Sinterizazio-atmosferaren eragina M graduko (ASP 30 ... - Euskara

elemental carbon in the form of graphite, 15 p.m mean size, were in some cases
made .
Compacts of 16 mm diameter were cold compacted uniaxially at a
pressure of 500 MPa, the walls being lubricated . The compacts were sintered either
in a flowing atmosphere composed of 90% volume N2-9%H2-1 %CH4 or in vacuum .
In this second case the vacuum was better than 5 Pa during the sintering . Density
after sintering was normally evaluated through the Archimedes' method although for
some samples this was calculated using the weight and the geometrical dimensions
after grinding the sintered specimen to a cylinder .
Transverse sections were mechanically polished . After etching with 5%
Nital these were observed with an optical _microscope so as to measure the austenite
grain size, using the mean linear intercept length technique . A minimum of 600
grains were measured . The amount of retained austenite was also determined by X-
ray diffraction techniques (17) . Quantitative metallography techniques were used to
measure the volume fraction and the distribution of primary carbides and/or
carbonitrides . For thin foil preparation 3 mm diameter and 200 µm thickness discs
were perforated by ion milling in a Gatan 600 ion thinning unit at 4 kV in order to get
a thin region transparent to the electrons . The thin foils were observed in a Philips
CM12 Scanning-Transmission Electron Microscope fitted with an EDAX PV 9900 X-
ray microanalyser . An accelerating potential of 100 kV was used in the transmission
mode and 40kV in the scanning mode . Fracture toughness on quenched and
tempered specimens were measured using 'short rod" specimens, and the details
are fully described elsewhere (15) .
RESULTS
Figs . 1, 2 and 3 summarize the densification results for T42, T15 and
Px30 respectively with and without the addition of 0 .2% C during the sintering under
the 90% N2, 9%H2 and 1 % CH4 and vacuum . Table II summarizes the C, N and 0
analysis, the densities and the temperatures at which the optimum densification is
reached (ie, the minimum temperature at which a density higher than 99% T .D . is
achieved) for the three steels . It is clearly apparent that the addition of 0 .2%
elemental carbon activates the sintering process and the optimum densification is
reached at lower temperatures, 20K and 30K for T42, 15K and 20K for T15 and 10K
and 30K for Px30 vacuum and gas atmosphere sintering respectively . An important
increase in the amount of N in the gas sintered samples in relation to vacuum