Microstructural study of duplex stainless steels obtained by powder injection molding

• The microstructural evolution of sintered PIM duplex stainless steels was studied.
• A destabilization of austenite occurs after sintering at high temperature.
• Electron backscatter diffraction (EBSD) revealed a remaining of 0.5% of austenite.
• Ferrite content was also determined employing a magnetic method.

This experimental work is focused on the study of microstructural evolution during sintering of duplex stainless steels (DSS) obtained by powder injection molding (PIM). Ferritic 430L and austenitic 316L stainless steel powders were previously premixed in a 50/50 volume ratio and afterward they were sintered in low vacuum at different temperatures for 1 h. Microstructural analysis of sintered samples was conducted by means of scanning electron microscopy (SEM) and a compositional analysis of the alloying elements along different phases was performed by energy dispersive analysis of X-rays (EDS). Phase transformations were evaluated by X-ray diffraction (XRD) experiments, and the magnetic phase content was measured with a ferritoscope. The intensity of the main austenite diffraction peak decreases as sintering temperature increases to finally disappear in the sample sintered at 1100 °C. This destabilization of the austenite is probably related to a high Nickel diffusion detected from austenite to ferrite particles. Moreover, electron backscatter diffraction (EBSD) data were collected to quantify microstructural properties. Several EBSD pattern maps were acquired in order to define the amount of austenite phase. Due to the advantages of this technique a 0.5% of austenite could be detected after sintering at 1200 °C. After sintering process, the austenite content in sintered duplex stainless steels obtained through this processing route was lower than expected. Finally, Bain mechanism was proposed as an explanation to this phase transformation takes place. EBSD technique has been proved to be the most suitable to monitor the microstructure of sintered DSS.