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Author(s)

Carlos Mario Garzon Ospina

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2004-12-08

Examining board members

Andre Paulo Tschiptschin; Maurizio Ferrante; Helio Goldenstein; Paulo Rangel Rios; André Luiz Vasconcellos da Costa e Silva

• Advisor: Andre Paulo Tschiptschin

Abstract

The phase transformations, as well as the crystallographic texture changes, that take place during high temperature gas nitriding of a dual-phase ferritic-martensitic steel UNS S41008 (13% Cr - 0,07% C), wt-%, and a duplex ferritic-austenitic steel UNS S31803 (22% Cr - 5% Ni - 3,0% Mo - 0,019% C), wt-%, were studied aiming to optimize the nitriding parameters. Steel samples were high temperature gas nitrided in high purity N2+Ar atmospheres under nitrogen partial pressures varying between 0.02 and 0.38 MPa for times between 3 minutes and 48 hours, in the temperature range 1273 - 1473 K. The nitriding treatments were carried out in academic prototype equipment. The microstructure of the specimens and the phase transformations were studied by optical and scanning electron microscopy, dilatometric techniques and by X-ray diffraction. Chemical composition measures at the surface of the samples as well as in the transverse section were carried out through wave-length dispersive spectrometry, energy dispersive spectrometry, optical spectroscopy and inert gas fusion spectroscopy. The microtexture of the samples was determined by electron backscatter diffraction, using a TSL-EBSD instrument interfaced to a scanning electron microscope. Steel samples with a martensitic case and a martensitic-ferritic core (UNS S41008 steel) and with an austenitic case and an austenitic-ferritic core (UNS S31803 steel) were obtained after high temperature gas nitriding treatments. Thecase depths obtained varied between circa 0.2 and 1.5 mm. Steel samples free of precipitates (with nitrogen contents up to circa 1.0 wt-%) as well as samples containing MN and M2N nitrides were obtained. The CALPHAD method was used for assisting the interpretation of chemical characterization and microstructure observations results, by numerical computations of the thermodynamics and the kinetics of high temperature gas nitriding processing. ) Thus, a mathematical model was developed to foresee the microstructure and the chemical composition of nitriding samples as a function of nitriding parameters: temperature, time and nitrogen partial pressure. In the range of the experimental conditions studied, the nitrided case depth was proportional to the square root of the nitriding time and to the logarithm of the N2 partial pressure but a non monotonic relationship between the case depth and the temperature was observed. Intensive austenitic grain growth in the nitrided case, as a consequence of the high temperature used during the nitriding treatments, toke place. Coarse columnar grains in the austenitic cases, with mean diameter circa of 0.1 to 0.2 mm, were formed in long-term treatments of the UNS S31803 duplex steel. As a general rule, at sample\'s surface of both studied steels it was observed occurrence of normal grain growth. However, for nitriding treatments of the UNS S41008 steel carried out at 1473 K occurrence of secondary recrystallisation was observed. Themain texture components of the austenitic cases obtained after nitriding the UNS S31803 duplex steel were and . The f(g) intensity of that components depended mainly on the nitriding time, being only slightly affected by nitrogen partial pressure or temperature variations. At the earliest stages of nitriding treatments a sharp transformation texture was developed, due to austenitic precipitation holding KS orientation relationship with the ferritic matrix. As a consequence of nitriding time increasing, the texture intensity weakens and different intermediate texture components were established. The maximum texture intensity in long-term nitrided samples was between circa 2 and 4 times random. Austenitic cases obtained after nitriding the UNS S31803 steel displayed high proportions of twin boundaries and moderated proportions of low angle boundaries. ) The grain boundary character distribution mainly depended on the nitriding time and on the N2 partial pressure. With nitriding time increasing the fraction of twin boundaries increased, tending up to a limit value (circa 60%), and the low-angle boundaries fraction decreased, almost to disappear. Furthermore, by increasing the N2 partial pressure the fraction of low-angle boundaries increased and a non-monotonic variation on the twin boundaries fraction was induced: the highest fraction was obtained for an intermediate pressure (0.065 MPa). The texture of martensitic cases obtained afternitriding the UNS S41008 steel was formed mainly by two texture fibers: // DN (gamma) and // surface. An intensity peak for orientations near of was observed in the // DN fiber. In addition, in the // surface fiber it was observed a intensity peak for orientations near . As a general rule, the maximum texture intensity of this martensitic cases was low, circa of 5 times random. Moreover, the grain boundary character distribution almost did not varied with variations in the nitriding parameters. The martensitic cases displayed moderated proportions of S-3 (~20%) and low-angle boundaries (~10%). After assessment of the effect of the nitriding parameters (nitriding time, nitrogen partial pressure and temperature) on the microstructure and on the microtexture of nitrided samples, a novel nitriding cycle that allows obtaining nitrided parts with optimized mean grain diameter, nitrogen content at steel\'s surface and microtexture, which aims to produce nitriding steel components that work under wear conditions in corrosive environments. That nitriding cycle consists on cycling the specimen between two different N2 partial pressures: a high-pressure stage and a vacuum one. ) During the high nitrogen pressure stage nitrogen is introduced in the specimen, being followed by a vacuum stage where ferrite grains precipitate at the surface and in a subsurface region of the specimen. The cycling leads to ferrite precipitation andre-dissolution, multiplying the number of austenite nucleation sites. (AU)