APPLICATION OF ATOMIC FORCE MICROSCOPY FOR STAINLESS STEEL MICROSTRUCTURE STUDY AT VARIOUS KINDS OF HEAT TREATMENT

Abstract

The paper presents the results of complex researches of 40Cr13 stainless steel. Using the methods of optical, scanning and atomicforce microscopy, micro- and nanostructures have been obtained, as well as has been given the comparison of the received images of structures and phase states of steel in three different states (after annealing, hardening and high-temperature tempering) with the results of electron and optical microscope investigations. The received optical images of a ferrite-pearlite structure of a valuable content of globular carbide with the composite of (Сr, Fe) 23 C 6 , received after annealing, are given in comparison with the research results of scanning and atomic-force microscope investigations. It has been shown that the use of atomic-force and scanning electron microscopy in this work allows to make conclusions on steel microstructure, which coincide not only with the data of optical metallography but also exceed the latter in specification of structural characteristics. The usage of scanning electron microscopy allows establishing that large carbides are situated along the borders of ferrite grains. There are also some number of carbides inside the fine grains of ferrite; the sizes of inclusions have been defined. After hardening there are some structures which consist of macroacicular martencite. The usage of the images of atomic-force microscopy allows receiving the structure with the expressed acicular structure in comparison with scanning electron microscopy with the possibility to build visual 3D-images. The form of undissolved carbides is also globular. The sizes of martensitic lamella have been defined. The steel structure after high-temperature tempering (secondary sorbite) is formed as the result of the cracking of martensite on ferrite-carbide mixture with the formation of carbides of right round shape. The formed single and line carbides contain strong carbide-former – chromium (Сr, Fe) 23 C 6 . It was confirmed by the results of spectrum analysis. Such structure differs from martensite in higher strength. For all the states the authors have given the results of mechanical tests by the scheme of axial extension, as well as have defined the HB hardness.