Local mechanical properties of intercritically reheated coarse grained heat affected zone in low alloy steel

Abstract

Steels applied in arctic climates are subjected to low temperature. Since they undergo ductile–brittle transition with falling temperature, their fracture toughness must be addressed, particularly after welding. To predict their behaviour requires knowledge on local properties. Thus, the present study concerns nanomechanical testing of typical microstructures present in the intercritically reheated coarse grained heat affected zone of a 490MPa forging. Such microstructures were achieved by weld thermal simulation of samples with 11mm×11mm cross section and 100mm length, using peak temperature of 1350°C in the first cycle and 780°C in the second cycle. Both cycles used cooling time Δt8/5 of 5 or 10s. This caused formation of M–A phases along prior austenite grain boundaries and mixture of bainite/tempered martensite in the bulk. Nanomechanical testing was performed by compression of nanopillars prepared in grain boundary located M–A phases and in the bulk of the grains. The results achieved showed significant that the grain boundary phase possesses much higher strength than the grain bulk. It is also shown that there is large scatter in the stress–strain data, depending on the actual local microstructure being tested.