Quantitative phase analysis of duplex stainless steels as a function of heat treatment

Abstract: During heat treatments the microstructure of metals changes and heat
treatment times, heating rates and cooling rates all affect the final
microstructure of metals. In the duplex stainless steels chromium nitrides
have been seen after quenching from higher temperatures. The precipitates
have negative effects on the material properties and decrease the fracture
toughness at lower temperatures and reduce pitting corrosion resistance. To
study the precipitates a method for quantification is needed and a method
for this will be developed.
The methods used are x-ray diffraction, magnetic balance, scanning
differential calorimetry and electron microscopy with image analysis. Due to
too small volume fraction the β-Cr2N phase could not be detected with
x-rays or in the differential scanning calorimetry. However, with electron
microscopes they could be quantified and this method together with image
analysis gave adequate results.
Samples of SAF3207HD were heat treated with different heat treatment
solution temperatures and cooling rates. Higher solution heat treatment
temperatures resulted in higher fractions of chromium nitrides, but an
increased cooling rate has an even larger effect on the volume fraction.
Phase fractions up to 2.33 vol% were registered. Depending on the solution
heat treatment temperature a minimum cooling rate exists which produces
β-Cr2N precipitates and these rates are strongly dependent in the
solution heat treatment temperature. The nitrides form in three shapes, as
needles, clouds or along grain boundaries. Faster cooling cause clouds to
form while at slower cooling the chromium nitrides are preferably formed in
grain boundaries.
Finally the results are plotted in two diagrams, where the heat treatment
temperature, the cooling rate and the fraction of precipitates are the three
parameters. If two of these three parameters are fixed, the third can be
predicted from these diagrams. These diagrams can be a very useful tool when
working with chromium nitrides.