Thixoforming or Semi-Solid Metal Forming offers many advantages in comparison with
casting and conventional forging. The purpose of the present study is to provide the basic
microstructure and deformation data for austenitic and ferritic stainless steel under mushy state. As
well known, the stainless steels solidify in different modes according to the different chemical
compositions. In this paper, microstructural evolution of austenitic stainless steel type 304 which
solidifies in FA mode ( L → L +δ → L +δ +γ →δ +γ →γ ),austenitic stainless steel type 310S
which solidifies in A mode ( L → L +γ →γ ), and ferritic stainless steel type 430 which solidifies in
F mode ( L → L +δ →δ )are investigated during partial remelting by way of SIMA (Strain Induced
Melted Activation). The results show that A and F mode of stainless steels melt directly at the grain
boundary without phase transformation during reheating. A banded structure, originating from the
primary dendritic segregation of the original ingots, is observed in type 310S steel during further
heating. On the other hand, a perfect globular and insegregative two-phase semi-solid structure L +δ
can be obtained while heated beyond the banded three-phase L +δ +γ semi-solid state in FA mode
austenitic stainless steel type 304. This spheroidization can be attributed to the peritectic reaction
occurred in the L +δ +γ semi-solid state. In addition, simple compression tests of these alloys in
semi-solid state for varied combination of deformation rate and deformation temperature are
conducted to examine the deformation behavior of stainless steel. Flow stress curves exhibit abrupt
change in various alloys, even though in the same alloy such as type 304, various flow stresses are
observed according to the difference in inner microstructure or morphology. Stress of type 310S steel
shows the most reduction as the deformation temperature increasing at the same strain rate condition.
The Liquid is centralized to periphery by the compression force in all deformed test pieces. Fracture,
observed in all alloys except type 304 steel in globular L +δ semi-solid state, should be resulted
from the lack of liquid in L +δ +γ state of type 304 steel and solidification crack in type 310S and
type 430 steel. Deformation of solid particles occurs only in L +δ +γ state of type 304 steel. Last in
this paper, various deformation mechanisms are proposed for various microstructures.
