Peculiarities of development of structural heterogeneity in the fusion zone of pearlite steel with austenitic nitrogen-containing weld metal

Abstract

The peculiar features of development of structural heterogeneity at high-temperature heating in the unstable fusion zone of the austenitic weld with the pearlite steel were considered. It was shown that one of the factors influencing the high nickel content in the weld and low heat input of welding on reduction of width of ferrite interlayer is the decrease in overheating and degradation of carbides in the HAZ metal. However, these changes do not prevent the film precipitation of carbides and embrittlement of metal between the fusion zone and the HAZ. The alloying of austenitic weld metal with nitrogen, formation of nitride particles being more stable than the carbides, and refining the grains facilitate the increase in stability of structure, inhibition of carbide reactions and more uniform distribution of carbides in the fusion zone with pearlite steel, which improves the properties of welded joints of dissimilar steels. 14 Ref., 7 Figures. K e y w o r d s : arc welding, welded joints of dissimilar steels, fusion zone, structural heterogeneity, diffusion of carbon, austenitic weld, nickel, nitrogen, decarburized zone, carburized zone, heat affected zoneDuring manufacture and repair of petrochemical, oil refining, power and other technological equipment the welded joints of dissimilar austenitic and pearlitic steels are applied, which operate at the high (above 350 °C) temperatures. To the joints of dissimilar steels the joints of heat-resistant alloyed steels with austenitic weld are also referred produced in case when the postweld heat treatment is impossible. The welding of such steels is carried out with the use of austenitic materials with a high (more than 13 %) nickel content. At the same time, the compliance of nickel content in the weld metal with the level of alloying of the pearlite steel and operating temperature is provided. The non-compliance of this condition results in development of structural heterogeneity in the fusion zone of the pearlite steel and the austenitic weld as a result of redistribution in carbon diffusion. Such a fusion zone is considered to be unstable [1].The characteristic features of structural heterogeneity in the unstable fusion zone is the presence of interlayers, i.e. the carburized one with a high hardness in the transition layer of austenitic weld and the ferrite one with a low hardness in the HAZ of pearlite steel. The diffusion of carbon is facilitated by the gradient of its thermodynamic potential between the austenitic weld and the pearlite steel [1][2][3]. In the austenitic weld it has lower values due to the presence of strong carbide-forming elements such as chromium, molybdenum, vanadium, etc. which bind carbon into carbides. In the pearlite steel a higher value of potential is caused by the presence of unstable carbides and free carbon in the solid solution.The transportation of carbon in the unstable fusion zone occurs according to the mechanism of both atomic and reactive diffusion [2]. During the atomic diffusion free carbon in the solid solution of metal is...

Intelligent Testing and Analysis of Dissimilar Steel Welds for Industrial Throttle Flowmeter

“…Vishniakas [11] found that the failure of austenitic electrode welds was viscous, and individual parts were quasibrittle failures, while the failures of ferritic and pearlitic electrode welds were moderate. Elagin et al [12] found that during long-term high-temperature heating, the formation of nitride particles and grain refnement help to improve the microstructure stability of the weld, inhibit the development of carbide reactions, and reduce the microstructure inhomogeneity of the weld. Terefore, the carbides are distributed more uniformly in the fusion zone with pearlitic steel, thereby reducing the hot brittleness of dissimilar steel welds.…”