Brittleness in ferritic Fe-Mn alloys.

The cause of intergranular brittleness in a number of alloys based on Fe-8%Mn has been investigated. The intergranular brittleness in as-water quenched alloys was mainly because of segregation of N to austenite grain boundaries, but some specimens did show the presence of P as well. A desegregation of carbon in the vicinity of the grain boundaries was also observed which may have also contributed to embrittlement. This embrittlement was further enhanced on air cooling, with segregation of N and Mn to prior-austenite grain boundaries. On ice-brine quenching the alloys, the DBTT was lowered. Intergranular fracture was avoided and the brittle fracture mode changed to cleavage. However, preliminary examination by AES indicated a rise in N content on the cleavage facets.Aging quenched alloys at 450&deg;C resulted in a rapid rise in DBTT and further embrittlement, with further segregation of Mn, N and at a later time P, to prior austenite grain boundaries. Reverted austenite was not detected until after 60h at 450&deg;C, and the hardness rose <12HV30 on aging. These factors were thought to have little effect on embrittlement.Analysis of segregation kinetics at 450&deg;C showed that the segregation of Mn and P was consistent with bulk diffusion of Mn and P in a-Fe, while measurement of the kinetics of embrittlement by tensile ductility tests indicated that diffusion of N in a-Fe was the main rate controlling factor in isothermal embrittlement.Above 450&deg;C reverted austenite formed rapidly on aging and was thought to be responsible for de-embrittlement.A thermal cycling treatment was devised to overcome embrittlement. Such treatment caused improvement in impact toughness through refinement of grain size and introduction of Y and e phases into the microstructure. The alloys showed resistance to embrittlement at 450&deg;C and evidence of deformation induced transformation on tensile testing at -78&deg;C.