Brittle and Ductile Fracture of X80 Arctic Steel

Abstract

This PhD work has focused on the effect of microstructure and changes in microstructure introduced by welding procedure, on the brittle to ductile transition properties of high strength low alloy steels which have been developed for application at low temperatures (Arctic regions). For this purpose, the experimental work was based on the new pipeline steel API X80 that has a low transition temperature. The relationship between the brittle to ductile transition temperature and the microstructure in the coarse grained heat affected zone, CGHAZ, and the intercritically reheated coarse grained heat affected zone, ICCGHAZ was studied by applying Smitweld TCS1405 weld simulation for producing the HAZ microstructures. During the weld thermal cycles, both single and double cycle, the specimens were resistance heated to 1350°C by a rate of 150°C/s. The cooling rate was adjusted to give a cooling time between 800°C and 500°C (Δt8/5) of 15 seconds. The second heating cycle had a peak temperature, Tp2, of 780°C, and the cooling rate was the same as during the first cycle to simulate the reheated intercritical coarse grained HAZ. Fracture toughness was measured at low temperatures down to -90°C by both tensile and CTOD testing. In order to study the slip system activity of this steel, low cycle three point bending fatigue tests were also performed on polished specimens in the same temperature range. Fracture surfaces and changes in microstructure were analyzed by scanning and transmission electron microscopy and by light optical microscopy.

In summary, the present work showed that, at each temperature the ICCGHAZ specimens had lower fracture toughness values than the CGHAZ specimens. Thus, the fracture mechanism is potentially more brittle in the case of ICCGHAZ specimens, and this is probably connected to the microstructure. The microstructure evalutions revealed that the ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries and stringer M-A constituents between the bainite laths. A more detailed investigation of the fracture surfaces by electron microscopy revealed also the existence of M-A constituents at the initiation points of the cleavage cracks. During deformation, the stress concentration is expected to increase due to the presence of the M-A constituents, and thus the level of stress and strain concentration around the M-A constituents become significantly larger than the nominal stress value and then causes initiation of cleavage fracture. Fracture surface analysis of fractured specimens documented that the cracks initiated either from debonded M-A constituents or from the region between two or more closely separated blocky M-A constituents where the transformation induced stress fields overlapped.

In the present work, the initiation of cleavage fracture occurred within the double CTOD distance from the crack tip. This implies that brittle fracture is easily initiated when M-A constituents are located near the fatigue crack tip, and it is also controlled by accumulation of continuum stress fields and local plastic strains.

The slip system analysis revealed that several slip systems are activated with a variety of Schmid factors within one grain, and in some cases the first activated slip system is not necessarily that with the highest Schmid factor. Thus, it can be concluded that the Schmid factor is not the only parameter that determines activation of slip.

Finally, the crystallographic facet analysis by EBSD illustrated that the {100} planes are the most potent cleavage facet planes in both CGHAZ and ICCGHAZ specimens. It was also shown that the {100} and {110} planes in the case of CGHAZ, and the {100} and {211} planes in the case of ICCGHAZ were the most favourable cleavage facet planes at subzero temperatures.