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

etter than 5 Pa during the sintering . In all cases the samples were heated to the
sintering temperature at a mean rate of 50K/min and soaked for 60 min . Specimens
were then cooled at a rate of 250K/min corresponding approximately to air cooling .
Density after sintering was normally evaluated through the Archimedes method (22)
although for some samples this was calculated using the weight and the geometrical
dimensions after grinding the sintered specimen to a cylinder . Specimens of 20 g
were used for carbon analysis using a Strohlein C .S. Universal apparatus . Nitrogen
and oxygen were analyzed in a Leco, DC-136 model .
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 . Specimens in the as-polished (with 1 µm diamond ) and
some in the etched( with a solution composed by 45cc of Nital, 45 cc of Picral and
5cc of HCI) conditions were observed in a 501B Philips Scanning Electron
Microscope fitted with an EDAX 9100 energy dispersive X-ray system, in order to
analyze the composition of the different phases . Micrographs were taken at high
magnification in order to obtain the carbide distribution and volume fraction . In each
case a minimum of 600 particles of each type of carbide (MC and M 6C) and
carbonitride were analysed . These were easily distinguished by atomic number
contrast, the M 6C, which is fundamentaly formed of W and/or Mo, being more
brilliant-than -the-MC with a significant V content and this last brighter than-the MX
even richer in V . The amount of retained austenite was also determined by X-ray
diffraction techniques (23) .
RESULTS
Figs. 1 and 2 summarize the densification results for Px30 and Px3OS
steels respectively with and without the addition of 0 .2% C during the sintering under
the 90% N 2 , 9%H 2 and 1 % CH 4 and vacuum. Table III summarizes the C, N and O
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
4