Investigation of Nanomechanical Test Methods On Arctic Steels

Sørås Rogne, Bjørn Rune (Department of Engineering Design and Materials, Norwegian University of Science and Technology) | Thaulow, Christian (Department of Engineering Design and Materials, Norwegian University of Science and Technology)

OnePetro 

ABSTRACT:

In this paper we apply a nanomechanical test approach on a steel developed for Arctic and cold conditions to quantify local material properties. The aim is to obtain quantitative mechanical data to investigate the ductile to brittle transition. The test approach uses a focused ion beam (FIB) as a machining process to prepare micron-sized specimens. After fabrication the specimens are tested with a nanoindentation device. Experimental results show that quantitative data can be obtained at the micron and submicron length scale, but further development are needed at this early stage testing.



INTRODUCTION

Steel undergoes a transition from ductile to brittle behavior as the temperature is decreased. In Arctic conditions an important concern is to obtain sufficient high fracture toughness of welded steel structure at low design temperature as -60 °C. The mechanisms of the brittleness are complex and not fully understood and there is a need to understand the deformation mechanisms in order to avoid failure at low temperatures. Obviously, the steels microstructure governs the properties at macro scale and it is generally believed that existence of hard and brittle particles or phases in a ductile surrounding matrix is one of the expected reasons for the brittleness. Numerous models have been proposed to predict the ductile to brittle transition (DBT), but there is a general lack of quantitative data at the scale where the mechanisms of fracture operate. In this paper a novel test method is investigated on an Arctic steel quality with the aim to determine local properties at the microstructural length scale. Focused ion beam (FIB) is applied to machine micron-sized specimens positioned into various microstructures of the base material. The force and displacement data from the experiment are analyzed to reveal the stress – strain relationship and the fracture toughness.