Stress-strain properties of ferritic steels using automated ball-indentation testing - pile-up effects included

Abstract

The automated Ball-Indentation (Bl) testing is based cm repeated indentation of a spherical indenter at a single location on a metallic sample. The indentation loads and the corresponding depths in an indentation are used to extract the key mechanical flow properties using well established mathematical relationships. The technique is almost nondestructive and an excellent substitute for the conventional tensile test especially when there is very little volume of specimen available for testing. The distortion at the original plane of surface caused by the material displaced by the indentation is referred to as the pile-up or sink-in effect. The pile-up behaviour alters the actual contact area and hence the indentation diameter which is used for calculating tire stress-strain parameters. It is well established that the extent of pile-up is related to the strain hardening coefficient of the material. This paper describes the methodology for deriving the plastic stress-strain properties by automated ball-indentation technique taking into account the usually ignored pile-up effects. Flow properties derived from ball-indentation tests using die proposed methodology were found to be in good agreement with those of tensile test results for various ferritic steels like AISI 1025 carbon steel, 2.25C:r-lMo steel, Mod 9Cr-lMo steel with different work hardening characteristics.