 MPa. Calculations show that an appropriate choice of heat treatment parameters can lead to optimal nitrogen absorption in the alloy. It was observed in the calculations that an increased pressure stabilizes CrN at expenses of Cr2N - type nitrides.

A method to estimate magnetic induction from texture in non-oriented electrical steels

Abstract A model to estimate magnetic induction in electrical steel sheets is presented. Considering only M s (magnetization of saturation) and K 1 (first-order magnetocrystalline anisotropy constant), it is possible to estimate the magnetic induction for a applied field H , depending on the texture of the sheet. Data from the model and experimental texture analysis by inverse pole figure method are compared with magnetic induction B 25 and B 50 measurements obtained from Epstein test.

Effect of Cooling Rate During Quenching on the Toughness of High Speed Steels

High speed steels are usually employed in cutting tools and forming dies. Heat treatment is an important step during the tool manufacturing process, being responsible for most of the final properties, mainly hardness and toughness. In this context, an especial interest relies on the effect of hardening variables on mechanical properties and tool performance. The present paper aimed to study the effect of cooling rate during the quenching process of standard designation AISI M2 high speed steel, under typical industrial conditions. The experiments were carried out in an industrial vacuum furnace, with high pressure nitrogen quenching. Several cooling rates were obtained by variation of the nitrogen pressure and by the use of test specimens with different dimensions. Toughness results were mainly evaluated through static bend test. Low cooling rates were shown to decrease material toughness and large parts presented a decrease in mechanical properties from surface to core regions. Carbide precipitation on grain boundaries are pointed as the main explanation for all these effects.