Modelling of Sigma Phase Dissolution by Solution Treatment in Super Duplex Stainless Steel

The modelling of the dissolution behavior of the sigma phase by solution treatment in duplex stainless steel was investigated to clarify the condition of heating temperature and time necessary to finish dissolving the harmful phase into the matrix. To determine that dissolution condition of the sigma phase experimentally, the heat treatment test at various temperatures and durations was conducted after pre-aging to form an amount of sigma phase using the super duplex stainless steels with 25%Cr-6/7%Ni-3/4%Mo-0/2%W-0.3%N. Using the experimental results, the model which considering the effect of initial situation of sigma phase on the condition of solution treatment to achieve the dissolution condition, was suggested. The solution treatment at higher temperature and for longer heating time was necessary to dissolve the retained sigma phase in the initial microstructure consisting of two phases of austenite and sigma without ferrite compared to that with ferrite phase. The critical condition by the experimental results was described as a linear function of the inverse of solution temperature and the logarithm of holding time. The effect of the chemical composition of the steel on that critical condition was reflected in a parameter in that function proposed.