Structural Transformations and Mechanical Properties of Metastable Austenitic Steel under High Temperature Thermomechanical Treatment

The effect of high-temperature thermomechanical treatment on the structural transformations and mechanical properties of metastable austenitic steel of the AISI 321 type is investigated. The features of the grain and defect microstructure of steel were studied by scanning electron microscopy with electron back-scatter diffraction (SEM EBSD) and transmission electron microscopy (TEM). It is shown that in the initial state after solution treatment the average grain size is 18 mu m. A high (approximate to 50%) fraction of twin boundaries (annealing twins) was found. In the course of hot (with heating up to 1100 degrees C) plastic deformation by rolling to moderate strain (e = 1.6, where e is true strain) the grain structure undergoes fragmentation, which gives rise to grain refining (the average grain size is 8 mu m). Partial recovery and recrystallization also occur. The fraction of low-angle misorientation boundaries increases up to approximate to 46%, and that of twin boundaries decreases to approximate to 25%, compared to the initial state. The yield strength after this treatment reaches up to 477 MPa with elongation-to-failure of 26%. The combination of plastic deformation with heating up to 1100 degrees C (e = 0.8) and subsequent deformation with heating up to 600 degrees C (e = 0.7) reduces the average grain size to 1.4 mu m and forms submicrocrystalline fragments. The fraction of low-angle misorientation boundaries is approximate to 60%, and that of twin boundaries is approximate to 3%. The structural states formed after this treatment provide an increase in the strength properties of steel (yield strength reaches up to 677 MPa) with ductility values of 12%. The mechanisms of plastic deformation and strengthening of metastable austenitic steel under the above high-temperature thermomechanical treatments are discussed.

Automated summary

The effect of high-temperature thermomechanical treatment on the AISI 321 type metastable austenitic steel was investigated. The treatment resulted in grain refinement, increase in strength properties, and changes in grain boundaries. The mechanisms of plastic deformation and strengthening were discussed.