Microstructure and properties of modern P/M super duplex stainless steels

Abstract

Precipitation of intermetallic secondary phases in duplex stainless steels is a well-known weakness of modern highly alloyed steel grades. Even relatively small amounts of these phases can severely deteriorate the mechanical and corrosion properties, the properties the duplex stainless steels are famous for. The problem can be handled by an appropriate heat treatment. However, finding and following the optimal treatment routine is challenging for modern industry often dealing with voluminous components and a careful separate study is required for each steel grade. Precipitation kinetics of secondary phases was studied in modern conventional duplex and super duplex stainless steels and in similar grades manufactured by power metallurgy (P/M) technology by hot isostatic pressing (HIP). This study is a part of a large research program on modern industrial duplex stainless steels, which has been carried out at the Laboratory of Engineering Materials, Helsinki University of Technology for about a decade. Extended collection of data was obtained on the microstructure, precipitation properties, mechanical and corrosion properties of Duplok 27, a novel grade of super duplex stainless steel designed and manufactured by Metso Oyj by P/M-HIP technology for the needs of the pulp and paper industry. Special attention was paid to the role of copper alloying. Optimal regimes of heat treatments, which take into account the size of the components and the capabilities of the modern industrial heat treatment equipment, were studied. It was found that the regime optimal for the material could hardly be achieved for a component because of the limitations of heat transfer in a massive component. Thermodynamic calculations with the Thermo-Calc software and TCFE3 database for steels and iron-based alloys showed that copper alloying has beneficial effect on the properties of duplex stainless steels. Copper decreases the high-temperature limit of precipitation of the harmful intermetallic sigma and chi phases and reduces the amount of precipitated chi phase. Copper precipitates as particles of FCC phase containing in addition to copper small amounts of some other elements, such as molybdenum and iron. The particles serve as pinning points for dislocations. This results in beneficial effect on tensile mechanical properties, which is, however, quite small due to the large size and the low volume fraction of the particles formed at high temperature during industrial heat treatment. Internal friction method was applied to studying the precipitation kinetics of secondary phases and hydrogen effects in duplex stainless steels for the first time. TTT diagram was supplemented with the region of precipitation of secondary austenite. Diffusion coefficients of hydrogen were determined separately for the ferrite and austenite phases in conventional and P/M super duplex stainless steel.