Nenad Milošević

Uticaj zaštitne atmosfere i brzine dovođenja žice na stabilnost mig/mag postupka zavarivanja

Rezime: Stabilnost procesa zavarivanja ima značajan uticaj na postizanje propisanog nivoa kvaliteta zavarenog spoja. Povećanje stabilnosti utiče na smanjenje mogućnosti pojave nesavršenosti, odnosno grešaka u samom spoju. Stabilnost procesa zavarivanja se ogleda u stabilnosti električnog luka. Promene dužine luka tokom zavarivanja utiču na varijaciju parametara zavarivanja, tačnije na jačinu struje i napon. Na osnovu praćenja parametara zavarivanja u realnom vremenu može se izvršiti statistička analiza dobijenih rezultata. U radu je prikazan uticaj brzine dovođenja žice i vrste zaštitne atmosfere na stabilnost MIG/MAG postupka zavarivanja pri upotrebi metalne punjene elektrodne žice.

Methods for calculating the preheat temperature welding the high strength steels

With preheating of steel the cooling speed of a heat affected zone decreases, which prevents appearance of structure prone to cold cracks. On the other hand, too small cooling speed, in this area, results with growth the metal grains which, also, leads as to the higher prone to cracks. Common used method for calculation of preheating temperature for high strength steels ar Itto - Bessyo method, and method by standard SRPS EN 1011-2. This paper presents the methods for calculating the preheating temperature of micro aloyed steel P460 NL1. Analysis of the results obtained from both methods shows significant differences in the amounts calculated preheat temperature. It was concluded that both methods have limitations that must be taken into account in their application in practice.

Characterization of Tube Repair Weld in Thermal Power Plant Made of a 12%Cr Tempered Martensite Ferritic Steel

The heat resistant tempered martensite ferritic steel X20CrMoV121 (DIN) has been extensively used within the last few decades as a material for boiler tubing systems and pipelines in thermal power plants (TPP). Long-term behavior of this steel is vastly researched and very well known, but main disadvantage is its poor weldability. In situ welding of martensitic steels is always challenging task and is usually quite difficult to implement properly in a short time, during forced outages of TPP. In this paper, characterization and mechanical properties of undermatch welded joint made during partial replacement of boiler outlet superheater (SH) in TPP by austenitic filler material, after 10 years of service are presented. Due to “cold” technique of welding, which does not required post weld heat treatment, this procedure were regular and widely used repair welding technique in two TPP (620 MW) units. In the purpose of comparison, two other type of matching welding joints of the same SH were also characterized: shop welded joint made by electrical resistance flash butt welding, as well as field welded joint made by gas tungsten arc welding during assembling of SH, which were both in service approximately 150,000 h.