MICROSTRUCTURE OF HAZ METAL OF JOINTS OF HIGH-STRENGTH STRUCTURAL STEEL WELDOX 1300

摘要

Investigation of weldability of high-strength WELDOX 1300 steel with more than 1300 MPa yield limit is conducted under joint Ukrainian-Polish project, in order to assess the prospects for its application in crane construction in Ukraine. The objective of the work consisted in investigation of initial microstructure of WELDOX 1300 steel in as-delivered condition, influence of welding thermal cycle parameters on it, as well as plotting the thermokinetic diagram of austenite decomposition in this steel. This will allow optimization of arc welding modes to ensure high performance of the metal of weld and welded joint as a whole. The work was performed with application of procedures of light metallography, scanning microscopy, simulation of austenite transformation in Gleeble 3800 system, and computational methods of investigation, It was established that the microstructure of high-strength WELDOX 1300 steel in as-delivered condition consists of bainite-martensite mixture with a large number of finely-dispersed (50-100 nm) differently directed acicular precipitates of carbides of niobium NbC, titanium TiC, and iron Fe_3C. A diagram of austenite transformation in this steel was plotted, and characteristic temperatures with new phase formation were determined. It is shown that preheating temperature should not be lower than 150℃, in order to prevent cold cracking in welding WELDOX 1300 steel. Results of this work can be used in development of new welding technologies.