Dilatometric Cycles in the Study of Precipitation of Intermetallic Phases in Duplex Stainless Steels

Abstract

Duplex stainless steels (DSS) are used in offshore platforms due to their good properties resulting from the combination of two phases, namely austenite and ferrite. Secondary intermetallic phases, such as alpha prime (α´) and sigma (σ), which are harmful, can be formed at temperatures above 400 °C, or by a casting process. This study investigates the formation of sigma phase by thermal cycles running in a dilatometer (800 °C, 850 °C, 900 °C and 1,000 °C) for 1h, and at 850 °C for 3h and 7h. The optical microscopy of the DSS microstructure subjected to 800, 850 and 900 °C for one hour revealed a small fraction of σ phase with nucleation in the contour of ferrite/austenite phases, and tests by ferritoscope indicated a decrease (~4%) in a magnetic fraction (ferrite). However, samples subjected to dilatometric cycles at 850 °C for 3 hours showed an 18% decrease in the magnetic phase and quantitative metallography revealed a 33% fraction of σ phase precipitated in the ferrite phase. Tests conducted at 850 ºC for 7 hours indicated a high fraction of σ phase precipitated (approximately 50%), starting in the grain boundary, austenite/ferrite, and advancing in ferrite grain.

Effect of Isothermal Holding at 750 °C and 900 °C on Microstructure and Properties of Cast Duplex Stainless Steel Containing 24% Cr-5% Ni-2.5% Mo-2.5% Cu

“…The presence of the σ phase and γ 2 austenite was associated with the eutectoid decomposition of the ferrite into the σ phase and secondary austenite (γ 2 ) [ 36 ]. The examinations that were carried out with a ferritoscope confirmed the drop in ferrite content along with the extension of the holding times at both temperatures ( Figure 4 ); thus, it was shown that extending the holding time from 1 to 3 h was the main factor that reduced the content of the magnetic phase (i.e., ferrite, at 750 °C and 900 °C) and promoted the formation of non-magnetic σ phase [ 37 ]. The obtained results demonstrated that in the manufacture of DSS steel castings, it is necessary to accelerate the casting cooling process in the mould.…”

“…Additionally, the use of deep etching on the cast steel allowed for confirming the presence of the secondary austenite (γ 2 ) with the Widmanstätten morphology in the SEM-EDS studies ( Figure 10 ). The SEM-EDS analysis of the chemical composition of the austenite and secondary austenite (γ 2 ) showed that the secondary austenite contained lower content of Cr and Mo and a slightly higher content of Ni ( Table 3 ) [ 37 ].…”