Strain Induced Martensitic Transformation at Low Temperatures

Abstract

As already mentioned in the previous chapters, the Fe-Cr-Ni stainless steels are commonly used to manufacture components of superconducting magnets and cryogenic transfer lines since they retain their ductility at low temperatures and are paramagnetic. The nitrogen strengthened stainless steels of series 300 belong to the group of metastable austenitic alloys. Under certain conditions the steels undergo martensitic transformation at cryogenic temperatures that lead to a considerable evolution of material properties and to a ferromagnetic behaviour. The martensitic transformations are induced mainly by plastic strain fields and amplified by high magnetic fields. Spontaneous transformations due to the cooling process — identified with respect to some alloys — are not observed in the most often used grades 304L, 304LN, 316L, 316LN. The stainless steels of series 300 show at room temperature a classical γ-phase of face centred cubic austenite (FCC). This phase may transform either to α′ phase of body centred tetragonal ferrite (BCT) or to a hexagonal ε phase. The most often occurring γ — α′ transformation leads to formation of martensite sites dispersed in the surrounding austenite matrix. In the course of the strain induced transformation the martensite platelets modify the FCC lattice leading to local distortions. The amount of the martensite depends on the chemical composition, temperature, stress state, plastic strains and exposure to magnetic field. It is well known that the solutes like Ni, Mn and N considerably stabilise the γ-phase. For instance the strain induced martensitic content in the grades 304LN, 316LN at low temperatures is much lower than in the grades 304L, 316L for the same level of plastic strain (Morris et al. 1992).