Structure and mechanical properties of HAZ metal of 10G2FB steel joints after welding and bending cyclic loading

Purpose. The structure and properties of the steel may vary, sometimes substantially, by thermal welding cycles. Further changes in the structure parameters of the metal, and therefore its properties occur already during operation of the structures under the influence of external forces. It was necessary to investigate how it happens in the metal heat affected zone of welded joints 10G2FB steel. Methodology. Researches were performed using treated by thermal cycle of welding samples. Structure and its parameters were studied by optical microscopy, and the parameters of the fine structure: the results of studies using the method of transmission electron microscopy. Testing of the samples carried out on the low power fatigue machine with brand UMM-1. Mechanical testing for a static tension and impact bending was carried out using standard samples: type II according to GOST 6996-66 and type IX according to GOST 9454-78. Findings. It is established that as a result of the effect of the welding thermal cycles, providing the cooling rate from 3 to 21°C/s of the metal heat affected zone in the temperature range 600-500°C its structure changes from grained ferrite-pearlite bainite to fine grained bainite. Thereby static parameters of the strength and impact resilience of the metal grow and its plastic properties almost no changed. Essential changes in the mechanical properties of the metal HAZ of 10G2FB steel take place due to the action on metal by the external bending cyclic loading. As the number of load cycles the structure parameters are reduced by 23...40%, and the dislocation density is increased by nearly 1.5 times. It led to an increase of yield strength of the metal by 20...22% and reducing its resistance to brittle fracture nearly 1.6 times. Originality. Broadens understanding about the influence of welding thermal cycles and external bending cyclic loading on the structure and mechanical properties of the HAZ of welded joints of low-alloy structural steels. Practical value. The research results can be used to develop technologies for the manufacture, hardening and repair of metal constructions of 10G2FB steel.