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

particle size smaller than 1 µm, while MC carbides after vacuum sintering have mean
sizes between 3 and 4 µm . The presence of these fine carbonitrides, rich in
vanadium, has been reported by other authors both after direct sintering in a
nitrogen rich atmosphere (12,13) and after nitrogenation and hipping (20) . The
presence of these carbonitrides, which are not dissolved at high temperatures,
explain the features observed in Figs 7, 8 and 9 . In these figures is shown the
austenite grain size evolution as a function of the deviation from optimum sintering
temperature . For the three steels, with a substantial amount of MX carbonitrides for
atmosphere sintering (see Table III), oversintering has only a minor effect in grain
growth, while in vacuum sintering, without the presence of these MX particles and
the dissolution of carbides as shown in a recent paper for T42 steel (13),
oversintering produces fast austenite grain size growth . In several paper (12-14) it
has been shown that the expression of Zener (25) for the relation between the
Limiting grain size (d) and the size (r) and volume fraction (f) of second phase
dispersion, can explain reasonably the different behavior in grain coarsening
observed for the steels sintered in vacuum and in the industrial atmosphere . Also the
limiting grain sizes worked out from Zener equation agree reasonably with those
found experimentally .
Tempering Behavior .
The peak in hardness are reached in the three present steels at higher
temperatures when sintered in the gas atmosphere in comparison with vacuum
sintering . For instance, the maxima in hardness in steels T42 and T15 are reached
after triple one hour tempering above -550°C and at-575=C - respectively; these values
are about 25°C above the 525°C, reported by Wright et al (3) for steel T42 sintered
under vacuum and 35°C above the 540'C found in the present work and reported by
Beiss et al (26) for vacuum sintered T15 . A similar effect is found also for Px30 (a M
grade steel), presenting, a peak of hardness at 575°C for gas sintering, 40-50'C
above the value observed for the vacuum sintered steel .
It is worth emphasizing that this increase in the temperature for the
peak hardness has a practical relevance in continuous cutting with considerable heat
generation and the increase in the peak hardness will allow these steel to be used as
substrates for harder coatings .