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

DISCUSSION
Densification Kinetics .
The nitrogen analysis results for specimens sintered using the gas
mixture show clearly that this element diffused into the steel . It is worth emphasizing
that the amount of nitrogen absorbed by steel T42 (0 .41-0 .48%), by steel T15 (0 .72-
0.78) and by steel Px30 (0 .71-0 .76%) is higher the higher the vanadium
concentration in the steel (2 .91 % V for T42, 4.7% V for T15 and 3 .32 %V for Px30) .
This effect, which is related with the formation of vanadium carbonitrides, has also
been found in other high speed steels(18) and is the base of a recent patent (19) .
At first approximation - the carbon in MC would be substituted by
nitrogen, giving carbonitrides . The carbon rejected due to the substitution by nitrogen
in MX, would have opportunity to form more M6C (that is for instance the case in
steels T42, T15 and Px30, see Table III) or to be free in the austenite ; in this second
case the amount of retained austenite after cooling would increase (that is for
instance the case in the three steels) . For the sake of relating the amount of retained
austenite with the amount of elements in solution in it, the concept of "excess of
carbon equivalent" used by Hirano and Kawai(20) has been used . This excess would
correspond to the amount of carbon and nitrogen free in the steel after the
precipitation of MX carbonitrides and M6C carbides . A definition of this "equivalent
carbon excess", used by Hirano and Kawai is :
ACeq = C+ 12/14 N-(0.19+0 .017(W+2Mo)+0 .22V) Ec .2
Fig . 14 shows the existence of a good correlation between the amount
of retained austenite and ACeq for steels T42, T15 and Px30 and also for the results
of several high speed steels (18) . This would indicate clearly either a direct effect of
nitrogen in solid solution retaining the austenite or an indirect effect of nitrogen by
substituting carbon in the carbides, transforming these into carbonitrides and
therefore leaving some free carbon to stabilize the austenite . In a recent work (18) it
has been postulated that the amount of carbon freed by the nitrogen during sintering
in the nitrogen rich atmosphere would be,
%C =(12/14)xO.5(%N)= 0.43(%N) Ec .3