THE EFFECT OF SUB-GRAIN FORMATION AND DEVELOPMENT ON CYCLIC RESPONSE IN ENGINEERING STEELS

摘要

A series of elevated temperature strain-controlled low cycle fatigue (LCF) tests has been performed on two steels, one which cyclic softens (2CrMoNiWV) and one which both hardens and softens during its lifetime (17Cr12Ni2Mo). Multi-specimen tests have been conducted at specific conditions of temperature, strain rate and strain amplitude. In addition to the determination of crack initiation endurance, tests have been taken to, and discontinued at, various life fractions to provide samples for TEM, EBSD and SEM-BSE microstructural examination. The development of sub-grain microstructures ultimately controls the way in which both steels respond to strain-controlled cyclic plastic loading, but in very different ways. In the case of the 2CrMoNiWV steel at 565°C, continuous cyclic softening from the first load reversal is the consequence of progressive dynamic recovery and sub-grain growth. The more complex cyclic hardening/softening response of 17Cr12Ni2Mo steel at room temperature is a result of the development first of vein and channel/wall structures which are subsequently broken down into sub-grains of progressively reducing size by cross-slip.