Welding Research Institute - Industrial Institute SR

Degradation of dissimilar metal welds in the piping systems of power plants is recognized as a significant ageing factor. Hence, close scientific attention is paid to various degradation mechanisms concerned with dissimilar metal welds, as well as to welding metallurgy. However, the process of dissimilar metal weld refurbishment consisting of the weld groove machining and subsequent repair welding is based on obsolete methods. Particularly, the machining stage has to rely on conventional portable machining or, more often, on manual grinding. Neither of these methods is adequate, especially when it comes to the special environment of the nuclear power plants. In this article, a novel design of an automatic orbital milling system intended for machining degraded welds in piping systems is presented. The system incorporates an orbital milling head designed to enable machining within the limited space, combined with a microcontroller-based control system providing full control over the geometry of the weld groove. Thus, the degraded dissimilar metal weld is removed, and simultaneously, the weld groove of desired geometry is prepared rapidly and flexibly, without the need for customized profile cutters, being the main contribution of the proposed design. The system prototype was verified by implementation in the Bohunice V2 nuclear power plant.

This work is focused on the austenite decomposition in Cr-Mn-N austenitic stainless steels. The experimental steels was solution heat treated (1100°C/30 min. followed by water quenching) and then annealed in the temperature range from 650 to 900°C for holding time 5 min. to 100 hours for the study of precipitation. The precipitation behaviour during isothermal aging was investigated using light optical microscopy, transmission electron microscopy, selected area diffraction and energy-dispersive X-ray spectroscopy. The results show that the phase composition of the precipitates depends on the chemical composition of the experimental steel. The critical temperature of precipitation is 825°C with the corresponding time 100 s.

It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.

This paper deals with a case study of a radiation tube failure in a fuel hydrogenation refining furnace. The tubes, made of Cr-Mn creep resisting steel, were attacked by intercrystalline stress corrosion cracking due to the formation of polythionic acid in the piping system during equipment shutdowns over 14 years of service.

Hot dip galvanizing is very effective means of protection against corrosion. Some recommendation concerning the steel quality are generally known and accepted. The process consists of cleaning (pickling or sand blasting) and dipping the structures or pieces into liquid zinc bath. The case study of hot dip galvanized steels is presented. Some recent failures of hot dip galvanized welded structures and hot dip galvanized high strength steel screws are presented. Structures were made of S355 grade steel and MIG/MAG process was applied for welding. Large cracks were observed in the vicinity of welds after hot dip galvanizing process. The presence of both hydrogen and liquid metal embrittlement was identified and associated mainly with higher hardness of HAZ or the quenched and tempered steels. Possible cracking mechanisms are discussed. The influence of chemical composition and production process (welding, heat treatment) was analyzed according to data published in literature. The solutions and recommendations for avoiding the failure in hot dip galvanized structures are proposed.