Effect of stress relief at 350 °C and 550 °C on the impact properties of duplex stainless steels

The production of massive duplex stainless steel castings weighting over 2 t, with thicknesses exceeding 5 in. represents a major challenge for the foundry industry. The difficulty in manufacturing such castings lies in the fact that thick sections experiment low cooling rates during the solidification process and during the solution annealing and water quenching heat treatment.As a result, intermetallic phases such as sigma phase (σ), Chi phase (χ), G phase, R phase, and complex carbides may precipitate, causing the material to be extremely brittle [Martins M, Casteletti LC. Effect of heat treatment on the mechanical properties of ASTM A890 grade 6A super duplex stainless steel. J ASTM Int 2005;2(1) [January]. [1]].After solution annealing and water quenching, the steel is, in principle, free of intermetallic precipitates, but will contain residual stresses resulting from rapid cooling on quenching. During and after machining, these stresses may produce dimensional distortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures, taking care to prevent the loss of mechanical properties, mainly impact toughness.The purpose of this study was to investigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact tests under the conditions of solution annealing and water quenching and stress relief at 350 °C for 4 h and at 550 °C for 2 h. Compared to CD4MCu the high nitrogen content of CD4MCuN stainless steel has a more balanced microstructure with similar ferrite and austenite contents, providing it with higher energy-absorbing capacity in impact tests. CD4MCuN fracture surfaces have predominantly fibrous structures typical of high toughness materials, while the CD4MCu steel's fracture surface shows cleavage facets typical of low toughness materials. The stress relief heat treatments reduced the impact toughness of the CD4MCu alloy but did not affect the CD4MCuN alloy.