Understanding Acoustic Emission Testing- Reading 1 Part B-A

The twinning causes a high value of the instantaneous hardening rate (n
value) as the microstructure becomes finer and finer. The resultant twin
boundaries act like grain boundaries and strengthen the steel. TWIP steels
combine extremely high strength with extremely high formability. The n value
increases to a value of 0.4 at an approximate engineering strain of 30% and
then remains constant until a total elongation around 50%. At the same time,
it hardens without breaking and it resists tensile pressures up to 1100 MPa
and it could be stretched to approximately 90 percent of its length without
breaking (Figure 2).
It is means in practice that when forces act on the steel, as in the deep draw
process, some of the austenite first transforms to the first martensite stage,
the hexagonal crystal form. When the steel is put under increasing stress, the
hexagonal lattice switches to the final, body-centered cubic form, similar to
conventional TRIP steel. This means that the steel retains a good part of its
ductility even after deep draw processing.
Austenite → ε martensite → γmartensite
Charlie Chong/ Fion Zhang
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