The Spinodal Decomposition of Ferrite in 2507 Biphasic Stainless Steels: Embrittlement and Possible Toughness Recovery

Abstract

Biphasic stainless steels provide an excellent combination of mechanical and corrosion properties. The occurrence of ferrite spinodal decomposition during processing and heat treatment can induce a dramatic drop in impact energy. In this paper, a forged rod of 480 mm diameter made of 2507 biphasic stainless steel was studied. The spinodal decomposition phenomenon upon aging at 475 °C (748 K) was investigated by macro- and micro-hardness tests on the constituent phases and by Charpy impact tests. Then, the influence of the microstructural constituents and their morphology on the crack path was studied by optical microscopy, noting possible correlations with the impact energy. Successively, the fracture surfaces of selected specimens were analyzed by a scanning electron microscope (SEM). Finally, reversion heat treatments at 550 °C (823 K) and 600 °C (873 K) were investigated to evaluate the possibility of recovering the detrimental effects of the α′ embrittlement. According to the literature, this procedure works well with some biphasic steels, but these steels are characterized by a chromium content lower than that of 2507 grade steel. Regarding the 2507 grade steel studied in this work, complete reversion was obtained by solution-annealing treatment, while reversion heat treatments at both 600 °C (873 K) and 550 °C (823 K) allowed only a partial recovery.

Keywords

• super duplex stainless steels
• 475 °C embrittlement
• transition curve
• crack path
• fractography