The investigation of peritectic solidification of high nitrogen stainless steels by in-situ observation

Abstract

High nitrogen stainless steel has wide application prospects in many fields such as aerospace, petrochemical industry, marine engineering, energy, and military. Nitrogen is added as an alloying element to replace the more expensive element, nickel, thereby reducing the cost. Since nitrogen serves as an austenite stabilizer and has a higher nickel equivalent, it can improve the corrosion resistance and other mechanical properties of the alloy, yet it could also cause porosity and cracks. As the most upstream process of material processing, the solidification process largely determines the structure and performance of the product. High temperature confocal microscope can provide in-situ observations of phase transformation in metals and alloys. Thermodynamic calculations and other research articles show that the solidification of high nitrogen stainless steel usually involves multiple phases, δ-ferrite, γ-austenite, and liquid (L). Present research utilizes a unique concentric solidification technique to manually create a solid/liquid interface for studying the coexistence of multi-phase. It was found that during the solidification process of a high nitrogen stainless steel, the γ grows along the L/δ interface (L + δ → γ), and then, the pre-formed γ through peritectic reaction grows into δ and L phase. This two-step solidification mode is a typical peritectic solidification. The intervention of N makes the solidification process of high nitrogen stainless steel extremely complicated, because N is a strong γ stabilizer, and its content dramatically affects the formation of the primary phase. The rarely reported peritectic solidification in high nitrogen stainless steel was observed, and these findings could help improve the continuous casting process of high N alloys.

Keywords

• high nitrogen stainless steel
• in-situ observation
• concentric solidification
• peritectic solidification