austenite as white areas depicting the interdendritic regions. a
For these reasons, maraging steels must go through homogenization A
heat treatment. This homogenization not only promotes a better ageing Lo
response but also improves toughness (8-11) Le
2. EXPERIMENTAL PROCEDURE a
An experimental heat of 18%Ni 1900MPa maraging steel was induction en
melted undervacuum with the following composition : C=0.007%;Si=0.03%; en
Mn=0.03%;Ni=18.15%;C0=9.01%;M0=4.97%;T1=0.70%;AR1=0.07%;P=0.007%; I
S=0.003%. A 140x140mm square section ingot of about 47kg was cast,
and hammer forged at 1180°C to a 90x90mm billet. Six specimens were oe
taken from the central region of this billet with relation to its
thickness and length. Only one of these specimens remained in the
as-forged condition and the other five were homogenized at 1197°C for Se
1.5 ,10,25,35 and 50 hours respectively, and air cooled. Under these Se
conditions, the six specimens were examined through optical So
metallography, X-ray diffractometry and electron microprobe. Then, Le
all specimens were aged at 480°C/3h and examined by optical “i
metallography and X-ray diffractometry. we
For metallographic tests the following etchants were used: 10%Nital, Dackets
4%Nital,10gFeCl;+30mLHC1+120mLH,0 and electrolytic 10% Chromic acid. EE
In the as~forged + aged condition the volumetric fraction of Ta
austenite was assessed by means of quantitative metallography in
accordance with the ASTM E562 standard (12) 120 random fields were Howe
counted using the 100-points grid and magnification 200x. The pa
microstructures were observed through a Zeiss ICM 405 optical So
microscope. RE
X-ray diffractometry was done to determine the austenite content by Eh
using a Rigaku-Strainflex MSF2M parallel beam X-ray diffractometer. as sh
The integrated intensities of the (220)y and (211)a reflections were Pl
used for the calculations of the austenite content. EN
The microsegregation degree of nickel,molybdenum and titanium was ” Ihr
assessed semi-quantitatively using a Jeol Superprobe 733 electron :
microprobe WDS. For each element, 100 X-ray intensities were
recorded along the lines perpendicular to the microsegregation
Prakt. Met. Sonderbd. 21 (1990)
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