The Paton Welding Journal, 2013, №09



2013 №09 (01)

2013 №09 (03)

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The Paton Welding Journal, 2013, #9, 9-17 pages

INFLUENCE OF CHEMICAL COMPOSITION OF MICROALLOYED STEEL AND COOLING RATE OF HAZ METAL OF PIPE WELDED JOINTS ON ITS STRUCTURE AND IMPACT TOUGHNESS

A.A. RYBAKOV, T.N. FILIPCHUK, V.A. KOSTIN and V.V. ZHUKOV

E.O. Paton Electric Welding Institute, NASU. 11 Bozhenko Str., 03680, Kiev, Ukraine. E-mail: office@paton.kiev.ua


Abstract
One of the most difficult tasks in manufacture of the gas-and-oil line pipes, from the point of view of technology of their welding, is fulfillment of the requirements of normative indices on impact toughness of HAZ metal of welded joint. The aim of present work is an investigation of effect of chemical composition of microalloyed steel and cooling rate of HAZ metal of pipe welded joints on its structure-and-phase state and toughness characteristics. Specimens of Kh70 grade steel of different chemical composition varying, mainly, in carbon content and different rate cooling conditions simulating HAZ metal of the pipe welded joints were studied using modern Gleeble-3800 complex. It is determined that a structure of bainite type, i.e. lamellar ferrite with strengthening second phase martensite-austenite-carbide - MAC or carbide phase), is mainly formed in the metal of studied chemical composition in sufficiently wide interval of the cooling rates. Density of distribution of the phase, its location (orientation), dimensions and morphology are determined, in preference, by chemical composition, and, to lesser degree, by metal cooling rate of the Vcool.8/5. In this connection, the possibilities of metallurgical factor influence should be use to greater extent in order to increase HAZ metal toughness. Strict limitation of weight fractions of the elements promoting formation of coarse ferrite packages with ordered carbide phase of lamellar morphology (for example carbon, niobium, molybdenum etc.) is also reasonable for formation of optimum structure and, respectively, improvement of HAZ metal toughness together with reduction of content of the detrimental impurities in steel (sulfur, phosphor and nitrogen) to minimum possible level. The results of investigations were used in production of pipes from low-alloyed steel of Kh70 grade for main gas-and-oil pipelines. 10 Ref., 3 Tables, 6 Figures.


Keywords: microalloyed steel, welded joint, heat-affected zone, cooling rate, microstructure, impact toughness


Received:                26.06.13
Published:               28.09.13


References
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