Neutron time-of-flight diffractometric investigation of duplex steel residual stresses

Abstract

The main problem with cast austenitic—ferritic duplex stainless steels is the formation of cracks after the solidification. The crack sensitivity is caused by volume differences between the microstructural components and coupled to an extreme embrittlement. Therefore the σ phase precipitation becomes important. It binds Fe, Cr and Mo and degrades the corrosion resistance. In addition, it decreases the toughness of the steel because of its embrittling character and by precipitating at grain boundaries. Both the absolute amount of σ phase and its distribution have an influence. During casting of components, the σ phase precipitation should be kept as low as possible. This is achieved by rapid cooling through the critical temperature range from 1000 to 600°C. Since the main cause of crack formation lies in microstructural stresses and incompatibilities, this process must be better understood. Neutron time-of-flight diffractometry can detect strains in different phases simultaneously from the same volume element of a specimen. Using this method, the microstructural strains as a function of the σ phase volume fraction in the steel have been determined. The results will be reported and discussed.

Key Words:

• Cast superduplex steel
• σ phase
• Refinement of primary microstructure
• Mechanical properties
• Neutron diffraction
• Neutron time-of-flight