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

primary carbides present in the microstructure . This correlation has been also found
by other authors (15) and have been interpreted in terms of fracture toughness
mainly related to the matrix properties and not to the amount, size and morphology
of the primary carbides . It is clearly apparent that the lower hardness values and
therefore the higher fracture toughness values are obtained for atmosphere sintered
specimens . This is a consequence of the increasing in the amount of retained
austenite produced by atmosphere sintered specimens, due to the release of carbon
by transforming the MC carbides to MX carbonitrides . The actual values of fracture
toughness are higher than those found for conventional wrought high speed steel
and similar to other values reported for vacuum sintered steels (15)
CONCLUSIONS
1 . Sintering high speed steel powders in a gas atmosphere allows to obtain full
density at lower temperatures than in vacuum sintering . The decrease in the
optimum sintering temperature, being higher the higher the amount of vanadium in
the steel .
2 . The "sintering gate" or the amount of oversintering without the formation or a
continuous deletorius layer of eutectic carbide is also elarged by gas atmosphere
sintering .
3 . By sintering in the nitrogen rich atmosphere massive MC carbides are replaced by
fine MX carbonitrides, very resistant to coarsening .
4 . The presence of these fine and very stable MX carbonitrides inhibits the fast grain
coarsening observed in vacuum sintering for oversintering temperatures and the
austenite grain size is fine even for important oversintering when sintering in the gas
atmosphere .
5 . The as-quenched microstructures in gas sintered high speed steels contain higher
amounts of retained austenite than the same steels vacuum sintered . This allows the
obtention, by simple tempering, of a broad variety of mechanical properties by the an
adequate combination of hardness and toughness .
6. The tempering peak hardness occurs in atmosphere sintered steels at higher
temperatures than in vacuum sintered steels . When the excess of carbon equivalent