Influence of the radial spacing between cathodes on the surface composition of iron samples sintered by hollow cathode electric discharge

The present work reports an investigation of the influence of the radial spacing between cathodes on the iron sintering process by hollow cathode electrical discharge, with surface enrichment of the alloying elements Cr and Ni. Pressed cylindrical samples of 9.5 mm diameter and density of 7.0 ± 0.1 g/cm³ were prepared by compaction of Ancorsteel 1000C iron powder. These samples, constituting the central cathode, were positioned concentrically in the interior of an external cathode machined from a tube of stainless steel AISI 310 (containing: 25% Cr, 16% Ni, 1.5% Mn, 1.5% Si, 0.03% C and the remainder Fe). Sintering was done at 1150 °C, for 120 min, utilizing radial spacings between the central and hollow cathodes of 3, 6 and 9 mm and a gas mixture of 80% Ar and 20% H2, with a flow rate of 5 cm³/s at a pressure of 3 Torr. The electric discharge was generated using a pulsed voltage power source, with a period of 200 mus. The radial spacing had only a slight influence on the quantity of atoms of alloying elements deposited and diffused on the surface of the sample. Analysis with a microprobe showed the presence of chrome (up to 4.0%) and nickel (up to 3.0%, in at. %) at the surface of the samples. This surface enrichment can be attributed to the mechanism of sputtering of the metallic atoms present in the external cathode, with the deposition of these elements on the sample surface and consequent diffusion within the sample.Universidade Federal de São Paulo (UNIFESP) Department of Mechanical EngineeringUniversidade Federal de Santa Catarina Department of Mechanical EngineeringUNIFESP, Department of Mechanical EngineeringSciEL

Martensite coarsening in low-temperature plasma carburizing

This work shows microstructural evidences of morphological transformation from as-hardened martensite to C-expanded martensite as a result of the low-temperature plasma carburizing treatment carried out in AISI 420 martensitic stainless steel. Treatments were carried out at different treatment temperatures (350, 400, 450, and 500 \ub0C, for a 12 h fixed time) and times (12, 24, 36, and 48 h, at 400 \ub0C fixed temperature). Confocal laser scanning microscopy and SEM results show martensite coarsening in the treated surfaces regarding the respective substrate bulks, which is related to the surface C-alloying, thus being dependent on the treatment time and temperature. Hardness measurement and XRD results strongly suggest that the martensite coarsening is related to the transformation from lath- to plate-type martensite due to the enhancement of the martensite C content, in accordance with the Zener ordering theory. For samples carburized at relatively low temperatures (namely 350, and 400 \ub0C), as C diffuses in paraequilibrium, strain-induced transformation mechanism would govern the changes in the initial martensite morphology as a result of its C content increase. On the other hand, as the chromium carbide precipitation occurs for samples treated at relatively high temperatures (higher than 450 \ub0C for 12 h), and long times (longer than 36 h at 400 \ub0C), C atoms would leave the supersaturated (expanded) martensite phase solid solution, leading the plate martensite to transform back to lath martensite

Dry sliding behavior (block-on-ring tests) of AISI 420 martensitic stainless steel, surface hardened by low temperature plasma-assisted carburizing

This paper reports on the dry sliding behavior (block-on-ring tests) of Low-Temperature Plasma Carburized (LTPC) AISI 420 martensitic stainless steel, treated under different times (4–16 h) and temperatures (350–500 °C), so as to identify the relationships between microstructure, micro-hardness and tribological behavior. In particular, LTPC was effective in decreasing wear when treatment conditions led to the highest diffusion layer thickness combined with absence of a thick outer layer (i.e. at 450 °C for a treatment time of 12 h). The dominant wear mechanism was mild tribo-oxidation, with the protective oxide tribolayer being partly removed by abrasive debris in samples treated at 500 °C