Cold brittleness of industrial purity iron-manganese alloys

Abstract

1. The effect of melt purity on the position of the e-solid solution boundary in the Fe−Mn system involves extension of the range for existence of e-phase. 2. The maximum amount of e-martensite (about 70%) is formed in industrial purity alloy with about 16% Mn (in high purity alloy with 17.5% Mn, about 85–90% of this phase is formed). 3. A reduction in alloy purity leads to an increase in the cold brittleness threshold and a reduction in impact strength for all Fe−Mn-alloys. The most marked increase in cold brittleness threshold occurs for γ-alloys, and a reduction in impact strength for e-alloys. 4. For γ-alloys of industrial purity, the lowest cold brittleness threshold is observed with manganese concentrations corresponding to the boundary of the metastable stability of γ- and e-phases (22–25 against 25–31% Mn for pure alloys). 5. Cold brittleness of γ-alloys of both industrial and high purity is not caused by structural transformations during deformation. 6. The nature of change in impact strength at normal temperature and at −196°C points to a sharp reduction in the deformation capacity of Fe−Mn-alloys of industrial purity with dynamic bending tests compared with pure alloys. 7. The effect of manganese on the cold brittleness threshold of single-phase α-, e-, and γ-alloys of industrial purity is different: an increase in manganese content in α-alloys leads to a reduction in the cold brittleness threshold; ductile fracture of these alloys is transcrystalline, and brittle fracture is mixed (intercrystalline and transcrystalline); in e-alloys increased manganese content also causes a reduction in the cold brittleness treshold; their ductile failure is transcrystalline, and brittle failure is intercrystalline with individual areas of transcrystalline failure; conversely, an increased manganese content in γ-alloys leads to an increased cold brittleness threshold; ductile failure for these alloys is transcrystalline, and brittle failure is intercrystalline. 8. Industrial purity alloys with the optimum manganese content of 22–25% may be recommended for operation at cryogenic temperatures, i.e., down to −150 to −160°C, and high purity alloys with an optimum manganese content of 26–31% are recommended for down to −196°C and below (−253°C).