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

Metallographic changes during sintering of
grade M high speed steels in industrial
atmosphere and vacuum
I. Urrutibeaskoa, R . Palma, V . Martínez, and J . J. Urcola
The metallographic changes taking place in four M
grade steels, M3/2, Px30 (a Powdrex SA water
atomised powder equivalent to ASP 30), M2, and
M42, during sintering at the optimum temperature
and oversintering in vacuum and in a nitrogen based
industrial atmosphere have been investigated using
SEM and EDS techniques . M6C and MC primary
carbides were observed after vacuum sintering and
MX carbonitrides instead of MC carbides were
found in the specimens sintered in the industrial
atmosphere . Small amounts of a eutectic carbide
rich in Cr and Fe were observed at the optimum
sintering temperature . On oversintering in vacuum
different types of eutectic carbides were observed -
MC, needle shape, and M 6C - but MC type eutectic
carbides were not observed on oversintering the
samples in the industrial atmosphere . PM/0535
©1990 The Institute of Metals . Manuscript received 9 July
1990 ; in final form 9 October 1990 . Dr Urrutibeaskoa,
Dr Palma, and Dr Martínez are in the Escuela Superior de
Ingenieros Industriales, Universidad de Navarra, San
Sebastián, Spain . Dr Urcola is in the Centro de Estudios e
Investigaciones Técnicas de Guipuzcoa, San Sebastián,
Spain .
Work carried out in the past 20 years on the sintering of
high speed steels (HSS) has allowed the development of
appropriate routes for the fabrication of HSS, which are
now well established .' -' Although it is well known that
direct sintering is through a supersolidus mechanism, 8-10
details of the different phases and carbides forming in such
a process are not entirely clear . Several carbides (M 6 C,
M2C, MC, M23C6i and M 3 C) found at room temperature
after sintering are assumed to form in the sintering process
. Two steels, T42'' 11 .12 and M2, '3 sintered in vacuum
have been studied in depth . In the present work, which
forms part of an extended programme studying the
influence of the sintering atmosphere on the microstructure
and the mechanical properties of several directly
sintered HSS, the metallographic changes taking place
when sintering M grade steels in vacuum and in nitrogen
based industrial atmosphere are analysed .
EXPERIMENTAL PROCEDURE
Annealed water atomised powders, with and without the
addition of 0 .2% of elemental carbon in the form of
graphite, were cold compacted uniaxially at a pressure of
500 MPa . This resulted in green densities of 67-70%
theoretical density . The compacts were sintered either in a
flowing industrial atmosphere of composition
90N29H2-1CH4 or under a vacuum better than 5 x
10 - ' Pa . The samples were heated to the sintering temperature
at a mean rate of 50 K min' and soaked for 45 min .
Specimens were then cooled at a rate of 250 K min' . The
sintered samples were sectioned and prepared according
to standard metallographic techniques . Specimens in the
as polished (with a 1 µm diamond) condition were
observed in a 501B Philips scanning electron microscope,
fitted with an EDAX9100 energy dispersive X-ray system
used to analyse the different carbides .
RESULTS
M3/2 steel
Figure 1 shows typical SEM micrographs at the optimum
sintering temperature for M3/2+0 .2%C steel sintered in
the gas atmosphere and in vacuum . (Analyses of the
metallic elements present in the various constituents
observed are given below all micrographs .) The grey MC
carbides, rich in V, observed in the vacuum sintered
specimens are substituted by the small black, square MX
carbonitrides, richer in V than the MC carbides . In both
micrographs the bright, rectangular M6 C carbides, rich in
W and Mo, are similar in appearance with almost the same
chemical composition . On increasing the sintering temperature
eutectic carbides tend to form (Fig . 2) . The following
eutectic carbides types are observed :
(i) eutectic carbide, present in small amounts in all
the samples at the optimum sintering temperature
(type I)
(ii) eutectic carbide type MC, present only in vacuum
sintered specimens, with composition and contrast
close to MC primary carbides
(iii) eutectic carbide type M 6C, similar in composition and
contrast to M6C carbide, with a clear dendritic shape
(iv) needle shaped carbide, found only in vacuum sintered
specimens after gross oversintering .
It is worth emphasising that eutectic carbides type I and
type M 6 C show the same composition whether the specimens
are sintered in vacuum or in an industrial atmosphere
and that MC type and needle type eutectic carbides
are observed only in vacuum sintered specimens and not in
those sintered in the industrial atmosphere . On the other
hand type I eutectic carbides are observed in both materials
at the optimum sintering temperature, MC type
eutectic carbides after only 10 K oversintering in vacuum,
M6C type eutectic carbides after 20 K oversintering in
vacuum and 30 K in the industrial atmosphere, and needle
type after 25-30 K oversintering in vacuum .
Px30 steel
Figure 3 shows two typical SEM micrographs of a Px30
steel sintered at the optimum sintering temperature in
vacuum and in the industrial atmosphere . It is obvious that
the M6C carbides have almost the same appearance and
composition independent of the sintering media used,
vacuum or industrial atmosphere (except in the minor
Powder Metallurgy 1990 Vol . 33 No . 4 305