HOMOGENIZATION HEAT TREATMENT AND MICROSTRUCTURES OF 18%Ni 1900MPa 
MARAGING STEEL 
Orsini, M.B., Falleiros, I.G.S. 
Villares Group Steel Division, S.C. do Sul, Sdo Paulo, Brasil 
ABSTRACT 
The effects of homogenization heat treatment on 18%Ni 1900MPa 
maraging steel have been studied by means of metallography. The 
degree of microsegregation after homogenization has been evaluated 
through electron microprobe analysis. Metallographic results showed 
that austenite reverts in microsegregated regions after normal ageing. 
The measured kinetics of homogenization compared to a fairly simple 
model allowed an assessment of the diffusion coefficient for nickel. 
1. INTRODUCTION 
The 18%Ni-maraging steels, also containing about 9% cobalt, 5% 
molybdenum and 0.6% titanium, in a solubilized condition, consist of 
a tough bec type lath martensite (1,2) Through ageing heat 
treatment, precipitation of intermetallic compounds occurs in the 
martensite, increasing its strength to the same level reached by high 
strength low alloy steels, but with greater toughness (1-3) The 
standard ageing of these materials is carried out at 480°C for 3 
hours (1-4) 
With overageing, a phenomenon occurs which is known as austenite 
reversion. The drop in hardness observed is due more to this 
phenomenon than to the precipitates growth (1-4) In homogeneous 
material, austenite reverts along the martensite laths for very long 
ageing periods, giving a lamellar microstructure with thickness below 
1 um (5-7) 
Maraging steels present interdendritic segregation of nickel, 
molybdenum and titanium (8-10) When the material is microsegregated 
the austenite reverts at enriched regions even during the normal 
ageing, because nickel and molybdenum accelerate the reversion 
kinetics (6) In this case, metallography shows the presence of 
Prakt. Met. Sonderbd. 21 (1990) 
143