Peculiarities of martensitic transformations and martensite structure in high nitrogen steels

The microstructure and strengthening resulting from martensitic transformations during cooling or cold working and subsequent tempering in carbon- and nitrogen-containing iron alloys were studied using magnetic measurements, X-ray diffraction and electron microscopy analyses as well as mechanical tests. Effects of alloying with nitrogen, a combination of carbon and nitrogen as well as with other elements have been investigated for steels having various compositions intended for different applications. The M-s-temperatures of the studied alloys were higher than - 196 degrees C, and the body-centered tetragonal martensite was formed on cooling. The tetragonality of nitrogen-containing martensites was estimated from X-ray diffraction data. During cold deformation, gamma -> alpha, gamma -> epsilon -> alpha or gamma -> epsilon transformations occur in dependence on the alloy composition. Carbon promotes the alpha-martensite formation, whereas nitrogen promotes the epsilon-martensite formation. Stress-strain diagrams for the studied steels are presented. Dissolution and precipitation of carbides and nitrides during heating and hot deformation of steels were studied. The thermomechanical treatments affect the martensitic transformations, martensite structure and tempering processes in dependence on steel composition, structure and treatment regime. Alloying of nitrogen-containing steels with vanadium retards the decomposition processes during tempering and aging and makes the dispersion hardening of the austenite and martensite more effective. The constructional strength diagrams for the thermally and thermornechanically treated nitrogen-containing and analogous nitrogen-free steels of the various structure classes and purposes are built. (c) 2007 Elsevier B.V. All rights reserved.