The influence of different factors on appearance of accicular ferrite structure in weld metal

Abstract

We are going to analyze a welded joint (weld metal) as a part that connects the segmentic constructions, because welded joints are critical spots in these constructions. Mechanical characteristics and impact ductility of welded joint depend upon microstructure of weld metal and HAZ. The mutual attitude of numerous researches is that the microstructure of accicular ferrite is the most desirable microstructure in weld metal of low-alloyed steels. The constitution of accicular ferrite microstructure is a complex process and depends upon large number of factors, whose influence is still being tested. Some of those factors are: type, size and number of inclusions, the size of austenitic grain, cooling rate, type of protective atmosphere during welding, added material, heat input, etc. The research is heading towards determination of mixture of protective gas that produces optimal microstructure during welding of two different types of microalloyed steels (with Ti, and Nb). We are also going to analyze the influence of input-heat on microstructure, in order to obtain the optimal characteristics and quality of welded joint on the whole.

U ovom radu biće razmatran zavareni spoj (metal šava) kao deo koji spaja segmente konstrukcije i kod čelika može biti vrlo kritično mesto. Mehaničke osobine i udarna žilavost zavarenog spoja zavise prvenstveno od mikrostrukture metala šava i ZUT. Zajednički stav mnogobrojnih istraživača je da je mikrostruktura acikularnog ferita najpoželjniji oblik mikrostrukture u metalu šava niskolegiranih čelika. Obrazovanje mikrostrukture acikularnog ferita je složena pojava koja zavisi od mnogobrojnih faktora čiji se uticaj još uvek izučava. Neki od tih faktora su: vrsta, veličina i broj uključaka, veličina austenitnog zrna, brzina hlađenja, vrsta zaštitne atmosfere pri zavarivanju, dodatni materijal količina toplote uneta pri zavarivanju itd. Istraživanje u ovom radu je usmereno ka određivanju mešavine zaštitnog gasa pri kojoj će se dobiti optimalna mikrostruktura prilikom zavarivanja dve različite vrste mikrolegiranih čelika (sa Ti i sa Nb). Pri tome se analizira i uticaj vrste uključaka na pojavu strukture acikularnog ferita. Takođe je razmatran i uticaj unete količine toplote na mikrostrukturu, a sve sa ciljem da se dobiju optimalne osobine i kvalitetan zavaren spoj kao celina.