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

in Table V the carbonitrides are richer in Vanadium than than the corresponding
carbides. This has also been observed in other high speed steels sintered in
nitrogen atmospheres (17,18) . It is worth emphasising that the MX carbonitrides in
gas sintered specimens present a mean particle size smaller than 1 um, while MC
carbides after vacuum sintering have mean sizes between 3 and 4 µ m, except for
Px30S +0.2%C with a mean size of around 2 pm . The presence of these fine
carbonitrides, rich in vanadium, has been reported by other authors both after direct
sintering in a nitrogen rich atmosphere (17,18) and after nitrogenation and hipping
(27) . The presence of these carbonitrides, which are not dissolved at high
temperatures, can explain the features observed in Figs 5a and 5b . In these figures
is shown the austenite grain size evolution as a function of the deviation from
optimum sintering temperature . For both steels, with a substantial amount of MX
carbonitrides for atmosphere sintering (see Table IV), oversintering has only a minor
effect in grain growth, while in vacuum sintering oversintering produces fast
austenite grain size growth . No appreciable effect has been found for carbon
addition. Using the expression of Zener (28) for the relation between the limiting
grain size (d) and the size (r) and volume fraction (f) of second phase dispersion :
d = 4r/3f
and using the data of Table IV refering to volume fraction and size of dispersed
particles the values, summarized in Table VIII _for the limiting grain size at_ the__
optimum sintering temperature are obtained . As shown in the table, in which also the
experimental values are given, the values proposed by the Zener equation are
around a half the values experimentally observed for Px30 steel ; the same proportion
is also observed for Px30S steel gas sintered, but the experimental values are
around three times bigger than those proposed by the Zener expression for vacuum
sintering . The important increment in the grain size taking place in vacuum sintering
(Fig . 5) is essentially due to a decrease in the volume fraction of M 6C carbides with
oversintering . These effect has also been reported for a T42 steel (17) . The finer
particles and the maintenance of the volume fraction of the carbonitrides with
oversintering in the atmosphere sintered specimens explain the small increase in
grain size with oversintering (see Fig . 5) .