Microstructural Features of Low-Alloy Pipeline Steels that Determine Impact Strength of Welded Joint Heat-Affected Zone

Microstructural mechanisms reducing the impact strength values of a coarse grained heat-affected zone are studied for two K60 microalloyed steels. Research is conducted on specimens subjected to simulation of a heat-affected zone. The microstructure and fracture surface are studied using scanning electron microscopy and backscattered electron diffraction. It is shown that titanium nitride inclusions have the greatest influence on impact strength of the heat-affected zone whose cleavage within large bainite packages may cause macro-brittle specimen fracture. The risk of such non-metallic inclusions depends on the their structural features. In particular, presence of aluminum oxide and calcium sulfide blocking contact of the ferritic matrix with nitride leads to an increase in cleavage stress and hinders macroscopic brittle fracture in the early stages of impact bending.