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

AVOIDING GRAIN GROWTH DURING THE SUPERSOLIDUS SINTERING
OF HIGH SPEED STEELS
J .I. SAN MARTIN, S .JAUREGI, LURRUTIBEASKOA, V.MARTÍNEZ, R.PALMA AND
J .J.URCOLA
CENTRO DE ESTUDIOS E INVESTIGACIONES TÉCNICAS DE GUIPÚZCOA (CEIT)
ESCUELA SUPERIOR DE INGENIEROS INDUSTRIALES DE SAN SEBASTIÁN
SAN SEBASTIÁN, (GUIPÚZCOA), BASQUE COUNTRY, SPAIN .
KEY WORDS : High Speed Steel, Grain Size, Supersolidus Sintering,
ABSTRACT
Grain growth during supersolidus sintering of high speed tool steels in vacuum and in an
atmosphere composed of N2-H2-CH4 has been investigated . An increase of temperature above the
optimum for sintering (minimum temperature at which the theoretical density is reached) in
vacuum causes a significant increase in the grain size, due to the dissolution of carbides and the
increased amount of the liquid phase. On the other hand, oversintering in the N2-H2-CH4
atmosphere, although increasing the amount of liquid phase, results in only a small increase in the
grain size . This different behaviour can be explained in terms of the formation, by reaction with the
atmosphere, of very fine vanadium carbonitrides (< 1µm), which are very stable and do not
dissolve even at high temperatures . This effect, responsible for better mechanical properties of
high speed steels oversintered in the N2-H2-CH4 atmosphere, in comparison with vacuum, can be
explained in terms of the Zener drag exerted by the fine and stable carbonitrides .
INTRODUCTION
It is widely known that oversintering, in vacuum, of high speed steels causes an accelerated
austenite grain growth (1), also resulting in the formation of eutectic carbides during cooling (2),
with the subsequent impairment of the mechanical properties (3) . In recent years investigations
carried out at CEIT have shown that this grain growth during oversintering can be avoided when an
atmosphere composed of N2-H2-CH4 is used (4) . In the present work a detailed analysis of the
microstructural changes taking place during sintering in this atmosphere, and their effect on grain
growth during the oversintering, have been carried out .