Impact and fracture properties of ADI, compared with SAE 4140 steel

摘要

In this work, Charpy impact testing is carried out on notched and unnotched austempered ductile iron (ADI) and SAF 4140 steel at room temperature. Fracture energy and contraction at the tension-strained face of the samples were measured. Testsshowed that a portion of the energy measured by the pendulum is converted into kinetic energy of the specimen fragments immediately after the collapse. This effect is more relevant for the unnotched specimens of the higher strength ADI grades. Slow bendtests are also carried out on samples similar to those used for impact testing. The influence of the stress concentration at the notch is analyzed and related to the material strength.Further tests were carried out, in order to identify the fracture toughness (K{sub}(IC)), using blunt notched aud jatigue precracked specimens. Critical crack size values were also calculated.The influence of load rate on the fracture energy was evaluated. Steel samples were found to be more sensitive to loading rate than ADI.Fracture results were used to calculate a preliminary correlation between impact energy and K{sub}(IC) for ADI This allows the use of simple and inexpensive impact tests to estimate values of fracture mechanics properties usually obtained by applying more expensive and complex tests.Standard Charpy tests indicate that ADI has inferior toughness compared to cast steel, while fracture toughness tests indicate a much less significant difference.The formation of shear lips is the main cause for the significant difference between the impact behavior of ductile iron and cast steel. The shear lips developed by the steel are responsible for a considerable fraction of its absorbed energy.