HEAT-AFFECTED ZONE STRUCTURE AND PHASE COMPOSITION IN STEEL 0.12C-18Cr-10Ni-1Ti-Fe DEPENDING ON DEFORMATION
The paper presents the transmission electron microscopy (TEM) investigations of the structure and phase composition of the heat-affected zone (HAZ) in welded joint obtained by electrode welding. The type 0.12C-18Cr-10Ni-1Ti-Fe steel is used for welding which then subjected to plastic deformation. Quasi-static tensile tests are performed on a test machine Instron-1185. The acoustic emission and acoustic decay are respectively recorded and measured using the acoustic surface wave technique before and after the load application. TEM investigations are carried out within HAZ between the deposited and base metal at a 1 mm distance from the weld joint at 0, 5 and 37% deformation degrees. It is found that at before deformation, g-phase represents four grain types consisting 1) defect structure with dense dislocation networks, 2) mechanical (deformation) microtwins with two developed systems, 3) e-martensite packets, and 4) both micrtotwin and e-martensite packets. The deformation of the weld joint leads to a gradual reconstruction of the crystal lattice within HAZ. Thus, grains comprising only dislocations disappear first, followed by grains with microtwins and e-martensite packets. And only grains with both micrtotwin and e-martensite packets remain. However, g-phase grains appear. They consist of fine a-martensite grains. Thus, it is shown that in the area of the heat-affected zone, the plastic deformation leads to the martensite transformation as follows: g ® e ® a. Along with phase transformations, plastic deformation results in the increase of the internal stresses and the bending-torsion amplitude of the crystal lattice. It is interesting to note that at e = 0, the bending-torsion amplitude is purely plastic, whereas at e = 37% it becomes elastoplastic.