Mechanical behavior and cumulative fatigue damage of vanadium-based microalloyed forging steel.

摘要

The use of microalloyed (MA) steels in the as-forged condition can result in the elimination of the costly quenching and tempering (Q&T) processes. MA steels can therefore serve as an economical alternative to the traditional Q&T steels, if equivalent mechanical properties can be obtained. This study is devoted to a systematic examination of this possibility. The material evaluated is a vanadium-based AISI 1141 microalloyed forging steel in the as-forged and Q&T conditions. It is the most commonly produced MA steel in the U.S.This study consists of two major parts: a fundamental experimental investigation to characterize deformation, fatigue, and fracture of the material and an advanced study on cumulative fatigue damage. The fundamental investigation includes deformation (both monotonic and cyclic), fatigue (both crack initiation and crack propagation), and fracture (both impact and fracture resistance) behaviors of the two MA steels. This is intended not only to provide designers with ready-for-use data for a reliable application of MA steels, but also to evaluate and compare the applicability of these steels under different loading situations. The cumulative damage study explores the fatigue damage accumulation process associated with both steels. Based on the different damage models reviewed and the information from both constant-amplitude and step-cycling fatigue tests, a new fatigue damage model is proposed. This model accounts for the damage caused by fatigue loading by the changes in the four material cyclic fatigue constants in the strain-based fatigue life equation. The effectiveness of this model as well as seven other models for life prediction are evaluated and compared with experimental data. The preliminary experimental study shows the proposed approach to be superior to the other methods and very promising for application to variable amplitude loading situations.To facilitate a comprehensive understanding, thorough reviews on both MA steel and cumulative fatigue damage are also provided in the appendices. First reviewed are metallurgical control and mechanical evaluation of mechanical behavior of MA steels. It is followed by a 70-year historical retrospect on cumulative fatigue damage studies. About 50 damage theories have been reviewed.