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

Referring now to oversintering, it is observed in figure 4 that even for 30°C oversintering in
the gas atmosphere, for steels T42 and Px30, the total amount of primary particles remains
constant. Taking into account also that the size of MX particles is nearly the same and that only a
small increase in size for M6C is produced, a similar or slightly larger grain size is predicted, which
is in agreement with experimental observation for gas sintering . For T42, an oversintering of 60°C
produces a significant dissolution and growth of M6C carbides, but the MX carbonitrides remain
and 3 .8% of 1µm MX carbonitrides predicts a limiting grain size of 35µm, in close agreement with
the 40µm found experimentally . For oversintering in vacuum, for T42, as shown in figure 4 , 10 °C
oversintering decreases the amount of primary carbides to around 6-8% which with a mean value of
1 .5µm for the mean particle size, predicts values of limiting grain size of 26-33 µm, in close
agreement with the 34 gm found experimentally . For 30°C oversintering in vacuum for the same
steel, volume fraction 2-6% of primary (non eutectic) carbides are found, predicting values of 33-
100 µm, the observed values being 50-60 µm . For oversintered Px30, the Zener equation predicts
smaller values than those observed experimentally . This could be due to the fact that, althought less
M6C carbides are dissolved for the same amount of oversintering, as shown in figure 4 , the
carbides observed are more clustered, which explains why they are less effective in preventing
grain growth .
CONCLUSIONS
1 . Oversintering of several high speed steels in a gas atmosphere composed of N2-H2-CH4
results in only small increases in grain sizes in contrast with the significant increases observed for
vacuum sintering .
2 . This difference in behaviour can be explained in terms of the Zener drag exerted by fine
and stable carbonitirides present after gas sintering, which do not dissolve even at high
temperatures .
3. Significant levels of oversintering are therefore allowable, without impairing the
mechanical properties of the sintered components produced in the gas atmosphere, this effect
having important industrial relevance .
REFERENCES
1 . I . Urrutibeaskoa, R . Palma, V . Martínez, J . J . Urcola . Powder Metall, 1990, vol . 33, 4, 305-312 .
2 . H . Fredriksson, M . Hillert, M . Nica . Scand . J . Metall, 1979, 8, 115-122 .
3 . V . Martínez . Ph . D . Thesis, ESTI, San Sebastián, Basque Country, Spain, 1990 .
4 . R. Palma . Ph . D . Thesis, ESII, San Sebastián, Basque Country, Spain, 1990 .
5 . Swedish Standars Institution . Powder Metallurgy MNC Handbook 6E, 1978 .
6 . R . L . Miller. Trans. ASM . 1964, pp 892 .