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

The reason for the need of higher tempering temperatures to reach
peak hardness in these steels is the high amount of nitrogen picked-up during their
sintering in the N2-H2-CH4 atmosphere used . As pointed out before, this has as
consequence the transformation of MC carbides into MX carbonitrides, leaving free
some amount of carbon and producing an appreciable increase in the amount of
retained austenite . The elimination of a much higher amount of -retained austenite,
also more stable, requires higher tempering temperatures for conditioning it to be
transformed into martensite in the cooling following tempering . It seems reasonable
to assume that the higher the excess of carbon equivalent -the larger the amount of
retained austenite, as shown in Fig .14- and the higher the tempering temperature
required to transform it . Table V shows the excess in carbon equivalent, defined
previously in Ec .2, the excess in stoichiometric carbon equivalent (SCE)(20),
SCE = %C+12/14%N -[0 .033(%W)+0 .063(%Mo)+0 .176(%V)+0 .060(%Cr)] Ec .3
and the ratio between carbide formers and carbon (20),
R = E atomic % carbide formers/atomic % (C+N) Ec .4
for the steels investigated in the present work and the standard grades . It is clearly
apparent that by sintering in the gas atmosphere important carbon equivalent
excesses and low R values are obtained in comparison with standard grades . The
experimental results found in the present work indicate that for excesses in carbon
equivalent around 0 .7 for Ec. and around 0 .5 for Ec. an increase in the peak
hardness can be obtained by appropriated tempering, but when these excesses are
very_ high,_ ie__for_ T15 steel (ACeq=0 .92 and ASCE = 0 .85) the supersaturation in
carbon is so high that for tempering at high temperatures a coarse (Fe,Cr)7C3 is
precipitated as shown in Fig . 16 and no additional increase in peak hardness is
produced .
Fracture Toughness .
In Fig . 13 has been shown that a clear decrease of toughness occurs
with increasing hardness . The fact that the results for the three steel could be
reasonably fitted to a single straight line, indicates that fracture toughness is only
related to hardness being independent of the steel composition and of the amount of