Strain Hardening of Bainitic and Martensitic Steel in Compression

Abstract

Martensite and bainite formed in steel on heat treatment are very complex structures, resisting quantitative analysis. Often such steels are used at high static and dynamic compressive stress. Thorough analysis of their structure after various types of treatment permits their effective use in manufacturing so as to ensure the required physicomechanical properties of the product. The properties of such materials are determined by the solid-solution structure; the presence of secondary-phase nanoparticles; dislocational substructure; the type and position of different types of boundaries; and internal stress fields. Successful control of structure and phase formation and hence of the mechanical properties of the material depends on a quantitative grasp of strain hardening of steels of different structural classes on active plastic deformation. In the present work, transmission diffractional electron microscopy is used to analyze the strain hardening of 38KhN13MFA steel with martensitic structure and 30Kh2N2MFA steel with bainitic structure in active plastic deformation (compression) by up to 26 and 36%, respectively. The contributions of strain hardening associated with intraphase boundaries, dislocational substructure, carbide phases, atoms of alloying elements, and long-range stress fields are considered. The main contributors to the strain hardening of quenched 38KhN13MFA steel are substructural hardening associated with internal long-range stress fields; and solidsolution strengthening associated with carbon atoms. For normalized 30Kh2N2MFA steel, the strain hardening may again be attributed to internal stress fields, the introduction of carbon atoms in the ferrite lattice, and also structural fragmentation when the strain exceeds 26%. The dislocational substructure and carbide particles make relatively small contributions to the hardening of such steels. The loss of strength of bainitic steel at deformation exceeding 15% is due to activation of deformational microtwinning.