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Transformation of Surface Oxides during Vacuum Heat Treatment of a Powder Metallurgical Hot Work Tool Steel

Sebastian Brust
Göteborg : Chalmers tekniska högskola, 2013. 94 s. Diploma work - Department of Materials and Manufacturing Technology, Chalmers University of Technology, 2013.
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Characteristics of surface oxide in case of gas atomized Hot Work Tool Steel powder X40CrMoV5-1 in asatomized condition and after heat treatment at different temperatures in vacuum conditions are discussed. Reduction of surface oxides on steel powder prior to consolidation is mandatory to achieve strong inter-particle connections in the final product. The efficient reduction requires knowledge concerning processes of the surface chemistry changes during heat treatment and sintering. Changes in powder surface composition was studied by X-ray Photoelectron Spectroscopy (XPS) for different temperatures after the heat treatment performed in a furnace attached to the XPS system in ultra-high vacuum (UHV). Morphology, chemistry and distribution of oxides on the powder particle surface were additionally analyzed by Scanning Electron Microscopy (SEM) combined with Energy Dispersive X Ray Analysis (EDX). The results show the presence of a heterogeneous oxide layer on the as-received powder surface, formed by a homogeneous iron oxide layer with a thickness of ~7 nm and particulate oxides rich in chromium, manganese, silicon and vanadium. The iron oxide layer is almost completely reduced after heat treatment at 600 °C. However, this leads to growth of Cr-, Mn-, Si- and V-rich oxides and segregation of sulfides on the powder surface. The reduction of chromium oxides starts around 700 °C, but further enrichment in Mn, Si and V is observed. Considerable decrease in the oxygen signal was found after heat treatment at 800 °C, indicating significant reduction in oxide amount. An almost fully oxide-free powder surface was detected after heat treatment at 900 °C. Only some vanadium compounds and sulfides were still observed on the powder particle surface. The amount of sulfides increases with increasing annealing temperature. The results indicate that high temperature vacuum annealing of the X40CrMoV5-1 powder before consolidation seems to be feasible to reduce surface oxides and thus improve the inter-particle bonding between the powder particles during further consolidation.

Nyckelord: Hot Work Tool Steel, Surface Oxides, Powder Metallurgy, Vacuum Annealing, Surface Analysis

Publikationen registrerades 2013-07-04. Den ändrades senast 2013-07-04

CPL ID: 179832

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