336 Prakt. Met. Sonderband 30 (1999)
2. Experimental procedure
2.1. Material and Heat Treatment
As pointed out above, the base material of this study is those employed in the previous studies [4,5],
containing (in mass.%) 22.5Cr-7.8Ni-2.3Mo-3.5Cu-0.1N-0.08C-bal Fe. It was received in the form
of sand cast block produced in the ordinary production process of an industrial foundry. The
microstructure of as-received cast block is illustrated in Fig.1a. It consists of a nearly continuous
austenite network and intragranular austenite particles in residual ferrite matrix, some amounts of
non-metallic inclusions, nitride and secondary carbide particles at 8/y boundaries. In accordance
with a Creg/Nigq ratio of 1.84 this steel solidifies in the primary ferrite with second-phase austenite
(FA) mode with the transformation sequences L—>L+3—>L+3+y—>3+y. The volume fraction of
residual ferrite is 33.3 vol.%.
In order to achieve a faster cooling during solidification and subsequent solid state transformation,
the locally surface melting was performed on individual samples 15x15x10 mm cut from the as-
received block. For surface treatment, a gas tungsten arc (GTA) welding technique was used
employing a power level of 1440 W for duration time of 20 seconds. After stationary remelting the
samples were air cooled to room temperature. Based on given data [10] the cooling rate was
estimated to be in the range of 102 to 10° °C/s. Resolidified samples were then isothermally
annealed at 900, 1050 and 1150°C for times ranging from 2 to 120 minutes and rapidly quenched
into room-temperature water. The transformation kinetics of the -ferrite were investigated in the
central zone of the upper parts of the surface melted layer.
2.2. Microscopy and Characterization
The microstructures were observed using light microscopy (LM), and in addition, some samples
were subjected to scanning electron microscopy (SEM) equipped with an energy dispersive X-ray
spectrometry (EDS). In order to reveal the microstructure, metallographic sections were
electrolytically etched at 5 V in a common multipurpose solution of 100 ml water and 20 g sodium
hydroxide. Etching for 20 seconds colors ferrite blue-brown, c-phase dark, while austenite remains
virtually uncolored. To identify the present phases further, scanning electron microscopy was
performed on a Philips XL-30 DX4i microscope operated at 20 kV. The local concentration of
major alloying elements was determined by quantitative EDS analysis with the aid of a ZAF-
corrected program. The size of an analysis spot was 0.35 pm, while X-ray counts were collected at
each position for 300 seconds.
The microstructural changes that occur in initial as-resolidified material were evaluated
quantitatively by image analysis. The characterization of the evolving microstructure as a function
of annealing time and temperature was accomplished by the measurement of the following
microstructural variables: the volume fraction of phases, Vv, surface area of the interface boundary
in unit volume, Sv, and the ratio of these two parameters, Sy/Vy. Particular attention has been paid
to the delicate assessment of the highly dispersed structure.
3. Results and Discussion
3.1. Unannealed material
Figure 1b is a light micrograph of the steel in the as-resolidified condition. The microstructure,
containing a same duplex (3+y) phases could be observed. Except a smaller and more round non-
metallic inclusions, no precipitates were found, which was confirmed by a careful SEM
examination. In contrast to the observation in a previous study [5], evidence of extremely fine
secondary phase particles within ferrite matrix was not found, presumably due to the existance of a
