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

EXPERIMENTAL PROCEDURE
Vacuum annealed, water atomized T42, T15 and Px30 powders were bought from Powdrex
Limited, Tonbridge, U .K. The compositions of the powders provided by the manufacturer are
given in Table I . Additions of 0 .2 weight % of elemental carbon in the form of graphite, 15 µm
mean size, were made in some cases .
TABLE 1.- Chemical Analyses of As-Received Steel Powders . Weight Percent .
C Si Cr a
(PPm)
Compacts of 16 mm diameter were cold compacted uniaxially at a pressure of 500 MPa, the
walls being lubricated . These compacts were sintered either in a flowing atmosphere composed of
90 volume %N2-9%H2-1%CH4 or in vacuum, better than 5 Pa during the sintering . Density after
sintering was normally evaluated through the Archimedes method (5), 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 under an optical microscope in order to measure the austenite grain size, using the mean
linear intercept length technique . A minimum of 600 grains was measured . The amount of retained
austenite was also determined by X-ray diffraction techniques (6) . Quantitative metallography
techniques were used to measure the volume fraction and the distribution of primary carbides
and/or carbonitrides .
RESULTS AND DISCUSSION
Co V Cu Mn Mo Ni p S W
PX30 1 .33 0 .31 4 .28 488 8 .7 3 .32 0 .08 0 .23 5 .14 0 .29 0 .024 0 .015 6 .63
T42 1 .43 0 .28 4 .18 478 9 .44 2 .94 0 .10 0 .21 3 .22 0 .12 0 .024 0 .013 8 .89
T15 1 .64 0 .25 4.37 757 4 .99 4 .70 0 .11 0 .24 0 .56 0 .13 0 .023 0 .019 12 .40
The variation of austenite grain size with the deviation of temperature from the optimum for
sintering is shown in figure 1 for the three steels with and without the addition of free carbon . It is
apparent that for vacuum sintering grain growth results rapidly at temperatures higher than the
optimum (positive deviation), but the grain size increases only very slowly above the optimum
sintering temperature for gas sintered steels. Grain growth is very similar for the three steels when
sintered under the same conditions .
Figure 2(a) shows the microstructure of a specimen vacuum sintered at the optimum
temperature . The presence of the products of decomposition of austenite is clearly seen mainly
surrounding the primary carbides and inside the original grains of austenite . Some retained
austenite is also present . The primary carbides are mainly MC and M6C . The carbides in the
grain boundaries are fundamentaly massive type MC (> 4 gm ) . For atmosphere sintering (figure
2(b)), the incorporation of nitrogen into the steel produces some important modifications to the
microstructure . Firstly a significant increase in retained austenite is observed and secondly the MC
type carbides change to MX carbonitrides of smaller size, as shown in figure 3 .