Mikrostruktura in lastnosti visoko temperaturnega območja zvara (HAZ) termo-mehansko obdelanega jekla S700MC z visoko mejo plastičnosti

Abstract

The aim of the study was to determine the properties and microstructure of the high-temperature heat affected zone (HAZ) of S700MC steel heated to a temperature of 1250 °C and cooled at different speeds. The simulation of the thermal cycles was performed using a welding thermal cycles simulator. Samples with a cross-section 10 mm × 10 mm × 55 mm were submitted to metallographic analysis, impact tests, hardness measurements and tensile tests. Welding thermal cycles with cooling times t8/5 = (3, 5, 10, 15, 30, 60 and 120) s and a maximum temperature cycle temperature of Tmax = 1250 °C were used. The welding thermal thermomechanical processing cycles differ significantly, especially with high rates of heating and cooling in the SWC, short time holding at the maximum temperature and frequent overlap of two or more cycles during the multi-layer welding. One of the elements in the evaluation of steel weldability is the analysis of the austenite phase transformation during cooling. Steel hardness tests on simulated HAZ regions cooling times increasing from 3 s to 120 s, showed reductions by approximately 40 HV, while, regardless of the length of the cooling time t8/5, the impact resistance was very low, at the level of a few J/cm. The tensile strength, hardness and toughness indicates a secondary role of austenite in the control of welded joints transformation strength and plastic properties, the analysis of the phase transition not shown to be a reliable basis for assessment of the weldability of this steel group.