Intermetallic phase formation in 25Cr-3Mo-4Ni ferritic stainless steel

Abstract

Two ferritic stainless steels, containing nominally (by weight) 25Cr-3Mo-4Ni and either Nb and Al or Nb and Ti as stabilizing elements, have been solution treated and subjected to annealing between 600 and 1000 °C for times ranging between 1 and 600 minutes. Selective etching and light microscopy, X-ray diffraction of extracted residues, and transmission electron microscopy have been used to identify intermetallic phases and to construct temperature-time-precipitation diagrams for sigma, chi, and Laves phase formation. In the Nb-Al stabilized alloy, sigma phase forms prior to the Laves and chi phases but over a somewhat lower temperature range. In the Nb-Ti stabilized alloy, Laves phase forms prior to chi and sigma, and the temperature range of formation of all the phases is about the same. Laves phase forms at ferrite grain boundaries, but its growth is limited by sigma phase envelopment. Once established, sigma allotriomorphs grow with a coarse dendritic morphology. Austenite, some of which transforms to martensite, forms between the dendritic branches of sigma phase.